Crank Length

[Jason_DuBose]

longer cranks = more power needed to turn them over.  you will definitely notice the change--which is not to say you shouldn't switch, but i'd wait until the season's over to do so.

Ok, so I'm new to all this cycling stuff, but wouldn't longer crank arms require less torque to turn them over, and thus a lower wattage, since T=F*r, and P=T*(angular speed) => Power to the wheel=F*r*(a.s.).  I guess I don't understand how it requires more power to turn over a longer crank arm since increasing r means decreasing F to keep the same Power? 

Please excuse my ignorance.

[architec]

does size REALLY matter?

[Vanessa]

jeff, you shouldn't have to ask.

jason, i have no idea what that means.  i got a d in physics.

there are a lot of theories about crank length, mostly involving whether or not body type should determine crank length.  some people recommend choosing cranks based on your height, some based on femur length, et cetera.

if you change your crank length, you change your hip angle, which affects your cadence.  if you are used to spinning 95 RPMs/minute on a 165 mm crankset and you switch to a 172.5, you'll need more power to maintain that cadence.  a longer crank produces more power, but it also requires more power if you're trying maintain your riding style.

[Jason_DuBose]

Ok, so I guess what you're saying is it's more an issue of biomechanical efficiency due to a larger radius of motion, not whether power to the wheels changes with respect to crank length?  All things being equal, pedaling at "X" rpm's with two different size cranks, the longer one would provide the higher power to the wheels, which means more speed at a given cadence.  The guy with the longer cranks could just change gears to go slower at that cadence, but he can't change the fact that he's pedalling in a bigger circle while trying to maintain an efficient cadence?

I think I see a little more light, but I do not claim to know a damn thing about biomechanics haha...

[ThePrez]

http://www.cptips.com/crnklth.htm

[ALF]

i think i need shorter arms but i need to finish a race with the pack first...

[olie05]

I changed from 170 to a 172.5mm crank length, and although everyone had a different opinion on whether it would actually make a difference or not, this is what i found:

I was spinning 5-10 rpm slower, at roughly the same effort

I had to raise my seat - due to my knee going to sharp angles at the top of the stroke. This is counter intuitive because you would think since the bottom of the stroke is lower you would just have to accept the higher knee angle at the top, well, in retrospect i probably had my seat too low with the 170's.

when I rode Athar's track bike recently (165mm cranks) I felt like i wasn't using all the muscles I had developed with the 172.5's which leads me to believe that you use more muscles when you allow your legs to go through a wider range during the pedal stroke.

Those are just a few of the things I have noticed.

Also, you can put less force into the pedal to get the same amount of torque with longer crank arms vs. shorter ones, but to go the same speed at that given torque, the pedal velocity will be higher (even though the rpm is the same). This explains why I was able to spin slower and use the same effort to maintain the same velocity using a bigger gear, after changing to longer arms.