The term PCD stands for (pitch circle diameter) and is the diameter of a circle drawn through the centre of your wheels bolt holes. PCD is measured in millimetres and also indicates the number of studs or bolts the wheel will have.
One of the most common fitment has 4 studs and a PCD of 100mm, hence the fitment 4x100. Check the fitment guide above to check the fitment of your car, if you are unsure please check with vehicle and wheel manufacturer before purchase and subsequent fitment.
OFFSET
Every car requires a unique offset. This is where the outside of the wheel needs to be in relation to the bodyline of the vehicle, realistically you can go 5-7mm outside these recommendations, but always check with vehicle and wheel manufacturer' if you are unsure, as there are often other factors that need to be considered.
Reasons For Fitting Alloy Wheels
The two main reasons for fitting Alloy Wheels, are to enhance Style, and reduce Weight. Alloy wheels are a lot 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 for when fitting after market wheels is the PCD (Pitch Circle Diameter, ) 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 ACTUAL meaning is the diameter of the studs from the centre of the wheel.
Spigot 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. On generic after market 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 proper 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'll break one or more studs when you drive the car hard or have to brake hard. 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.
The important thing is to keep the same overall tyre diameter. This is done by using a lower profile tyre. Increasing the tyre width and reducing the profile height will give the vehicle different handling characteristics. The car will be a lot more responsive when cornering, but a lot more sensitive to bumps and irregularities in the road.
Wheel Size
Two measurements, the Rim Diameter and the Rim Width, normally determine the wheel size. This is best explained with reference to the diagram:
The ‘Offset’ of a wheel measures the distance between the wheel centreline and the wheel mounting face and is measured in millimetres. It is extremely important that wheels of the correct offset are used in order to maintain the correct track of the vehicles.
Offset can be Positive or Negative and is best illustrated in the two diagrams below.
Pitch Circle Diameter (PCD)
The P.C.D. can be defined as the diameter (in millimetres) of an imaginary circle drawn through the centre of the stud holes on the wheel and/or the vehicle wheel hub.
When new wheels are required, it is essential that they have the correct P.C.D. for the vehicle concerned.Just because a wheel from one vehicle has the same PCD and offset as the wheel from another does not mean they are interchangeable - the centre bore of the wheel and hub must also be the same to ensure centralisation of the wheel, and the shape of the spokes must ensure clearance of the brake calipers. Many manufacturers use the same wheel fitments as others, but some are unique.
Clearance
It is, of course, extremely important that the clearance Pitch Circle Diameterof both the vehicle body work and steering/suspension components are maintained. Failure to do so would most likely cause damage to both the tyre/wheel assembly as well as the body of the vehicles. Clearance is achieved by selecting a wheel of the correct size and type of tyre. Quality alloy wheel suppliers can give such information.
Aspect Ratio
Aspect ratio is the ratio of section height to section width. It should be noted that in general, tyres with a lower section height (i.e. where the aspect ratio is lowered) have a higher cornering force and therefore improved performance-handling characteristics.
A wheel offset calculator is available on THIS WEBSITE.
A standard Subaru chart:
Source websites:
http://www.nice-whee...dandoffset.html
http://www.1010tires...tCalculator.asp
