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Influence of unsprung masses (wheels and tyres)

Does the weight of the wheels have the same impact on vehicle peformance as weight inside the car? In other words, Is it the same to have wheels 5kg heavier as having 5 kg inside the car? The answer is no, and that is because the wheel is part of the unsprung mass of the car. It is submitted to two different forces:

  • Energy of translation: the wheel moves forward.
  • Energy of rotation: the wheel spins.

The unsprung mass follows all the irregularities of the road, An extra pound in unsprung mass corresponds to several more pounds of sprung mass.

In order to reach optimal performance in summer, Unsprung masses should be reduced to a minimum in order to get the best of acceleration, braking, road handling, comfort and fuel consumption .

In order to reach optimal performance in winter, when road grip is paramount, it is advisable to mount heavier wheels to enhance grip and road handling. That is why many drivers opt for steel wheels in winter , often heavier, and alloy wheels in summer.

Acceleration and braking

The heavier the wheel, the more force is required to set it in motion or to bring it to an halt.

A comparativily heavier wheel will require more torque from the engine, and affects acceleration negatively. Conversely, a lighter wheel contributes to better acceleration

It is easier to stop a 1kg wheel than a 100kg wheel since it develops less inertia. Braking loses efficiency at the same time as weight of wheel increases.

Fuel consumption

Fuel consumption is partly determined by the amount of energy required for acceleration.

As we have already seen, a lighter wheel requires less energy to gain speed.

Less torque is needed with lighter wheels so that you can save on fuel..

Other performance criteria

Other elements can contribute in reaching better performance.

  • Rigidity of the wheel : The more rigid the wheel, the better since it reduces deflection (the difference in the radii between constrained and unconstrained parts of the tyre).
  • Heat dispersal : The materials used in the manufacturing of alloy wheels are great at getting rid of the heat produced by the brake discs