Drivetrain Power Transfer

Why can’t we just use force transfer?

In our previous section, we found force transfer is dependent on gearing and transforms fractions of the original Pedal Force into ForwardForce. So why is that not the end of the discussion?

Forces answer the question of what is happening at a particular point in time. They also can enable us to estimate the total effort required to complete a given task. But forces require an additional variable to be useful in estimating what it would take to complete the task in a given timeframe.

That is where Power comes into the discussion. It tells us how much effort we need to expend per unit time to complete a task in a a total time interval. It is not like we are discarding force transfer as useless. Power is force discussions with time brought in.

Forces enable us to discuss dynamically what is going on at a particular instance of time. By for a cyclist, they are interested in accomplishing some cycling objective or scenario in a given rate of time. At this point, the discussions shift to discussing power rather than forces.

Cycling Scenario Modeling

When discussing a particular cycling scenario such as riding a sprint at a given speed or climbing the Alpe d’Huze in a fixed time, we start by estimating the total effort, and then determine from the time how much effort must be expended per unit time to achieve that.

This becomes our scenario power output requirement. For the cyclist to achieve this goal, they must be capable of generating the power output requirement, not at the pedals, but at the Rear Wheel.

Pedaling Power and Cycling Power

Here are the two equations for PedalPower and CyclingPower. The point is that each is computed similarly, but with different physical elements, so we have no a priori reason to expect them to be equal.

CyclePower = TireForce * TireRotation * TireCircumference /60

PedalPower  = PedalForce * Cadence * PedalCircumference/60

Drivetrain Power Transfer

This brings us to the Drivetrain Power Transfer discussion. If a cyclist needs a certain power output to show up at the rear wheel, how much pedaling  power must they produce. The same, more, or less. In this section, we answer this question and will get one of those “sweet” insights into the cycle design. The cycle drivetrain transforms Pedaling Power equivalently into Forward Power. The only losses are small and due to friction.

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