Weight saving

Reducing un-sprung mass (Wheels, discs etc) has a far greater performance benefit than reducing sprung mass (bodywork). In racing, some designers use a factor of eight. For example, saving 1kg on a wheel weight gives the same performance gain as saving 8kg of sprung weight. This is due to the following benefits :

Braking/Acceleration

The force required to start a wheel rolling (moment of inertia) increases with the mass of the wheel. The same is true of the force required to stop a wheel, and so reducing the wheels weight, improves acceleration and braking efficiency. Since less energy is required to start and stop the bike, fuel efficiency also improves, but is not often noticed as the rider simply goes faster!.

Gyroscopic effect

Reducing the wheel mass, also reduces the gyroscopic effect of the wheel. When a wheel is spinning, it is very difficult to make it change direction, but the lower the mass of the wheel, the easier it is to flick from side to side. Try holding a front wheel at arms length via a spindle, spin it and then simulate diving in and out of a corner. The effort required is considerable, but it becomes significantly easier as the wheel mass reduces. Consequently, this has a major effect on the handling of a motorcycle.

Radius of Gyration

This is more a function of wheel design rather than pure weight reduction, but the two are inextricably linked. The mass of a wheel is deemed to act at a certain radius from the centre of the wheel (radius of gyration). In order to improve the performance of the wheel, the designer will try to reduce this radius, so that it acts as close to the wheel centre as possible.

To demonstrate this, try swinging a weight on a piece of string around your head. The shorter the string, the easier it is to swing. Hence reducing the radius of gyration, improves the manoeuvrability of the motorcycle. This is achieved by lightening the rim as this is on the extreme radius, and designing the spoke system with minimal weight at the extreme, whilst maintaining the required strength and stiffness.

Materials

In order to achieve the ultimate wheel, material choice is also crucial. Magnesium has the highest strength to weight ratio of any metal, and is only surpassed by Carbon Fibre. Hence all modern high performance wheels tend to use these materials. Dymag carbon wheels have achieved the tough German TUV standard for road use. This is a testament to the correct use of materials and design, enabling Dymag to make a race weight product, which conforms to British, Japanese and German roads standards.

Summary

In summary, the two main features of a high performance wheel are weight and stiffness. Both are important as the wheel needs to be as light as possible for the above reasons, but it also needs to be stiff to give crisp handling. If the wheel flexes, the tyre contact patch also changes, which can result in less predictable handling. These two features need to be married to a good design, which follows established engineering principles, to produce the optimum product for any given application.