Wheel sizing
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The appropriate wheel size for a car or truck wheel depends on a number of factors, including:
- The bolt pattern
- The diameter of the bolt circle
- The offset
- The size of the wheel
- The centerbore
- the x-factor
- the load capacity
- the GVWR
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[edit] 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. In contrast, some smaller French cars, including the Citroën 2CV, Renault 4 and some Peugeot 106's and Citroen Saxos only have three.
[edit] 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 to the right, 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. Alternatively, the radius can be found by dividing the distance between any two adjacent holes by 1.176.
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(180/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).
[edit] Lug nuts or bolts
Another thing considered when new wheels are purchased is proper lug nuts or bolts. They are usually either flat, tapered (generally at 60 degrees and referred to as conical seat), or ball seats, meaning the mounting surfaces are flat, tapered, or spherical respectively. For example, most Mercedes have ball lug seats from the factory while most aftermarket wheels have a tapered lug design. If you buy aftermarket wheels for a Mercedes make sure you get the proper lug nuts for the wheel or the wheel will not be properly centered. Some manufacturers (e.g. Toyota and Lexus) have used taper lug nuts for steel wheels and flat seated lug nuts for alloy wheels.
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 bit style to multisided or spline drive , and are sometimes lightweight for performance purposes. A variation is the "locking wheel nut", which is almost universally used for alloy wheels in the United Kingdom. One standard lug nut on each wheel is replaced with a nut which requires a special and unique key (typically a computer-designed, rounded star shape) to fit and remove the nut. This helps to discourage theft of wheels. However, universal removal tools are available which grip the head of the locking nut using a hardened left-hand thread. The success of these depends on whether there is room to use it in the lug hole, and whether the manufacturer has incorporated a free-spinning outer casing to the lock. Keeping an appropriate tool to lock and unlock aftermarket nuts, and a spare set of nuts, with the spare tire in the boot of the car is recommended by manufacturers.
[edit] Offset
The offset, measured in millimeters, can be negative or positive, and is the distance from the hub-mounting surface to the rim's true centerline. A positive offset means the hub-mounting surface is closer to the outside edge of the wheel, i.e. the wheel wraps around the hub and brake hardware more deeply; a negative offset means the hub-mounting surface is closer to the inside edge of the wheel and wheel sticks outwards more than inwards. When selecting aftermarket wheels, a wheel with too little positive offset will be closer to the fender, and one with too much positive offset will be closer to the suspension components. Wheel width, offset and tire size all determine the way a particular wheel/tire combination will work on a given car. Offset also affects the scrub radius of the steering and it is advisable to stay within the limits allowed by the vehicle manufacturer. Because wheel offset changes the lever-arm length between the center of the tire and the centerline of the steering knuckle, the way bumps, road imperfections and acceleration and braking forces are translated to steering torques (bump-steer, torque-steer, etc) will change depending on wheel offset. Likewise, the wheel bearings will see increased thrust loads if the wheel centerline is moved away from the bearing centerline.
[edit] Wheel size
The wheel size is the diameter of the wheel in inches where the beads of the tire seat on the wheel. This measurement does not include the rim flange. Modern road tires have several measurements associated with their size as specified by tire codes like 225/70R14. The first number in the code "225" represents the nominal tire width in millimeters, this is followed by the aspect ratio "70" which is the height of the side wall expressed as a percentage of the nominal width. R stands for radial and relates to the tire construction. The final number in the code "14" is the rim size measured in inches. It should be noted that the overall circumference of the tire will increase by increasing any of the tires specifications. For example increasing the width of the tire will also increase its circumference, because the side wall height is a proportional length. Increasing the aspect ratio will increase the height of the tire and hence the circumference. Off-road tires may use a different measurement scheme: outside diameter times width, followed by rim size (all in inches) - for example 31x10.50R15.
[edit] Centerbore
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. Some 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 center 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 equal to or greater than that of the hub or the wheel cannot be mounted on the car. Many aftermarket wheels come with "hubcentric rings" that lock or slide into the back of the wheel to adapt a wheel with a larger centerbore to a smaller hub. These adapters are usually made of plastic but also in aluminium.
[edit] X-factor
Caliper Clearance (X-factor): The amount of clearance built into the wheel to allow for the vehicle’s brake rotor and caliper assembly.
[edit] Load capacity
Load capacity is the amount of weight a wheel will carry. This number will vary depending on the number of lugs, the PCD, the material used and the type of axle the wheel is used on. A wheel used on a free rolling trailer axle will carry more weight than that same wheel used on the drive or steering axle of a vehicle. All wheels will have the load capacity stamped on the back of the wheel.
[edit] GVWR
This is the Gross Vehicle Weight Rating. In the United States this information is required to be on the vehicle's door placard. The load capacity of the total number of wheels on the vehicle combined must meet or exceed the vehicle's GVWR.
[edit] See also
- Wheel
- Alloy wheel
- Offset (wheel)
- Plus sizing
- Speedometer#Error - handy tire diameter formula, using tire code
- Tire code
