I conceed. You are spot on here. I enjoy making things more complicated! But I like my bike simpler. I believe having the best rotors and...
I conceed. You are spot on here. I enjoy making things more complicated! But I like my bike simpler. I believe having the best rotors and pads with the least unsprung, rotating weight is a worthy balance to strive for. Mostly everyone likes sensitive suspension and any unsprung weight reduction will help that. Some of these thicker rotors are brutally heavy on top of the additional spacers and bolts required to mount them. Older calipers also might not be compatible. I am also mullet, so my 180 rotor has more leverage on my little wheel than a 29er so I effectively have a "larger" rotor on the rear relative wheel size. I am certainly being a contrarian here, and it is a much simpler (and possibly cheaper) solution to simply click 'buy' on larger rotors and be done with it for people, especially those that are riding 99% with gravity rather than against it or competing against a clock. "Enduro racing still includes pedalling in places on many courses much to everyone's dismay.
You got me thinking about moto in terms of scraping area. Scraping area doesn't necessarily equal power but it does aid consistency and modulation. (measurements are rough and not accounting for radiused edges on the pads)
2020 CRF450 front pad surface area: 1809sq.mm per pad, 3618 for the pair
Rear pad SA: 2220 (pair)
Shimano 2-pot: 400sq.mm per pad, 800 pair
Formula Cura 4: 552sq.mm per pad, 1104 total
Assuming the average system weight for a moto is 240lbs bike, and 160lbs rider (400 total), and 35lbs bike, 160 rider (195 total) for mtb we get the following:
CRF450 front brake: 9.045sq.mm of scraping surface per pound of system weight.
CRF450 rear brake: 5.55sq.mm of scraping surface per pound of system weight
Shimano 2-pot: 4.1sq.mm per pound of system weight
Cura 4 (similar pad size to code): 5.66sq.mm per pound.
With MTB, rider weight more heavily skews this ratio so that's a big deal. These numbers would somewhat explain the perceived consistency of moto brakes, however, a quick scan of moto forums reveals alot of bitching about their brakes as well. My experience with moto (to a lesser extent) and sportbikes, is that the braking is very predictable in all situations.
Buuuut, to increase scraping area we need to increase caliper piston size which means we also need to increase master cylinder piston size as well as the reservoir size and then our teensy-weensy brake lines (a size standard established by cable brake housing sometime during the caveman days) are the choking point for this increased fluid flow requirment so our hoses need to be larger but then the stupid internal routing ports in all our frames are too small so we all need to buy new bikes...dammit!
Fun stuff.
It's been a long time since I rode a moto...unfortunately.
I have been watching quite a bit of hard enduro and those guys simply lock the rear wheel and skid down anything remotely steep. Found that interesting, and wondered if part of it was not having as fine dexterity using a foot you're also standing on to modulate it.
Added point, the same sized rotor is less effective on a 29 inch wheel compared to smaller wheels. Look at the rotor size in relative terms...
Added point, the same sized rotor is less effective on a 29 inch wheel compared to smaller wheels. Look at the rotor size in relative terms to the wheel diameter instead of absolute sizes of 180, 200, etc. mm.
@jeff.brines considering you're also a moto guy, how is braking done on a moto? One finger or two finger? What are pad clearances like on motor bikes? Etc.
Sorry dude - missed this. David answered it already and is dead on. I should note a few small things to clear up a tiny bit of confusion.
1) When I'm saying "moto brakes" I'm really referring to Brembo or Magura. Brembo in dirt bike world is kind of like Codes in that they are on majority of the bikes people ride off road.
2) I HAMMER my moto brakes when I'm riding trail. Sometimes there are descents as part of a race or similar that are literally 3K+ of super gnarly steep craziness and I'm an hour or more into the race. In these situations I'm so thankful I'm on a moto not a MTB lol. Being able to reliably find the edge of traction without fatiguing me any further on a 235 pound machine is pretty nice.
3) To the above posters point, you aren't wrong with respect to your ability to really modulate the brake with your foot, but its not like you have zero touch. That said, when I am racing hard enduro and its mega steep I do just lock up the back and hope for the best. ITS TERRIFYING.
I'm not suggesting we take dirt bike brakes and put them on a mtb. But I do think we can built a bigger brake and I'd be a lot happier. I'm talking something that is significantly larger with more fluid. A true gravity brake - not a slightly larger XC brake. Just an idea.
Added point, the same sized rotor is less effective on a 29 inch wheel compared to smaller wheels. Look at the rotor size in relative terms...
Added point, the same sized rotor is less effective on a 29 inch wheel compared to smaller wheels. Look at the rotor size in relative terms to the wheel diameter instead of absolute sizes of 180, 200, etc. mm.
@jeff.brines considering you're also a moto guy, how is braking done on a moto? One finger or two finger? What are pad clearances like on motor bikes? Etc.
Sorry dude - missed this. David answered it already and is dead on. I should note a few small things to clear up a tiny bit...
Sorry dude - missed this. David answered it already and is dead on. I should note a few small things to clear up a tiny bit of confusion.
1) When I'm saying "moto brakes" I'm really referring to Brembo or Magura. Brembo in dirt bike world is kind of like Codes in that they are on majority of the bikes people ride off road.
2) I HAMMER my moto brakes when I'm riding trail. Sometimes there are descents as part of a race or similar that are literally 3K+ of super gnarly steep craziness and I'm an hour or more into the race. In these situations I'm so thankful I'm on a moto not a MTB lol. Being able to reliably find the edge of traction without fatiguing me any further on a 235 pound machine is pretty nice.
3) To the above posters point, you aren't wrong with respect to your ability to really modulate the brake with your foot, but its not like you have zero touch. That said, when I am racing hard enduro and its mega steep I do just lock up the back and hope for the best. ITS TERRIFYING.
I'm not suggesting we take dirt bike brakes and put them on a mtb. But I do think we can built a bigger brake and I'd be a lot happier. I'm talking something that is significantly larger with more fluid. A true gravity brake - not a slightly larger XC brake. Just an idea.
Do you really think power is the limit factor on brakes though? All the top of the line brakes have more than enough (Code's, Magura, Hayes, Formula etc) when paired with a 200mm+ rotor to lock a wheel with 1 finger.
I ride Hayes brakes and I would never struggle to lock a wheel on almost any terrain if i wanted to, I find braking is more grip limited (i.e tyre grip) than brake limited.
Do you really think power is the limit factor on brakes though? All the top of the line brakes have more than enough (Code's, Magura, Hayes...
Do you really think power is the limit factor on brakes though? All the top of the line brakes have more than enough (Code's, Magura, Hayes, Formula etc) when paired with a 200mm+ rotor to lock a wheel with 1 finger.
I ride Hayes brakes and I would never struggle to lock a wheel on almost any terrain if i wanted to, I find braking is more grip limited (i.e tyre grip) than brake limited.
Power on a 100 foot descent? No. Power when my brakes are wicked hot after trying to slow down my 215 pounds of rumbling bumbling stupidness down a mountain? Yes.
You are right, brakes by and large have plenty of up front power. What they lack is power when they get really hot. This is the big variable that changes rider to rider, location to location.
I'm heavier, not scared to try and go fast (which is worst than truly being fast) and can find descents north of 2K vert. While the brakes you just listed are all good, I have to pull the lever significantly harder at the bottom of a descent than at the top.
I think we can all agree Dak is on a different level than anyone in this forum and he has also mentioned how much he'd love more braking power citing something somewhat similar.
