WHEEL TECH

Dr Tim talks us through the ins and outs of offset, backspace and track

WHEELS can make or break a car. They’re often the first thing to be purchased for a car build, regardless of the design of the project. There they sit against the wall, with a huge dish that you can put your head into. They look sexy, but will they fit, and will they be in track? For a tough-looking ride, owners often want rear wheels with big outer offsets, but that usually means they’ll be illegally over-track.

So what exactly is wheel track? Track is measured from the centre of the tyre on one side of the car to the centre of the corresponding tyre on the opposite side. So when you bolt your chosen wheels onto your build, if the track varies by more than an inch from the factory track, you’re in trouble. The Department of Transport sets down a ‘one inch rule’, where wheel track must not be increased by more than 25mm beyond factory. The intent of limiting track variation is to minimise the risk of axle and bearing failure. As track increases with wheel offset, the bending moment on the axle also increases, as the wheel load is moved further away from the bearing.

It is possible in some jurisdictions to argue your case for larger tracks with the Department of Transport via an engineer, based on the lighter weight of the vehicle compared to that of the donor differential, or by the use of floating hubs, but this is by individual application.

So let’s have a look at what goes into choosing wheels and a diff that will give your project the desired look while still being within s the rules.