The weight of a rider acts on a bike in one manner. But do to the rider being dynamic as well as the system of bike and rider being dynamic, energy/forces are added into the equation. You guys are right, the rider’s center of gravity doesn’t change, but there is still energy/forces present in the equation that have to be accounted for. Where the new energy or “weight” or virtual mass is “directed has to matter, and we know that as riders we can control where it goes to an extent. Though this doesn’t change our center of gravity, in effect there is a center of the rider, bike, and forces present that will move and change depending on a lot of factors. The center of the equation between you pushing down on your hands, and the tires pushing down on the dirt is a very different center than your feet pushing into the pedals and your tires pushing into the dirt. There is extra force being applied beyond where your center of gravity is, as the totally amount of energy (potential, kinetic) has changed in that instance, and where that force is directed will change the overall center of that equation.
Wanted to add this little anecdote to the forum, as I found it a pretty staggering illustration of how the balance shifts across different sized bikes. I...
Wanted to add this little anecdote to the forum, as I found it a pretty staggering illustration of how the balance shifts across different sized bikes.
I have been riding an S2/Medium Ibis HD6 this year. By today's standards, its certainly moderate geo-wise, if not a tad conservative. The wheelbase is 1228 with a 435 chainstay, putting the FC/RC ratio at about 1.82 and making the chainstay about 35.4% of the total wheelbase (435/1228). Out of curiosity, I did the same basic math with one of Paul Aston's enormous custom steel bikes, which had a 1375 wheelbase and 490 chainstay (far longer than any production XL bike I know). The numbers on that bike had chainstays that equated to 35.6% of the wheelbase and a 1.81 FC/RC ratio! It blew me away that Aston's XL bike that is perceived to be so radical has such similar proportions to my medium Ibis, which is seen as fairly vanilla.
As a shorter rider, this was also upsetting as I now have fewer excuses for not being faster, lol.
Coincidentally, the Ibis is one of my all time favorite bikes, alongside my 2018 Transition Sentinel. Both bikes are quite close geometry wise (450/454 reach, 435 chainstays, and 64 head angles). Both bikes provided me with that easy "stand there and ride through the feet" feeling with very intuitive, no-nonsense handling.
Interesting data point.
Although it doesn't discredit your point at all, it's worth noting as general matter that those metrics can be misleading. Making up numbers to make the math easy, 440/1250=35.2% and 450/1260=35.7%. What most people would consider a huge change in rear center and ride characteristics produces a .5% change in RC as a percentage of wheel base. At least to me, that .5% number makes a very significant change seem insignificant unless you've thought about it and played with the math.
Wanted to add this little anecdote to the forum, as I found it a pretty staggering illustration of how the balance shifts across different sized bikes. I...
Wanted to add this little anecdote to the forum, as I found it a pretty staggering illustration of how the balance shifts across different sized bikes.
I have been riding an S2/Medium Ibis HD6 this year. By today's standards, its certainly moderate geo-wise, if not a tad conservative. The wheelbase is 1228 with a 435 chainstay, putting the FC/RC ratio at about 1.82 and making the chainstay about 35.4% of the total wheelbase (435/1228). Out of curiosity, I did the same basic math with one of Paul Aston's enormous custom steel bikes, which had a 1375 wheelbase and 490 chainstay (far longer than any production XL bike I know). The numbers on that bike had chainstays that equated to 35.6% of the wheelbase and a 1.81 FC/RC ratio! It blew me away that Aston's XL bike that is perceived to be so radical has such similar proportions to my medium Ibis, which is seen as fairly vanilla.
As a shorter rider, this was also upsetting as I now have fewer excuses for not being faster, lol.
Coincidentally, the Ibis is one of my all time favorite bikes, alongside my 2018 Transition Sentinel. Both bikes are quite close geometry wise (450/454 reach, 435 chainstays, and 64 head angles). Both bikes provided me with that easy "stand there and ride through the feet" feeling with very intuitive, no-nonsense handling.
Interesting data point. Although it doesn't discredit your point at all, it's worth noting as general matter that those metrics can be misleading. Making up numbers...
Interesting data point.
