Energy – Nothing happens without it. For this section, we'll be looking at Kinetic Energy, and how that energy relates to power requirements. Kinetic Energy is a measure of the moving energy of an object. When a motorbike changes speed, its total kinetic energy changes. The formula for this is:
To show how the energy changes, I'll go through a simple example. For our example, let's assume that we're trying to accelerate a 270kg motorbike (incl rider) from 50km/h to 100km/h
KE (50km/h) = 0.5 * 270kg * 13.89m/s^2
= 26,045 J
KE (100km/h) = 0.5 * 270kg * 27.78m/s^2
= 104,183 J
So, as you can see, a doubling of speed results in a quadrupling of the kinetic energy.
So the increase in Kinetic Energy = 104.2 kJ – 26.0 kJ = 78.2 kJ
Let's relate this to power. Power is energy over time, which can be seen if you break down the units
Power (watts) = Energy (Joules) / Time (Seconds)
So to calculate the power needed to be expended in our 50-100km/h case above; let's look at two cases – Accelerating to 100km/h in 3 seconds, and doing it in 6 seconds.
Power (3 seconds) = 78.2 / 3
= 26.1 kW
Power (6 seconds) = 78.2 / 6
= 13.0 kW
This correlates with our known experience. You need a lot more power to accelerate quickly, than to accelerate slowly. Note that this is not the engine power that you need, but the additional power to the wheels that will need to be delivered just to accelerate.
These equations just look at the energy difference between a vehicle travelling at 50km/h vs one travelling at 100km/h. Noticeably absent is discussion about the drag caused by wind resistance and the like.
In the next instalment, I'll be looking at Maintenance Power, which is the power that you need to maintain a given speed on the road.