Lowering vehicle height
Acceptable 'eyebrow heights' (the measurement from the centre of the wheel vertically upward to the edge of the mudguard) or 'bump rubber clearance heights' (the clearance between the bump rubber and corresponding contact point) for most passenger vehicles can be found on the Passenger Car Track List for 1970 and later vehicles .
Other requirements for lowering a vehicle include:
- either front or rear, the suspension travel must not be reduced by more than one third of that specified by the vehicle manufacturer
- the vehicle must also retain 100 mm ground clearance which is measured between any part of the vehicle and the ground
- all measurements must be carried out with the vehicle on level ground in its normal operating but unladen state
- coil springs must not be shortened by cutting, and no suspension component is to be subjected to heating
- lowering blocks may be used on leaf spring suspensions provided they are manufactured from steel, aluminium, or metal and to be positively located to the axle spigot hole and the spring centre bolt. The axle mounting 'U' bolts must not be below the lower rim height.
Replacement coil springs must have:
- clearance between the coils when the suspension is at full bump
- the same or greater load capacity as the original springs
- sufficient force between the spring and its seat for it to remain correctly seated when the suspension is at full droop
- the same end shape as the original springs.
Lowering blocks may be used on leaf spring suspensions provided they are manufactured from steel, aluminium, or a metal of equivalent strength and are positively located to the axle spigot hole and the spring centre bolt. It should be noted that the axle mounting 'U' bolts are not permitted to be below the lower rim height.
Rubber or other resilient bump stops must be provided where the suspension or axle are likely to bottom out or result in the spring being coil bound (ie no clearance between coils at full bump).
When the ride height of a vehicle is lowered, the vehicle must still comply with other legal requirements - eg lowering the vehicle will affect the height of the vehicle's lights and reflectors and these have to remain within specific legal boundaries ie headlamps must be at least 500 mm above the ground, measured from the centre of the light.
Raising vehicle height
Vehicle ride height can be increased by a total of 50 mm without a roadworthiness inspection.
The ride height can be raised by one of three methods or a combination of them up to a maximum of 50mm.
- Suspension lift - fitting longer travel springs
- Body lift raising the body of the chassis
- Tyres fitting of larger diameter tyres
A total lift of more than 50 mm requires prior approval in the form of a statement of requirements and a report from a Light Vehicle Engineering Signatory (LVES). A roadworthiness inspection will also be required.
Raising a vehicle is not generally recommended due to cornering and braking instability that may result. Often vehicles are raised because the owner wants to fit large wheels and tyres that would otherwise contact the mudguards. In these cases the wheels and tyres may be unacceptable.
When raising the height of your vehicle's suspension, you must ensure the the original amount of rebound travel is maintained. The rebound must be limited by the same method as the original manufacturer, ie limit straps or shock absorber full extension.
The body of a vehicle can be raised by up to 50 mm by fitting spacer blocks between the vehicle chassis and body at their mounting points. The blocks must be manufactured from steel, aluminium, or a metal of equivalent strength.
In some cases the steering shaft may have to be extended and longer brake hoses fitted. If the original steering components are altered or replaced, you will need to:
- submit an MR620 Application to Modify a Motor Vehicle to Vehicle Standards.
- have a roadworthiness inspection
- a report from a LVES may also be required.
Vehicle ride height cannot be raised by:
- wedges or blocks located between the coils.
- relocating a leaf spring on the opposite of an axle to that of the vehicle manufacturer
- spacer blocks mounted above or below the coils spring or mounted between the top of the suspension strut and vehicle body.
- extended or adjustable shackle plates on leaf springs.
Fitting tyres and rims with a larger diameter to those listed on the vehicle's tyre placard may also affect vehicle ride height.
It is acceptable provide that the overall diameter of the alternative rim/tyre combination is not greater that 15 mm over the largest combination or 15 mm smaller than the smallest combination specified on the vehicle's tyre placard.
In four wheel drive vehicles an increase in overall diameter of up to 50 mm is acceptable.
Auxiliary suspension devices
Auxiliary suspension devices may be fitted. Sway bars may be fitted to your vehicle. Sway bars add roll stiffness at the front which may increase under-steer, and additional roll stiffness at the rear may create an increase in over-steer that could lead to unpredictable handling.
Track rods/traction bars may be fitted to control rear spring 'wind-up' provided that they meet the minimum ground clearance requirements.
Strut braces may be fitted between suspension strut and spring mounting towers.
Adjustable coil-over suspension
Aftermarket adjustable coil-over suspension components are suspension units that incorporate an external thread on the main body and corresponding threaded spring saddle that allows the vehicle's suspension height to be varied.
The report must specify and include the following points:
- the suspension travel must have at least two thirds of the amount of suspension travel of the original suspension system fitted buy the manufacturer of the vehicle
- fitment of a rubber bump stop to limit travel to ensure there is clearance between the coils when the suspension is at full bump
Airbag suspension replaces the conventional coil or leaf springs with an airbag or an air strut. Each airbag or strut is connected to a valve to control the amount of air allowed into it which is activated by a compressor.
This pressurizes the air, using compressed air as a spring. The purpose of airbag suspension is to provide a smooth ride quality, and in some cases, self-levelling.
Airbag suspension is acceptable provided that:
- the front and rear suspension is fitted with the original suspension bump rubbers in the original manufacturer's positions
- the vehicle maintains a 100 mm minimum ground clearance at all times
- if the airbag system fails and the air is vented, no part of the vehicle must fall below rim height
- the airbag line pressure and vehicle height cannot be adjusted from within the cabin of the vehicle
- there is no transfer of air between one side of the vehicle and the other - fore and aft movement is acceptable
- the vehicle must be fitted with a front to rear automatic leveling device that does not incorporate a manual override - this value is to operate at all times when the vehicle is running, not just on engine crank
- the system must incorporate a lock-out function that prevents the vehicle being moved or driven without the air spring being correctly inflated and the vehicle being at its normal ride height.