As we ride along the road, a certain percentage of our engine’s power is used just to maintain our current speed, which I call Maintenance Power. Maintenance Power is made up of two components – Drag Power and Rolling Resistance.
Drag is by far the largest part of the Maintenance Power equation. Drag is caused by the displacement of air that the vehicle passes through, kind of like an artificial wind. Any motorcyclist is well acquainted with drag forces, particularly those on naked bikes!
The formula for drag is:
There are two important elements to these equations. The first is the symbol 'v', which represents velocity. The second is 'Cd.A'. Cd is commonly known as the drag coefficient, and A is the frontal surface area. Manufacturers of cars in recent years have been putting a lot of effort in reducing the drag coefficient of vehicles, as a good reduction will lead to improved fuel economy. A good car will have a Cd value of < 0.3.
The drag coefficient for a motorbike is fairly bad, as they don’t have a fairly smooth surface for the wind to flow over. A good bike will have Cd of 0.5, and a poor bike will be 1. For our example, we’ll pick a middle-of-the-road value of 0.70, and a frontal area of 0.5m2.
Note that the formula for Drag Power had the velocity cubed! This means that to double your speed, you need eight times more power just to overcome drag forces. To look at this in a graph:
Rolling Resistance makes up the other half of the Maintenance Power equation. For this example, we'll ignore the rolling resistance. It only makes up a small part of the overall force resisting the bike's forward momentum. It's value is more than overwhelmed by the 'fudge factor' of our 0.7 drag coefficient.
In the next installment, I'll combine the maintenance power and the kinetic energy to generate what I call Power Differential.