When it comes to modifying your 4X4, it’s easy to get caught up in the excitement of new upgrades - the bigger tyres, the suspension lift, or the bar work that makes your vehicle look like it’s ready to conquer the Simpson Desert. But here’s the thing: not all modifications are created equal, and some will need to be signed off by an engineer to keep your 4X4 legal. While the rules and requirements vary from state to state, some modifications almost always need engineering approval, no matter where you’re based.
The penalties for skipping this step can be steep. We’re talking fines, defect notices, or worse - a rejected insurance claim after an accident. Let’s take a look at the most common 4X4 modifications that require engineering and why it’s crucial to tick the legal boxes before hitting the tracks.
1. Suspension Lifts Over 50mm
The holy grail of 4X4 modifications—getting your vehicle higher for better clearance and a tougher look. But as soon as your suspension lift goes beyond 50mm, you’re likely in engineering territory. The reason? Lifting a vehicle changes its centre of gravity, which affects stability and handling. A poorly executed lift can lead to decreased handling, especially when cornering or navigating uneven terrain.
Here’s where state laws come into play. In NSW and the ACT, for example, you’re allowed up to a 75mm lift without engineering through a combination of tyre diameter and suspension lift, so a 25mm suspension lift and 50mm lift from the tyres would be legal(ish). Anything above 75mm needs engineering sign-off.
In Victoria, the rules are similar, but that 75mm has to be a combination of suspension (max 50mm) and tyres (max 25mm). Queensland has the same 75mm rule as Victoria with a 50mm limit for anything with Electronic Stability Control (ESC). You can bump this up to 125mm with engineering. NT will let you go up to 100mm over stock without needing engineering, while our mates in SA, WA, and Tassie are all capped at 50mm suspension lifts before they’ll need to get engineering sorted.
It’s worth mentioning, though, that it’s not just the springs that can cause issues here. In some states, the tyres count towards the lift too, so if you’re anywhere close to the limit, it’s worth double-checking with a local engineer. An engineer ensures that the components you’ve installed (springs, shocks, control arms) work together safely and that your vehicle still meets Australian Design Rules (ADRs).
It’s not just the vehicle height that may need engineering, supporting components like control arms or radius arms in solid axle 4X4s can require a sign off too
2. Tyres Larger Than 50mm Over Standard Diameter
Bigger tyres are often the first modification for many 4X4ers, giving your vehicle improved traction, clearance, and that iconic off-road stance. But fitting tyres more than 50mm larger than your factory size is another modification that typically needs to be engineered. Larger tyres can impact speedometer accuracy, braking distances, and steering geometry - all safety concerns that need addressing.
An engineer will confirm the basics, like ensuring your tyres fit safely within the guards, won’t rub against suspension components, and won’t compromise the braking system. They’ll also check that your drivetrain can handle the additional strain. Ignore this step, and you could be looking at fines, defect notices, or trouble with your insurer if something goes wrong.
3. GVM Upgrades
Gross Vehicle Mass (GVM) upgrades are popular for those planning heavy touring setups, like decked-out canopies or simply loading up with camping gear. A GVM upgrade increases the legal weight your 4X4 can carry, ensuring you’re not overloading the chassis, suspension, or brakes. However, this is a significant legal change and always requires engineering approval.
The upgrade typically involves installing heavier-duty shocks and springs, sometimes chassis reinforcing, or even upgraded differentials. An engineer will test the setup to ensure your vehicle can safely handle the extra weight under all conditions. Without this approval, your 4X4 could be deemed unroadworthy, even if it seems to handle the load fine behind the wheel.
4. Pre-Rego Mods: Why It Pays to Start Early
When you get your 4X4 engineered is just as important as what you get engineered. Having a brand-new vehicle modified before it even hits the road can save you a fortune and simplify things if you ever go to sell it. If you’re planning significant modifications, such as chassis extensions, portal axles, or other heavy-duty changes, consider doing them before your 4X4 is registered.
This is known as the Second Stage of Manufacture (SSM) process. Under SSM, the modifying workshop can make changes under their engineering umbrella, ensuring your vehicle complies with Australian Design Rules (ADRs) and remains legal nationwide, not just in the state where your engineer is signed off. You’ll benefit from lower costs and less red tape compared to getting the same modifications approved on a registered vehicle. It’s worth asking your workshop if SSM is an option for your build.
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5. Seat Upgrades
Upgrading your 4X4’s seats is a popular way to boost comfort and support for long hours on the tracks, especially in an older vehicle. But there’s a hidden risk - how those seats are mounted. Even if the seat itself is ADR-compliant, there’s no guarantee the mounts holding it in place are strong enough to withstand a crash. This is where an engineer’s expertise becomes critical.
Think about it: a poorly mounted seat could turn into a deadly projectile in an accident. An engineer ensures that your seats and mounts are made from proper materials, are correctly installed, and can withstand the forces involved in a collision.
6. Bull Bars and Brush Bars
Not all bull bars need engineering, but if you’re installing a custom setup or something that deviates from ADR-compliant designs, you might need approval. Bull bars and brush bars affect how your vehicle behaves in a crash, particularly with airbags and crumple zones. If the bar isn’t designed with these safety systems in mind, it could prevent the airbags from deploying properly.
An engineer will check that your barwork is mounted correctly, doesn’t obstruct vision, and meets all ADR requirements. This is especially important if you’re running a setup that includes brush bars connecting to your side steps, as these can change the structural dynamics of your vehicle.
With modern 4X4s being so capable you really don’t need big lifts and big mods any more to have a capable tourer
7. Chassis Modifications
Any change to your chassis—whether it’s lengthening, shortening, or adding additional cross-members—will need engineering. Chassis modifications are sometimes done for custom tray setups, GVM upgrades, or even chassis extensions for a longer wheelbase. These changes are highly regulated because they directly impact the structural integrity of your 4X4.
Engineers will assess the materials used, the quality of the work, and how the modifications affect the overall strength of the vehicle. Poorly executed chassis modifications can lead to catastrophic failures on the road or in the bush, so this isn’t a step to skip.
8. Brake Upgrades
Upgrading your brakes is a smart move if you’ve increased your vehicle’s weight, swapped engines, or added larger tyres. However, modifying or replacing brakes with anything other than factory-spec parts usually requires engineering approval. This is because your braking system must meet strict performance standards under ADRs.
An engineer will test your brakes under load to ensure they perform safely, whether you’re towing a trailer or tackling a steep descent. It’s not just about stopping power—it’s also about balance and preventing brake fade under heavy use.
Why Engineering Matters
The legal side of 4X4 modifications isn’t just about avoiding fines or defect notices - it’s about safety. Engineering approval ensures that the modifications you’ve made don’t compromise your vehicle’s handling, braking, or crash performance.
Getting your modifications engineered might feel like a hassle, but it’s a small price to pay for peace of mind. After all, the last thing you want is for your dream vehicle to become a liability - on or off the tracks. Check your state’s regulations, keep your modifications legal, and enjoy the confidence that comes with knowing your 4X4 is up to code.
Keep the shiny side up!
Pat
There’s a reason so many 4X4s all run a similar formula, it’s a cost-effective way to build a capable tourer without running afoul of the law, if you do get a bit carried away though it’s surprising how much has to be engineered
