Quest for flat back.... saddle height and crank length
 

Howdy

I've got stubby little legs with a 31" inseam. I'm pretty borderline for 170 or 172.5, have been using a 172.5, and I've recently been wondering if I couldn't improve cadence and efficiency by dropping to a 170mm crank. Then, on a long bomb down mountain roads last night, it also occurred to me that this would have the side (and even more pronounced/beneficial) effect of giving me another 2.5mm of saddle height and thigh clearance to get tilted even farther forward more comfortably...

Sound reasoning, no? Any reason I shouldn't expect to be able to flatten out a little better after shortening my stroke diameter a full 5mm??

Thanks

-B

I'm not following your math or your reasoning. FWIW I am 5'10 and my inseam is around 33". I have 165mm pedals on one of my bikes. Most of them have 170 but a couple have 175. None have 172.5. If you get from that that I don't think that there is a magic crank length for a given inseam you would be right. Nor do I think the average person could tell the difference between a 170 and a 175 crank. But lets say it is true. How does the 2.5mm difference in crank length result in a 25mm reduction in seat height?

H

Not reduction- increase.

But you're right- my numbers are ********ly off. Obviously mixed up my CMs and my MMs in a spot or two.

But what I'd get is a 2.5mm INCREASE in saddle height, if I wanted it (not 25mm), and a 5mm overall decrease in pedal stroke diameter.

I don't know what 5mm comes out to by the time you get out to the shoulders, but if it's an extra inch, it's an extra inch in the right direction.

I have no idea what that's supposed to mean. What shoulders? And where do you get an inch from?

Flattening your back has to do with core strength, flexibility and whether you can comfortably roll your pelvis forward on your saddle, 2.5 mm of added saddle height won't make a lick of difference in this regard.

If you have a hard time spinning 172.5 mm cranks a 170 isn't much of a change, maybe try a 165.

If your thighs are hitting your belly or ribs, and that is preventing you from getting your back more horizontal, you might also try moving your saddle forward, raising your saddle, or losing some girth around the midsection.

I think you get a decrease in saddle height (not that I think 2.5mm decrease/increase would be noticeable) by using a longer crank. Your pedal would be 2.5mm lower at the bottom of the stroke than where it is now so you would need to lower the seat, granted it is also 2.5mm higher at the top of the stroke. I don't know of anyone who sets their seat based off of where the pedal is at the top of the rotation though.

EDIT: My bad, I need to learn to read, yes a shorter crank would theoretically need a seat raise.

This. Its what I was saying earlier. Because the pedal circle shrinks away from you the saddle must be lowered to meet it. But 2.5mm is not a perceptible difference. I don't even think 5mm is. A couple of degrees difference in the pointing of the toes makes much more difference.

H

He's talking about going to a SHORTER crank so he could raise his saddle because at the bottom of the stroke the pedal will be 2.5 mm higher.

It still won't make any noticeable difference.

I'm the same way: short legs and my thighs hit my ribs when I'm down. Manufacturers don't offer enough choice in crank lengths.

Here's a crank length calculator:
Optimum Bicycle Crank Length Calculator

I think the OP's reasoning is sound, but also agree that an extra 2.5mm at the top of the stroke is probably going to be undetectable, although it won't hurt anything, either.

According to that thing, I need 163mm cranks.... It's been a while since I've investigated methods, but I don't think any I've used have ever put me at that short.

I will need to investigate this... coming down to a 165 or 160 would obviously have even greater effect on angle relief

165's are fairly easy to find.

My inseam is 81.5 cm, about 32, and I just switched back from 172.5 to 170. Feels like home, and I can spin well, again.

I think a flat back is based on good stretch-out, strong core, and body balanced over the BB, with the saddle height not too high. With all that, you can find the comfortable bar height. But if it doesn't feel natural it's not likely to be right.