Without getting really technical what determines the length of the cranks that you would use on your bike? I was wondering if longer cranks without being ridiculous would act as a fulcrum and make climbing easier in a higher gear ratio.
As an opener to your question....
Femur length. Theoretically. The longer the femur - the longer the crank length. The old pros used to favour the longer crank lengths 175mm to 180mm being common. Iirc Pantani who was a short rider even favoured a slightly longer 172.5mm for reasons of leverage as you mention above. There has been a trend towards shorter crank lengths - which lend themselves to a slightly faster cadence.
Natural cadence speed. Most riders find when they switch to shorter cranks, their cadence increases.
Flexibility. A longer length will bring the upper leg at the top of the stroke closer to the abdominal cavity.
You are correct that a longer crank arm acts as a longer lever resulting in greater peak torque output on the power stroke of the pedaling cycle. That might lead us to think that longer cranks are better. However, the optimum crank arm length is that which enables a rider to produce maximum power which does not necessarily correspond to maximum peak torque.
The most important factor determining optimum crank arm length is obviously the length of the femur (between hip and knee joints), but there are other physiological factors that mean a given rider may prefer a slightly longer or shorter crank arm.
Really, there's no simple way of determining optimum crank arm length. Pro riders who really want to do the optimization will do controlled tests with different crank arm lengths and a power meter.
You have two questions there.
What determines crank length?
- Personal preference
- Equipment availability
- Drive train type
- Stories you read on the internet
Not necessarily in that order.
Do long cranks make climbing easier on high gear?
Increasing crank length increases torque and reduces pedaling speed (gears can be used to convert RPM to torque and vice versa). Whether your body accepts such trade decides if it makes your climbing easier. You will still need to output the exact same power.