The wheels on your car must be strong enough to resist the many forces acting upon it but light enough to allow it to be steered. The rolling motion of the wheel reduces friction and when two are used in conjunction with an axle, it allows for heavy loads to be carried safely. A load is at its most stable when it is within the area bounded by the wheels.
In your car, passengers sit within the area that is bounded by the wheels as this provides the greatest comfort and protection.
Wheel Construction and Size
Your wheel is made up of a hub at the centre of the wheel which is where the axle connects joining 2 wheels. A set of bearings allow the wheels to rotate around the axle. The hub is then connected to the rim by either wires or spokes.
Your wheel can be dished which means that the hub and wires or spokes are set in a bit from the rim, in order to protect them from damage. The rim is the outside edge of the wheel that holds the tyre.
The width of a tyre should never be greater than the width of the wheel. A tyre that is wider than the wheel will distort when cornering. A tyre that is much narrower than the wheel will not flex properly, resulting in excessive vibration and a very uncomfortable ride.
A wheel larger than the standard size for your vehicle will mean the vehicle is going faster than the speedometer shows. Put simply, this is because your speedometer is measuring how many times the wheels are turning. If the wheels are bigger than the standard, they will be travelling further and therefore faster for a given number of turns.
Most vehicles carry a spare wheel but, in order to create more space in the vehicle, a much thinner spare wheel is often used. Spare wheels are for emergencies only and are sometimes known as 40/40 wheels. This is a reference to the fact they should not be driven faster than 40 mph over a distance not greater than 40 miles. If your vehicle has a space-saver wheel you must consult The Manufacturer’s Handbook for the correct use of it.