Although it doesn't discredit your point at all, it's worth noting as general matter that those metrics can be misleading. Making up numbers to make the math easy, 440/1250=35.2% and 450/1260=35.7%. What most people would consider a huge change in rear center and ride characteristics produces a .5% change in RC as a percentage of wheel base. At least to me, that .5% number makes a very significant change seem insignificant unless you've thought about it and played with the math.
For sure. Those numbers don't tell you everything, and there are a lot of variables they don't account for, but I thought it was interesting to see the differences. I should have included some more common bikes for context as it helps to have a reference point.
For example, the Megatower in a Medium is 1.82 FC/RC and 35.4% chainstay/wheelbase. In the XL is 1.92 and 34.2%. The XXL is 1.98 and 33.6%. While those aren't the biggest nominal changes, I think its fair to say it shows how different the proportions are between sizes and how an 'extreme' bike like one of Paul Aston's customs actually has fairly similar proportions to a traditional manufacturers medium-sized bikes.
1) On an academic level, someone who has a strong understanding of physics and mechanical engineering can chime in regarding how forces are distributed through the chassis into the ground via rider feedback. I'm not super interested in this, only because I already know the "levers" I can pull on the change this for the better (or worse) via frame geometry. The big takeaway here we all know to be true is riding through your hands is hard, riding through your legs is much easier. This hypothesis underlies this entire thread.
2) To reiterate, we all know the biggest variable here is rear center vs front center (or wheelbase). However, there are other ways to play with this distribution too, including stem length, and fork offset, stack/handlebar height and amount of sag f/r.
As to the "best" numbers, this isn't really possible to objectively define without putting a rider on the bike, having them get into their "attack" position and measuring weight distribution on scales front to rear. Even better, measure their weight distribution as they ride. Biometrics, rider technique, etc also all play into this massively. While I have my ideas here (based off dirt bikes), I'd like to see top tier racers and riders in DH and enduro get weighed, log a bunch of measurements pertaining to their bikes and have each of them fill out a survey to start to hone in on what might work best/worst.
The final step, once we have some idea what ratios we're looking for, would be for a progressive/innovative frame manufacturer to offer a lot more rear center adjustment. I'm talking like 50mm. I'd also like to see as much reach adjustment as we could put into a frame, too.
This would allow a "modern mountain bike fitter" to put a rider on the scales after taking a number of measurements and get the bike to actually fit the rider for the descent while not compromising on other handling/fit components like stem length, handlebar height and suspension tuning. This fit would also take into account average steepness of terrain and if the rider has any biases. Obviously, this won't be an end all be all, but get a rider far more closer to "good" than just randomly pedaling a bike around and making sure the seat height "feels okay".
If done properly, this would open the door to a level of bike handling many of us can only dream about, or randomly stumble into once in a blue moon.
...heck, maybe I'll write a white paper on this and publish it to my blog (LOL).
EDIT, I probably wrote all this in a prior post a few years back. No way I'm going back to read all this, however - Ha.
What came first? A forward-biased attack position or a front center-biased frame geo?
Ha.
One thing I should note, just because I'm a disgruntled larger reach riding rider doesn't mean all bikes handle poorly. For years smaller and medium sized bikes have worked really really well. To add, don't forget back in the day, a size large was like 425mm, which would balance a shorter rear end fairly well.
Once we strated stretching things out is where we went off the proverbial rails...
What clicked for me is that reaches got longer because HTA got slacker. But not for a handling reason: for uphill performance/seated riding position/front wheel lift, etc. DH bikes didn’t go through the same transformation because no one cared about front wheel lift when pedaling, no one cared about seated pedaling position, and so raw downhill handling and traction and overall performance was unmolested.
I am frustrated that we compromised “enduro bikes” because reviewers like to comment on how much weight is on the front end when climbing. That’s so dumb to me! I don’t pedal my bike uphill for fun, I’m doing it for the downs, baby!!! So now I’m pushing outwards to figure out how far into DH bike territory we can go before a bike is just plain miserable to ride uphill. At this point I don’t actually see an issue pedaling a full DH bike uphill. If you look at the comments on the Status 2 170 DH people just want that bike with a wide range cassette. So do I.