Its not how good the brakes can work. Its how good they work at the bottom of a huge descent with a fatigued rider who has less hand strength after a day of riding.
If we're comparing MTB and moto... How large are moto wheels (outer tyre diameter)? About the diameter of a 24-inch bike? What are rotor sizes? I'm not up to speed on moto stuff, but I'd guess the rear rotor is roughly 200-ish mm in diameter? So a motocross bike has a larger brake rotor, which gives it more power. Then the added benefit is that the rotor is MUCH larger in relative terms to the outer tyre diameter again. So a double whammy.
Also, pushing more fluid I doubt will give a reasonable benefit as it's a closed loop system (once squeezed shut) and the oil is not flowing. We're not talking about dampers, where flowing more oil will improve thermal management.
TL;DR: I doubt using moto brakes (lever and caliper) on a mountain bike will give a meaningful improvement over the current state.
If we're comparing MTB and moto... How large are moto wheels (outer tyre diameter)? About the diameter of a 24-inch bike? What are rotor sizes? I'm...
If we're comparing MTB and moto... How large are moto wheels (outer tyre diameter)? About the diameter of a 24-inch bike? What are rotor sizes? I'm not up to speed on moto stuff, but I'd guess the rear rotor is roughly 200-ish mm in diameter? So a motocross bike has a larger brake rotor, which gives it more power. Then the added benefit is that the rotor is MUCH larger in relative terms to the outer tyre diameter again. So a double whammy.
Also, pushing more fluid I doubt will give a reasonable benefit as it's a closed loop system (once squeezed shut) and the oil is not flowing. We're not talking about dampers, where flowing more oil will improve thermal management.
TL;DR: I doubt using moto brakes (lever and caliper) on a mountain bike will give a meaningful improvement over the current state.
Front moto wheel has an outside diameter of 28in which is the same as mtb 23" wheels (also known as "27.5") with a 2.5 inch tyre. The rear wheel is slightly smaller at 27in or less. 260mm front rotor and 240mm rear rotors.
If we're comparing MTB and moto... How large are moto wheels (outer tyre diameter)? About the diameter of a 24-inch bike? What are rotor sizes? I'm...
If we're comparing MTB and moto... How large are moto wheels (outer tyre diameter)? About the diameter of a 24-inch bike? What are rotor sizes? I'm not up to speed on moto stuff, but I'd guess the rear rotor is roughly 200-ish mm in diameter? So a motocross bike has a larger brake rotor, which gives it more power. Then the added benefit is that the rotor is MUCH larger in relative terms to the outer tyre diameter again. So a double whammy.
Also, pushing more fluid I doubt will give a reasonable benefit as it's a closed loop system (once squeezed shut) and the oil is not flowing. We're not talking about dampers, where flowing more oil will improve thermal management.
TL;DR: I doubt using moto brakes (lever and caliper) on a mountain bike will give a meaningful improvement over the current state.
On any of my dirt bikes one of the first things I do is a Braking 270mm rotor and bracket on because they look trick lol, stock are usually 240-250 depending on the manufacturer, 260mm if you got a husky/ktm/gasgas with Brembo's (don't need to upgrade those)
If we're comparing MTB and moto... How large are moto wheels (outer tyre diameter)? About the diameter of a 24-inch bike? What are rotor sizes? I'm...
If we're comparing MTB and moto... How large are moto wheels (outer tyre diameter)? About the diameter of a 24-inch bike? What are rotor sizes? I'm not up to speed on moto stuff, but I'd guess the rear rotor is roughly 200-ish mm in diameter? So a motocross bike has a larger brake rotor, which gives it more power. Then the added benefit is that the rotor is MUCH larger in relative terms to the outer tyre diameter again. So a double whammy.
Also, pushing more fluid I doubt will give a reasonable benefit as it's a closed loop system (once squeezed shut) and the oil is not flowing. We're not talking about dampers, where flowing more oil will improve thermal management.
TL;DR: I doubt using moto brakes (lever and caliper) on a mountain bike will give a meaningful improvement over the current state.
Just so I'm again clear here, I am not advocating for Monster Garaging a dirt bike setup onto a mountain bike. What I am suggesting is the relative performance of a dirt bike braking system to the size of the machine/rider/etc is significantly better than a mountain bike.
Worth pointing out here, I do not feel this is a hard problem to solve from an engineering perspective. Its a cultural problem. One where too many people care about 1/2 to 1 pound in the form of stopping performance. Every company trying to build a top tier brake is trying their damnest to keep weight down because they know a big part of the market only rides their bike "on the internet" (in a performance vacuum).
If we get to the real world of it all and accept a weight penalty, I guarantee any brake company out there could build what I'm asking for .
On the other hand, I weigh 80ish kilos naked, so just over 85 kg kitted (180 to 200 freedom units?) and haven't noticed I need more brake over what I get with Code RSCs on old centerline or new HS2 rotors in 200 200 mm on a 29er. This includes everything from short, 300 m vertical descents to 15 minute 1000 m vertical top to bottom runs (where a friend of mine on an identical bike setup other than the rear rotor overheated a 180 mm rear rotor and immediately upgraded to a 200 mm rotor), two Megavalanches, multiple multi day enduro races, etc. And I'm a notorious brake dragger.
TL;DR: how much of an impact on braking performance is rider weight dependant? If a lot, how much of the market actually requires a substantially larger amount of braking performance compared to what we have now on the market?
Also, @jeff.brines , why don't you try out Trickstuff brakes (whichever is the most powerful)? Or as a cheap alternative the Chinese knock off in the form of the Lewis? Everybody that has tried them that I know raves about the power.
Do you really think power is the limit factor on brakes though? All the top of the line brakes have more than enough (Code's, Magura, Hayes...
Do you really think power is the limit factor on brakes though? All the top of the line brakes have more than enough (Code's, Magura, Hayes, Formula etc) when paired with a 200mm+ rotor to lock a wheel with 1 finger.
I ride Hayes brakes and I would never struggle to lock a wheel on almost any terrain if i wanted to, I find braking is more grip limited (i.e tyre grip) than brake limited.
Power on a 100 foot descent? No. Power when my brakes are wicked hot after trying to slow down my 215 pounds of rumbling bumbling stupidness...
Power on a 100 foot descent? No. Power when my brakes are wicked hot after trying to slow down my 215 pounds of rumbling bumbling stupidness down a mountain? Yes.
You are right, brakes by and large have plenty of up front power. What they lack is power when they get really hot. This is the big variable that changes rider to rider, location to location.
I'm heavier, not scared to try and go fast (which is worst than truly being fast) and can find descents north of 2K vert. While the brakes you just listed are all good, I have to pull the lever significantly harder at the bottom of a descent than at the top.
I think we can all agree Dak is on a different level than anyone in this forum and he has also mentioned how much he'd love more braking power citing something somewhat similar.
Its not how good the brakes can work. Its how good they work at the bottom of a huge descent with a fatigued rider who has less hand strength after a day of riding.
Yeah that's totally fair.
I ride at Maydena quite a bit (820m top to bottom) and the brakes are cooked after doing long runs, but i guess i'm just used to losing braking force by the bottom of the runs by now? Not sure if I notice it that much, over being absolutely knackered from 400m of chunder.
