How to: Off-road tyre pressures and improving traction

ByMark AllenMay 8, 2020
6 MINUTE READ
How to: Off-road tyre pressures and improving traction

Getting off-road tyre pressures right to suit the terrain you’re driving on can dramatically improve your vehicle’s traction. Here’s everything you need to know.

Put your thinking caps on as Unsealed 4X4 tackles off-road tyre pressures, ink prints and tractive force with a 10-tonne load cell and a calculator to show you the advantages of getting it right.

It’s no secret that a tyre tread footprint (on the ground) gets longer, not wider, as pressures decrease. Sure, the tyre sidewall does bulge and will touch the ground in some circumstances, like deep, soft sand, but it’s the increased footprint length that makes all the difference to vehicle performance.

This increased length, and therefore tread area touching the ground, results in gains in both flotation (on soft surfaces) and tractive force (on all surfaces) via both increased contact patch area as well as allowing the tyre to conform to the shape of the terrain.

To prove this, we’ll calculate the kilograms-per-square centimetre (or pounds-per-square-inch) of force that the vehicle exerts on the surface of the ground as we vary off-road tyre pressures.

Are there any negatives to lowering tyre pressures?

Yes. Before we get too deep into the advantages of getting off-road tyre pressures right, note that there are some downsides. These include loss of ground clearance as your tyres slightly flatten, overheating of the tyres if you drive too fast (so don’t), and a greater chance of popping a tyre off a bead (so don’t corner too quickly). Other than that, getting off-road tyre pressures right is all positive.

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The tyre conforms to the terrain with lower pressures allowing more grip and drive.

Lowered pressures aren’t just for sand driving

Lower pressures in sand provide superior ‘floatation’ and allow your tyres to remain on top of the sand instead of digging in. It’s the larger tread area touching the ground that helps here to return improved kilograms per square centimetre ratios.

Rock driving is also greatly enhanced with lower tyre pressures. The added flexibility of your tyre’s tread provides increased grip and reduces the risk of punctures. This is the same on any uneven surface if driven slowly.

There are also benefits from lowering tyre pressures when driving at medium speeds on gravel or dirt roads, but not nearly as low as for slow rock driving. Lowering tyre pressures allows the tread to conform to the stones and sharp protrusions, which minimises the chance of puncturing. Remember, the lower your pressures are set, the lower your speeds should be to prevent overheating (from sidewall flex) and having a tyre pop off the rim.

Allowing your tyres to conform to the terrain will help your tyres maintain as much contact with the ground as possible, returning improved grip and therefore forward drive. This is most evident on steep uphill climbs on slippery, loose, wet or rocky surfaces. With lower pressures, you’ll also minimise track damage, as your tyres are less likely to lose grip and spin.

Given the recordings of our load cell at different tyre pressures, there is no doubt lower pressures aid in grip, ultimately allowing more drive, or tractive force. And it is ‘free’ grip, so give it a shot next time you’re out four-wheel driving; you’ll be amazed at how much further you’ll get along tracks and how much easier it will be on your vehicle.

Disclaimer…

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One sturdy tree, overly rated rigging, load cell and V8 power – we went nowhere

We probably don’t need to say it but we will anyway: this yarn is only talking about reduced tyre pressures when driving off-road. You should never drive on a sealed surface, or at high speeds, when you’ve lowered tyre pressures on your rig. Please see your vehicle’s handbook for safe inflation levels for on-road driving.

So, how low can you go off-road? There’s no one-pressure-fits-all for 4X4s. The heavier your vehicle, the less pressure you should let out and the more care you should take when doing so.

As an example, I often drop to about 18psi to 20psi on soft sand. I then drop lower to get out of trouble but I would be extremely careful not to drive too fast or turn too sharply to prevent tyre damage or rolling a tyre off the rim.

How did we measure all this? With a 10-tonne load cell

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For this test, we parked the LandCruiser on flat ground in two-wheel drive, with the rear locker engaged and in low range. We went with two-wheel-drive, so I didn’t risk bursting a CV if the traction got too high, and the rear ELocker engaged to ensure drive was distributed equally between both rear wheels. If the locker wasn’t on, there would be a chance that drive would swap from one side to the other, affecting the readings from our load cell. Low range was used to help avoid clutch slip and to allow a gentle, even pull to ensure accurate load-cell readings.

