Crank length/bb height . Is shorter/lower the future?

Well, ignorance is bliss they say. Good lucky to you…

I don't think it's necessarily about knowing how to pedal or not, it's about bike-body fit and opening up options. If someone feels more comfortable spinning a smaller circle and gets to approach features with a wider range of spots to fit a half-pedal in, it seems like a valuable change to investigate! Not to mention lowering the likelihood of getting stripped off your bike by a well camouflaged rock hiding in the grass/leaves.

Hmm, I don’t remember that warning being there when I bought mine - mine are still in the box, I might have to get in touch and swap them for 155s, or put them on my daughter’s bike

Completely agree! Good luck to you as well good sir

I went full dummy mode to 155mm Canfields. It was a hard comparison to my other bike as I was also switching bikes/wheel size (‘16 134 27.5 to ‘22 153 29). I had 175s on the 134. Supposedly you “should” drop a chainring size for the wheel size change AND for each 10mm drop in crank length. I stuck with the 30t (same as my 134) and it was too high, noticeably higher/harder granny gear compared to my other bikes. I even went from 46t to 48t granny in the rear. So I’ve dropped to a 28t oval which def helped, but isn’t quite the stump pulling ratio I had before, according to a gearing calculator. I’ve ordered a Ali express 26t ring by passquest to try but I refuse to go any lower than that - my 26” fatbike has a 26t front ring and it’s crazy low gearing. Great for climbing and snow with no top end. But the calculator doesn’t lie… So I’ll try it.

That all being said - I’ve had little to no pedal strikes where I thought for sure I’d hit or have hit in the past. And downhill, cornering, feels better than ever on shorter cranks. It’s all manageable with a smaller chainring, and the benefits in technical climbing far outweigh the extra grunt required, for me at least! I’m not done with them. 

EDIT 2024-01-05: Going to try 165s, so I can up my chainring size to 28t oval. Low end gearing will be the same, but 165s should help me not spin out so much on the downhills. But curious to see how much worse my pedal strikes are. I had close to zero with the 155s. I also need to up my springrate anyway F&R so might be a net-zero. We'll see...

Well I didn’t read everyone’s post so there may be some redundancy in this but hey, here’s your warning.  I think there is such thing as too low for a BB, especially for longer wheel bases because there will eventually be a moment that you will high center the heck out of something and ruin your day. That said, short cranks have my interest still.  I worked at a shop with a pretty accomplished bike fitter who works mainly with triathletes, and he is big on short cranks because it opens up the hips and allows for us mere mortals (read inflexible) to ride a more aerodynamic position on a TT bike.  While aero gains aren’t that sought after in mountain biking, hip impingement is definitely still a thing that can happen to us.  I have some pretty significant hip pain following a leg injury, which I have been treating with PT, but longer rides have left me in pain for days after and I’m not gonna say no to something that might help this. (for me a longer ride is 20-25 miles, Normal rides are 10-15) I haven’t been able to try short cranks on trail yet, but I have been able to hop on a fit bike recently and feel the adjustments in real time and I think there is some merit there.  All that said, SR suntour is flying under the radar making a 155 and 160mm Raidon crank that uses a 24mm style shimano spindle, it uses a non-boost chain line/Q-factor but you can get boost offset rings for it, so provided the Q-factor clears your frame it is a low-investment, experiment friendly option for those that want to dip their toes in without spending a premium.

Short cranks are huge in the road/tri/track world right now for that exact reason, and not just for mere mortals. Bradley Wiggins is as close to an immortal as they come, and he sized down from 177's to 170's and then 165's for the last Olympics to get more aero. He said opening up his hips allowed him to drop his front end by 30mm, which has an enormous effect on aerodynamics.

I think that getting your bars lower won't be a big selling point for mountain bikers who aren't racing cross country, because most enduro/trail/downhill riders run relatively high handlebars and upright pedaling positions, so hip impingement isn't a normal problem riders are butting up against. Obviously riders recovering from hip injuries will be a little different.

I think the bigger selling points for short cranks in MTB is what others have mentioned, minimizing rock strikes with your pedals and cleaning up your pedal circle, especially for people with shorter legs or impingements who are ill-matched to longer cranks.

