Tire and Wheel Installation
In order to size a wheel for an automobile, an owner needs to know a few fundamental things: more...
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The bolt pattern;
The diameter of the bolt circle;
The offset;
The size of the wheel;
The centerbore;
the x-factor;
Bolt pattern
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 U.S. passenger cars have 5 bolts while compact models sometimes have only 4 and pick-up trucks can have as many as 6 or 8.
Bolt circle
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 "pitch circle diameter" (PCD), 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 the midpoint of 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.
The PCD can be calculated for any wheel from the number of bolts (n) and the measured center distance between two adjacent bolts (d) as; PCD = d / SIN(π/n).
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 114.3 mm bolt circle.
The most common PCD values are 100 mm and 114.3 mm. This difference arises from the manufacturers' measurement convention - whether they are designing around metric values (100 mm) or imperial values (4.5 inches, i.e. 114.3 mm).
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 (generally at 90degrees), 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 manufacturers (e.g. Nissan) have used taper lug nuts for steel wheels and flat seated lug nuts for alloy wheels.
Read more at Wikipedia.org
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