How heavy is a Motocross brake? Talking to a superbike/motoGP enthusiant/mechanic he says their brake drag is about 1HP the brakes are set so close to the rotor and to maintain temprature in the carbon disk, obviously those types of braking systems are not even in the same universe as MTB
I'd say any catastrophic failure of a solid stainless steel braking track will involve a component of bending, so it will jam the caliper. Even if not, it will likely occur during braking, so squeezing in the brake pads will cause the remaining rotor to suddenly stop once it hits the closed up pads.
I'd say wearing through the stainless layer on an ice tech rotor is a catastrophic failure and I have seen done that during riding with no major consequences other than very diminished braking performance. But not enough for the guy to not finish the multi day stage race :D
I'd say any catastrophic failure of a solid stainless steel braking track will involve a component of bending, so it will jam the caliper. Even if...
I'd say any catastrophic failure of a solid stainless steel braking track will involve a component of bending, so it will jam the caliper. Even if not, it will likely occur during braking, so squeezing in the brake pads will cause the remaining rotor to suddenly stop once it hits the closed up pads.
I'd say wearing through the stainless layer on an ice tech rotor is a catastrophic failure and I have seen done that during riding with no major consequences other than very diminished braking performance. But not enough for the guy to not finish the multi day stage race :D
I've worn through my ice tech rotors in patches and they still felt as rigid as new, only info I could find at the time was to change when they start to wear through. I think the only way you're going to bend a rotor is a rock strike. 🤷🏻
Added point, the same sized rotor is less effective on a 29 inch wheel compared to smaller wheels. Look at the rotor size in relative terms...
Added point, the same sized rotor is less effective on a 29 inch wheel compared to smaller wheels. Look at the rotor size in relative terms to the wheel diameter instead of absolute sizes of 180, 200, etc. mm.
@jeff.brines considering you're also a moto guy, how is braking done on a moto? One finger or two finger? What are pad clearances like on motor bikes? Etc.
Sorry dude - missed this. David answered it already and is dead on. I should note a few small things to clear up a tiny bit...
Sorry dude - missed this. David answered it already and is dead on. I should note a few small things to clear up a tiny bit of confusion.
1) When I'm saying "moto brakes" I'm really referring to Brembo or Magura. Brembo in dirt bike world is kind of like Codes in that they are on majority of the bikes people ride off road.
2) I HAMMER my moto brakes when I'm riding trail. Sometimes there are descents as part of a race or similar that are literally 3K+ of super gnarly steep craziness and I'm an hour or more into the race. In these situations I'm so thankful I'm on a moto not a MTB lol. Being able to reliably find the edge of traction without fatiguing me any further on a 235 pound machine is pretty nice.
3) To the above posters point, you aren't wrong with respect to your ability to really modulate the brake with your foot, but its not like you have zero touch. That said, when I am racing hard enduro and its mega steep I do just lock up the back and hope for the best. ITS TERRIFYING.
I'm not suggesting we take dirt bike brakes and put them on a mtb. But I do think we can built a bigger brake and I'd be a lot happier. I'm talking something that is significantly larger with more fluid. A true gravity brake - not a slightly larger XC brake. Just an idea.
3) To the above posters point, you aren't wrong with respect to your ability to really modulate the brake with your foot, but its not like you have zero touch. That said, when I am racing hard enduro and its mega steep I do just lock up the back and hope for the best. ITS TERRIFYING.
You have never ridden with a Rekluse Auto Clutch which allows a left hand rear brake. No need to stomp on the rear brake coming down hills. You can feather at the edge of traction and hang off the back as needed. All as the bike idles in a sort of coast mode. It is a revelation on a Moto. But that is another forum.
Do you really think power is the limit factor on brakes though? All the top of the line brakes have more than enough (Code's, Magura, Hayes...
Do you really think power is the limit factor on brakes though? All the top of the line brakes have more than enough (Code's, Magura, Hayes, Formula etc) when paired with a 200mm+ rotor to lock a wheel with 1 finger.
I ride Hayes brakes and I would never struggle to lock a wheel on almost any terrain if i wanted to, I find braking is more grip limited (i.e tyre grip) than brake limited.
Power on a 100 foot descent? No. Power when my brakes are wicked hot after trying to slow down my 215 pounds of rumbling bumbling stupidness...
Power on a 100 foot descent? No. Power when my brakes are wicked hot after trying to slow down my 215 pounds of rumbling bumbling stupidness down a mountain? Yes.
You are right, brakes by and large have plenty of up front power. What they lack is power when they get really hot. This is the big variable that changes rider to rider, location to location.
I'm heavier, not scared to try and go fast (which is worst than truly being fast) and can find descents north of 2K vert. While the brakes you just listed are all good, I have to pull the lever significantly harder at the bottom of a descent than at the top.
I think we can all agree Dak is on a different level than anyone in this forum and he has also mentioned how much he'd love more braking power citing something somewhat similar.
Its not how good the brakes can work. Its how good they work at the bottom of a huge descent with a fatigued rider who has less hand strength after a day of riding.
This post answers the thread in a nutshell.
I'm super super fussy with brake feel and input.
I'm not a great rider, so tend to over brake compared to the really fast riders.
Descents are regularly 500-700m and 1-3km long. So plenty steep.
Ground conditions aren't always ideal, so heavy braking required when you can, to get through the chutes where there's less control.
I ride the same tracks repeatably and have swapped pads, rotors etc quite often (I generally only run a rotor for 2 sets of pads, replace them as soon as theres any tracks or concavity), and I'll replace pads when half worn. I set calipers up without pads, using a feeler gauge, then only adjust pistons as the pads wear. (told you I'm fussy)
Running trickstuff brakes on the enduro and Radic Kaha on the eeb, the biggest performance changer is pad. Metallic ones are good on longer descents, but eat rotors. I've settled on the Galfer reds for a balance, of wear and bite.
Then rotor choice makes a difference, but not in an obvious way.
I've only considered 200/203's and gone through hope, shinano ice, sram, trickstuff and galfer sharkfin. The galfers on really long descents have been the only ones to not affect bite point or fade on a 10min descent. And they are the same thickness as the sram and trickstuff.
In assuming it's down to the fact there's way less material, but a huge surface area that sheds heat. They do wear a bit quicker, but thats a compromise I can live with.
Different subject that was touched on was wheel size.
We all used to run 26" mostly with 203mm rotors and the weak link in the chain was grip at the tyre.
Now that 29 is normal and the grip is leagues higher, means more power is required for slowing as the wheel continues to rotate instead of lock up.
And it's a heavier wheel acting on a bigger radius. Rotational inertia is squared in regards to size and weight, so we're order of magnitude more force to slow a wheel, disregarding any rider input.
The more heat management that can be employed, the better.*
*If you need it.
If you're somewhere flat and dry, there's a reason xtr v brakes were still available until recently.
while on rotors, anyone had/seen a catastrophic rotor failure, did it jam in the calliper or did the wheel keep rolling?
I witnessed a guy catch a front rotor on a stump hidden by some brush on the side of the trail. It bent the rotor outward, causing the front wheel to almost instantaneously eject him OTB. I also came across a guy coaching a handful of teenagers, one of whom had bent a rear rotor in a crash so badly they couldn't straighten it enough to pass. The coach wanted to just take the fucking thing off so they could roll back to the van, except it was centerlock...
And to double post, so it's not too long .
Different subject that was touched on was wheel size.
We all used to run 26" mostly...
And to double post, so it's not too long .
Different subject that was touched on was wheel size.
We all used to run 26" mostly with 203mm rotors and the weak link in the chain was grip at the tyre.
