Road Cycling - Crank arm length?

Bikeforums.net is a forum about nothing but bikes. Our community can help you find information about hard-to-find and localized information like bicycle tours, specialties like where in your area to have your recumbent bike serviced, or what are the best bicycle tires and seats for the activities you use your bike for.



PDA

View Full Version : Crank arm length?

gringorio
06-12-04, 04:07 PM
Hello,

can anyone describe in laymans terms the benefits or drawbacks to longer crank arms for time trialing or hill climbing? are there ergonomic benefits to longer crank arms? do any benefits depend on ones body type/leg extension (leverage)?

thanks!

on2wheels
06-12-04, 04:49 PM
Hello,

can anyone describe in laymans terms the benefits or drawbacks to longer crank arms for time trialing or hill climbing? are there ergonomic benefits to longer crank arms? do any benefits depend on ones body type/leg extension (leverage)?

thanks!

I remember reading somewhere that recommended longer crank arms for cyclists that rode bigger gears and a lower cadence - the longer crank arms give more leverage. Conversely, shorter crank arms are better for cyclists that like to use a higher cadence and smaller gears. Lance went down from a 175mm to a 172.5mm crank arm length on his time trial bike to compliment his unusually high cadence.
sch
06-12-04, 04:53 PM
Unless you are very strong, (say capable of constant power output over 300watts
for several hours), you will find that higher rpms are more efficient than high torque
in generating high power outputs needed for TT or hill climbing. There are a few
cyclists who do ok at cadences in the 40-60 range but if you watch pro cyclists or
ride with good cyclists you will note cadences in the 80-100/min range and occasionally faster. Longer cranks interfere with your ability to spin and thus
generate high cadences. In theory they reduce torque felt by the legs, but in practice most people are more comfortable using the shifter to do this. Long cranks
force a larger circle that the legs must move through, and the human leg (for most
people) works better with a smaller circle. Taller people benefit from longer cranks
and ATB are typically specified with 175mm cranks compared to the road standard
of 170mm. So a 6' tall rider might be better fit with 172.5 or 175mm road cranks.
This is not a simple question and there are more parameters involved than I have
mentioned, but generally where they have been tried shorter cranks are more comfortable than longer cranks, above and below 170mm. (Track bikes have 165mm
cranks to facilitate spinning, trackies spin like crazy...) Steve