PCD stand for Pitch Circle Diameter is the diameter of the circle which passes through the centre of all the studs, wheel bolts or wheel rim holes.
One of the easiest way to calculate the PCD is to measure the distance (S) between two adjacent studs from the middle of each hole.
Different cars have different PCDs, but often similar cars within the same manufacturers group have the same PCD. Many of the same manufacturer cars in the VAG range have the same PCD and other wheel dimensions, meaning that wheels are often interchangeable between these cars.
The PCD has connection with the size of the diameter of a circle that the lug holes create. The center of every bolt/lug lies on the circumference of a circle that they create. PCD is usually measured in millimeters (mm).
The most common fitment has 4 studs and a PCD of one hundred mm, hence the fitment 4×100. Check the fitment of your car, if you are not sure , you may consult a experience mechanic.
Two reason for fitting alloy wheels are Weight and Style.
Alloy wheels are much lighter than the equivalent size of steel wheels, so the unsprung weight of the car is reduced. They also look 10 times smarter than a steel wheel, even with a fancy plastic trim covering it.
Two things to look out for when fitting aftermarket wheels is the PCD and spigot size.
The PCD is easy to match as this relates to the number of studs you need to hold the wheel on the car. The real meaning is the diameter of the studs from the centre of the wheel.
pigot size is a bit trickier….the spigot is the bit in the centre of the hub that you rest the inside centre of the wheel on whilst aligning the studs and screwing back the wheel nuts. Aftermarket wheels, the spigot hole inside the wheels is a lot bigger than the spigot on the car. So what you need to do in this case is fit spigot locating rings. These are just rings of aluminium or hard plastic, that fit over the spigot on your car and then have a correct fit with the spigot hole on the wheel. If you don’t have the spigot taking all the weight of the car, chances are you are likely to break one or more studs when you drive the car hard or have to e-brake . Remember the wheel nuts are simply there to hold the wheel on, NOT support the weight of the car. Also, as there is nothing to centre the wheel, you’ll notice the wheels go in and out of balance because as you drive around, they’ll move around on the hub.