Now that 29 is normal and the grip is leagues higher, means more power is required for slowing as the wheel continues to rotate instead of lock up.
And it's a heavier wheel acting on a bigger radius. Rotational inertia is squared in regards to size and weight, so we're order of magnitude more force to slow a wheel, disregarding any rider input.
The more heat management that can be employed, the better.*
*If you need it.
If you're somewhere flat and dry, there's a reason xtr v brakes were still available until recently.
Does anyone know how to do the math of braking with different wheel sizes?
Because to my mind, a 15kg Enduro bike, with an 80kg rider, slowing from 30-15km/h, the difference between a 26" and 29" wheel is surely going to be like 2-3% at MOST, compared to the total energy required to slow down a 95kg system by 15km/h over a 10m distance.
I think the while difference isn't as much as what people think it to be, I think its because simply people are riding faster, harder and longer than previously is the reason we want better and better brakes
Does anyone know how to do the math of braking with different wheel sizes?
Because to my mind, a 15kg Enduro bike, with an 80kg rider...
Does anyone know how to do the math of braking with different wheel sizes?
Because to my mind, a 15kg Enduro bike, with an 80kg rider, slowing from 30-15km/h, the difference between a 26" and 29" wheel is surely going to be like 2-3% at MOST, compared to the total energy required to slow down a 95kg system by 15km/h over a 10m distance.
I think the while difference isn't as much as what people think it to be, I think its because simply people are riding faster, harder and longer than previously is the reason we want better and better brakes
So looking at it from a conservation of energy standpoint, yes you have to slow down a rotating wheel. Where it gets interesting is when, assuming no slipping, you write out the wheels energy in terms of your velocity. Bigger wheel rotates slower. In the end the radius of the wheel is not included in the energy equation. This is because angular velocity (ω) in rad/s is v/r. Kinetic energy of a rotating mass is (1/2)*I*(ω^2). Since I is proportional to m*r^2, assuming hub is negligible for simplicity, the r^2 term is canceled out by ω^2, which is (v^2)/(r^2). In the end you get (1/2)*mtot*(v^2). What wheel size has an impact on, beyond just bigger wheels are heavier, is ratio of wheel radius to brake rotor radius. If you maintain this ratio, brake force generated by the caliper is constant. If you grow wheel radius without growing rotor radius, the caliper then has to generate more force to creat the same moment. So going from 26 to 29 and keeping the same rotor size would require about 12% more force from the caliper, which is 12% more line pressure, and at least in the vicinity of 12% more finger force. That depends on what exactly the leverage curve of your lever is exactly.
Does anyone know how to do the math of braking with different wheel sizes?
Because to my mind, a 15kg Enduro bike, with an 80kg rider...
Does anyone know how to do the math of braking with different wheel sizes?
Because to my mind, a 15kg Enduro bike, with an 80kg rider, slowing from 30-15km/h, the difference between a 26" and 29" wheel is surely going to be like 2-3% at MOST, compared to the total energy required to slow down a 95kg system by 15km/h over a 10m distance.
I think the while difference isn't as much as what people think it to be, I think its because simply people are riding faster, harder and longer than previously is the reason we want better and better brakes
So looking at it from a conservation of energy standpoint, yes you have to slow down a rotating wheel. Where it gets interesting is when, assuming...
So looking at it from a conservation of energy standpoint, yes you have to slow down a rotating wheel. Where it gets interesting is when, assuming no slipping, you write out the wheels energy in terms of your velocity. Bigger wheel rotates slower. In the end the radius of the wheel is not included in the energy equation. This is because angular velocity (ω) in rad/s is v/r. Kinetic energy of a rotating mass is (1/2)*I*(ω^2). Since I is proportional to m*r^2, assuming hub is negligible for simplicity, the r^2 term is canceled out by ω^2, which is (v^2)/(r^2). In the end you get (1/2)*mtot*(v^2). What wheel size has an impact on, beyond just bigger wheels are heavier, is ratio of wheel radius to brake rotor radius. If you maintain this ratio, brake force generated by the caliper is constant. If you grow wheel radius without growing rotor radius, the caliper then has to generate more force to creat the same moment. So going from 26 to 29 and keeping the same rotor size would require about 12% more force from the caliper, which is 12% more line pressure, and at least in the vicinity of 12% more finger force. That depends on what exactly the leverage curve of your lever is exactly.
Science!
Thanks Cascade!
So to infer from this, a larger wheel requires more force at the caliper, but the brake system isn't actually dissipating more heat (as more energy isn't required).
So the argument that 29" bikes are overheating brakes more just doesn't hold up
Does anyone know how to do the math of braking with different wheel sizes?
Because to my mind, a 15kg Enduro bike, with an 80kg rider...
Does anyone know how to do the math of braking with different wheel sizes?
Because to my mind, a 15kg Enduro bike, with an 80kg rider, slowing from 30-15km/h, the difference between a 26" and 29" wheel is surely going to be like 2-3% at MOST, compared to the total energy required to slow down a 95kg system by 15km/h over a 10m distance.
I think the while difference isn't as much as what people think it to be, I think its because simply people are riding faster, harder and longer than previously is the reason we want better and better brakes
So looking at it from a conservation of energy standpoint, yes you have to slow down a rotating wheel. Where it gets interesting is when, assuming...
So looking at it from a conservation of energy standpoint, yes you have to slow down a rotating wheel. Where it gets interesting is when, assuming no slipping, you write out the wheels energy in terms of your velocity. Bigger wheel rotates slower. In the end the radius of the wheel is not included in the energy equation. This is because angular velocity (ω) in rad/s is v/r. Kinetic energy of a rotating mass is (1/2)*I*(ω^2). Since I is proportional to m*r^2, assuming hub is negligible for simplicity, the r^2 term is canceled out by ω^2, which is (v^2)/(r^2). In the end you get (1/2)*mtot*(v^2). What wheel size has an impact on, beyond just bigger wheels are heavier, is ratio of wheel radius to brake rotor radius. If you maintain this ratio, brake force generated by the caliper is constant. If you grow wheel radius without growing rotor radius, the caliper then has to generate more force to creat the same moment. So going from 26 to 29 and keeping the same rotor size would require about 12% more force from the caliper, which is 12% more line pressure, and at least in the vicinity of 12% more finger force. That depends on what exactly the leverage curve of your lever is exactly.
Science!
Thanks Cascade!
So to infer from this, a larger wheel requires more force at the caliper, but the brake system isn't actually dissipating more heat...
Science!
Thanks Cascade!
So to infer from this, a larger wheel requires more force at the caliper, but the brake system isn't actually dissipating more heat (as more energy isn't required).
So the argument that 29" bikes are overheating brakes more just doesn't hold up
More force at the caliper = more heat.
Brakes convert kinetic energy into heat energy.
So for the same braking effect over the same distance equals more force. Then this energy required is increased.
And like cascade said, this doesnt include wheel slip. 29s grip way harder than a 26, so are rotating at a higher rate before they break traction. Hence the more heat at the caliper.
Another way to think of this, isnt braking to stop, but braking to maintain speed whilst countering the acceleration due to gravity. You can brake harder for longer before skidding. Therefore more energy/heat
So looking at it from a conservation of energy standpoint, yes you have to slow down a rotating wheel. Where it gets interesting is when, assuming...
