Wheels

Function

Wheels support the chassis and allow the car to roll. Powered wheels also transfer a force to the ground that can make the car move.

Materials for wheels

Look at materials that are round or that can be cut into circular shapes. An effective wheel for your car will be rigid enough to keep its shape under the weight of the car. A lighter wheel needs less energy to start it turning (less inertia) but it must still be strong enough.

Some materials that you could use are:

• CDs
• thin plywood
• balsa wood
• foam core
• stiff plastic sheet
• cardboard tubes
• plastic toothed gear
• plastic bottle top
• thin disc stuck to plastic toothed gear
• card or plastic or wood disc cut out with a pair of compasses
• cotton reel
• toy/model wheel
• tape spool
• plastic pipe

Design decisions

The choice of materials for wheels and wheel alignment can make a big difference to the performance of the car.

• There should be minimal rolling resistance.
• Try different diameters and widths of wheels on your car.
• Remember also that the diameter of the drive wheels affects the final transmission ratio of your car – a larger wheel diameter gives a higher overall gear ratio.
• Low weight in the wheels reduces the energy needed to accelerate the car.
• A narrow wheel has a smaller ‘footprint’ than a wider wheel of the same diameter. This means that you can alter the area of contact between wheel and running surface and see how this may affect car performance.

Fixing a wheel

Some manufactured gears and wheels have a centre hole (or bore) that is fractionally smaller than the diameter of the axle on which they are intended to fit. This is known as a ‘push-fit’ fixing. A way of adjusting the bore size of a wheel is to use a plastic reducer. The wheel and axle are securely attached so that turning force is transferred between them.

Tyres

Tyres are not always necessary, used to increase grip, change the ‘footprint’ of the wheel or offer a little shock absorption at the wheel.

Design decisions for tyres:
narrow and not too soft to keep rolling resistance low, suitable material to create traction (useful friction to transfer a force)

Materials for tyres:

• Rubber ‘O’ ring
• Flat rubber band
• Rubber from a balloon
• Rubber sheet
• Cloth tape

Maths for Building Wheels

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