If you’re reading this, you’ll almost certainly want to replace the stock tyres on your 4x4 for offroad use. Typically, this tyre will be taller and of a different, tougher construction with a more open tread pattern. That’s a different tyre from the standard, and therefore, it is a modification. This means you must fit the tyre within the limits of the regulations. Where do you turn for advice on tyre regulation? You can find all the tyre regulations on your state government road authority website. The tricky thing about the tyre regs is that there’s not just one, there are several, and the ones you need to comply with don’t always specifically relate to tyres. It’s complex, so let’s break it down. Diameter The first limit is the diameter. The additional diameter permitted varies depending on class of vehicle, but for 4x4s it’s typically plus 50mm, for cars, 15mm. The definition of a 4WD? Here are the words from Vehicle Standards Bulletin (VSB) 14, Section LS: “4WD passenger vehicles specifically designed for off-road use”. That doesn’t include utes, but they’re actually allowed such mods anyway as they’re in a commercial vehicle classification, the N category of vehicles as opposed to passenger cars which are M category. There are plenty of tyre diameter calculators on the web, so pick one. They will work out the diameter based on the tyre specs, such as 265/65/17. However, larger tyres cannot foul any part of the body when the suspension flexes and/or the steering wheels are turned, so that needs to be checked. Here are the words from VSB14: “No part of the wheel must touch any part of the body, chassis, steering, braking system or suspension under any operating condition.” The 75mm rule There is also the “75mm rule”, which means you can lift a 4WD by 75mm with a combination of 50mm suspension and 50mm tyre. But 50 plus 50 is 100, right? Not in this case, as a 50mm tyre diameter increase means a 25mm increase in height, so it’s 50+25=75mm. The existence of this 75mm lift rule is a maximum, and like all other limits, if another rule kicks in first, then that’s as far as you can go. Width Then we come to width. The limit for width for 4WD is 1.5x the widest original tyre as per the placard. But that’s a perfect example of an indirect regulation, as chances are you’ll run afoul of other limits first. For example, there are limits on how much of a track increase you can have, which is the width between wheel centres on an axle, and that dictates the maximum offset you have chance (distance from wheel hub to centre of wheel). That, in turn, starts to limit the width of the rim, which limits the width of the tyre as every tyre has a maximum and minimum rim width it can be fitted to. We’re dealing with tyres here, not rims though. That’s a related but separate subject. BIGGER TYRES, BIGGER PROBLEMS? UNDERSTANDING OFFROAD TYRE REGULATIONS
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