So looking at it from a conservation of energy standpoint, yes you have to slow down a rotating wheel. Where it gets interesting is when, assuming no slipping, you write out the wheels energy in terms of your velocity. Bigger wheel rotates slower. In the end the radius of the wheel is not included in the energy equation. This is because angular velocity (ω) in rad/s is v/r. Kinetic energy of a rotating mass is (1/2)*I*(ω^2). Since I is proportional to m*r^2, assuming hub is negligible for simplicity, the r^2 term is canceled out by ω^2, which is (v^2)/(r^2). In the end you get (1/2)*mtot*(v^2). What wheel size has an impact on, beyond just bigger wheels are heavier, is ratio of wheel radius to brake rotor radius. If you maintain this ratio, brake force generated by the caliper is constant. If you grow wheel radius without growing rotor radius, the caliper then has to generate more force to creat the same moment. So going from 26 to 29 and keeping the same rotor size would require about 12% more force from the caliper, which is 12% more line pressure, and at least in the vicinity of 12% more finger force. That depends on what exactly the leverage curve of your lever is exactly.
Science!
Thanks Cascade!
So to infer from this, a larger wheel requires more force at the caliper, but the brake system isn't actually dissipating more heat...
Science!
Thanks Cascade!
So to infer from this, a larger wheel requires more force at the caliper, but the brake system isn't actually dissipating more heat (as more energy isn't required).
So the argument that 29" bikes are overheating brakes more just doesn't hold up
More force at the caliper = more heat.
Brakes convert kinetic energy into heat energy.
So for the same braking effect over the same distance equals...
More force at the caliper = more heat.
Brakes convert kinetic energy into heat energy.
So for the same braking effect over the same distance equals more force. Then this energy required is increased.
And like cascade said, this doesnt include wheel slip. 29s grip way harder than a 26, so are rotating at a higher rate before they break traction. Hence the more heat at the caliper.
Another way to think of this, isnt braking to stop, but braking to maintain speed whilst countering the acceleration due to gravity. You can brake harder for longer before skidding. Therefore more energy/heat
One of us hasn't understood what Cascade said, and I'm inclined to think its you.
The formula he wrote out clearly states
In the end the radius of the wheel is not included in the energy equation.
The larger wheel doesn't require more energy than a small one to brake.
The reason it requires more force at the caliper is leverages, but that doesn't mean more energy overall. Therefore it doesn't produce more heat, as the energy is the same.
So for the same rotor size to stop a 29 over a 26" wheel in the same time, takes 12% more force. (Its the ratio of wheel size to rotor)
Force is an application of energy with a direction.
For the sake of being repetitive there's two forces here. Kinetic and heat (some kinetic will be sound) therfore, 12% more force becomes a greater energy transfer into heat.
That's not even touching the "assuming no slip" discussion, where 29s with a far larger contact patch means that wheel can experience much higher ground forces before skidding. (As soon as skidding occurs- caliper friction stops)
Force is not energy as you are alluding (by saying more force means more energy). Force over distance is energy. A larger wheel rotates more slowly (in RPM), so the same sized rotor on a bigger wheel will make less rotations covering the same distance.
The force is increased because the braking path of the pad traveling over the rotor decreases.
I'm guessing you did actually read the equation?
So for the same rotor size to stop a 29 over a 26" wheel in the same time...
I'm guessing you did actually read the equation?
So for the same rotor size to stop a 29 over a 26" wheel in the same time, takes 12% more force. (Its the ratio of wheel size to rotor)
Force is an application of energy with a direction.
For the sake of being repetitive there's two forces here. Kinetic and heat (some kinetic will be sound) therfore, 12% more force becomes a greater energy transfer into heat.
That's not even touching the "assuming no slip" discussion, where 29s with a far larger contact patch means that wheel can experience much higher ground forces before skidding. (As soon as skidding occurs- caliper friction stops)
But this is just plain incorrect.
The simplest way I can explain it to you is this.
Roll from a 10m hill down, you convert the gravitational potential energy into momentum, that energy is fixed. If you have 29" or 26" wheel do don't get more or less energy from rolling down the hill.
Now once you have gained momentum, you hit the flat and start braking, converting this momentum into primarily heat via braking.
Since a 29 and 26 wheel have gained exactly the same amount of energy going down, to come to a stop they must require exactly the same amount of energy to brake.
Your mistaking leverages with energy of the system. Primoz explained it very simply above.
Reviving the thread as something has been bugging me the last 3 summers.
Context: 75kg (165lbs) rider, loads of racing experience, done a few EWS'. I never really had any issues with Brakes from 2010-2022, mostly on shimano saints with 200mm rotors. About 3 years ago both my bikes had come with Code R's so I decided to stick with them to simplify spare pad situation, but I didn't love them. So I bought Galfer 223mm rotors (not the shark ones), much preferred the feel. Note that I currently only ride my ebike which is 26kg with a 1.4kg rear tyre.
My issue: There are 2 specific sections of track (one backcountry trail, the other one bikepark red), where I can guarantee that from cold I will overheat my rear brake to failure within 30 seconds. This has only been happening since I moved to the 220mm rotors, I have ridden the same track at the bikepark for 10 years. First I replaced the rotors with HS2's 220mm, which seemed like an improvement to me, I also then changed my main bike brakes to Magura MT5s. I'm just back from the bike park, and as usual, on that one track my rear brake just stops working in one particular section forcing me to stop for about a minute.
The question: What is going on?
Hypothesis 1: Do the larger and thicker rotors actually store more heat? I could also be down to the pads, galfer purple providing much more friction?
Hypothesis 2: My ebike is a bit heavier and the rear wheel especially so considering the tyre is much heavier than a few years ago. But we are only talking about 90kg->100kg for me and the bike.
Hypothesis 3: Shimano brakes were much better than SRAM/Magura entry level stuff?
Hypothesis 4: I'm just so much faster and radder now!
I'm mostly confused about how heavier DHers are able to do full runs down WC tracks like Champery without their brakes exploding the entire time. Although I used to race 10-20 minute enduro stages without ever having my brakes completely stop working, and that was with 29er rear wheels.
Hmm, curious. What is the rear brake doing when you say failure? Are you using the front brake a lot in that section too? I might venture that pro riders are using much more front brake in those intense slowdown sections, where possible.
Reviving the thread as something has been bugging me the last 3 summers.Context: 75kg (165lbs) rider, loads of racing experience, done a few EWS'. I never...
Reviving the thread as something has been bugging me the last 3 summers.
Context: 75kg (165lbs) rider, loads of racing experience, done a few EWS'. I never really had any issues with Brakes from 2010-2022, mostly on shimano saints with 200mm rotors. About 3 years ago both my bikes had come with Code R's so I decided to stick with them to simplify spare pad situation, but I didn't love them. So I bought Galfer 223mm rotors (not the shark ones), much preferred the feel. Note that I currently only ride my ebike which is 26kg with a 1.4kg rear tyre.
My issue: There are 2 specific sections of track (one backcountry trail, the other one bikepark red), where I can guarantee that from cold I will overheat my rear brake to failure within 30 seconds. This has only been happening since I moved to the 220mm rotors, I have ridden the same track at the bikepark for 10 years. First I replaced the rotors with HS2's 220mm, which seemed like an improvement to me, I also then changed my main bike brakes to Magura MT5s. I'm just back from the bike park, and as usual, on that one track my rear brake just stops working in one particular section forcing me to stop for about a minute.
The question: What is going on?
Hypothesis 1: Do the larger and thicker rotors actually store more heat? I could also be down to the pads, galfer purple providing much more friction?
