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AUTOMOBILE WHEELS AND TYRES



The automobile wheels support the total weight, resist the strains created during turning and transmit the driving torque for propelling and braking torque for retarding. The wheels must have well-balanced construction particularly for running at high speeds. Unbalanced wheel assemblies cause excessive vibration, which ac­celerates tyre and king-pin wear.

Automobile control, acceleration and braking occur through the tyres and their contact on the road surface. The tyres must be large and strong enough to support the vehicle on the road. The traction, the force required to make the tyre slip on the contact patch, is the same on the wheel in all directions whether it is accelerating, cornering, braking, or any combination of these. The tractive force to control the automobile drops rapidly when a skid starts so that control is lost. The tyre must absorb, by deflecting, part of the shock from road irregularities.
During normal operation, passenger car tyres rotate approximately 500 revolu­tions for each km travelled. The chief factors affecting tyre life are inflation pressure, vehicle speed and rate of acceleration, temperature, tyre interchanging, and tyre and wheel balance. A tractor tyre normally is expected to last twenty years, a truck tyre 80,500 km running, a passenger car tyre 48,500 km and a racing tyre 800 km.

Structure.

Wheels must be rigid enough to retain their shape under all operating condi­tions. When subjected to abnormal impact, they should preferably buckle and must not collapse. The dimensional tolerances of the wheel should be accurate enough for carrying out wheel alignment and balancing.
Weight. Wheels must be light so that the unsprung weight is reduced. Light wheels and tyres also follow the road surface contour more accurately so that wheel bounce is minimized, resulting in improved road contact and reduced tyre wear.

Tyre Attachment.

Since the wheel-tyre combination is responsible for the transmission of traction to the road or for steering reaction, the tyre must be suitably located and rigidly secured on the wheel. Additionally, the design of the wheel should be such that the tyre can be fitted easily.

Wheel Mounting.

The wheel attachment must be designed properly for locating, securing, and supporting the wheel. Also, the wheel should be easily fitted or removed from its axle-hub.

Cost.

Wheels should be made out of cheaper materials that can easily be fabricated, cast, or forged, with the minimum machining. It should also have a better finish and appearance and should not easily deteriorate with age and weathering.

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