In order to size a wheel for an automobile, an owner needs to know a few fundamental things:
- The bolt pattern
- The diameter of the bolt circle
- The offset
- The size of the wheel
- The centerbore
The bolt pattern is merely the number of bolts on the wheel. As the bolts will be evenly spaced, the number determines the bolt pattern. For example, most cars have 4 bolts while larger models have 5.
The bolt circle is the circle determined by the positions of the bolts; the center of every bolt lies on the circumference of the bolt circle.
The important measurement is the diameter of the bolt circle, usually expressed in millimeters, although inches are sometimes used. For a 4- or 6-bolt car, this measurement is merely the distance between the center of two diametrically opposite bolts. In the 4-bolt picture below, this would be the distance between holes #1 and #4. Some basic geometry is needed to find the center of a 5-bolt pattern: draw a line between any two neighboring bolts, and draw a line from this line to the opposite bolt. Repeat with a different set of three bolts, and the two long lines will cross in the center, thereby making the distance between this intersection and the center of a bolt the radius of the bolt circle.
A 1974 MG B is a 4/4.5" (4/114.3) car, meaning it is, again, a 4-bolt pattern with a 4.5" or 114.3mm bolt circle.
This measurement is also known as "pole center diameter" or PCD, and the most common PCD values are 100mm and 114.3mm. This difference arises from the manufacturers' measurement convention - whether they're designing around metric values (100mm) or imperial values (4.5 inches, i.e. 114.3mm).
Lug nuts or bolts
Another thing to consider when new wheels are purchased is proper lug nuts or bolts. They are usually either flat, tapered, or ball seats, meaning the mounting surfaces are flat, tapered, or spherical respectively. For example, most Hondas have ball lug seats from the factory while most aftermarket wheels have a tapered lug design. If you buy aftermarket wheels for a Honda make sure you get the proper lug nuts for the wheel or the wheel will not be properly centered.Some aftermarket wheels will only fit smaller lug nuts, or not allow an ordinary lug nut to be properly torqued down because a socket will not fit into the lug hole. Tuner lug nuts were created to solve the problem. Tuner lug nuts utilize a special key to allow removal and installation with standard lug wrench or socket. The design of tuner lug nuts can range from spline drive to multisided, and are sometimes lightweight for performance purposes.
- Make sure to keep a set of lugs that fit your spare tire.
- Keep a key to aftermarket lug nuts in the car in case of a flat.
The offset, measured in millimeters, can be negative or positive, and is the distance from the mounting surface to the rim's true centerline. A positive offset means the wheel is front of the mounting surface, closer to the center of the fender; a negative offset means the wheel is away from the mounting surface and projecting from the fender.
The wheel size is the diameter of the wheel, in inches, not counting the tire. Modern tires in North America have several other measurements associated with their size. A tire designated as a "225/70R14" denotes a tire with the following dimensions: width of tread: 225 mm; ratio of tire height (rim diameter vs tread height): 70 percent; rim diameter: 14". Therefore a 185/70R14 has the same rim size and aspect ratio, but a narrower tread face than the 225/70R14.
The centerbore of a wheel is the size of the hole in the back of the wheel that centers it over the mounting hub of the car. Factory wheels have a centerbore that matches exactly with the hub to reduce vibration by keeping the wheel centered. Wheels with the correct centerbore to the car they will be mounted on are known as hubcentric.Hubcentric wheels take the stress off the lug nuts, reducing the job of the lug nuts to holding the wheel to the car. Wheels that are not hubcentric are known as lugcentric, as the job of centering is done by the lug nuts assuming they are properly torqued down.Centerbore on aftermarket wheels must be greater than or equal to that of the car or the wheel will not physically mount to the car. Many aftermarket wheels come with hubcentric rings that lock into the back of the wheel to center it like a factory wheel, usually made of plastic.