Anyways. That’s a good transition to my next question: DH bikes have recently gone over to the dark side of front-center biased. Any ideas why this is? Are DH riders getting better at riding off the front of the bike? Are tracks getting smoother and faster?
It’s why I am obsessed with DH bikes.What clicked for me is that reaches got longer because HTA got slacker. But not for a handling reason...
It’s why I am obsessed with DH bikes.
What clicked for me is that reaches got longer because HTA got slacker. But not for a handling reason: for uphill performance/seated riding position/front wheel lift, etc. DH bikes didn’t go through the same transformation because no one cared about front wheel lift when pedaling, no one cared about seated pedaling position, and so raw downhill handling and traction and overall performance was unmolested.
I am frustrated that we compromised “enduro bikes” because reviewers like to comment on how much weight is on the front end when climbing. That’s so dumb to me! I don’t pedal my bike uphill for fun, I’m doing it for the downs, baby!!! So now I’m pushing outwards to figure out how far into DH bike territory we can go before a bike is just plain miserable to ride uphill. At this point I don’t actually see an issue pedaling a full DH bike uphill. If you look at the comments on the Status 2 170 DH people just want that bike with a wide range cassette. So do I.
Anyways. That’s a good transition to my next question: DH bikes have recently gone over to the dark side of front-center biased. Any ideas why this is? Are DH riders getting better at riding off the front of the bike? Are tracks getting smoother and faster?
Steeper the track the more a rearward bias makes sense. YMMV.
Just drank some coffee so I'll play (ha).1) On an academic level, someone who has a strong understanding of physics and mechanical engineering can chime in...
Just drank some coffee so I'll play (ha).
1) On an academic level, someone who has a strong understanding of physics and mechanical engineering can chime in regarding how forces are distributed through the chassis into the ground via rider feedback. I'm not super interested in this, only because I already know the "levers" I can pull on the change this for the better (or worse) via frame geometry. The big takeaway here we all know to be true is riding through your hands is hard, riding through your legs is much easier. This hypothesis underlies this entire thread.
2) To reiterate, we all know the biggest variable here is rear center vs front center (or wheelbase). However, there are other ways to play with this distribution too, including stem length, and fork offset, stack/handlebar height and amount of sag f/r.
As to the "best" numbers, this isn't really possible to objectively define without putting a rider on the bike, having them get into their "attack" position and measuring weight distribution on scales front to rear. Even better, measure their weight distribution as they ride. Biometrics, rider technique, etc also all play into this massively. While I have my ideas here (based off dirt bikes), I'd like to see top tier racers and riders in DH and enduro get weighed, log a bunch of measurements pertaining to their bikes and have each of them fill out a survey to start to hone in on what might work best/worst.
The final step, once we have some idea what ratios we're looking for, would be for a progressive/innovative frame manufacturer to offer a lot more rear center adjustment. I'm talking like 50mm. I'd also like to see as much reach adjustment as we could put into a frame, too.
This would allow a "modern mountain bike fitter" to put a rider on the scales after taking a number of measurements and get the bike to actually fit the rider for the descent while not compromising on other handling/fit components like stem length, handlebar height and suspension tuning. This fit would also take into account average steepness of terrain and if the rider has any biases. Obviously, this won't be an end all be all, but get a rider far more closer to "good" than just randomly pedaling a bike around and making sure the seat height "feels okay".
If done properly, this would open the door to a level of bike handling many of us can only dream about, or randomly stumble into once in a blue moon.
...heck, maybe I'll write a white paper on this and publish it to my blog (LOL).
EDIT, I probably wrote all this in a prior post a few years back. No way I'm going back to read all this, however - Ha.
“...heck, maybe I'll write a white paper on this and publish it to my blog (LOL).“
I have a theory that adding stack is the counter to long rear centers making a bike hard to manual/lift without sacrificing the benefits of longer chainstays.
Adding stack means that the rider creates a larger moment (rotating force) when pulling back and lifting the bike into a manual.
It’s why I am obsessed with DH bikes.What clicked for me is that reaches got longer because HTA got slacker. But not for a handling reason...
