The term PCD refers to “pitch circle diameter,” and is a useful way of working out how to fit different wheels to a car.
Pitch circle diameter basically refers to the diameter of the centre a set of studs, into which a new wheel will need to be attached. Most cars feature a pitch circle diameter of 4 x 100, which needs to be measured in millimetres to the most exact figure possible. Some PCDs can extend up to 130 millimetres in diameter for larger cars, as well as adapting to use 5 studs rather than 4. It should be seen as denoting the diameter distance of the studs from the centre point of a wheel, with information provided by the manufacturer for the exact dimensions.
How to Measure PCD
Actually measuring PCD is relatively simple. You need to measure between the centres of opposite studs, which will typically produce a figure such as 100mm. While most PCDs will follow this rule, it is usually worth measuring for an exact figure. Deviations can result in ill fitting wheels, which will cost you significant amounts of time and money to adjust for.
There are some exceptions to measurements, particularly when dealing with three or five studded parts. In this case, you can measure the diameter of the centre bore, and the distance from the edge of the centre bore to the centre of any stud. For this equation, you add the diameter to the centre stud distance, and then double the latter figure.
Uses of PCD Measurements
A PCD measurement can be used to correctly fit new steel wheels, or alloy replacements. Alloy wheels are particularly recommended for their lighter weight and stylish look. Fitting alloy wheels to the right PCD is essential to making the right conversion. At the same time, any PCD measurement needs to be considered alongside the offset measurement of a wheel and hub.
Offset basically refers to the distance between the wheel and the body line of a car, and should provide enough depth to prevent any overlap with brake calipers. Positive offsets tend to be front facing, while negative offsets refer to rear facing mounts.
PCD and offset are the two main components for deciding on the installation of new wheels, and any measurement of the former should be used with the latter to make a decision.
Installation and Safety
PCD measurement can be made if you have experience of changing tires and fitting alloy wheels. You will also need to be able to jack up a car to properly and safely remove a tire and wheel for proper fitting. This process can be dangerous if you aren’t properly trained. If in doubt you should ask for a PCD measurement and fitting to be taken at a local garage. Getting the right PCD for your car will make it much easier to arrange your own fittings and replacements, and will also allow you to work out whether you can rely on standard measurements or more unique figures when ordering and specifying new wheels.
Guest Post by Martin Roche – Car enthusiast with a passion currently blogging for Alloy Wheel World