Frequently asked questions. Click on the + or – at the end of the Questions
It’s pretty common to see big 4WD’s driven daily on the road. Bigger tyres and lift kits will make a 4WD more capable off-road, but it also makes them illegal on the road, unless you have jumped through a number of hoops. A vehicle that is illegal has a number of very serious risks attached to the driver, which we will go into below.
What is legal?
For starters, let’s consider what is legal, and then you will know where you stand right now. In WA, VSB 14 has been adopted, which is a document that goes through the large majority of modifications done to vehicles, and what is legal or illegal.
If you have modified any of the following items on your 4WD it would be a very good idea to do some reading up:
- Tyre size
- Wheel track
- A vehicle that could be over weight
Lifting your 4WD
In VSB 14, It states that your roof height may only be lifted by 2” (or 50mm) from the factory (or standard height) before it needs to go through a lane change test. This 50mm is the total combination of body lifts, suspension lifts and bigger tyres.
You are also not allowed to fit tyres that are more than 50mm bigger than the factory tyre diameter. For a lot of four wheel drives, 31’s are standard tyres, so 33’s are the biggest you can legally fit (or 285’s from 265’s).
In reality then, you are limited to:
1” lift and 2” bigger diameter tyres (together)
2” lift and the same size tyres.
For those of you out there that are running bigger than a 2” lift, your vehicle is illegal. If you have bigger tyres than standard and a 2 inch lift (or bigger), your vehicle is also illegal. The only way this does not apply is if it has been signed off by an engineer.
For more information regarding VSB14 click on or paste the link below to your browser
Lowering of vehicles
The road clearance of a fully laden vehicle must not be
less than 100mm measured at any part of the vehicle
other than the wheel rim or brake backing plates.
(This does not apply where a lesser clearance has been
specified by the manufacturer.)
When lowering or raising a vehicle body (front or rear),
the following additional restrictions are based on the
manufacturer’s dimensions for the standard unmodified
vehicle while unladen:
• The ride height measured between the rubber bump
stop and the corresponding metal stop may be reduced
by no more than one third.
• The rebound travel measured between the rubber
rebound stop and the corresponding metal stop
(or the extension of the shock absorber for vehicles
without a rebound stop) may be reduced by no
more than one third.
• Coil springs are to remain in locating seats on full
suspension droop without forcibly being removed.
In addition, the normal relationship between the front
and rear suspension heights must not be unduly affected.
Replacement springs (shorter or taller) must have the
same or greater load capacity as the original springs.
Suspension coil springs must not be shortened by cutting
Leaf spring suspensions must not be raised
by the use of extended shackles, adjustable metal plates
or by placing the leaf springs to the opposite side of the
If lowering blocks are used, they must be either
steel or aluminium.
Replacing some or all of the suspension system
with an air or hydraulic suspension requires specific
Airbags are designed to ASSIST with your leaf springs they are not designed to replace them!
What we mean is that manufacturers fit soft leaf springs as the end use of the vehicle may change once sold. For example:
Brand new Navara Ute with factory well body. A plumber buys the vehicle takes it to a body builder fits a canopy to it. Then takes it home fills it with tools, fills it with pipe fittings, fills it with nuts and bolts and Gas bottles etc!
Nett result is this vehicle now has a constant load of 400 kilograms 24 hours a day and the rear of this vehicle is now sagged. Whilst an airbag can lift this load the airbag has to be filled to such capacity that the chassis now sits predominantly on top of the airbag and not the springs leaves. You can bend your chassis doing this!
Ideally the leaf packs need to be strengthened either by replacing them or resetting the existing leaves and adding in more leaves so that the spring leaves will now cope with the weight on top of them.
If this plumber pulls a trailer then you can fit airbags to assist with towing.
If you dont have too heavy a constant load then it may be possible to fit airbags but again to assist primarily with towing.
Always contact us for advice. Its FREE
GVM (Gross Vehicle Mass)
GVM is the maximum weight that the vehicle ortruck can carry, including its own weight, as measured where the tyres contact the road.
The kerb weight is the weight of the basic cab and chassis before any specific body has been added. It does however include an allowance for some fuel and lubricating oils, and may include the weight of a spare wheel and tyre.
Available Payload Capacity = GVM – kerb weight – tare weight
In normal life you will face more situations of towing a trailer, than when only driving a truck. The GVM is the total load the truck can carry on its own wheels, whereas GCM includes trailer weight.
GCM (Gross Combination Mass)
GCM is the total weight of the truck, trailer and their loads combined.
GCM = Weight of vehicle (truck) + equipment + weight of trailer + payload
The GCM on the ground should not exceed the manufacturer’s GCM rating, and trucks can not be loaded to exceed the truck’s GVM or individual axle capacities, even if the total combination weight is lower than the rated GCM.