It’s why I am obsessed with DH bikes.
What clicked for me is that reaches got longer because HTA got slacker. But not for a handling reason: for uphill performance/seated riding position/front wheel lift, etc. DH bikes didn’t go through the same transformation because no one cared about front wheel lift when pedaling, no one cared about seated pedaling position, and so raw downhill handling and traction and overall performance was unmolested.
I am frustrated that we compromised “enduro bikes” because reviewers like to comment on how much weight is on the front end when climbing. That’s so dumb to me! I don’t pedal my bike uphill for fun, I’m doing it for the downs, baby!!! So now I’m pushing outwards to figure out how far into DH bike territory we can go before a bike is just plain miserable to ride uphill. At this point I don’t actually see an issue pedaling a full DH bike uphill. If you look at the comments on the Status 2 170 DH people just want that bike with a wide range cassette. So do I.
Anyways. That’s a good transition to my next question: DH bikes have recently gone over to the dark side of front-center biased. Any ideas why this is? Are DH riders getting better at riding off the front of the bike? Are tracks getting smoother and faster?
I don't really keep up with DH bikes at all, but, just to be sure, are you accounting for high pivots and rearward axle paths when looking at geo charts? Could that be part of it?
Thanks for writing this all out, Jeff. Another thought to back up your argument is that it's not just reach length that's gotten longer over the last 10 years. Reach length has grown at the same time that head angles have gotten slacker. And head angles have gotten slacker on all bikes, from DH bikes to XC rigs. This means that the front center is getting doubly longer, from the longer reach and the slacker head angle. A head angle that's one degree slacker translates to roughly an inch of additional front center length.
Alright what about this:
The weight of a rider acts on a bike in one manner. But do to the rider being dynamic as well as the system of bike and rider being dynamic, energy/forces are added into the equation. You guys are right, the rider’s center of gravity doesn’t change, but there is still energy/forces present in the equation that have to be accounted for. Where the new energy or “weight” or virtual mass is “directed has to matter, and we know that as riders we can control where it goes to an extent. Though this doesn’t change our center of gravity, in effect there is a center of the rider, bike, and forces present that will move and change depending on a lot of factors. The center of the equation between you pushing down on your hands, and the tires pushing down on the dirt is a very different center than your feet pushing into the pedals and your tires pushing into the dirt. There is extra force being applied beyond where your center of gravity is, as the totally amount of energy (potential, kinetic) has changed in that instance, and where that force is directed will change the overall center of that equation.
Interesting data point.
Although it doesn't discredit your point at all, it's worth noting as general matter that those metrics can be misleading. Making up numbers to make the math easy, 440/1250=35.2% and 450/1260=35.7%. What most people would consider a huge change in rear center and ride characteristics produces a .5% change in RC as a percentage of wheel base. At least to me, that .5% number makes a very significant change seem insignificant unless you've thought about it and played with the math.
For sure. Those numbers don't tell you everything, and there are a lot of variables they don't account for, but I thought it was interesting to see the differences. I should have included some more common bikes for context as it helps to have a reference point.
For example, the Megatower in a Medium is 1.82 FC/RC and 35.4% chainstay/wheelbase. In the XL is 1.92 and 34.2%. The XXL is 1.98 and 33.6%. While those aren't the biggest nominal changes, I think its fair to say it shows how different the proportions are between sizes and how an 'extreme' bike like one of Paul Aston's customs actually has fairly similar proportions to a traditional manufacturers medium-sized bikes.
Just drank some coffee so I'll play (ha).
1) On an academic level, someone who has a strong understanding of physics and mechanical engineering can chime in regarding how forces are distributed through the chassis into the ground via rider feedback. I'm not super interested in this, only because I already know the "levers" I can pull on the change this for the better (or worse) via frame geometry. The big takeaway here we all know to be true is riding through your hands is hard, riding through your legs is much easier. This hypothesis underlies this entire thread.
2) To reiterate, we all know the biggest variable here is rear center vs front center (or wheelbase). However, there are other ways to play with this distribution too, including stem length, and fork offset, stack/handlebar height and amount of sag f/r.
