Car Parts and Accessories

Variations in the three main Wheels designs

It is not enough for a wheel to be round: it must also be strong, light, well- balanced, resilient to some forces, stiff against others, and not too expensive for a mass market.

The basic types of wheel in use today – pressed-steel discs, wheels with steel-wire spokes, and light-alloy castings–meet all these requirements, though the last two are more expensive to produce.

Stiffness is necessary in a wheel in order for it to respond accurately to the demands made on it in steering; but the wheel must not be brittle. Resilience is necessary too, so that the wheel will absorb the hard knocks it inevitably takes.

The wheel has to have what is called a well-base rim to enable the tire to be fitted or removed; if the bead (or edge) of the tire is pushed down into the well at one point, the diametrically opposite part can be eased over the flange of the rim without serious difficulty.

These days cars are being fitted with safety rims which discourage the tire from dropping into the well, where it could be torn off the wheel in the event of the sudden loss of air pressure caused by a blowout. The safety rim thus reduces the degree of possible loss of control of the car.

Width of the steel rim is an important factor in the handling characteristics of a car. A rim that is too narrow in relation to the tire width, for example, will allow the tire to distort sideways under fast cornering. On the other hand, unduly wide rims on an ordinary car tend to give rather a harsh ride because the sidewalls have not enough curvature to make them flex properly over road irregularities.

Steel disc wheels

Most cars today are fitted with pressed- steel disc wheels. They are light, strong, stiff and resistant to accidental damage. They are also easy to produce in large numbers at low cost.

It was once a disadvantage of this design that the disc had to be liberally perforated to allow for the passage of cooling air to the brakes; and piercing holes in the disc could weaken it.

By using a slightly more expensive technique, modern designers can turn the disadvantage into an advantage. The holes are swaged—that is, their edges are turned smoothly inwards—and this may actually increase the strength of the wheel. Swaging of this kind is now a standard technique on production cars.

How the Wheel is secured to the axle

The most common type of wheel mounting consists of either four or five threaded studs equally spaced in a circle around the hub flange. These studs pass through holes in the wheel, which is secured by nuts screwed on to the studs.

The holes through which the studs pass are not simply pierced through the disc. The area around each hole is pressed out to form a tapered seating which allows the nut to centre the wheel correctly. Each nut has a corresponding taper.

Some manufacturers provide nuts with both ends tapered: others make nutswhich are tapered only at one end and flat- faced at the other.

Wheel nuts should be fitted so that the tapered face engages with the tapered seating in the wheel; otherwise the nuts will not centralize the wheel on the hub and the wheel will be likely to work loose.

It is dangerous to reverse a wheel so that it is mounted back to front. The centre area is designed to engage with the hub or brake drum over a considerable area and it is friction between these faces that transmits the drive.

Never leave nuts only finger-tight after replacing a wheel. They should be tight enough to ensure that the two faces are held firmly together.

One recent wheel-mounting design has the wheel positioned by the hub itself, which engages with a precisely-machined hole in the centre of the nave. Studs secure rather than locate the wheel.

Centre-lock disc wheels on splined hubs are fitted to some sports cars. The centre lock, or ‘knock-off nut, was evolved many years ago to enable racing-car wheels to be changed quickly by knocking off the nut with a soft-headed hammer. The wheel is positioned by matching tapered faces within the wheel centre and on its hub, the splines enabling driving or braking forces to be transmitted from one to the other.

In some versions, the single securing nut is shaped for tightening and loosening with a soft-headed hammer: others require the use of a special spanner.

Centre-lock mountings have an additional advantage now that radial-ply tires are in general use: radials are intolerant of inaccuracies of wheel centring, and the precise location of the centre-lock wheel helps to eliminate tire vibration.

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