Crank arm length
 

Trying to think something through.

I haven't been comfortable on my wife's mountain bike; it's frame is a bit too small, which is a good though for an MTB; but I've always felt like my knees are going into my chest if I set it where I don't over extend at full leg extension.

Now, I have my road bike fitted to me, all feels good; but as a roadie I'm used to spinning--and this MTB feels wrong. Long gentle climbs just feel awful to take seated. I'm starting to wonder if it's due to the crank arm length. I don't have my road bike in front of me but I think it has 170mm cranks while the MTB has 175's. So I'm wondering, would it make sense to drop the MTB down to 170's or maybe even 165's?

In my head, at full leg extension, the saddle needs to be in the same spot regardless of crankarm length; and dropping crankarm length means the saddle has to go upwards. Which seems wrong, for mount/dismount. But at the same time, at the top of the stroke my knee won't be so high, which might fix the problem I'm having.

In the same vein, does foot position matter in this? I ride clipless on road, which puts pedal axle under the ball of my toes. MTB, I use platforms and ride with the pedal under my arch. I think, I don't really pay attention to exact location, but definitely farther back. My old 3 speed I think has 165mm cranks, platforms, and no problems with pedaling.

I have crank sizes 165 to 175 on the different bikes I have, and I feel comfortable on all of them. Buying them and trying it out is really honestly the only way to tell if it'll fit you or not.

On the road bike you may be using your ankle to extend your reach at the bottom, and flexing at the top, making the crank feel shorter. So changing the crank length may make some difference, but it probably has more to do with foot position.

I suspect it has something to do with bottom bracket height. Higher bracket, thus the saddle has to go up. But being a mountain bike I need to be able to drop a foot and not fall over. So I can't go above a certain point and safely ride.

That's my suspicion, anyhow.

That is how it is, but you shouldn't be seated in the saddle when mounting/dismounting. Better to be standing on one pedal, then you can have more room to work.

Somehow I missed learning that, years ago. Well, I can do that, it's just not what I normally do. As I play with saddle height on my road bikes it appears I will wind up doing this more often.

But when I was doing MTB years ago I got into the habit of dropping a foot pronto when I'd snag on a rock or root. Hence I "needed" a low seat then. I think I might have copied that to road bikes when I switched over several years ago--but I left my saddle alone after getting a fit at a bike shop. Which left me quite able to easily stop with one foot down.

Anyhow, as I come to realize this, I suspect I'm not going to get back into MTB'ing, as I'm not sure how I'd be able to come to abrupt stops while still sitting (which is how I do most of my biking).