As to the "best" numbers, this isn't really possible to objectively define without putting a rider on the bike, having them get into their "attack" position and measuring weight distribution on scales front to rear. Even better, measure their weight distribution as they ride. Biometrics, rider technique, etc also all play into this massively. While I have my ideas here (based off dirt bikes), I'd like to see top tier racers and riders in DH and enduro get weighed, log a bunch of measurements pertaining to their bikes and have each of them fill out a survey to start to hone in on what might work best/worst.
The final step, once we have some idea what ratios we're looking for, would be for a progressive/innovative frame manufacturer to offer a lot more rear center adjustment. I'm talking like 50mm. I'd also like to see as much reach adjustment as we could put into a frame, too.
This would allow a "modern mountain bike fitter" to put a rider on the scales after taking a number of measurements and get the bike to actually fit the rider for the descent while not compromising on other handling/fit components like stem length, handlebar height and suspension tuning. This fit would also take into account average steepness of terrain and if the rider has any biases. Obviously, this won't be an end all be all, but get a rider far more closer to "good" than just randomly pedaling a bike around and making sure the seat height "feels okay".
If done properly, this would open the door to a level of bike handling many of us can only dream about, or randomly stumble into once in a blue moon.
...heck, maybe I'll write a white paper on this and publish it to my blog (LOL).
EDIT, I probably wrote all this in a prior post a few years back. No way I'm going back to read all this, however - Ha.
What came first? A forward-biased attack position or a front center-biased frame geo?
Ha.
One thing I should note, just because I'm a disgruntled larger reach riding rider doesn't mean all bikes handle poorly. For years smaller and medium sized bikes have worked really really well. To add, don't forget back in the day, a size large was like 425mm, which would balance a shorter rear end fairly well.
Once we strated stretching things out is where we went off the proverbial rails...
It’s why I am obsessed with DH bikes.
What clicked for me is that reaches got longer because HTA got slacker. But not for a handling reason: for uphill performance/seated riding position/front wheel lift, etc. DH bikes didn’t go through the same transformation because no one cared about front wheel lift when pedaling, no one cared about seated pedaling position, and so raw downhill handling and traction and overall performance was unmolested.
I am frustrated that we compromised “enduro bikes” because reviewers like to comment on how much weight is on the front end when climbing. That’s so dumb to me! I don’t pedal my bike uphill for fun, I’m doing it for the downs, baby!!! So now I’m pushing outwards to figure out how far into DH bike territory we can go before a bike is just plain miserable to ride uphill. At this point I don’t actually see an issue pedaling a full DH bike uphill. If you look at the comments on the Status 2 170 DH people just want that bike with a wide range cassette. So do I.
Anyways. That’s a good transition to my next question: DH bikes have recently gone over to the dark side of front-center biased. Any ideas why this is? Are DH riders getting better at riding off the front of the bike? Are tracks getting smoother and faster?
Steeper the track the more a rearward bias makes sense. YMMV.
“...heck, maybe I'll write a white paper on this and publish it to my blog (LOL).“
If you do make sure to post a link back here!
https://jeffbrines.com/2024/07/22/a-novel-way-of-approaching-mountain-bike-fit/ I'm sure I'll add/proofread/change this when I wake up tomorrow. I desperately need an editor.
I have a theory that adding stack is the counter to long rear centers making a bike hard to manual/lift without sacrificing the benefits of longer chainstays.
Adding stack means that the rider creates a larger moment (rotating force) when pulling back and lifting the bike into a manual.
I don't really keep up with DH bikes at all, but, just to be sure, are you accounting for high pivots and rearward axle paths when looking at geo charts? Could that be part of it?
Thanks for writing this all out, Jeff. Another thought to back up your argument is that it's not just reach length that's gotten longer over the last 10 years. Reach length has grown at the same time that head angles have gotten slacker. And head angles have gotten slacker on all bikes, from DH bikes to XC rigs. This means that the front center is getting doubly longer, from the longer reach and the slacker head angle. A head angle that's one degree slacker translates to roughly an inch of additional front center length.
Post a reply to: The Internet Was Wrong: Short Chainstays Suck*