Sit back and relax as we tackle tyre pressures, ink prints, and the tractive force with a 10-tonne load cell and a calculator to show you the advantages of lowering your tyre pressures off-road.
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.
This increased length, and therefore tread area touching the ground, aids the 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 our claims, 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. Someone pass me my lab coat.
TYRE PRESSURES
Setting the correct pressures to suit the terrain when you're driving off-road can dramatically improve traction. Here's everything you need to know about tyre pressures for off-road driving.
WORDS, IMAGES AND VIDEO BY MARK ALLEN
FOR OFF-ROAD DRIVING EXPLAINED
GUIDE Tyre pressures
Are there any negatives to lowering tyre pressures?
Yes. Before we get too deep into the advantages of lowering your tyre pressures for off-road driving; note that there are downsides. Lose of ground clearance as your tyres slightly flatten, overheating of tyres if you drive too fast (so don't) and the chances of popping a tyre off a bead (so don't corner too quickly). Other than that, it's all positive.
Lowered pressures aren’t just for sand driving
Lower pressures in sand provide superior 'flotation' 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 too. The added flexibility of your tyre's tread provides increased grip and reduces the risk of punctures. Same too for any uneven surface if driven slowly.
Driving at medium speeds on gravel or dirt roads can also benefit from lower pressures (but not as low pressures as for slow rock driving) by allowing the tread to conform to the individual stones and hard, sharp protrusions to help prevent puncturing. Remember, the lower your pressures are set, the lower your speeds should be to prevent overheating 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. That is most evident in steep uphill climbs, slippery, loose, wet or rocky surfaces. With lower pressures, you'll also do less track damage.
Given the recordings of our load cell at the varying tyre pressures, there is no doubt the lower pressures aid in grip, ultimately allowing more drive or tractive force. 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 the tracks and how much easier it is on your vehicle.
ABOVE The tyre conforms to the terrain with lower pressures allowing more grip and drive.
“For this test, we parked the 'Cruiser on flat ground in two-wheel drive, the rear locker on and in low range.”
Disclaimer…
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 18 to 20psi on soft sand. I'd then drop lower to get out of trouble but would be extremely careful not to drive too fast or turn too sharply to prevent tyre damage or rolling a tyre off the rim.
BELOW One sturdy tree, over-rated rigging, load cell and V8 power – we went nowhere
How’d we measure all this? With a 10-tonne load cell
For this test, we parked the 'Cruiser on flat ground in two-wheel drive, the rear locker on 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 ELocker on 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 altering the readings from our load cell. Low-range was used to help avoid clutch slip and 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 into a recovery hitch slid into the tow receiver. The ratings of all equipment 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.
ABOVE Four-wheel drive engaged makes a huge difference, but tyre pressures remain important. // BELOW 10-tonne load cell takes all the guesswork out of the job.
ABOVE AND BELOW Tyre pressures set at 40psi above and 20psi below
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 extra grip, or tractive force, is one of the main reasons we lower pressures on most off-road terrain, to gain as much go-forward grip as possible - it's just the exact grip level had never been measured.
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 too much grip.
Okay, I'd 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; 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 4X2, 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.
All tyre ink prints were made using a 285/75R16 tyre, with the 'Cruiser lightly loaded, by jacking up one wheel, painting ink over the surface of the tyre tread and slowly lowering the tyre onto the cardboard. The vehicle was not rolled forwards or backwards.
Lengths and widths of the ink image have been measured and used to calculate the kilograms per square centimetre the 'Cruiser is exerting on the ground. We’ve then repeated the exercise at decreasing psi readings to show the differences in tread width and length.
The primary variable will be load via the camping gear, tools and all luggage packed into your vehicle. The heavier you are loaded, the more the tyre print will vary. Tyre brands and specifications will also alter the tyre footprints.
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 1.254 to just 0.722kg/cm² representing 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.
ABOVE Jacked up, painted and lowered without rolling the wheel to make ink imprints
ABOVE Tyre pressures were dropped from 40 to 15psi in 5psi increments
Do sidewalls get wider?
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 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 – no point chancing your expensive tyres. Remember the inner side too.
ABOVE AND BELOW At 40psi, with the Cruisers full weight resting on it, the tyre sits up on top of the cricket ball with comparatively little tread flex.
ABOVE Yep, there's a cricket ball under this tyre which has been lowered to 15psi
Rock and roll
Here's another way of showing how the lowering of tyre pressures can help the tread to 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 deforms. When lowered to 15psi, the tyre completely wraps around the ball, allowing the tread to touch the ground, which in the real world would aid traction.