Canfield recommends about 20 percent of inseam as a starting point. 

https://canfieldbikes.com/pages/mtb-crank-chainring-faq-canfield-bikes

I'm 6'4" on an XL frame, but with a 32-inch inseam I'm now running 165. My only regret is not going shorter sooner.

it's a weird balance in my opinion. The minute you drop your bb by 20 mm, now you have a new set of problems - smashing your chainring/bash guard and feet into things you previously didn't have an issue with. Assuming your trails are technical. Now if you're just smashing out bike park laps, then lowering bb and shorter cranks could be a good option.

If your running shorter cranks to reduce rock strikes, then lowering the bb for some reason, aren't you just going to be striking you cranks as much as before as well as smashing you bash guard/chain ring more? Seems like your taking a step forward and 2 back......

Add Trailcraft to the list. I spoke with him while making my decision to go shorter, and found him to be quite nice. he has some other sizes besides the multiple of 5 you see from others. Just mounted my Canfields and am excited to get out for a ride. 

https://www.trailcraftcycles.com/product/trailcraft-direct-mount-cranks/

I want longer cranks. I love the idea of a low BB, but you get more leverage on the chain with longer cranks, period. I can't afford to lose any pedal power, as I am a weakling. 

What would be really cool is some kind of telescoping crank that gets shorter at the bottom of the rotation so it clears the ground. you could then have a longer arm for the pedal stroke. 

Track racers are still putting out 1800+ watts with 165mm cranks, where as before they were running 177mm cranks or longer.  You aren't losing any power, that myth has been dispelled long ago. There is much more involved in producing power on a bike than just the length of the lever arm.

I think the foot stance length is more interesting for descending. Gwin talks about going between 175mm -> 165mm on the DH bike based on how far apart he wants his feet.

BB's have already gotten lower and as someone who rides XL, or sometimes now a L, I really don't want the BBs any lower.  If anything, custom geo is the future.  We all have different preferences regarding bb heigh, cs length, reach, sta, etc.  Not really a right or wrong answer.  

I've tried 170's a couple of times and have been unable to get used to the feeling of spinning clown circles.  I actually preffered 177.5 on my roadbike and am happy with 175 on the mtb.  Going to give 170 another try so I can run the bike in the low setting (for the more progressive suspension curve, not the BB height).  

Back in the day Santa Cruz used to have 14 7/8” bb heights probably at sag. Pedaling out of a turn is quite a bit better with shorter cranks on modern bikes that have low bbs. Flat pedals are 4 1/2” wide and clips are 3 1/2” so if you are not badass and on flats like Sam Hill then short cranks are going to be a lot less mind blowing. 

long cranks for people with a low cadence and shorter cranks for riders with higher cadence seem to work best.  

6'2" here, ride an XL and have a 33"ish inseam.  I had been running 175's on my trail/xc bikes and 170 on my enduro bikes and ebike for years.  Recently, I switched to 165's on my enduro bike, 160mm on my ebike and 170 on my trail/xc bike.  

When I initially built up my enduro bike back in 19', I kept getting tripped up going from 175's on my other bikes and getting on that bike and feeling like my leg/foot speed was so fast and yet it felt like the bike was sluggish, like I was pedaling through molasses.  That of course, went away after about 17-10 rides.  I got the same phenomenon when I put the 165's on my enduro bike and just now, after say 10 rides, it's starting to normalize and pedaling feels normal.  

Overall, I prefer the shorter cranks from a downhill perspective most.  It seems to me that the timing is easier on jumps/drops and popping off of features. I also have noticed that turning in my un-natural direction (right for me since I ride left foot back) seems to be easier and it's easier for me to twist my hips in that direction with the shorter crank length.  Pedaling is more or less the same as before and other than on the ebike, which I am prone to rock strikes on because of the nature of doing sketchy climbing things... the rock strikes are all about the same.  

Overall, I think it's a positive change for me.  I also do like that It's given me the chance to go to a 10mm longer dropper on one of my bikes where the seat tube length is stupid long (orbea rise in size XL).  

