A wheel is made up of a rim and centre member, known as a disc or spider. The rim supports the tire and the spider (disc) connects the vehicle with the rim.
Wheels are usually of two types-the Drop Center (DC) and the Semi-Drop Center (SDC). Drop centre wheels are used on all cars and light trucks; semi-drop center wheels are usually only used with large multi-ply, heavy-duty tires on over-the-road trucks. The SDC wheel has a removable outer ring that allows easier installation and higher inflation pressure. Above six ply's, tires would be extremely rigid in the bead and be very difficult to mount on a single-piece wheel without damaging the bead.
Most passenger vehicle wheels fall into two types. The all-steel wheel is the type found on many vehicles as original equipment from the factory. Custom or "MAG" wheels were named for their resemblance to magnesium racing wheels. True magnesium wheels are too porous to hold the air pressure of a street tire and never should be used on the street. Custom wheels are a cast aluminium alloy, a steel rim with cast aluminium alloy spider or a two-piece steel wheel.
Fig. 1: Two types of wheel construction. The semi-drop center has removable flanges and does not need the severe drop in the center of the rim. These wheels are used on heavy equipment.
Just as tires have a maximum load capacity and inflation pressure, so do wheels. Any wheels you install should have a greater load capacity and inflation pressure capacity than the tires, or you could have problems. Obviously, the load-carrying capacity of the vehicle is only as strong as the weakest part. If you have selected your tires to carry an anticipated load of, say, 1500 lbs ., then the wheel should be capable of carrying at least that, preferably more.
Wheel sizes are determined by three measurements-rim diameters, rim width and flange height. A typical wheel size might be 14 x 7J. Rim diameter and rim widths are always expressed in inches, so this wheel is 14 inches in diameter and has a rim width of 7 inches . The letter combination following the rim width indicates the flange height in inches. A J rim has 0.68 inch high flange while a K rim has 0.77 inch high flange. The circumferences on which the centres of the wheel bolt holes are located is the bolt circle. It is usually shown as a double number: 5-5 1 / 2 . The first number indicates the number of holes, and the second, the diameter of the bolt circle.
The rim width will be dictated by the tire section width and/or the tread width. The general rule is that the flange-to-flange width of the rim should be a minimum of three-quarters of the tire section width. The maximum flange-to-flange wheel width should be equal to the width of the tire tread. Narrow tires on wide rims tend to make the outer edges of the tire curl in toward the center. The result is less tread on the road, increased tire wear and a harsher ride. At high speeds, centrifugal action can pull the tire beads away from the bead seat on the rim.
Wide tires on narrow rims create a poor bead seal and force the tread to assume a convex shape causing abnormal tire wear, reduction of control with a somewhat smoother ride.
The general rule is that the tire and wheel combination is satisfactory if, when the tire is flat, no part of the underside of the vehicle touches the ground. This will prevent a shower of sparks, should a blowout occur.
Fig. 1: Wheel rim measurements
Fig. 2: The dotted line indicates the bolt circle
Fig. 3: The flange-to-flange wheel width should never be more than the tread width of the tire
Another important dimension to be considered when looking for wheels is offset. Offset is the distance from the mounting face of the wheel spider to the rim centreline. Offset is positive when the mounting face (lug circle) is outboard of the centreline and negative if the lug circle is inboard of the centreline. All wheels are designed for either positive, negative or zero offset, usually for disc brake clearance or for handling characteristics.
Generally, you should not increase the offset more than 1 / 2 inch (12mm) or tire width by 1 inch (25mm), or you'll create further problems. Increasing offset 1 / 2 inch (12mm) or tire width 1 inch (25mm) will put the entire extra tire width 1 / 2 inch (12mm) to the outside, where it may not clear the wheel well. Increasing the offset also has the effect of loading the front wheel bearings past their design limits and can actually "cock" the bearings causing rapid wear or premature failure.
Occasionally, disc brakes cause a mounting problem; some wheels were not designed for use with disc brakes and will not clear the brake calliper or will interfere with the disc. Be sure to check before buying wheels, especially used wheels, that they will fit your vehicle. Be sure that the tires on wider wheels will clear the wheel wells, especially when turned at full lock, and that the tires do not interfere with suspension travel.