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Tyre deflation comparison

We compare the footprints of 16 popular All-Terrain tyres at different pressures.

What happens to your tyre’s footprint and sidewall as pressure is dropped to gain extra traction? SA4x4 partners with Tiger Wheel & Tyre to compare the stats of 16 popular all-terrain tyres

Angus Boswell

There are myriad all-terrain tyre choices out there. As an overlander and 4×4 enthusiast, you want to ensure your purchase hits the sweet spot between toughness, longevity, and price. But what really is tough, and how do you know how the tyre will perform for your specific needs?
First up, you want to be honest with yourself about where the tyre will be used. Bombing the occasional sand dune on weekends and using the vehicle during the week to commute and get kids to school is one thing. It’s not the same as being a regular 4×4 trail driver and overlander traversing 60% rocky gravel roads with a load close to the vehicle’s GVM.

All-terrains mark the popular middle ground between beefy (and noisy) mud-terrain tyres and the more road-oriented P-metric tyres most 4x4s come with direct from the factory. The latter offer a quiet ride, low rolling resistance, and good handling mostly on tar. They won’t necessarily have the toughest sidewalls and will probably perform well in sand when deflated. If you are into more regular trips to rocky terrain with a load, and you drive a hardier 4×4, you want to go the Light Truck (LT-metric) route.

Now for why we did this comparison in the pages ahead. Deflating a tyre is the first step to improving ride on rough ground and enhancing traction on rough gravel, in mud, and in sand. Deflation lengthens the tyre’s footprint, extending the grip patch from something the size of a hand at standard pressure (typically 2.2- to 2.4-bar), to something longer and flatter as pressure drops to 1.2-bar and lower.

This can only be done for short periods, and at low speeds. Deflation causes tyres to heat up rapidly, with the potential for catastrophic delamination. Manufacturers don’t recommend a minimum pressure but advise that one goes no lower than 1.2-bar at a push. When the situation calls for it, many 4×4 drivers will routinely go lower than that. What we cannot emphasise enough, is that if you do deflate to sub 1 bar pressures, the onus is on the driver to re-inflate as soon as solid ground is reached.

But what is the ideal pressure to drop down to? First, it depends on the vehicle and the load. A Jimny might need less of a pressure drop than a Land Cruiser 70 Series. Similarly, a lightly-loaded Ford Ranger double cab might need to go down to 0.8-bar to clear a section, while a more heavily-loaded Ranger using the same tyres might get away with just 1.0- or 1.2-bar because the tyre has a bigger footprint. Crucially, the construction of a tyre will determine what the best pressure is for the terrain, the load and the vehicle.

In theory, the softer P-metric tyre the vehicle was supplied with will deflect a lot at 1.2-bar, and the sidewalls will bulge out, becoming vulnerable to damage from stray rocks and roots. An equivalent LT tyre with hardier sidewalls will bulge less, but will also need to go down to a lower pressure to achieve the same footprint and similar levels of traction. Is this true? We devised a deflation comparison to test this theory for a spectrum of all-terrain tyres supplied through Tiger Wheel & Tyre outlets, and for the most part, the results confirm this.

THE METHOD

At SA4x4 we do not pretend to be scientists. We like to keep it simple, so we have used basic tools one might find in the typical home workshop. Rather than pin-sharp accuracy, we were aiming here for a ballpark indication that could be used by anyone to gauge how their specific vehicle, load, and tyre combination works together.

We compared 16 all-terrain tyres from a variety of manufacturers, in the size 265/65 R17. This is the most popular choice among 4WD enthusiasts. All were fitted to Black Rhino Sidewinder aftermarket rims for the purpose of this exercise.

Two right-angle rulers were used together to show the changing length of the tyre footprint with the tyre on a flat sheet of Supawood. To show sidewall bulge, we used one of the right-angle rulers held vertically against the tyre at standard pressure of 2.4-bar and then at 1.2-bar, with the depth of offset to the standard rim measured using a Vernier caliper.

After initially using the Vernier to measure tread depth, and finding inconsistent results due to slight variations in the depth under different parts of the typical tread pattern, we swapped to an analogue tread gauge lent to us by TWT.

Tyre weight is something we wanted to compare directly, and for this, we used a Safeway digital bathroom scale. Yes, scales do lie, and we won’t be held responsible for a missing gram or two here and there, but each tyre was placed on the scale two to three times to get to a rough measure of the tyre’s mass.

Tyre pressure was a key part of the mix, and here we used a Schrader calibrating gauge used by TWT to set the accuracy of their workshop gauges. Also analogue, it did occasionally produce readings that didn’t make sense, so we’d re-measure in the interest of repeatability.

Finally, for the tyre equivalent of the Rorschach test at different pressures, we roped in a sponge, a tin of black boot polish, and a few sheets of rough paper.

To approximate a typical travel load scenario, we placed four 50kg sandbags in the load bin, over the rear axle of our long-termer Isuzu D-Max, which is fitted with some 200kg in accessories – an RSI Smartcanopy with a SmartKitchen unit on the left-hand side, plus an Eezi-Awn K9 roof rack, and Blade rooftop tent.

