Weight & Moment of Inertia
When comparing wheels the two most important figures that contribute to their performance is weight and their moment of inertia, weight is fairly self-explanatory but moment of inertia is slightly trickier. Below is a guide on how these two elements affect motorcycle wheels;
Weight or mass comes in two forms on a motorcycle sprung and un-sprung. Sprung mass is weight supported by the bikes suspension such as the chassis, engine, and exhaust. Un-sprung mass is weight that is directly linked to the suspension but is not supported by it for example the tires, wheel axles, and wheels. Wheels and tires are the bikes contact points with the road and directly influence the handling and performance of the bike. Lighter wheels improve handling because it requires less force to move a lighter object than a heavier one. When a wheel is rotating it is generating rotational inertia which increases the force required to change direction, the faster the rotation is the more force is required. If you reduce the wheel mass you also reduce this inertia.
Moment of inertia
This is the measure of torque required for a change in angular velocity about an axis of rotation. The larger the moment of inertia the more torque required. Weight, its location, and the distance of the weight to the axis directly influences the moment of inertia. The closer the weight is to the axis the less of an effect the weight has and vice versa. So how does this relate to wheels? Well first off let’s look at the 3 frequently used materials for motorcycle wheels;
- Aluminium – either cast or forged aluminium is the densest material used for wheels and carries the most weight.
- Magnesium – either cast or forged magnesium is less dense than aluminium and is therefore lighter.
- Carbon Fibre – is the least dense of the three materials so it is also the lightest.
On any wheel the furthest distance from the axis is always the rim, so if we are to manufacture a carbon fibre wheel to the exact same size and dimension as an aluminium wheel not only will the carbon wheel weigh less (because of its density) but because it has less weight further away in the rim its moment of inertia will be lower too. This is why lightweight aluminium wheels that may weigh very similar to carbon fibre wheels still have a higher moment of inertia, because the bulk of that weight is in the rim where in the carbon fibre wheel it is in the hub area. The combination of low total weight and a low moment of inertia is the sweet spot for saving torque.
The benefits of losing weight and improving your MOI in your wheels include;
- Increased acceleration and deceleration
- Quicker and more responsive handling at lower steering forces
- Lower forces impacting suspension leading to a better ride
- Less fatigue and improved confidence when riding
- Improved lap times (see below)
Below is a chart showing weights and MOI comparisons between Dymag wheels, stock OEM wheels, and some competitor aftermarket wheels to give you an idea of the kind of savings you can expect.