Wheel Information.

Okay. If you want to change the wheels on your car, you need to take some things into consideration.

• Number of bolts or studs
  It goes without saying that you can't fit a 4-bolt wheel onto a 5-bolt wheel hub. Sounds obvious, but people have been known to fork out for an expensive set of wheels only to find they've got the wrong number of mounting holes.
• Pitch Circle Diameter
  Right. So you know how many holes there are. Now you need to know the PCD, or Pitch Circle Diameter. This is the diameter of the invisible circle formed by scribing a circle that passes through the centrepoint of each mounting hole. If you've got the right number of holes, but they're the wrong spacing, again the wheel just won't fit.
   

  4 stud (bolt) PCD	5 stud (bolt) PCD

• Inset or outset
  This is very important. Ignore this and you can end up with all manner of nasty problems. This is the distance in mm between the centreline of the wheel rim, and the line through the fixing face. You can have inset, outset or neither. This determines how the suspension and self-centring steering behave. The most obvious problem that will occur if you get it wrong is that the steering will either become so heavy that you can't turn the car, or so light that you need to spend all your time keeping the bugger in a straight line. More mundane problems through ignoring this measurment can range from wheels that foul parts of the bodywork or suspension, to high-speed judder in the steering because the suspension setup can't handle that particular type of wheel. This figure will be stamped on the wheel somewhere as an ET figure.
   

  No offset	Inset wheel	Outset wheel

• A real example
  They say a picture is equivalent to a thousand words, so study this one carefully. It's one of the wheels off my own car. Enlarged so you can read it is the wheel information described above. You'll notice it reads "6J x 14 H2 ET45". The "6J x 14" part of that is the size of the wheel rim - in this case it has a depth of 6 inches and a diameter of 14 inches (see the section directly below here on wheel sizes for a more in-depth explanation). The "J" symbolises the shape of the tyre bead profile. (More on that later)
  The "H2" means that this wheel rim is designed to take "H" speed-rated tyres. The "ET45" figure below that though symbolises that these wheels have a positive offset of 45mm. In other words, they have an inset of 45mm. In my case, the info is all stamped on the outside face of the wheel which made it nice and easy to photograph and explain for you. On most aftermarket wheels, they don't want to pollute the lines and style of the outside of the wheel with stamped-on information - it's more likely to be found inside the rim, or one one of the inner mounting surfaces.