Originally Posted by
slyjackson
Whereas I have read in replies here that going from a 175 mm crank to a 172 mm or 170 mm crank would allow you to spin faster. Well you will not spin any faster just by reducing your crank length 1/8" or 3/16”. You want to spin faster work on the engine….
If you think less than a 1/8" or 3/16" of crank length will make you a lance Armstrong or enhance your performance and add 2-3 mph on your average speed, you are mistaken.
The primary reason to move to a smaller crank arm is comfort. If you use too long of a crank arm and adjust your seat height so as for the angle of your knee to be optimal at the bottom of the stroke, then the angle at the top of the stroke will be more severe. For many people this can result in more pain in their knees over extended usage.
It can also affect performance by making it difficult for the cyclist to maintain their optimal spinning motion. Obviously this would vary from person to person.
This seems to be even more of an issue for recumbent riders, where one's leg position is fixed relative to the pedals.
I use 170mm cranks on my upright bikes and those have worked well. However on my recumbent, 170 was too long. I was either losing some contact with the pedal on the downstroke, or it was coming up too high for comfort on the upstroke. I switched to 155's and my cadence went up by 10 and my speed by 1 mph immediately ... because I was then able to apply power throughout the stroke. And just cruising along is now so much easier than it was with the 170's.