Many cars today come equipped with “mag wheels’ of aluminium alloy. Makes the car look sporty, has a hint of prestige, and the variety of designs sets them apart from the pack. But there’s another reason why they are favoured by car designers and engineers. It’s called the MacPherson strut front suspension.
The late Colin Chapman , designer of the Formula 1 winning cars of the sixties, chased down the sources of unsprung weight on his racing cars to extremes. ”If it didn’t break, it was too heavy”, was a statement attributed to him. Unsprung weight is all the components that go to make up the suspension, brake, and drive components. The remainder of the car sits on springs. Springs oscillate at particular frequencies, so that if a vibration is fed into a sprung system, the oscillations can compound and become quite noticeable, even destructive. For example, the low speed vibration caused by an army of marching men on a long bridge, can get quite a rock up in the bridge surface, so they are told to break stride. The amplitude of the oscillation generated by something out of balance, or out of round, feeding up into the sprung suspension of a motor car can cause a vibration felt by the occupants-the vibration resonates.
The late unlamented Falcon XB series had a vibration at 117 km/h, which became quite noticeable. Like many modern cars, it too had a MacPherson strut front suspension. Keep in mind that the wheel assembly had already been balanced off the car, yet there was still a vibration. Much development work went into developing a fix. It was realised that the steel wheel, centred on the hub by the wheel nuts, was one of the causes of imbalance. So the bore hole in the wheel was used to locate the wheel centrally on a machined hub, the wheel nuts being relegated to their real role of holding the wheel on. Today, this hub fit can be quite tight, and if the wheel has been on a while, hard to dislodge.
Investigation then turned to the wheel itself. A steel wheel is heavier, stronger, particularly laterally, than a modern alloy wheel, though alloys have stringent safety standards to meet for lateral impact resistance. But being a punching made in a die, with the hub assembled into the rim in a secondary process, there exists a greater potential for it to be out of round than a mag wheel. If the press die is out of round, the wheels are out of round. Mag wheels are cast, then machined on a computer controlled machine, which drills the bore hole and machines the bead ledges in the one operation. The machining tolerances can be set much tighter than for a steel pressing. Also, being lighter, the spinning mass may not generate a noticeable vibration until the car reaches a higher speed- you may never notice it.
So that got rid of two sources of vibration. That only leaves the tyre, which is a composite lay-up of many components, with plenty of scope to be out of round, and out of balance. This will be subject of another article. However, once the car engineers tracked these sources down, tyres came under even greater scrutiny. Car design engineers specialise in tracking down these sources of noise, vibration, and harshness. It’s a very specialised world of engineering.
As to Colin Chapman’s mag wheel, the first time you pick up a real magnesium alloy wheel from a true race car, you will be very surprised indeed to find out how light the whole assembly is! However, it will never be subjected to the everyday abuse that a road wheel will be subjected to, so it is not surprising that it is much lighter.
A final word on cleaning mag wheels. Some are coated with a clear protective finish, some are not. Using a specialised cleaner to spray on, hose off brake dust is recommended for periodic maintenance and detailing but make sure that you buy the correct type of cleaner for the wheels on your car.