GUIDE Tyre pressures
Sit back and relax as we tackle tyre pressures, ink prints, and the tractive force with a 10-tonne load cell and a calculator to show you the advantages of lowering your tyre pressures off-road.
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.
This increased length, and therefore tread area touching the ground, aids the 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 our claims, 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. Someone pass me my lab coat.
TYRE PRESSURES
Setting the correct pressures to suit the terrain when you're driving off-road can dramatically improve traction. Here's everything you need to know about tyre pressures for off-road driving.
FOR OFF-ROAD DRIVING EXPLAINED
WORDS, IMAGES AND VIDEO BY MARK ALLEN
Are there any negatives to lowering tyre pressures?
Yes. Before we get too deep into the advantages of lowering your tyre pressures for off-road driving; note that there are downsides. Lose of ground clearance as your tyres slightly flatten, overheating of tyres if you drive too fast (so don't) and the chances of popping a tyre off a bead (so don't corner too quickly). Other than that, it's all positive.
Lowered pressures aren’t just for sand driving
Lower pressures in sand provide superior 'flotation' 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 too. The added flexibility of your tyre's tread provides increased grip and reduces the risk of punctures. Same too for any uneven surface if driven slowly.
Driving at medium speeds on gravel or dirt roads can also benefit from lower pressures (but not as low pressures as for slow rock driving) by allowing the tread to conform to the individual stones and hard, sharp protrusions to help prevent puncturing. Remember, the lower your pressures are set, the lower your speeds should be to prevent overheating 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. That is most evident in steep uphill climbs, slippery, loose, wet or rocky surfaces. With lower pressures, you'll also do less track damage.
Given the recordings of our load cell at the varying tyre pressures, there is no doubt the lower pressures aid in grip, ultimately allowing more drive or tractive force. 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 the tracks and how much easier it is on your vehicle.
ABOVE The tyre conforms to the terrain with lower pressures allowing more grip and drive.
Disclaimer…
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 18 to 20psi on soft sand. I'd then drop lower to get out of trouble but would be extremely careful not to drive too fast or turn too sharply to prevent tyre damage or rolling a tyre off the rim.
BELOW One sturdy tree, over-rated rigging, load cell and V8 power – we went nowhere
ABOVE Four-wheel drive engaged makes a huge difference, but tyre pressures remain important. // BELOW 10-tonne load cell takes all the guesswork out of the job.
ABOVE AND BELOW Tyre pressures set at 40psi above and 20psi below
How’d we measure all this? With a 10-tonne load cell
For this test, we parked the 'Cruiser on flat ground in two-wheel drive, the rear locker on 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 ELocker on 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 altering the readings from our load cell. Low-range was used to help avoid clutch slip and 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 into a recovery hitch slid into the tow receiver. The ratings of all equipment 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 extra grip, or tractive force, is one of the main reasons we lower pressures on most off-road terrain, to gain as much go-forward grip as possible - it's just the exact grip level had never been measured.
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 too much grip.
Okay, I'd 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; 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 4X2, 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.
ABOVE Jacked up, painted and lowered without rolling the wheel to make ink imprints
ABOVE Tyre pressures were dropped from 40 to 15psi in 5psi increments
All tyre ink prints were made using a 285/75R16 tyre, with the 'Cruiser lightly loaded, by jacking up one wheel, painting ink over the surface of the tyre tread and slowly lowering the tyre onto the cardboard. The vehicle was not rolled forwards or backwards.
Lengths and widths of the ink image have been measured and used to calculate the kilograms per square centimetre the 'Cruiser is exerting on the ground. We’ve then repeated the exercise at decreasing psi readings to show the differences in tread width and length.
The primary variable will be load via the camping gear, tools and all luggage packed into your vehicle. The heavier you are loaded, the more the tyre print will vary. Tyre brands and specifications will also alter the tyre footprints.
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 1.254 to just 0.722kg/cm² representing 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.
ABOVE AND BELOW At 40psi, with the Cruisers full weight resting on it, the tyre sits up on top of the cricket ball with comparatively little tread flex.
ABOVE Yep, there's a cricket ball under this tyre which has been lowered to 15psi
Do sidewalls get wider?
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 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 – no point chancing your expensive tyres. Remember the inner side too.
Rock and roll
Here's another way of showing how the lowering of tyre pressures can help the tread to 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 deforms. When lowered to 15psi, the tyre completely wraps around the ball, allowing the tread to touch the ground, which in the real world would aid traction.