Bicycle Mechanics - Bike fit question: Crank size
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05-24-11, 07:24 PM
So I was contemplating going from 30-42-52 to a 28-38-48 earlier.
My current crank is a 175mm crank.
Now, my seat's a bit higher because of the seat post. I changed it out, and now when I push all the way down I feel like I lose power. At BDC, my leg is just barely not bent; I come 1/2cm or less off the seat straightening it, but just the natural spring in my heel can let me straighten my leg fully without lifting. It's a natural shift; my heel isn't flexing enough for me to notice, much less care, that it's not straight.
That's easy, right? Cut the bottom 2cm off the post (it bottoms out in the seat tube), slide the seat down.
Well, my knees are pretty high at TDC too. It's not a stretch for me, but I think it's sub-optimal. The knee itself is closed at around a 60-70 degree angle, and of course my femur is not parallel to the ground at all. Angled up from hip to knee.
I've seen 170mm and 173mm cranks. Does it really make that much of a difference? I'm thinking I don't want another 175 ... a bit smaller seems better for me. Not sure though. I might have to poke around a bike shop I guess.
05-24-11, 10:00 PM
Sounds like your seat is too high. At the bottom of the stroke, the ankle-knee-hip angle should be only about 140-145 degrees for road bikes.
(That is, 140-145 is best for max effectiveness. If your knees object, you should of course raise the saddle.)
I don't think crank length makes a huge difference. One reason for that is that the differences between commercially available cranks are TINY. You can get 165mm through 175mm and there's a few 180mm ones available, but that's pretty much it. Going from a 165 to a 175 adds only 1 centimeter to the radius of the circular shape your legs traverse. It's noticable, but won't affect bike fit tremendously. A 5mm difference is even less noticable. I'd say it's not worth the money (cuz crank replacement is a pretty serious and pricey procedure).
05-24-11, 11:18 PM
http://www.peterwhitecycles.com/fitting.htm Go down to crank length. On a "racing" triple you can change the 30 for a 24 and the 52 for a 50 and the middle to a 38.
Foxie, what is your height and inseam?
bluefoxicey. From what I've read and experianced in decades of riding a bike, the crankarm length is the most easily adaptable cycling related item for the recreational rider to become accustomed to, barring range of motion issues. I read excerpts from a study that proclaims that 170 mm is the most suitable length for most recreational riders. My four roadies have four different crankarm lengths and I can tell a difference if I ride them back to back, but then I quickly adapt.
Cadence is perhaps the main issue. A shorter crankarm is more suitable to a cyclist that spins, or wants to spin at a higher RPM and the longer crankarms are a good alternative for those that spin slower, producing a tad more torque.
All-in-all it won't hurt anything to try the 170 mm length crankarms.
05-25-11, 06:52 AM
I tend to spin around 100-ish RPM, but I'll sprint .... I've got a gear that's ~12mph at 90 (according to Sheldon's calculator) and I hit over 25mph in it. By the way, I never stand; the muscles needed for standing are weak, and they fatigue in seconds, so I've never bothered. Yes I climb long, steep hills entirely seated. So yeah guess I'm doing everything wrong xD
In any case, shorter crank arm, faster cadence, okay, that should make my normal more comfortable.
I'm going to find a hacksaw and chop off the bottom of the seat post, then polish the end smooth again I guess. We'll start there. I'll be mindful of the crank arm length in the future though, didn't know it let you spin faster.
05-25-11, 08:51 AM
Using a shorter crank will mean you pedal at a higher cadence for the same overall gear, but due to the pedals moving in a smaller circle, pedal speed will be the same. Crank length should not be used to adjust your gearing- that's what the front and rear sprockets are for. You should use the crank length that feels most comfortable, and adjust the gearing accordingly. I like the use of gain ratios rather than gear inches/meters development since it takes this into account. That said, I use cranks as short as 140mm on my commuter and though I notice a difference, find no real problem when I ride my other bike with 170mm cranks. Depends on the rider I guess- some people find a 2.5mm difference (1/10") to have an effect.
05-25-11, 10:51 AM
Well yeah I know that, I'm looking at gear ratios; but my crank seems to raise my legs fairly high if I'm on the seat, and if I drop the seat 2cm it's going to be ridiculously high... maybe 1cm would be good, I dunno, we'll see.
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