Hypothesis 2: My ebike is a bit heavier and the rear wheel especially so considering the tyre is much heavier than a few years ago. But we are only talking about 90kg->100kg for me and the bike.
Hypothesis 3: Shimano brakes were much better than SRAM/Magura entry level stuff?
Hypothesis 4: I'm just so much faster and radder now!
I'm mostly confused about how heavier DHers are able to do full runs down WC tracks like Champery without their brakes exploding the entire time. Although I used to race 10-20 minute enduro stages without ever having my brakes completely stop working, and that was with 29er rear wheels.
Describe the failure of the rear brake? Lever going soft or still firm lever but no bite/ not slowing down? How they fail should dictate the direction you should go to address the failure.
What color is your rear rotor? You can tell a lot about temperature ranges by the color of the rotor.
Thicker rotors store more energy. That means it takes more energy to get them to the same temperature, but they take longer to cool off too.
Heavier bike, rider , larger diameter and heavier tires all put more energy into the brakes.
Pros and better riders get by going much faster without overheating brakes because they brake better. They brake harder for shorter periods of time. This lets the brakes cool off between sections of braking. They don't drag brakes. Dragging brakes puts energy into the brakes without giving them a chance to cool off.
Reviving the thread as something has been bugging me the last 3 summers.Context: 75kg (165lbs) rider, loads of racing experience, done a few EWS'. I never...
Reviving the thread as something has been bugging me the last 3 summers.
Context: 75kg (165lbs) rider, loads of racing experience, done a few EWS'. I never really had any issues with Brakes from 2010-2022, mostly on shimano saints with 200mm rotors. About 3 years ago both my bikes had come with Code R's so I decided to stick with them to simplify spare pad situation, but I didn't love them. So I bought Galfer 223mm rotors (not the shark ones), much preferred the feel. Note that I currently only ride my ebike which is 26kg with a 1.4kg rear tyre.
My issue: There are 2 specific sections of track (one backcountry trail, the other one bikepark red), where I can guarantee that from cold I will overheat my rear brake to failure within 30 seconds. This has only been happening since I moved to the 220mm rotors, I have ridden the same track at the bikepark for 10 years. First I replaced the rotors with HS2's 220mm, which seemed like an improvement to me, I also then changed my main bike brakes to Magura MT5s. I'm just back from the bike park, and as usual, on that one track my rear brake just stops working in one particular section forcing me to stop for about a minute.
The question: What is going on?
Hypothesis 1: Do the larger and thicker rotors actually store more heat? I could also be down to the pads, galfer purple providing much more friction?
Hypothesis 2: My ebike is a bit heavier and the rear wheel especially so considering the tyre is much heavier than a few years ago. But we are only talking about 90kg->100kg for me and the bike.
Hypothesis 3: Shimano brakes were much better than SRAM/Magura entry level stuff?
Hypothesis 4: I'm just so much faster and radder now!
I'm mostly confused about how heavier DHers are able to do full runs down WC tracks like Champery without their brakes exploding the entire time. Although I used to race 10-20 minute enduro stages without ever having my brakes completely stop working, and that was with 29er rear wheels.
It's 2 and 4.
Larger rotors do store more heat (as there is more mass it takes more energy to heat them up) but this is a good thing. Your ebike and heavy tires will require more energy to slow down than other setups. If your talking about a system weight of 90 -> 100kg that's 11% more energy required to decelerate, which could push your breaks over the temperature window.
The other part is the pad and rotor combo your using could have a lower temperature window than previous setups. If your now using organic pads and previously metallic pads on the saints, it's possible this descent would overheat the pads till the fade badly whereas metallic pads are fine in that window.
How do DH racers not destroy their brakes? Well they actually do, but they don't fail. Firstly they tend to use metallic pads or are least very high quality organic pads that can take the heat generated. They actually don't brake as much as you'd think, they hit corners so fast the suspension takes some of the energy in deceleration. Because they're moving so fast the rotors do a very good job at cooling as the pump effect increases at higher speeds. Lastly they burn through parts incredibly fast. The old Sram centerline rotors because of their design with very little metal in the middle of the brake surface, would last only 1 race run before becoming too thin to run. So qualy and race run was 2 rotors. A team I know could use 4+ sets of rotors for one rider in a weekend
I'll check for sure, but I have in a drawer 5 year old Centerline rotors that were subjected to ~450.000 vertical meters of descending and were run I think exclusivelly on Sram sintered pads (lets say 1,5 sets per season). They are worn down to 1,65-ish mm (they are half way worn). I will check the bridges, but I'm fairly certain it is not a problem on my set of rotors.
I do know what you are talking about as this exact issue happened to me on an old set of Formula rotors that had many holes on two separate diameters, causing the centre to centre bridge between the holes to get worn down much faster than the solid track in between the two hole diameters.
It's been a long time since I rode a moto...unfortunately.
I have been watching quite a bit of hard enduro and those guys simply lock the rear wheel and skid down anything remotely steep. Found that interesting, and wondered if part of it was not having as fine dexterity using a foot you're also standing on to modulate it.
Sorry dude - missed this. David answered it already and is dead on. I should note a few small things to clear up a tiny bit of confusion.
1) When I'm saying "moto brakes" I'm really referring to Brembo or Magura. Brembo in dirt bike world is kind of like Codes in that they are on majority of the bikes people ride off road.
2) I HAMMER my moto brakes when I'm riding trail. Sometimes there are descents as part of a race or similar that are literally 3K+ of super gnarly steep craziness and I'm an hour or more into the race. In these situations I'm so thankful I'm on a moto not a MTB lol. Being able to reliably find the edge of traction without fatiguing me any further on a 235 pound machine is pretty nice.
3) To the above posters point, you aren't wrong with respect to your ability to really modulate the brake with your foot, but its not like you have zero touch. That said, when I am racing hard enduro and its mega steep I do just lock up the back and hope for the best. ITS TERRIFYING.
I'm not suggesting we take dirt bike brakes and put them on a mtb. But I do think we can built a bigger brake and I'd be a lot happier. I'm talking something that is significantly larger with more fluid. A true gravity brake - not a slightly larger XC brake. Just an idea.
Do you really think power is the limit factor on brakes though? All the top of the line brakes have more than enough (Code's, Magura, Hayes, Formula etc) when paired with a 200mm+ rotor to lock a wheel with 1 finger.
I ride Hayes brakes and I would never struggle to lock a wheel on almost any terrain if i wanted to, I find braking is more grip limited (i.e tyre grip) than brake limited.
Power on a 100 foot descent? No. Power when my brakes are wicked hot after trying to slow down my 215 pounds of rumbling bumbling stupidness down a mountain? Yes.
You are right, brakes by and large have plenty of up front power. What they lack is power when they get really hot. This is the big variable that changes rider to rider, location to location.
I'm heavier, not scared to try and go fast (which is worst than truly being fast) and can find descents north of 2K vert. While the brakes you just listed are all good, I have to pull the lever significantly harder at the bottom of a descent than at the top.
I think we can all agree Dak is on a different level than anyone in this forum and he has also mentioned how much he'd love more braking power citing something somewhat similar.
Its not how good the brakes can work. Its how good they work at the bottom of a huge descent with a fatigued rider who has less hand strength after a day of riding.
If we're comparing MTB and moto... How large are moto wheels (outer tyre diameter)? About the diameter of a 24-inch bike? What are rotor sizes? I'm not up to speed on moto stuff, but I'd guess the rear rotor is roughly 200-ish mm in diameter? So a motocross bike has a larger brake rotor, which gives it more power. Then the added benefit is that the rotor is MUCH larger in relative terms to the outer tyre diameter again. So a double whammy.
