Custom Tire Selection – Size

    Published: 06-16-2009
    Views: 16,654
    Automotive Expert Rick Robinson discusses custom tire size.

    Rick Robinson: Hi! I am Rick from Springfield Motorsport and we're here today to talk about selection and installation of custom wheels and tires for your car. In this segment, we're going to talk about specific size parameters that you have to take into consideration when choosing that right wheel.

    One of the first parameter to consider is the diameter of the wheel. Now we talked earlier about how that affects the overall size in the sidewall of the tire itself. There are some other things that you can take into consideration too. What kind of brakes do you have on a car, is it the stock brakes that came on the car or have you upgraded the brakes to the larger size? If you have, that brake has to fit inside this wheel.

    Larger brakes require a larger wheel or you're going to have problems in here in clearance between the wheel and the brake. For example, a 13 inch cover brake requires at least a 17 inch wheel to clear it, a 14 inch brake requires an 18 inch wheel and so forth. So first thing to consider from both, appearance and a fit perspective is the diameter of the wheel itself. The second factor that we're going to consider is the width of the rim itself.

    Again the use of the car comes into play here, how am I going to use this car? What size tires do I want on a car in order to support that particular use? Now if I want to be able to rotate tires on a daily driver to maintain proper wear patterns, I have to have the same size wheel on all four tires. The typical production car comes with either a seven or an eight inch wide rim on it.

    To support larger tires for performance use, for example, we're going to need a wider rim to support a wider tire to give us more of a footprint on the ground. In many performance cars today, the owners like to have a staggered look. A wider tire in the rear than they do upfront So you might put a nine inch wide rim upfront and a ten inch wide rim in the back, giving more grip on the rear wheels for acceleration and a better overall look to the car.

    Now the third and most misunderstood factor in wheel selection is a parameter called offset and a complimentary parameter called backspace. Both of these measurements are in relation to the hub face of the wheel, which is this portion right here, where the wheel mounts to the axle of the car. We're going to move to a diagram of the wheel to give you a better idea of what these parameters look like.

    In this offside diagram, you can see how these parameters are measured. First of all we start with a center line of your wheel and how it relates to the hub face that we saw in the earlier frame. The distance between he hub face and the center line of the wheel is what's referred to as offset. The distance from the center line to the very back edge of the wheel, is what's referred to as backspace. Now if the hub face is on the back side of the center line, in other words, closer to the interior of the car, then the exterior is referred to as negative offset.

    You saw this a lot back in the 60's with the Deep Dish Chrome wheels. It was very popular look back then. Today, most of our cars have positive offset. In other words, the hub sits outside of the center line of the wheel and the spokes of the wheel sit more towards the outside of the wheel. So when considering the choice of wheels for your car, you have to have the proper offset or the wheel is not going to fit on your car. Now before you can in get excited about a particular wheel and how it will look on your car, the first parameter you have to look at is the bolt circle. The bolt circle determines whether or not that wheel has even got a bolt up to your car and the bolt circle, of course, is the circle of the bolts that attaches the wheel to your hub. This particular wheel is for Ford Mustang. It's the standard 5 bolt, 41/2 inch bolt circle. So make sure you check your manufacturer's handbook and you know what bolt circle is required for your car before you even start shopping. Now in the next segment, we're going to switch to new tires and the selection of the proper tire to go on your new wheel.