A tree-trunk protector was wrapped around a suitably large and sturdy tree, attached to a drag chain, which was then connected to a recovery hitch in the tow receiver. The ratings of all the equipment we used far exceed the forces that would be applied, so all was safe.

After each pull, we slightly lengthened the drag chain to ensure even terrain (undisturbed ground) was used for each attempt, making for a fair comparison.

Starting at 40psi in the rear tyres, our first readings saw 1030kg of tractive force applied to the load cell. Dropping down to 30psi returned 1130kg, and at 20psi we saw 1360kg applied to the load cell.

This massive gain in measured tractive force proves that lower tyre pressures improve grip. We knew this would be the case, but didn’t realise just how improved the forces would be. In our case, the truly astonishing 32 per cent of additional grip, or tractive force, is one of the main reasons we lower pressures on most off-road terrains, to gain as much go-forward grip as possible – it’s just the exact grip level had never been measured before.

A little warning though; lowering tyre pressures too much, without some form of bead lock (illegal on non-competition 4X4s), can allow your tyres to rotate on the rim via excessive drive forces. Snapped axles, busted diffs and even wheel standing on ultra-steep tracks are all downsides of having too much grip.

Okay, I said I would stay in two-wheel-drive to protect my Cruiser’s CV joints, but I couldn’t resist the urge to give it a crack in four-wheel-drive, so I locked the front hubs, engaged the front ELocker and recorded a massive 1870kg of tractive force. Four-wheel-drive will always provide more drive than two-wheel drive, but lowering tyre pressures on all four tyres is paramount.

For my impromptu test, the fronts were at 38psi and the rears were still at 20psi. Imagine the tractive force if the front tyres had been lowered to 20psi as well.

Tyre pressures and pretty pictures

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Tyre Pressure Chart
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All the tyre ink prints were made using a 285/75R16 tyre, with the Cruiser lightly loaded. We jacked up one wheel, painted ink over the surface of the tyre tread and slowly lowered the tyre onto the cardboard. The vehicle was not rolled forwards or backwards.

Lengths and widths of the ink image were measured and used to calculate the kilograms per square centimetre the Cruiser was exerting on the ground. We then repeated the exercise at decreasing pressures to show the differences in tread width and length.

The primary variable in the real world will be the load onboard a vehicle in the form of camping gear, tools and luggage. The heavier it’s loaded, the more the tyre print will vary. Tyre brands and specifications will also alter the tyre footprints.

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Referring to the table; the tyre tread length has increased from 19cm at 40psi to 33cm at 15psi – an increase of 14cm, which is just short of double the tread length.

Comparatively, the tyre tread width that touches the ground stays the same at 23.5cm right through the pressure range.

Using the measurements and weights of the 4X4 (at one wheel) we’ve calculated the pressure the tyre is putting on the ground has decreased from 1.254kg/cm² to just 0.722kg/cm², which represents a 42.4 per cent decrease in ground pressure. This is what allows us to ‘float’ over sand and provide increased traction to climb slippery surfaces and crawl over uneven terrain.

Do sidewalls get wider?

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As can be seen from these sidewall photos, the width of the tread touching the ground remains unchanged, although the sidewall does flex out considerably as the tyre pressure drops. If driving in soft sand, this sidewall flex may help a little with reducing the pressure of the entire tyre footprint, but only marginally when compared to the increase in tread length that’s in contact with the ground.

It’s this sidewall bulge at low pressures that can allow a tyre to be staked or cut from sharp rocks or other protrusions, so be careful with wheel placement.

While the softer or more compliant sidewall may ride over or deform around an obstacle, my advice would always be to steer clear of an object if possible – there’s no point chancing your expensive tyres. Remember the inner sidewall too.

Rock and roll

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Here’s another way of showing how the lowering of tyre pressures can help the tread stay in touch with the ground.

We placed a cricket ball on our test board. At 40psi, with the full weight of the Cruiser on the cricket ball, the tyre hardly deformed. When lowered to 15psi, the tyre completely wrapped around the ball, allowing the tread to touch the ground, which in the real world is improved traction.

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At 40psi, with the Cruiser’s full weight resting on it, the tyre sits up on top of the cricket ball with comparatively little tread flex.

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