Car Parts and Accessories

Spokes criss-cross to give Wire Wheel Strength

The earliest type of wheel still in production is the wire-spoke variety, a light but very strong wheel now used mostly for sports and racing cars.

All loads on the wheel are transmitted from the rib to the hub, by spokes made of steel, which are much stronger in tension than in compression.

Spokes individually have little resistance to bending stresses, so they have to be laced in a complex pattern, criss-crossing in three planes. This ensures that all the complex loads fed into a wheel are resolved into tensile loads (that is, pulling rather than pressing or bending loads), evenly distributed among an adequate number of spokes.

Assembling a wire-spoke wheel is a skilful operation. Each spoke is hooked at one end into the hub and its other end is pushed through a hole in the wheel rim, where a tapered nut (the nipple) is screwed down to pull the spoke tight. If any spokes are too loose or too tight, the relatively flimsy rim will distort.

The pierced rim of a wire wheel makes it impossible to fit tubeless tyres, as they need airtight rims.

Wire wheels are expensive because of their complicated construction. This expense was justified in the days when the alternatives available were not so strong or light; but, strictly from an engineering point of view, there is little justification for wire wheels today.

A wire wheel is fitted in the same way as a centre-lock disc wheel. The wheel hub and axle shaft are both splined; when the wheel is slid on to the stub shaft, the splines engage, so that when the shaft revolves it turns the wheel.

The wheel is secured by a large six-sided nut or a wing nut which can be tightened or loosened by a soft-headed hammer.

Coping with Stresses from different Wheel Directions

The car wheel is subjected to extreme loads and stresses even in normal road use.

It has to withstand the car’s weight and forces of acceleration, braking and cornering; and it often has to cope with several of these stresses at once. Cornering, for instance, can be combined with acceleration or braking, which subjects the wheel to different stresses at the same time.

For steering control the wheels must be of rigid construction. There is no difficulty in obtaining the necessary stiffness with a cast light-alloy wheel, because the short ‘spokes‘ can be cast in the form of substantial radial ribs.

With a pressed-steel wheel, the ’spoke’ portion is usually of near-conical shape for extra lateral (side-to-side) stiffness.

In the traditional wire wheel, where the spokes are the only connection between the hub and the rim, this essential sideto-side stiffness is achieved by using a relatively wide hub with two or three sets of spokes arranged at different angles. The spokes are arranged in pairs, inclined so that, with the hub, they form a series of rigid triangles for resisting sideto-side forces when the car is cornering.

Backward and forward weight transfer, when the car is braking or accelerating, is absorbed by the steel spokes acting alternately in tension. Properly designed disc or alloy wheels cope readily with these forces.

The Cast Light-Alloy Wheel

At present the number of production cars fitted with these wheels as standard is relatively small, but it is increasing.

Because of their light weight compared with steel, aluminium and magnesium alloys permit the use of thicker sections, which promote stiffness and distribute stresses over a wider area.

The wider rim possible with light-alloy wheels is the main reason for their use on some sports cars. They allow wider tires to be fitted, so improving road-holding, especially on corners.

Light alloys are also good conductors of heat, and therefore disperse the heat generated by brakes and tires more quickly than steel does. Some precautions are necessary with light-alloy wheels. They react badly to salt spray and must be checked regularly for signs of corrosion.

There is also a danger of electrolytic corrosion, which may occur when steel comes into contact with a light alloy. To prevent this, the holes through which retaining or locating studs pass should be greased; and if balance weights are used, avoid those which are held in place by steel clips or screws.

Wheel Balancing

A wheel that is not properly balanced may set up alarming vibrations which can affect steering control even at moderate speeds.

Wheels, tires and tubes are usually checked for balance within certain limits before they leave the factory; but the balance of a wheel can be upset by damage, and that of a tire by wear, especially on a car driven continuously at high speeds. The whole unit, therefore, should be checked every few thousand kilometres.

There are two types of balancing: static and dynamic.

Static balancing can be done, as its name implies, with the wheel stationary. Lead weights are clipped to the wheel rim to counterbalance any local heaviness.

The weights rarely exceed 50-80 g, and consist of slugs of lead increasing in 15 g units.

A wheel that is in static balance can still need dynamic balancing.

The effect of a wheel being dynamically out of balance is as if it had a concentration of weight at one point on the edge of its rim so that, as the wheel spins, it wobbles rapidly from side to side. The wider the rim the greater the chances of this imbalance being troublesome.

The only way to discover the exact degree of a tire’s imbalance is to have it checked on special equipment which most large garages have. A new tire or wheel should always be properly balanced before being fitted to the car.

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