Want to know what wheel offset and what PCD means? Well you’re in the right place.
WHAT IS WHEEL OFFSET?
Offset is the distance between the mounting surface of the back of the wheel and the wheels true centre line and it’s important for two reasons. First, if the offset of your aftermarket wheels varies too much from the manufactures standard specification it can compromise steering response and directional stability as well as putting excess load on components like wheel bearings. Secondly the wrong offset can make the wheels or tyres may foul suspension, brake components or the inner arches.
Offset is expressed as an ET number, (ie is ET38) and the ‘ET’ is a shortened version of the German word ‘Einpresstiefe’ which means ‘insertion depth.’ The number is the distance in millimetres between the centre line and the mounting surface.
It sounds confusing but the important thing to remember is wheels with a positive offset will have the mounting surface towards the front of the rim and those with a negative offset will have the mounting surface towards the back often giving the wheel more ‘dishey’ appearance at the front. For most aftermarket wheels the offset helps compromise between increased wheel width and the space available under the standard arches.
WHAT IS WHEEL CENTRE BORE?
The diameter of the hole in the back that fits onto the flange on the car’s hub is known as the centre bore. Most modern wheels are hub-centric which means the centre bore will fit tightly onto the hub, transferring the load of the car to that component. This is the most common set-up, and with it, the studs or nuts do nothing other than hold the wheels on.
The other type of wheel is the lug-centric version where it’s the studs or bolts that take the load of the vehicle because the centre bore doesn’t always fit snugly onto the hub. These are much less common and require regular replacement of the studs or wheel bolts.
WHAT IS WHEEL PCD?
PCD explained (Pitch Circle Diameter)
Car manufacturers use different stud or bolt hole patterns for the fitment of their wheels and these must line up exactly with your new ones enabling them to seat properly against the hub. PCD fitments are a measurement showing first the number of studs/bolt holes and secondly the distance that they are spaced out. Common ones are 4×108, 4×100, 4×114.3, 5×100 and 5×114.3.
The PCD on a 4-stud hub is simply a straight line from the centre of one stud, through the centre of the hub and onto the centre of the stud opposite. Put simply a 4×108 PCD would be 4 bolt holes/studs evenly spaced around an imaginary 108mm circle.
The principle is the same with 5 stud wheels, a 5×100 PCD would be five evenly spaced studs/holes around a 100mm circle, although it’s a bit harder to measure because there’ll be no stud or hole directly opposite the stud you’re measuring from. Instead a straight line must be drawn from the centre of the first stud, through the centre of the hub and on to the edge of the imaginary circle between the near opposite two studs. Pretty complex eh?
Some wheel manufactures produce multi-fit alloys that have a multi-stud fitment enabling them to be installed on a number of different PCDs. These are much more cost effective for manufacture.