Just wanted to share my recent experience with 5dev customer service.  I had ordered a set of 145mm enduro cranks from them last fall purely out of short crank curiosity.  With the never ending winter we had in Utah I hadn't gotten around to installing them and then someone in this thread noted they were actually for youth riders.  I contacted them a couple days ago and they were super cool, sent me out a set of the new freeride cranks and a return label for the old ones.  I'm aware of the whole wheelie guy debacle, just wanted to provide a little counterpoint.

I'm sure there are lots and lots of variables when we're talking about bicycle cranks. However, a longer lever with the same force applied to the end of it produces more torque, period. People have been failing to disprove that since Archimedes' time. Proper pedaling form, athletic output and foot placement preferences aside, even a slightly longer crank would produce slightly more output. 
Now could I ever tell the difference? Maybe not. You may have me there. 

"The same force applied to the end of it" is where it gets tricky. In an extreme example, if you swap out 175 cranks for 155s, the first thing you have to do is raise your saddle 20mm to maintain proper pedaling position. That means at the top of the stroke as you begin to apply power, the pedal spindle is about 40mm further away from your hip joint, a very noticeable biomechanical advantage. At the 3 o'clock position, the spindle is 20mm closer to the vertical plain of the hip joint, similar to having a more forward seat position over the cranks so that power is applied more in direct opposition to the gravitational force acting on your mass. These are noticeable biomechanical differences that work well for some people (me!) and out weigh the simple mechanical advantages of a longer crank arm.

I think we might be arguing across purposes on this thread as people are using torque and power interchangeably. You're absolutely correct that a longer lever will produce more torque. No argument, and this is one reason that I personally like longer cranks on my mountain bike, because I like being able to notch up tricky technical stuff at low RPM and the long cranks allow me to pick my way up things more methodically instead of spinning with abandon. The question that a lot of people are asking is "which crank length produces more power?" where power refers to work over time. Or they're fretting "will a shorter crankarm negatively affect my power?"

To produce X amount of power, a person can push a longer crankarm at a lower RPM (moving their feet around a bigger circumference circle), or can produce the same power by spinning a smaller crankarm at a higher RPM (moving around a circle of smaller circumference). The circumference of the circle represents the distance of work, and the RPM captures the time aspect. And it turns out that, even though people generally have a pretty narrow RPM power band for a given crank arm length, they adapt to different crankarm lengths pretty easily by shifting their RPM power band slightly up or down. And if for some reason you're dead set on keeping an exact RPM number as you change crankarm lengths, you can just shift gears to accommodate your desired RPM.

Barring physiological limitations, different crank lengths are surprisingly comparable in power, and the torque thing only matters for rapid acceleration events like sprinting or my example of notching up a techy climb. It also turns out that a lot of people have physiological limitations (knee pain, hip mobility, back pain, etc) that make shorter crank arms more efficient.

Regarding length and torque. I can't remember where I read it (seb stott on pb maybe?), and apologies if it's already been discussed in this thread. Anyways, the gist is that when going shorter/longer a rider keeps the same foot translational speed not the same crank rotation speed. So going to shorter crank arm your foot will be still be doing the same m/s but around a circle with a smaller circumference, hence the rpm will be higher. And you'll naturally shift to a lower gear, so while the torque at the cranks will be lower, the torque applied to the rear wheel (which is what actually matters) will be the same, or roughly the same depending on exact gear ratios. If you regularly use your lowest gear with long crank arms, then you would have to go to a smaller front chainring (or a cassette with a bigger dinner plate) if you go to short cranks to preserve the same torque at the wheel. 

Hope is making 155 cranks.

https://www.instagram.com/p/CtXLaaJsjPx/

Pretty sure Laurie has been running those for over a year (albeit w/ a Saint sticker on them)...

Great point- As a longtime snowboarder I've always messed with stance based on riding style, and I've always wonder what the goldilocks stance would be in terms of strength/endurance, weight distribution, and when you combine Q factor, crank length, with the cockpit (i.e. how upright and forwards body positioning will be).