Words by Grant Spolander Pictures by Grant Spolander & Andrew Middleton
A few years back, we did a comparison test between a Ford Ranger 3.0 TDCI and a Hilux 3.0 D-4D. I was hugely excited about the shootout and was expecting big results. But, shortly after hitting our test trail (Klein Tafelberg), I was miserably disappointed.
It seemed that, no matter what the obstacle, both bakkies were on par with each another. Sure, they had unique engines and torque curves, and as a result we used different gears for each bakkie, but, aside from that, both vehicles performed equally well and there were no revelations.
However, one particular obstacle did reveal a variation, but it had nothing to do with the 4x4s themselves and everything to do with their tyres.
It was a blistering hot morning, the sand was softer than I’d ever seen it at Klein Tafelberg and we’d spent the better part of the day trying to get up a sandy incline. After deflating both sets of tyres, we found the Ranger 74performed better at 0.6 bar, while the Hilux was happy at 0.8 bar. (It must be said that both bakkies were carrying equal loads in their bins.)
It was interesting to note how big a difference 0.2 bar made to both sets of tyres. In the Ranger’s case, its off-road performance greatly improved the moment we dropped the pressure from 0.8 to 0.6 bar. And the Hilux became a different beast when going from 1.0 to 0.8 bar.
So, if such a small increment can play a big role in off-road performance, the question is: What’s the right pressure to use on sand, gravel and rock? Unfortunately, like all great questions, there’s never a straightforward answer. In fact, the above question is quite similar to the “how long is a piece of string” puzzle.
The problem starts with tyre size, then moves on to sidewall profile, compound properties, vehicle payload, and, of course, sidewall rating. All these factors play a major role in determining your 4×4’s optimum off-road tyre pressure, and, because these factors are so vastly different from one vehicle to the next, there’s no solid answer.
However, there are guidelines you can follow in terms of percentages. Several 4WD guide books quote a 10 to 15% reduction on gravel, a 25% reduction in mud, a 50 to 60% reduction in sand, and anywhere between 15 and 50% on rocks – depending on the rock type.
If you’re tackling smooth rocks and boulders, you can go as low as 50%. But, on sharp jagged terrain, keep your reduction to 15 or 20% to minimise sidewall bulging and the risk of a sidewall puncture. (Remember: a tyre damaged on its sidewall cannot be repaired and will have to be thrown away!).
Another possible risk exists on sandy terrain, where there’s always the temptation to lower your pressure too much and potentially de-bead the tyre off the rim. This happens when your tyre pressures are significantly low and you turn a sharp corner, causing pressure on the sidewall to de-bead the tyre..
The fitment of bead-locks will allow you to run tyre pressures as low as 0.4 bar without the risk of de-beading a tyre. However, bead-locks ain’t cheap and can cost anything up R12 500 per set of five..
On the plus side, keep in mind that your 4×4’s rear tyres are not subjected to the same hard turns and sidewall pressure, so you can generally go lower at the rear than you would up front – assuming that your vehicle isn’t heavily laden, which would require additional pressure at the rear.
The obvious downside to tyre deflation is the simultaneous decrease in ground clearance. During a recent tyre-deflation test, we noticed an 18 mm ground clearance reduction between 2.0 and 1.0 bar. However, the traction benefits of a deflated tyre far outweigh the risks of an 18 mm ground-clearance sacrifice.
Aside from better performance and traction, the other major advantage of a deflated tyre is comfort. This applies to all off-road surfaces, from rock and sand to corrugated tracks; a deflated tyre will substantially improve your vehicle’s off-road comfort levels.
However, it’s important to remember that you should never drive your 4×4 at speed with deflated tyres. Doing so will increase the tyre’s friction area and internal heat, potentially leading to a blowout. If you’re unsure of what tyre pressure to use on gravel (and you’re the sand-driving performance of a Goodyear Wrangler ATSA against a BFGoodrich, when both tyres are set to the same pressure – let’s say 1.0 bar.