Car Doctor: Technical Aspects Allow Wheels

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 shown on the right:
Offset

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 on the left. Most Modern vehicles are front-wheel drive, and as such generally require positive offset wheels. The main exception to this rule is 4x4 vehicles, which often use negative offset wheels.
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.
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.

Wheel Location on Hub
Adaptor / Spigor Ring Arrangement Motor vehicle manufacturers use a central location collar on the stub axle hub in order to accurately locate the wheel. The wheel collar diameter varies, depending on the make of vehicle.
Many replacement allow wheel manufacturers use an adapter (spigot ring) to vary the diameter of the locating hole. In this way, a particular wheel can, by changing the spigot ring, be used on a variety of vehicles.
The adapter/spigot ring arrangement is illustrated in the diagrams to the left.
As an example, tsw spigot rings should always be fitted without he tapered edge facing the vehicle hub. This is to allow easy and accurate location of the ring onto the hub collar.

Upstepping to High Performance
Legal and illegal wheel/tyre combinationsVehicle performance can be maximised by selecting a lower profile tyre and larger wheel diameter combination. Care must be taken to select the appropriate replacement wheel / tyre combination to avoid problems.

For example, it is a legal requirements that the tyre and wheel assemblies stay within the body of the car.
The fitting of larger diameter replacement wheels is referred to as ‘up-stepping’. Up-stepping‘Plus One’ or ‘Plus Two’ denotes how much larger the wheel diameter is.

This approach of ‘up-stepping’ allows the tyre section width to be increased whilst maintaining the correct overall diameter. This allows the optimisation of cornering force and grip whilst maintaining the original gearing of the vehicle. One example of this is:
195/60 R 14 14 inch diameter Original size
205/50 R 15 15 inch diameter Plus One
205/45 R 16 16 inch diameter Plus Two
205/40 R 17 17 inch diameter Plus Three
All of the above have an overall rolling diameter which is within an acceptable tolerance.
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.