Also, pushing more fluid I doubt will give a reasonable benefit as it's a closed loop system (once squeezed shut) and the oil is not flowing. We're not talking about dampers, where flowing more oil will improve thermal management.
TL;DR: I doubt using moto brakes (lever and caliper) on a mountain bike will give a meaningful improvement over the current state.
Front moto wheel has an outside diameter of 28in which is the same as mtb 23" wheels (also known as "27.5") with a 2.5 inch tyre. The rear wheel is slightly smaller at 27in or less. 260mm front rotor and 240mm rear rotors.
On any of my dirt bikes one of the first things I do is a Braking 270mm rotor and bracket on because they look trick lol, stock are usually 240-250 depending on the manufacturer, 260mm if you got a husky/ktm/gasgas with Brembo's (don't need to upgrade those)
I always leave the rear rotors alone.
They never bend or warp. They last years too.
Just so I'm again clear here, I am not advocating for Monster Garaging a dirt bike setup onto a mountain bike. What I am suggesting is the relative performance of a dirt bike braking system to the size of the machine/rider/etc is significantly better than a mountain bike.
Worth pointing out here, I do not feel this is a hard problem to solve from an engineering perspective. Its a cultural problem. One where too many people care about 1/2 to 1 pound in the form of stopping performance. Every company trying to build a top tier brake is trying their damnest to keep weight down because they know a big part of the market only rides their bike "on the internet" (in a performance vacuum).
If we get to the real world of it all and accept a weight penalty, I guarantee any brake company out there could build what I'm asking for .
Makes it more clear.
On the other hand, I weigh 80ish kilos naked, so just over 85 kg kitted (180 to 200 freedom units?) and haven't noticed I need more brake over what I get with Code RSCs on old centerline or new HS2 rotors in 200 200 mm on a 29er. This includes everything from short, 300 m vertical descents to 15 minute 1000 m vertical top to bottom runs (where a friend of mine on an identical bike setup other than the rear rotor overheated a 180 mm rear rotor and immediately upgraded to a 200 mm rotor), two Megavalanches, multiple multi day enduro races, etc. And I'm a notorious brake dragger.
TL;DR: how much of an impact on braking performance is rider weight dependant? If a lot, how much of the market actually requires a substantially larger amount of braking performance compared to what we have now on the market?
Also, @jeff.brines , why don't you try out Trickstuff brakes (whichever is the most powerful)? Or as a cheap alternative the Chinese knock off in the form of the Lewis? Everybody that has tried them that I know raves about the power.
Yeah that's totally fair.
I ride at Maydena quite a bit (820m top to bottom) and the brakes are cooked after doing long runs, but i guess i'm just used to losing braking force by the bottom of the runs by now? Not sure if I notice it that much, over being absolutely knackered from 400m of chunder.
How heavy is a Motocross brake? Talking to a superbike/motoGP enthusiant/mechanic he says their brake drag is about 1HP the brakes are set so close to the rotor and to maintain temprature in the carbon disk, obviously those types of braking systems are not even in the same universe as MTB
while on rotors, anyone had/seen a catastrophic rotor failure, did it jam in the calliper or did the wheel keep rolling?
I'd say any catastrophic failure of a solid stainless steel braking track will involve a component of bending, so it will jam the caliper. Even if not, it will likely occur during braking, so squeezing in the brake pads will cause the remaining rotor to suddenly stop once it hits the closed up pads.
I'd say wearing through the stainless layer on an ice tech rotor is a catastrophic failure and I have seen done that during riding with no major consequences other than very diminished braking performance. But not enough for the guy to not finish the multi day stage race :D
I've worn through my ice tech rotors in patches and they still felt as rigid as new, only info I could find at the time was to change when they start to wear through. I think the only way you're going to bend a rotor is a rock strike. 🤷🏻
3) To the above posters point, you aren't wrong with respect to your ability to really modulate the brake with your foot, but its not like you have zero touch. That said, when I am racing hard enduro and its mega steep I do just lock up the back and hope for the best. ITS TERRIFYING.
You have never ridden with a Rekluse Auto Clutch which allows a left hand rear brake. No need to stomp on the rear brake coming down hills. You can feather at the edge of traction and hang off the back as needed. All as the bike idles in a sort of coast mode. It is a revelation on a Moto. But that is another forum.
This post answers the thread in a nutshell.
I'm super super fussy with brake feel and input.
I'm not a great rider, so tend to over brake compared to the really fast riders.
Descents are regularly 500-700m and 1-3km long. So plenty steep.
Ground conditions aren't always ideal, so heavy braking required when you can, to get through the chutes where there's less control.
I ride the same tracks repeatably and have swapped pads, rotors etc quite often (I generally only run a rotor for 2 sets of pads, replace them as soon as theres any tracks or concavity), and I'll replace pads when half worn. I set calipers up without pads, using a feeler gauge, then only adjust pistons as the pads wear. (told you I'm fussy)
Running trickstuff brakes on the enduro and Radic Kaha on the eeb, the biggest performance changer is pad. Metallic ones are good on longer descents, but eat rotors. I've settled on the Galfer reds for a balance, of wear and bite.
Then rotor choice makes a difference, but not in an obvious way.
I've only considered 200/203's and gone through hope, shinano ice, sram, trickstuff and galfer sharkfin. The galfers on really long descents have been the only ones to not affect bite point or fade on a 10min descent. And they are the same thickness as the sram and trickstuff.
In assuming it's down to the fact there's way less material, but a huge surface area that sheds heat. They do wear a bit quicker, but thats a compromise I can live with.
And to double post, so it's not too long .
Different subject that was touched on was wheel size.
We all used to run 26" mostly with 203mm rotors and the weak link in the chain was grip at the tyre.
Now that 29 is normal and the grip is leagues higher, means more power is required for slowing as the wheel continues to rotate instead of lock up.
And it's a heavier wheel acting on a bigger radius. Rotational inertia is squared in regards to size and weight, so we're order of magnitude more force to slow a wheel, disregarding any rider input.
The more heat management that can be employed, the better.*
*If you need it.
If you're somewhere flat and dry, there's a reason xtr v brakes were still available until recently.
I witnessed a guy catch a front rotor on a stump hidden by some brush on the side of the trail. It bent the rotor outward, causing the front wheel to almost instantaneously eject him OTB. I also came across a guy coaching a handful of teenagers, one of whom had bent a rear rotor in a crash so badly they couldn't straighten it enough to pass. The coach wanted to just take the fucking thing off so they could roll back to the van, except it was centerlock...
Does anyone know how to do the math of braking with different wheel sizes?
Because to my mind, a 15kg Enduro bike, with an 80kg rider, slowing from 30-15km/h, the difference between a 26" and 29" wheel is surely going to be like 2-3% at MOST, compared to the total energy required to slow down a 95kg system by 15km/h over a 10m distance.
