Pedal and Forward Forces

The Cyclist and Pedaling

As magnificent as a cycle might be, it is not going anywhere without a cyclist. But once you put the two together, the cyclist has a role to play in apply force to the pedals, and the cycle has a role in transforming that pedaling into a forward force applied by the rear tire to the road.

Rear Tire and Forward Forces

We already understand the cycle drivetrain converts pedal force into rear tire force applied where the  rear tire meets the road in the backward direction. By Newton’s Third Law, the road pushes back in the forward direction ultimately moving the CyclistCycle forward.

Computing Forward Forces From Pedal Forces

We have already seen how the cycle drivetrain transforms pedal force into rear tire force. One is applied at the pedals along a small circle, while the other is applied at the outer edge of large cycle. For that reason, the forward force is a fraction of the pedal force. Knowing which gear you are in, you can determine what that fraction is.

Computing Forward Power from Pedaling Power

This is where things get really easy. While the rear tire force is less than the pedal force, when it comes to power, the distance rotated by the rear tire when compared to the distance covered by the pedals is in reciprocal of the force transfer fraction. The net result is when each power is calculated, it turns out to be the same.

If you have a scenario that requires a given forward power ability, you therefore know that the cyclist will have to generate Pedaling Power that is just a bit more to account for friction.

The Front Wheel Role in Forward Forces

It has none. It is simply along for the ride. Bicycles are rear-wheel drive vehicles. In the early days when the pedals were directly connected to the front wheel, it was the Big Shot. But today, its role is to help support the cyclist and to enable turning.

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