Greenpower Car Maths - Chipping Sodbury School

Terry Barnaby - Version 1.0 - 2009-01-14 - Information and Help
This is an interactive, Greenpower car forces and Motor temperature calculator.It uses the basic mathmatics outlined in the Greenpower Car Maths and Science document to estimate the main steady state forces and other parameters of a Greenpower Car when racing.

To use enter the parameters for your car in the form below and click on the 'Calculate' button.Some info is available on the following link:

There is information on selecting gear ratios at:

Car All Up Weight (Kg)Rolling Drag Coefficient
Frontal Area (m sq)Air Drag Coefficient
motorGearingWheel Diameter (m)
Measure of motor case cooling (degrees C/W)

Current: 16.411599403 Amps, Speed: 34.776 Km/H

Where the black arrow crosses the green line will be the steady state, where the forces balance. The car will accelerate until this point is reached.This will be the cars maximum speed on the flat with no wind.Where the black arrow crosses the purple line will be the steady state current being used by the motor at the cars maximum speed.

The motor temperature calculations are rudementary and could be wrong, but the basic trends are correct.Motor temperture is proportional to the square of the motor current plus an additional component due to winding resistance increasing with temperature.Motor current increase is proportional to the gearing ratio used.Note that for a given gear setting the Motor temperture is, approximately, inversely proportional to the square of the cars speed.This means that if a car ends up by be driven more slowly than the full speed that the gearing would allow (power on,off,on ...), possibly due to driver experience, the heat generated is much much greater.