A guide to wheel rims

There are three fundamental elements to a rim. The first is the offset, which is stamped on the wheel as an ET value. It defines the distance between the hub-bearing surface and the centre line of the rim. It is zero when the surface of the hub is at the mid-point of the rim. It is positive if the hub is closer to the outside of the rim and negative when nearer to the inside. The second element is the barrel, which is expressed in inches (1 inch = 2.54 cm). This is the part of the rim on which the tyre is mounted. The third element is the pitch circle diameter (or PCD), which indicates the number of lug holes and the spacing between them. 

By playing around with these elements and using your common sense, you can make the wheel protrude more or less. For instance, by increasing the barrel width, the wheel will stick out more, but if it is increased too much there is a risk the tyre could start to slip out of its housing. A larger offset will move the rim closer to the axle, but a smaller offset will position the rim further from it, which means there is a risk it could scrape against the bodywork. A larger wheel is an option, but if you exaggerate the size then the wheel could touch the brake cable or the shock absorbers. For an optimal choice, consult the wheel/tyre equivalence table, which reveals the ideal models for each minimum and maximum tyre width. If in doubt, of course, an expert's advice is your best bet.

Alloy wheels vary a great deal, but in general they significantly improve vehicle performance by reducing the unsprung weight and enhancing fuel economy. Steel wheels, meanwhile, are cheaper and have another important advantage – they are far easier to mend in the case of damage. A knock with a hammer can fix a dent and make them good as new again. In contrast, a light alloy rim can be irreparably damaged after a close encounter with a pothole or the curb. Even minor deformations may require regeneration and straightening work that can only be carried out in a specialist workshop. 

Proper maintenance will keep your rims looking good and help ensure they last a long time, too – corrosion is the main enemy of all rims, light alloy and steel alike. Do not hesitate to have them checked by an expert the moment you notice even the smallest blemishes. Cleaning, however, can be tricky because everyday grime from the road mixes with dust from the oven-hot brakes and becomes baked on to the wheels. A pressure washer is perfect for getting rid of all loose dirt, then apply a specialist wheel-cleaning solution.