Some change can achieve improvements in cornering stability, however other change, including the selection of unsuitable tyre and rim can lead to dangerous situations.
"Vehicle Owners should consider the following when wishing to change the appearance of their vehicle"
The fitment of wider rims and tyres usually involves altering the steering "scrub radius". This can result in unpredictable steering response characteristics. Such as tyres contacting th body and suspension components can reduce the vehicles turning circle.
Road Holding and Handling
The road holding and handling qualities of a modified vehicle must not be adversely affected.
Some non-standard rims and tyres fitted to vehicles with diagonally split braking systems can cause reduced directional stability in the event of brake failure. Larger diameter tyres reduce braking capacity.
The vehicle must have a sufficient ground clearance to meet the Australian Design Rule and Australian Vehicle Standards Rules requirements and ensure safe operation of the vehicle on the road.
The vehicle must have a sufficient turning circle in each direction and must meet all Australian Design Rule dimension requirements.
Any modification to suspension and steering, including the replacement of tyres and rims, must ensure that the vehicle's body, exhaust system, axles, suspension or steering components do not contact the road when tyre(s) deflate. Therefore, if one or more of the tyres deflate when the vehicle is on a level road, the wheel/rims and tyre must be the only part of the vehicle in contact with the road.
When you change or modify the wheels and tyres of a vehicle this can alter the drivability of the vehicle on the road. Because of this situation there are specific limits to the changes permitted. While these changes may be permitted, however it is still the "responsibility of the vehicle owner" to ensure that their vehicle remains safe even though it is modified within the specification limits. The vehicle owner is responsible prior to the modification being undertaken to contact their insurance company to check and see if this is permitted and acceptable under the terms of their insurance policy.
Wheel Track is the measured distance between the vehicles wheels; it is the measure distance between the rim centre lines. The fitment of wider wheels will probably increase the wheel track and this can be associated with the change in wheel "Offset". This in turn can increase the load on bearings, axles, suspension joints and steering tie rods.
The wheel track must not be increased by more than 25mm beyond the maximum specified by the vehicle manufacturer for that particular moxel. This means that the rim offset must not be changed by more than 12.5mm per wheel.
The reduction in wheel track is not permitted without approval of the relevant registration authority.
Wheel Sizes and Axles
All rims that are fitted to the front or rear axle must be of the same diameter, offset, width and mounting configuration (except spare wheels used in an emergency situation or where the vehicle manufacturer has specific variation as standard for front and rear fitment on specific models).
High Performance Specifications
When a vehicle owner considers converting a passenger car's wheels and tyres to those fitted as standard to a high performance version of that model, then the matching suspension components such as spring, shock absorbers and sway bars from the high performance model should be used and fitted.
Wheel Attachment to the Vehicle
The replacement wheels should be designed for the particular hub/axle and have the same bolt/stud pitch circle diameter (P.C.D.) and the same centre location method. The wheel nuts or bolts must have the same tapers as the wheel. Wheels with slotted bolt/stud holes are not allowed.
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Wheel Offset Measurement and Explanation
The offset of a wheel is the measured distance in mm from the hub mounting surface to the centreline of the wheel. The offset can be one of three types.
Is where the hub mounting surface is even with the centreline of the wheel.
Is where the hub mounting surface is towards the front or the wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and later model rear drive cars.
Is where the hub mounting surface is towards the back or the brake side of the wheels center line. The "Deep Dish" wheels are a typical example of negative offset.
is measured in mm followed by the capital letter P or N - e.g 35P is 35mm positive offset from the mating hub to the centre line.
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Wheel P.C.D (Pitch Circle Diameter)
Pitch Circle Diameter
is the diameter measurement of the circumference of the centre line of the drilled holes on the wheel mounting face.
this can be four, five or six stud holes and can be measured as shown in the following diagrams. The easiests way of measuring four or six stud wheels is to take the two diagonal holes and measure from the outer edge of one hole to the inner edge of the second hole.
With a five stud wheel it is not possible to utilise the same measurement practice as for four studs as there is not two diagonal holes. The five stud diagram shows one of the ways to measure.
Ref: 1997-2008 Njoy Training & Consulting