When it comes to changing the wheels/rims on a 4WD, its important that you consider their offset.
Wheel offset refers to the position of the centre part of the wheel (where the wheel bolts on). As the centre moves, the wheel position also moves. If the wheel has a negative offset, it sticks out further from the differential (or centre of the car), and if it is positive it sits closer to the differential. The more offset your wheels are, the greater your wheel track is, and the more likely your tyres are to stick outside of the guards of your 4WD.
Take our Isuzu Dmax, which has positive 33mm aluminium rims from the factory. If you were to fit zero offset rims, they would stick out 33mm further on each side, thus increasing the total wheel track by 66mm.
What’s the importance of offset?
Wheel offset affects a number of things, and taking a minute to get the right offset is a wise move, or you might end up with an illegal 4WD, wheel bearings that fail early, poor vehicle handling or nasty chipping up the side of your vehicle.
Wheel track increase legalities
By law, you are only allowed to increase your wheel track by a certain amount. For 4WD’s in Western Australia, this is 50mm. This may vary from state to state, but exceeding this means your vehicle is not roadworthy, and you risk having problems with your 4WD insurance and the police.
Wheel bearing stress
The further your wheels stick out, the more stress you put on your wheel bearings. If you widen the wheel track your wheel bearings have to work harder, and the more you move the wheels out the harder the more load is applied. While you can fit -55 offset wheels, its going to come at the cost of wheel bearings that wear out faster, need more maintenance or fail more often. It’s simple physics, and there’s no disagreeing with it.
If you think about it, there is a lot of flexibility when it comes to changing offset. If we put -55mm rims on our Dmax, we’d have the rim sticking out 88mm further on each side. That’s nearly 180mm increase in wheel track, plus any extra width you gain if you fit bigger tyres. Obviously, doing this would cause other issues with scrubbing tyres so its not always practical but I have seen some pretty significant changes in wheel offset.
Will the tyres stick outside of the guards?
The most obvious thing when you change offsets is that your tyres can sit outside of the guards. In Australia, looking down the side of your vehicle, no part of the tyre is allowed to protrude past the wheel arches. This includes the sidewall. A lot of people refer to this as ‘poke’; how much the wheel pokes outside of the vehicles guards.
If it sticks out, you’ll get nasty chips in your paint work from gravel roads (unless you have flares or sidesteps), and likely get pulled over by the police and given a yellow sticker (or an unroadworthy).
If they stick outside of the guards, the way to get around it is to install some flares. You can go the el cheapo way and install Bunnings garden edging, or get rubber flares from Bushranger with spring steel to keep them nicely shaped, or buy fibreglass or plastic flares from aftermarket locations.
Be aware that just because you have flares doesn’t mean the vehicle is legal; it still cant have a wheel track increased by more than what is allowed in your state. The flares will reduce the attention you get by the police, which is often the main aim.
Stability increase or decrease
4WD’s often tip over. Sometimes its just unlucky, and other times its poor line choice, or from doing something stupid (like ripping doughnuts in a paddock, or on the beach). Regardless, your chances of going over are greatly increased when you make changes to your vehicle that affect its stability. If you install a lift kit, or bigger tyres, or a roof rack, or you add lots of weight up higher your stability decreases considerably.
If you increase your wheel track you’ll gain some stability, but only to a point; its hard to overcome a 60kg roof top tent and 30kg roof rack!
Likewise, if you reduce the wheel track you’ll also reduce the stability and have a much greater chance of tipping your vehicle over. If you go up with a lift, its not a bad idea to counter it by going out with the wheel offset (to a point).
I mentioned above that scrubbing is a big problem, and its super common from those who either fit bigger tyres, or who change the offset.
In general, your tyres will rub on the inner footwell of your 4WD. To test this, park on level ground, and turn your tyres full lock to one way. Then, hop out of your vehicle, and look at the gap between the edge of your tyre and the footwell (near where your accelerator would be, but from the outside).
This is the most common place for scrubbing, and normally on the inner side of the vehicle. You can chop body mounts, or do panel work to give you more clearance, but this can be hard work and require engineering to be legal.
Alternatively some people relocate the front differential further forward, but that’s a whole new kettle of fish.
On flat ground, you’ll see where the tyre is most likely to hit, but when you are off road and that wheel flexes upwards, it will get even closer, and scrub even harder to the point where you can damage your tyres and panels.
Check your offset
The takeaway from this is to ensure that the rims you are buying are going to suit your vehicle. Some tyre shops will trial it for you, but many will not. If you blindly buy a new set of rims and find they come with the wrong offset, it can be a problem trying to sort it out.
I would stick within the increase in wheel track laws, ensure your tyres are covered properly and drive accordingly, and you’ll be just fine.
I’ve owned 4WD’s in the past where the tyres stuck outside the guards, and the bottom door panels got absolutely chipped to bits.
To find out exactly what the offset is, you can either measure it, or look for a stamp on the rim (normally on the inside). This will tell you exactly what it is.
What else do you need to check
If you are getting new wheels, you need to make sure that the stud pattern is the same (5 or 6 stud), that the hole centre is supported by your hubs and that you are happy with the wheel in general.