Using a Reich Caravan Weight Control load gauge, we determined the left rear of the Isuzu weighs 730kg all in, and the right rear, which we used for each of the tyres, weighed 685kg. In other words, a typical overland-ready rear weight.

CONSTRUCTION

There is a wealth of information imprinted on the sidewall of every tyre. Find it in the fine print and you will have a good idea of the tyre’s construction and design emphasis. You will quickly see whether it is an 80/20 or 50/50 on-/off-road tyre. However, be aware that the bare number of layers of different materials do not tell one about how or where the layers are placed, how the plies work together, or about the thickness of the beading.

More layers on the tread and sidewall generally equal a stronger tyre, and the LT-metric tyres invariably have added material, but as manufacturers get better at integrating fewer and lighter ply materials with better compounds, a stronger and lighter tyre can be produced. Only real-world use will prove this point, or not.

WEIGHT

Manufacturers know the exact weight of their tyres for each size. Our Safeway bathroom scale isn’t accurate to the last gram, so take this measure in this spirit. We found an 8.5kg difference between the heaviest and lightest tyre we compared. As expected, the LT tyres with their beefier construction, extra plies, thicker beads, and more rubber, topped the scale, led by the Dunlop Grandtrek AT3 G, Discoverer ST Maxx, and BF Goodrich KO2.

The lightest tyres were the GT Radial Adventuro AT3 and Bridgestone Dueler 693 AT, as expected due to their more road-biased two-ply construction and less aggressive tread. These would be easy OE replacements, having little effect on ride and handling. By contrast, a heavier tyre with stiffer sidewalls and bigger tread voids not only adds to unsprung weight but will in all likelihood be noisier and offer a slightly less compliant ride. It’s a necessary trade-off for toughness.

TREAD DEPTH

Adding extra tread depth adds more rubber, more weight, provides additional traction, and, all things being equal, should offer longer tyre mileage. Treated properly, which means regular rotation and balancing, one expects a tyre with close to 14mm of tread to last almost double the useful life of a tyre with just 8mm of tread.

The details of this, of course, depend on the rubber compound, the proportion of hard-wearing silica versus other elements in the compound, and many more highly-technical aspects to compound design that the layman is not expected to understand. Manufacturers anyway keep these details close to their chests, just like Colonel Saunder’s fried chicken recipe.

The beefier tyres scored highest on tread depth, led by the ST Maxx at 13.5mm, with the Bridgestone Dueler and Continental Cross Contact dipping down to the 8mm mark. The budget Velocity Raptor AT was a surprise 12.5mm, but the average for these all-terrains was around 10-11mm. Again, real-world use will determine which one actually fares better across a spectrum of uses and over time.

THE POLISH PRINT

Try this one at home, with an accurate tyre gauge on hand and a handlanger to work the jack. Use a sponge to get a layer of boot polish on the tread, and lower the polished section over a large sheet of paper. It’s like those potato prints that were so much fun in Grade 1.

We made a print set for the Cooper ST Maxx (LT) to compare directly to those of the General Grabber AT3, at five pressure intervals: 2.4-bar, 2-bar, 1.6-bar, 1.2-bar, and 0.8-bar. The kinds of increments one might use to improve grip off-road.

Each print shows significant footprint extension, with the softer Grabber extending further from higher pressure. In other words, the ST Maxx needs more air let out to achieve a longer footprint.

Also clearly demonstrated is that the tread print changes from a round format to one where the shoulders of each of the tyres are more distinct. As pressures drop then, it’s not just the centre blocks in contact with terra firma, but also the outer edges of the tyre which come into play to enhance traction.

In addition, it clearly demonstrates why under-inflation generates wear on the shoulder of a tyre, while over-inflation wears the centreline.

Boot polish print Cooper ST Maxx
2.4 – 60mm/ 2.0 – 180mm/ 1.6 – 205mm/ 1.2 – 235mm/ 0.8 – 260mm

Boot polish print General Grabber AT3
2.4 – 172mm / 2.0 – 185mm / 1.6 – 205mm / 1.2 – 240mm / 0.8 – 260mm

SIDEWALL BULGE

Each tyre was fitted to a standard Black Rhino sidewinder alloy rim, in a pretty neat matte black finish. This was our benchmark for a sidewall bulge test, using a Vernier to measure the distance from the outermost point of the tyre to the rim first at standard 2.4-bar pressure and then again at 1.2-bar.

We expected the softer compound tyres to deflect the most, exposing more of their sidewalls to potential damage. The results do not bear this out. While the actual deflection value is not huge by any means, those showing by far the most bulge of 10mm were one of the harder LT tyres (Discoverer ST Maxx) and one of the softer tyres (Hankook Dynapro RF10). Those with the least deflection were not subjectively the toughest tyres either, being the Scorpion ATR (just 1mm) and the Bridgestone Dueler 693 (2.1mm). The General Grabber AT3 with it two-ply walls exhibited the same 3mm change as the tough 3-ply-walled Dunlop Grandtrek AT3 G (both 3mm), while most of the othere hovered in the 4-6mm range.