I think the while difference isn't as much as what people think it to be, I think its because simply people are riding faster, harder and longer than previously is the reason we want better and better brakes
So looking at it from a conservation of energy standpoint, yes you have to slow down a rotating wheel. Where it gets interesting is when, assuming no slipping, you write out the wheels energy in terms of your velocity. Bigger wheel rotates slower. In the end the radius of the wheel is not included in the energy equation. This is because angular velocity (ω) in rad/s is v/r. Kinetic energy of a rotating mass is (1/2)*I*(ω^2). Since I is proportional to m*r^2, assuming hub is negligible for simplicity, the r^2 term is canceled out by ω^2, which is (v^2)/(r^2). In the end you get (1/2)*mtot*(v^2). What wheel size has an impact on, beyond just bigger wheels are heavier, is ratio of wheel radius to brake rotor radius. If you maintain this ratio, brake force generated by the caliper is constant. If you grow wheel radius without growing rotor radius, the caliper then has to generate more force to creat the same moment. So going from 26 to 29 and keeping the same rotor size would require about 12% more force from the caliper, which is 12% more line pressure, and at least in the vicinity of 12% more finger force. That depends on what exactly the leverage curve of your lever is exactly.
Science!
Thanks Cascade!
So to infer from this, a larger wheel requires more force at the caliper, but the brake system isn't actually dissipating more heat (as more energy isn't required).
So the argument that 29" bikes are overheating brakes more just doesn't hold up
More force at the caliper = more heat.
Brakes convert kinetic energy into heat energy.
So for the same braking effect over the same distance equals more force. Then this energy required is increased.
And like cascade said, this doesnt include wheel slip. 29s grip way harder than a 26, so are rotating at a higher rate before they break traction. Hence the more heat at the caliper.
Another way to think of this, isnt braking to stop, but braking to maintain speed whilst countering the acceleration due to gravity. You can brake harder for longer before skidding. Therefore more energy/heat
One of us hasn't understood what Cascade said, and I'm inclined to think its you.
The formula he wrote out clearly states
In the end the radius of the wheel is not included in the energy equation.
The larger wheel doesn't require more energy than a small one to brake.
The reason it requires more force at the caliper is leverages, but that doesn't mean more energy overall. Therefore it doesn't produce more heat, as the energy is the same.
I'm guessing you did actually read the equation?
So for the same rotor size to stop a 29 over a 26" wheel in the same time, takes 12% more force. (Its the ratio of wheel size to rotor)
Force is an application of energy with a direction.
For the sake of being repetitive there's two forces here. Kinetic and heat (some kinetic will be sound) therfore, 12% more force becomes a greater energy transfer into heat.
That's not even touching the "assuming no slip" discussion, where 29s with a far larger contact patch means that wheel can experience much higher ground forces before skidding. (As soon as skidding occurs- caliper friction stops)
Force is not energy as you are alluding (by saying more force means more energy). Force over distance is energy. A larger wheel rotates more slowly (in RPM), so the same sized rotor on a bigger wheel will make less rotations covering the same distance.
The force is increased because the braking path of the pad traveling over the rotor decreases.
But this is just plain incorrect.
The simplest way I can explain it to you is this.
Roll from a 10m hill down, you convert the gravitational potential energy into momentum, that energy is fixed. If you have 29" or 26" wheel do don't get more or less energy from rolling down the hill.
Now once you have gained momentum, you hit the flat and start braking, converting this momentum into primarily heat via braking.
Since a 29 and 26 wheel have gained exactly the same amount of energy going down, to come to a stop they must require exactly the same amount of energy to brake.
Your mistaking leverages with energy of the system. Primoz explained it very simply above.
Reviving the thread as something has been bugging me the last 3 summers.
Context: 75kg (165lbs) rider, loads of racing experience, done a few EWS'. I never really had any issues with Brakes from 2010-2022, mostly on shimano saints with 200mm rotors. About 3 years ago both my bikes had come with Code R's so I decided to stick with them to simplify spare pad situation, but I didn't love them. So I bought Galfer 223mm rotors (not the shark ones), much preferred the feel. Note that I currently only ride my ebike which is 26kg with a 1.4kg rear tyre.
My issue: There are 2 specific sections of track (one backcountry trail, the other one bikepark red), where I can guarantee that from cold I will overheat my rear brake to failure within 30 seconds. This has only been happening since I moved to the 220mm rotors, I have ridden the same track at the bikepark for 10 years. First I replaced the rotors with HS2's 220mm, which seemed like an improvement to me, I also then changed my main bike brakes to Magura MT5s. I'm just back from the bike park, and as usual, on that one track my rear brake just stops working in one particular section forcing me to stop for about a minute.
The question: What is going on?
Hypothesis 1: Do the larger and thicker rotors actually store more heat? I could also be down to the pads, galfer purple providing much more friction?
Hypothesis 2: My ebike is a bit heavier and the rear wheel especially so considering the tyre is much heavier than a few years ago. But we are only talking about 90kg->100kg for me and the bike.
Hypothesis 3: Shimano brakes were much better than SRAM/Magura entry level stuff?
Hypothesis 4: I'm just so much faster and radder now!
I'm mostly confused about how heavier DHers are able to do full runs down WC tracks like Champery without their brakes exploding the entire time. Although I used to race 10-20 minute enduro stages without ever having my brakes completely stop working, and that was with 29er rear wheels.
Hmm, curious. What is the rear brake doing when you say failure? Are you using the front brake a lot in that section too? I might venture that pro riders are using much more front brake in those intense slowdown sections, where possible.
Describe the failure of the rear brake? Lever going soft or still firm lever but no bite/ not slowing down? How they fail should dictate the direction you should go to address the failure.
What color is your rear rotor? You can tell a lot about temperature ranges by the color of the rotor.
Thicker rotors store more energy. That means it takes more energy to get them to the same temperature, but they take longer to cool off too.
Heavier bike, rider , larger diameter and heavier tires all put more energy into the brakes.
Pros and better riders get by going much faster without overheating brakes because they brake better. They brake harder for shorter periods of time. This lets the brakes cool off between sections of braking. They don't drag brakes. Dragging brakes puts energy into the brakes without giving them a chance to cool off.
It's 2 and 4.
Larger rotors do store more heat (as there is more mass it takes more energy to heat them up) but this is a good thing. Your ebike and heavy tires will require more energy to slow down than other setups. If your talking about a system weight of 90 -> 100kg that's 11% more energy required to decelerate, which could push your breaks over the temperature window.
The other part is the pad and rotor combo your using could have a lower temperature window than previous setups. If your now using organic pads and previously metallic pads on the saints, it's possible this descent would overheat the pads till the fade badly whereas metallic pads are fine in that window.
How do DH racers not destroy their brakes? Well they actually do, but they don't fail. Firstly they tend to use metallic pads or are least very high quality organic pads that can take the heat generated. They actually don't brake as much as you'd think, they hit corners so fast the suspension takes some of the energy in deceleration. Because they're moving so fast the rotors do a very good job at cooling as the pump effect increases at higher speeds. Lastly they burn through parts incredibly fast. The old Sram centerline rotors because of their design with very little metal in the middle of the brake surface, would last only 1 race run before becoming too thin to run. So qualy and race run was 2 rotors. A team I know could use 4+ sets of rotors for one rider in a weekend
I'll check for sure, but I have in a drawer 5 year old Centerline rotors that were subjected to ~450.000 vertical meters of descending and were run I think exclusivelly on Sram sintered pads (lets say 1,5 sets per season). They are worn down to 1,65-ish mm (they are half way worn). I will check the bridges, but I'm fairly certain it is not a problem on my set of rotors.
I do know what you are talking about as this exact issue happened to me on an old set of Formula rotors that had many holes on two separate diameters, causing the centre to centre bridge between the holes to get worn down much faster than the solid track in between the two hole diameters.
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