It goes without saying that a properly pimped out car will be incomplete without a set of the blingiest and biggest alloy wheels on the market. Alloy wheels are much more aesthetically pleasing than their steel counterparts with their plastic hub caps. In addition fitting larger alloy wheels with low profile tires can give your car a mean, tar eating look and it seems the bigger those alloys are, the better they look, but before you rush out to buy that set of 24" alloys you need to consider a few things, as well as the impact bigger wheels will have on the handling of your car.
As we mentioned in our section on suspension tuning, the major advantage of fitting alloy wheels is the reduction in weight that they provide. Wheels and tires form part of the unsprung mass of a car and alloy wheels are generally about 30% lighter than steel wheels if the same size. Reducing the weight of the unsprung mass is better in terms of handling as the unsprung mass is not supported by the suspension and is most susceptible to irregularities in the road surface and cornering forces. Reducing the unsprung mass not only provides more precise steering and better cornering characteristics; it also allows the suspension to react faster to irregularities in the road surface and to follow the road more closely. In addition, reducing the unsprung mass also provides better acceleration and improves car safety as it provides better deceleration and braking. While fitting wider wheels will improve grip and cornering ability as they have larger contact areas but wider wheels will be a bit heavier and will reduce the improvements in acceleration and braking that alloy wheels provide.
However, fitting larger wheels will adversely affect handling by causing tramlining, a bumpier ride and poorer cornering characteristics. They will also shorten the final drive ratio resulting in inaccurate speedometer reading and poorer acceleration (so it's not a good idea if you've got a turbocharged car)! It would thus be better to maintain the same rotational diameter of the stock wheels. One way of accomplishing this is to fit a low profile tire. Thus, if you want the big wheel look, I'd suggest you get a larger alloy wheel with an ultra low profile tire; although low profile tires are noisier but they also have better handling characteristics due to the reduced flexing of the tire wall.
The Pitch Circle Diameter (PCD)
You should also check the PCD (Pitch Circle Diameter), which is the diameter of a circle drawn through the center of the wheel's stud holes and measured in millimeters. The PCD also indicates the number of studs the wheel has; thus a PCD of 5x114.3 indicates that the rim had 4 stud holes that are drilled through the center of a 114.3 mm (or 4.5") diameter circle. The PCD will ensure that the stud hole pattern on the rim matches the stud pattern on your rotors. If the PCD is out, the wheel will not fit the studs on your car!
The Wheel Offset
The wheel's offset will also affect fitting and appearance of the wheel as it is the distance between the mounting face of the hub and the center line through the width wheel. The wheel with a negative offset (that is a wheel with a mounting face that is closer to the back of the wheel) will have a deep dish appearance while a wheel with a positive offset (that is a wheel with a mounting face that is closer to the front of the wheel) will have a shallow dish appearance. A wheel with a negative offset will also fit will more of the wheel protruding out from under the car and may rub against the wheel arches during cornering while a wheel with a positive offset will have more of the wheel under the car and may give the appearance that the wheels are too narrow; it may also result in the wheel knocking up against the coil spring or other suspension parts or the brake caliper! To avoid these problems it is best to check the offset of your stock wheels. You can check the offset of your stock wheels by placing a straight edge across the back of the wheel rim (not the tire!) and measuring the backspace between the straight edge and the mounting face of the hub. Then measure the width of the wheel rim. Finally, subtract half the width of the rim from the backspace or use the formula: offset = backspace - ½ width. Thus, if the wheel is 203 mm wide and the backspace is 141 mm, your formula would be: 141 mm – (203 mm ÷ 2) = 40 mm offset.
If you are going to purchase alloy wheels that are the same width as the stock wheels, then it would be best to purchase alloy rimes with the same offset; but if you're going to purchase a wider alloy wheel, you'd want the alloy wheel to have the same backspace, so that the back of the wheel is the same distance from your brake caliper and suspension parts. So you're going to need to calculate the backspace from a given width and offset using the formula: backspace = offset + ½ width. Thus, if the wheel is 203 mm wide with a negative offset of 20 mm (- 20 mm), your formula would be: -20 mm + (203 mm ÷ 2) = 80 mm backspace.
Now that you have your PCD and offset worked out, all you need to remember is that low profile tires will not protect your alloy rims from hitting the curb so you should either steer clear of curbs and potholes, or choose an alloy wheel that does not protrude too much beyond the tire. Finally, when you have your tires balanced, ask the technician to attach the weights to the inside of the wheels and remember to have your tracking realigned and the camber and toe reset whenever you change your wheels or tires to avoid uneven and premature tire wear.