How do you improve traction off-road? You deflate your tyres, of course. But why exactly does this work so well?
If you’re a regular Leisure Wheels reader, you undoubtedly know that you can improve off-road traction by deflating your tyres. But have you ever asked yourself why? How exactly is it that a drop in tyre pressure can have such a profound improvement on off-road performance? We’ll do our best to explain but before we do, here’s a caveat: we’re going to stick to the generally accepted theory. There are plenty of opinions out there; some have merit, some don’t. This, however, is the explanation that most people agree on.
Every vehicle has a recommended tyre pressure at which it should operate. This number is not randomly selected, it is based on the weight and application of the vehicle. Most vehicles spend 99% of their time on tar, usually at pretty high speeds, so that’s what manufacturers base their selection on. Let’s look at the 2.4-litre SRX Hilux, which I’m driving at the moment, as a typical example. Like all vehicles, you can find the tyre-pressure information on the inside of the doorframe, on the driver’s side. Simply open the door and inspect the area where the door latches to the frame. You’re sure to find the tyre pressure info there. With the SRX Hilux, Toyota recommends a pressure of two bar at all four corners. However, when you put a heavy load on the back, the company recommends upping air pressure in the rear tyres to 2.5 bar. Why is this? Well, it’s all about efficiency.
Traction isn’t an issue on tar, so you don’t want too much contact between the tyre and the road. You want the four contact patches to be small, since it will reduce fuel consumption and wear on the tyres, and make the vehicle easier to turn. Even at two bar, a heavy load will increase the contact patches to the point where efficiency is compromised. The weight will cause the rubber to bulge, increasing resistance and preventing the tyres from rolling lightly over the surface of the tarmac. A properly inflated tyre will sit evenly on the road surface, and therefore wear evenly over the entire tread of the wheel. An underinflated tyre, meanwhile, will bulge at the sides and cause more wear to take place at the edges of the tread, instead of in the centre. Lastly, an overinflated tyre will have too little contact with the road, and will largely show tread wear at the centre.
When you venture off-road, things change considerably. Suddenly, it’s all about traction. In fact, a vehicle’s off-road prowess is really nothing more than an illustration of its ability to maintain traction in difficult situations. In order to keep moving forward off-road, a vehicle needs to be able to transfer the power and torque generated, to the road. Excellent wheel articulation and differential lockers are useful precisely because they allow you to do this. As should hopefully be obvious, tyres are also crucial, since they are the only items linking your pukka 4×4 to the ground. Adjust the tyre pressures even a little, and the change in performance can be significant. To illustrate this effectively, let’s look at one of the most troublesome off-road surfaces: thick sand. When sand is wet and compacted, it’s easy to drive on, since it offers a hard surface that allows for the transfer of power. When it’s dry and loose, though, you lose traction. Instead of your vehicle being propelled forward over the surface, it stays put while sand is kicked out from under it.
It’s very easy to loose traction and momentum in sand. Flooring the throttle at low speeds can result in your vehicle digging itself into the sand.
Why does this happen? There are two reasons. Firstly, there isn’t enough contact between the surface and the tyre. Because they are too firm, your tyres can’t ‘grip’ the sand and dig their way out. Secondly, your vehicle is too heavy. Remember, with your tyres at normal pressures, all the weight of your 4×4 sits on four very small contact patches. The solution is to deflate your tyres, which effectively deals with both issues. When you let some air out of a tyre, the bottom flattens out and increases the contact patch. So, instead of sitting above the sand and spinning uselessly, it can now grip a wider area and fight its way out. In addition, increasing the contact patch also spreads the weight of your 4×4 over a wider area. In the same way that a person can lie on a bed of nails because the pressure is spread over a massive area, deflating your tyres reduces the weight per square centimetre.
Let’s get practical
What does all this theory look like in practice? Well, when it comes to sand, it means deflating your tyres quite a lot. Manufacturers will typically warn you not to go any lower than 1.2 bar, but when you’re dealing with hot, loose and very deep sand, you’ll probably need to drop pressures all the way down to 0.7 bar. This doesn’t come without risks. As a tyre is deflated and starts to bulge, its sidewall is exposed to sharp roots and stones. This isn’t always in an issue in sand, but it can be on rock and gravel. A heavily deflated tyre can also be driven straight off the rim if you’re not careful. Driving quickly and turning sharply will often see the tyre popping off the rim. Jacking up the vehicle and sorting out the issue in deep sand can be a huge hassle…
To deal with sand effectively, you often need to lower tyre pressure to below one bar.
Once back on tar or hard gravel, it’s very important that you inflate your tyres. Driving with flat tyres on a hard surface will result in a massive amount of heat build-up, which will quickly damage a tyre. Deflating a tyre is a risky strategy in general, but it is often the only solution when travelling off-road. There’s a popular saying in 4×4 circles when it comes to off-road obstacles and speed: “Drive as slow as possible, but as fast as necessary.” The same approach holds true for tyre pressures. When driving off-road, always deflate your tyres as much as is necessary, but as little as possible.
Text: GG van Rooyen