Therefore this comparison was not as conclusive or as clear-cut as we imagined. Temperatures will no doubt have an effect, but from this test alone, ride comfort could not be accurately predicted, nor does it show that one tyre or another will be prone to sidewall damage when deflated. In the field the Discoverer ST Maxx has a proven record of handling rocky and stony conditions, while the same could not be said for the Bridgestone, which prefers to keep away from those Grade 4 and 5 trails.

Our guess? There will always be a trade-off between ride comfort, road noise and sidewall thickness, with the more road-oriented patterns being a little more flexible and mostly quieter. At pressures much lower than 1.2-bar, for example, most of the tyres on test showed great flex – precisely when delamination damage threatens.

FOOTPRINT LENGTH

This was the core of our comparison and makes interesting reading. The pressure intervals that were chosen echo those typically needed for different terrain (2-bar for gravel, 1.6-bar for the rockier, tough stuff, 1.2-bar for deep sand, and 0.8-bar when needed to get up a steep dune or across a rough section).
The longest, fattest footprint at 0.8-bar came from the Geolander GO15 (33.1mm), followed by the GT Radial Adventuro AT3 (32.8mm) and General Grabber AT3 (32.4). Also expected, was the softer Goodyear Wrangler Adventure AT (29.8), Continental Cross Contact (29.8) Velocity Raptor (29.6) and Discoverer AT3 4S (31).
What surprised was how close behind the LT tyres were at this below recommended pressure, with the BF Goodrich KO2 at 28.5, Cooper Discoverer ST Maxx at 28.2 and the Cooper Discoverer AT3 LT at 28.0. Again, the Dunlop Grandtrek AT3 G moved very little, to just 23.5.
The specific extension values reflect a lot more about individual tyre constructions than showing a clear distinction between LT and P-metric, in most instances. Exceptions were the Pirelli Scorpion ATR, which stayed relatively rigid at 27.2, along with the more beefily constructed Wrangler Duratrac (27.4). What surprised us was that the Bridgestone 693 AT, touted as a more highway-oriented OE replacement tyre, showed a 27.4mm footprint – the fourth-lowest rating.
We have added a new table for footprint gain, to better illustrate which tyres extend the most when pressure was dropped from 2.4-bar to 0.8-bar. No surprise at the top three, the GT Radial Adventuro AT3 with 10.8mm and the Yokohama Geolander GO15 AT and Cooper AT3 4S, both at 9.8mm. What surprised was that the closest LT tyre, the Cooper ST Maxx, was right behind with 8.2mm. Hence a good combination of strength and footprint extension at lower pressures. As expected, mid-rankers hovering around 7.5mm included the Wrangler Adventure AT and Continental Cross Contact, though we did not expect the Discoverer AT3 LT (7.4mm) to exhibit a bigger gain than the General Grabber AT3 (6.7mm). The stiffest of the lot were the remaining two LT-construction tyres, the Dunlop Grandtrek AT3 G (4.1mm) and BF Goodrich KO2 (3.9mm).
When tyres are hotter, with use off-road, we expect all these values to shift a great deal. Our static comparison indoors, while repeatable, does not showcase how individual tyres will react under actual outdoor conditions in a variety of terrains.

CONCLUSIONS?

As in everything, there are horses for courses. One should choose a tyre with the right kind of construction and an appropriate load and speed rating for your purposes and vehicle. This is the bottom line.

While tyre footprints are key to off-road performance, this is not the only yardstick by which to judge a tyre. As ever, things like price and warranty come into the picture. In addition, take a good look at the tyre off the rim. Feel the sidewall, evaluate the tread pattern.

Be aware too, that tyres built for different types of use (e.g. 80/20 or 60/40 on-/off-road construction) will affect characteristics such as ride, handling, noise levels, and grip.

In this exercise, we were aware of the limited scope of the comparison. Importantly, we were not trying to prove one tyre is better than another, simply aiming to show that carcass construction differences affect the optimum tyre pressure points for different terrains and different loads.

What we did not and could not test using this comparison, is actual grip in different terrain. Rather than trying to find a perfect all-terrain, which does not exist, our intent is to encourage all owners to think about your tyre choices, first of all, and to test the set of tyres you have fitted now at different vehicle loads.

Truth is, as one of our tyre experts commented, “When you have to get up a steep dune, or over a muddy patch, you will tend to deflate by increments until you have cleared the section.” Just remember the golden rule: reinflate as soon as possible, close to the standard recommended pressure, or risk permanent damage to the tyres.

THANKS: We are grateful to Chris Farrar of TI Auto for getting the comparison into action, to TWT Western Cape area manager Willem van der Westhuizen for collecting the tyres in one place, and to all the staff at the very upmarket Tiger Wheel & Tyre Tygervalley outlet for changing all those tyres and lending us equipment over two days of testing. A final thanks to Grant Spolander for decoding sidewalls and ensuring our methods were up to scratch.

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