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There are three main solutions to the rear axle, in all cases it is only necessary to drive one wheel as a Greenpower car has very little power and its handling will not be affected by the uneven drive force.
The axle is a single piece of steel or aluminium rigidly fixed to the chassis, the wheels rotate on the axle on their own bearings and the driven wheel has the drive sprocket/pulley attached directly to it.
Pros: Simple to build, very strong and contributes to the car's overall strength, ensures perfect wheel alignment across the axle.
Cons: Can be heavy, a 700mm long 20mm diameter steel axle weighs 1.8kg, takes up space in an area often used for battery mounting.
As above a single piece axle but the axle turns in bearings fitted to the chassis. The driven wheel is fixed to the axle and the non-driven wheel is free to rotate.
Pros: Ensures perfect wheel alignment
Cons: Heavy (as above), requires a suitable hub to attach the drive sprocket to the axle, takes up space in an area often used for battery mounting.
Two short, fixed, axles on which the wheels rotate on their own bearings.
Pros: Up to 1kg lighter than a one piece solid axle, does not encroach on potential battery area.
Cons: Must be accurately built to ensure perfect wheel alignment, the structure holding the stub axles has to be stronger than for the one piece axle.
For wheels with 20mm bearings it is possible to use, in order of both increasing strength and weight, aluminium bar, 20mm x 3mm steel tube, or 20mm mild steel bar. Because the rear axle is such a critical part of the car it is worth adding the weight of a solid steel axle as this guarantees perfect wheel alignment.
Other common axles sizes that could be used with bicycle wheels, and are suitable for supporting on just one side, range from 12mm to 17mm. All of these must be steel, and if under 15mm then it is essential to use a high tensile steel to avoid bending and distortion on bumpy tracks. It is not advisable to use any axle smaller than 12mm if it is to be supported only on one side of the wheel.