Originally Posted by
max5480
you're switching to longer cranks so you can spin your legs faster going downhill?
what's your logic?
longer radius = larger circumference = longer distance your legs have to travel in the same amount of time (assuming cog and chainring remain the same) = more work
What you are referring to is footspeed, which is the actual speed the foot travels in the circle.
For a given cadence, the bike with the longer cranks will require the rider to have faster footspeed.
So, if you can move your feet in a circle at x meters/second, you will have a higher cadence (and therefore higher bike speed) on shorter cranks....but you give up leverage, so it requires more torque to get the cranks moving. This is really evident on hills, which is why longer cranks are recommended for hills. There are no hills on the velodrome, so shorter cranks are normal.