Size, Width, ET, Bore and PCD are the key parameters which characterize every automotive wheel, and are responsible for proper wheel fitment. Familiarity with these terms will help buyers better understand the wheel selection process.
Wheel size refers to a wheel’s diameter and is specified in inches. This is the rare case where an imperial measurement system is accepted worldwide. Wheel diameter is usually provided in inches, even in countries that otherwise rely on a metric system.
Appropriate wheel sizes may vary from one modification of the same car model to another. They are specified by the manufacturer, but in some cases auto tuners may choose to step away from the official requirements and opt for a larger (or smaller) size–please refer to the article on choosing wheel size for more details.
Wheel width term is actually self-explanatory. Same as wheel size or diameter, the width is specified in inches, which is an international standard. The width is closely related to a diameter. Larger wheels are often wider, which improves a vehicle’s performance. Driving with large narrow wheels may lead to a lack of stability on the road.
Wider wheels help create a more dramatic overall look.
ET / Wheel offset
Offset refers to a position of a wheel (the distance from its hub mounting surface to the centerline of the wheel.). It is usually described by the term ET, which stands for the German word ‘Einpresstief,’ meaning ‘insertion depth.’. ET can be positive, negative or zero. Most cars today have positive ET, but it is important to pay attention to a + or – sign in front of the ET number. The higher the ET, the deeper the wheel “seats” into the body of a car, and the lower the ET, the more the wheel “pops out” of a car.
Offset variations are used to affect the way wheels look on the vehicle and may also be used to accommodate non-standard wheel sizes. For example, decreasing an offset is required to fit in extra-large and wide wheels.
Bore, which may also be called central bore, is the hole in the center of a wheel. Diameter of a bore may be provided in either imperial or metric system. Modern wheels are usually manufactured with a larger bore diameter to fit any vehicle. These types of wheels are than sold with special rubber coverups to match a specific model. Therefore, bore parameter can be ignored for most car owners unless you have a non-standard vehicle.
PCD stands for pitch circle diameter. It represents the bolt pattern and is often stated as A x B, where A is a number of bolts and B is a diameter of a bolt circle. The latter can be specified in inches or millimeters. Unlike automotive wheel size and width where car owners usually have several options to explore, PCD is a fixed parameter that must be matched exactly, to the millimeter. At SMW we produce wheels for any PCD, but some manufacturers offer a limited number of options per design, so this needs to be checked in advance.
If a PCD of the wheel slightly mismatches the vehicle’s standard pattern it may still fit, but will negatively affect performance and driving safety.
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