Wheel Specification Selection

Most people see the wheels as just the four rounds bits of metal the tyres are fitted to and are then bolted to the car. This could not be further from the truth, a team has to balance grip from the tyres (wider rubber = more grip) v handling (wheel off-set effects suspension setting) & speed (wider tyres means more power and torque required to accelerate the car).

So, to help understand how ZRT Motorsport selected their wheel size (8.5” x 17” ET35), the following information was investigated:

Tyre Profile:

The tyre profile was taken from the event tyre supplier (Hankook in the case of the Dubai 24hr). Through comparing the wheel arch dimensions with the tyre profiles offered, the possible tyre sizes can be determined. A tyre with a great wall depth will result in a small wheel diameter being required.

Wheel 3

Wheel Width:

The sporting and technical regulations dictate that no part of the tyre can be visible when looking vertically down on the car. Thus, using the depth of the wheel wells and the edge of the bodywork, the possible tyre widths can be identified.

The team now has a shortlist of possible tyre sizes and thus the corresponding wheels sizes.

PCD/number of holes

Pitch Circle Diameter (PCD) and the number of holes are set by the wheel hubs that are fitted to the car. The diagram below shows this visually. The number of holes is pretty self-explanatory really! Generally, PCD will be expressed as a single number, followed by an ‘x’ symbol and then another, larger number. An example would be 5×100. This simply means that the wheel has five lug holes spaced around a circle 100 mm in diameter.

Wheel 1


Every wheel has an offset number (this is usually stamped into it somewhere) such as ET15 or ET45. This refers to the distance in mm between the axle pad of the wheel (the flat plane that mounts against the hub) and the imaginary centre line of the wheel. In simple terms, wheels with a lower offset will stick out more than those with a neutral or positive offset.

Conversely, if you were to fit a set of wheels with an offset much higher than your standard rims, you would find that the wheels would be closer to the hubs – possibly fouling on your suspension components, wheel arches, etc. Clearly, offset is something that you don’t want to mess up!

Wheel 2

Of course is you order wheel with too much offset, you can fit spacers between the hub and the wheel (effectively increasing the negative offset). The problem then is that the wheel may stick out beyond the bodywork resulting in the car not complying with the regulations…

Centre bore

The centre bore of a wheel is something fairly self-explanatory, but not everyone realises just how important it is. On a hub-centric fitment (which describes the majority of cars), the wheel studs or bolts do not bear the weight of the vehicle itself. Rather they’re simply there to hold the wheel on. The weight of the vehicle is held up by the spigot at the centre of the hub – which protrudes out and fits into the centre bore of the wheel. On most OEM (Original Equipment Manufacturer) wheels, this bore will be a close fit – meaning that it simply slips over the spigot. But on most aftermarket wheels, this bore is oversized to allow for a range of fitments to be accommodated. It is vital that in such cases a suitable spigot ring is fitted to increase the diameter of the spigot and ensure a tight fit. Quite simply, without one of these simple devices fitted, you run a real risk of your wheels falling off!

Wheel nuts/bolts

The type of fixings you use on steel wheels are not suitable for use with alloy wheels – because they’re much harder. This means that they can eventually chew through the bit of the alloy responsible for holding them to your hubs! Not good at all! So the team has to make sure they use wheel nuts designed for alloy wheels.


So after taking all of the above into consideration (and as ever technical advice from Jeff Owen), the team run on Enkei RPF1 supplied by Special Projects Motor Sport.