Three mounting systems
The most commonly encountered mounting systems are bolt-centred, hub-centred and double-centred. It is essential to use appropriate components for each type of mounting system and to mount wheels on the correct hubs. If components for the various mounting systems are used inter-changeably, this can lead to bolt failure, reduced torque, tyre punctures and poor traction.
- Bolt-centred systems use bolts in the wheel hub to guide and centre the wheel. Wheels with this system have bevelled lug holes. These wheels should be centred by placing conical or spherical nuts on the bolts.
- Hub-centred systems use the actual wheel hub to guide and centre the wheel. Hub-centred wheels normally have straight lug holes without a ball seat, i.e. for nuts with a flat ring (flange nuts).
- Double-centred systems use both the hub and bolts to guide and centre the wheel. These wheels have recessed lug holes and a centre hole with minimum tolerance. They normally use spherical or conical nuts.
The offset is one of the most important dimensions on the wheel disc. This is the distance from the imaginary centre of the rim to the inside of the connecting surface. The offset of a wheel disk can be positive, negative or '0 offset'.
In double assembly applications, the offset is equal to 'half the double distance'. In this case, the double distance is the distance between the imaginary centre lines of the two rims. Double distance is used to determine values for tyre clearance, the total vehicle width of tyres, vehicle clearance and, in some cases, the total width of the vehicle. That is why values for minimum double distance are regulated by several norms, including ETRTO and TR&A.