The Density and Inertia of Forged Wheels

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While we’re no metallurgists, the average density of each wheel can also say something about its strength. Each wheel is made of aluminum, and the more densely the metal is compacted together or forged, the stronger the wheel is. But because no one would give us exact material properties or the computer-generated solid model files of each wheel that would allow us to determine wheel volume precisely, we resorted to dunking each wheel inside a tub of water and figuring out how much water it displaces. Divide the wheel volume by its weight, and we have a ballpark density for each wheel, even though this included the paint on the wheel. Either way, the forged wheels should have a higher density than the cast wheels.

One other physical aspect of a wheel is its mass moment of inertia (more commonly referred to as rotational inertia). Rotational inertia basically indicates how hard it will be to start and stop a wheel from spinning.In theory, this will affect braking and acceleration performance. But rotational inertia is affected not only by a wheel’s weight and shape, but more importantly how the weight is distributed throughout the wheel and its overall size. Two same-weight wheels could have vastly different amounts of rotational inertia if the weight or metal in the wheel was distributed differently. A wheel with a heavy hoop will have more rotational inertia than an identically heavy wheel whose weight is mostly in the hub.