Drivetrain Force Transfer

Pedal Force into RearTire Force

A cyclist is pushing on the pedals with a given force. We know from experience, that somehow that results in the rear wheel pushing against the road. How does Pedal force get transformed into RearTire force?

Computing RearTire Force from Pedal Force

The computation here is how much pedal force is translated to the rear wheel. This seems to ignore gearing, but we will discover gearing plays an important role in cycling force transfer. Pedal Force becomes RearTire Force through seven distinct steps:

  1. The cyclist pushes on the pedals
  2. The pedals through the crankshaft apply a turning force to the front chainwheel hub.
  3. The chainwheel hub in turn takes the turning force and uses it to pull on the chain.
  4. The chain force in turn pulls on the rear cassette.
  5. The force on the rear cassette becomes a turning force on the rear cassette.
  6. The rear cassette turns the force on its hub into a turning force on the rear wheel.
  7. The rear wheel takes its force and applies it to the road which in response produces a forward force on the cyclist-cycle.

Fast Tracking through Force Transfer:

The drivetrain discussions though straightforward can be intimidating. You should understand the following points even if you do not wish to work through the steps:

  1. Rear Tire forces are a fraction of Pedal forces in magnitude. But the reason for this difference is you are doing two different things. Turning a large wheel is different from turning a pedal crankshaft.
  2. Bicycles are extremely efficient and upward of 98% of the pedal force is transformed into forward motion.
  3. The gearing configuration is a part of the computation, and each possible configuration transfers force to the rear wheel with its own factor.
  4. The resultant force transformation estimate is determined by the current cycle configuration including the gearing.
  5. Transformation factors are computed using four radii. Two fixed: crankshaft and RearWheel. Two gearing: front chainwheel and rear cassette.
  6. If you do not wish to work through the details, feel free to jump to the final result.

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