4WD MODIFICATIONS - TYRES & WHEELS
No matter how many tyre stories we run we still come across many myths. With the help of Bridgestone, Kumho, Maxxis and Cooper Tyres, we conducted some real-world testing to dispel some of these tyre myths – hopefully forever.
Myth 1: “High tyre pressures prevent punctures”
The high-pressure, low-puncture rate myth was easily dispelled by a simple rig that Cooper Tyre’s Terry Smith had made up: a flat steel plate with a 45mm-long, 8mm-diameter stud welded to it.
We mounted three different, worn 4WD tyres – a Cooper S/T-C, an OEM (original equipment manufacturer) Dunlop Grandtrek and an OEM Bridgestone Dueler – on a vehicle and drove them in turn over the plate, forcing the stud repeatedly into the same position in the tyre tread centre.
We started the test with 260kPa (38psi) in each tyre and recorded the number of passes over the plate before the tyre punctured. We then plugged the resulting hole; pumped the tyre up to 190kPa (28psi); repositioned the plate so that a fresh section of tyre tread was aligned with the stud and repeated the fore and aft rolling of the tyre over the stud until it punctured again.
We repeated the repair and re-alignment process, pumped the tyre to 165kPa (24psi) and did the fore and aft rolling once more. The same tests were done at 140kPa (20psi) and 110kPa (16psi) inflation pressures.
The results showed clearly that even in the case of the lightly constructed Grandtrek and Dueler tyres the puncture resistance increased dramatically as the pressure dropped.
Number of passes before puncture
Pressure kPa (psi) Cooper S/T-C OEM Grandtrek OEM Dueler 693
260 (38) 2 1 1
190 (28) 3 1 1
165 (24) 5 1 1
140 (20) 16 2 2
110 (16) 25 (No puncture) 4 4
Fact 1: Lower pressures reduce the incidence of low-speed tread punctures.
Typical ‘zipper’ failure seen on many outback roads
Myth 2: “Tyre sidewalls are the weak link”
This tyre was given a through-tread puncture and refitted…
Any tyre repair facility can tell you that most punctures are through the tread and what looks like sidewall failure in a ‘zipper’ blow-out usually began as an undetected through-tread puncture, resulting in a gradual pressure drop that caused the tyre to flex excessively and overheat – ultimately resulting in total sidewall failure.
…and this is the subsequent blow-out result that looks like sidewall failure, but which started as aa tread puncture.
To check sidewall vulnerability at different pressures we moved the ‘puncture plate’ to the tyre shoulder position and ran the three test tyres over the stud, dropping pressure after each sequence, just as we’d done with the centre-tread puncture tests.
It was almost physically painful to watch each shoulder and sidewall distorting as the tyres ran over the stud, but all three test tyres managed 10 passes over the stud at each pressure setting, without more than superficial damage. We reckoned we’d still be doing it after dark, trying to get a tyre to pop.
There is a rider to this result, however: we know it’s possible to pinch a sidewall between the rim and a rock or slash the sidewall on a sharp tree root, but these punctures are statistically much rarer than through-tread punctures.
Yep, sidewall punctures also happen!
Fact 2: The sidewalls of 4WD LT tyres aren’t as puncture-prone as most people think.
Myth 3: “I don’t need light truck (LT) tyres to go bush”.
LT tyre cutaway compared with SUV tyre construction
Most new 4x4s are purchased by people who never go off-road and that’s the determinant for the tyres that are fitted. The brief from the 4×4 wagon maker to the tyre supplier is for rubber that can run cool at high speed, with good on-road grip, have low noise and deliver good fuel consumption.
It’s a common misconception that high-speed tyres are more heavily built than low-speed tyres – the truth is the other way around.
Heavily-made earthmoving equipment tyres hardly ever have punctures, but they’re limited to low-speed operation. High-speed tyres are lightly made, to reduce the heat build-up that occurs as the various components of the tyre flex.
A tread design that gives good on-road grip isn’t necessarily the ideal off-road shape. Widely-spaced tread blocks that give good off-road grip and self-clean to some extent can be noisy on- road and suffer ‘heel and toe’ wear that causes vibration.
That said, the latest Kumho ‘muddies’ make very little noise and our testing shows minimal heel and toe wear.
OEM tyres don’t like stony surfaces
Nearly all new 4WDs roll out of the showroom on rubber that is ideal for sealed-road, small-stone dirt road and sand driving. (‘Small-stone’ dirt roads are surfaces with no larger than marble-sized pebbles.)
As an illustration of the different puncture resistance abilities of LT and standard-issue 4WD tyres check the table in Myth 1. At all pressure settings the Cooper ST showed much more puncture resistance than the OEM-fitted tyres.
Fact 3: Light truck tyres are many times more puncture resistant than standard-issue 4WD tyres.
Myth 4: “Light truck tyres don’t handle”.
We ran four identical-spec’ and identical-load 4WDs on bitumen and dirt roads, and we gave them more severe cornering and braking tests on some private roads.
The vehicles were fitted with Highway terrain H/Ts, all-terrain ATRs, medium-block-tread M/Ts and large-block, heavy-sidewall M/Ts, respectively.
All were 265/70R17 size, with 118/121 load ratings. The H/Ts and the A/Ts had an ‘R’ speed rating and the ‘muddies’ had ‘Q’ ratings.
We ran all the tyres on bitumen and dirt roads at 210kPa (30psi).
This array shows the difference between typical pure highway (left) and extreme mud and rock (right) tyres.
The handling of the four tyre types was predictable on all surfaces. On bitumen the muddies made more noise than the ‘quieter’ patterns of the H/Ts and the A/Ts, and were slightly less directionally ‘sharp’.
At press-on, but not ridiculous speeds on bitumen and dirt surfaces we couldn’t detect any handling or braking differences between the H/Ts and the A/Ts. The muddies felt similar to each other, exhibiting more understeer than the road-oriented tyres, but still giving quite predictable handling.
We’re sure that the differences in handling between these tyre types would be much more noticeable on high-performance posts cars, with their finely tuned balance and taut suspension, but on the average 4WD vehicle, the differences are marginal.
Fact 4: Light truck tyre handling characteristics match 4WD road handling capabilities quite well.
Myth 5: “Mud tyres don’t work on sand”
We took our four test 4WDs to the beachfront and ran them in identical conditions with a range of tyre pressures. We progressively dropped pressures from a starting point of 210kPa (30psi) to 110kPa (16psi), with mid-pressure stops at 165kPa (24psi) and 140kPa (20psi).
Yes, we had one driver do all the testing and he was ‘blind’ to what tyres were fitted to each vehicle.
On firm, wet sand there was no performance difference, even at highway pressures. On flat, soft sand there was also no discernible difference, with the power of the 4WDs able to cope with the possible differences in tread pattern and ground pressure.
Many people get stuck on the uphill soft-sand climb from a beach to a road, so that’s where we set up a test that we hoped might separate the different tyres. The sand was dry on top, but became progressively damper underneath.
We faced each vehicle up a uniform slope and attempted a standing start. None of the tyres could manage a lift-off with 210kPa (30psi) on board and the supposedly sand-oriented, smoother-pattern H/Ts and A/Ts couldn’t manage a start at 165kPa (24psi), nor could the less aggressive M/Ts.
The surprise was the aggressive M/Ts that lifted off slowly at 165kPa (24psi) flailing away at the sand, but moving the vehicle nonetheless. Conventional wisdom says they should have dug in, but they didn’t.
At 140kPa (20psi) the H/Ts, A/Ts and M/Ts managed to lift-off, but crawled slowly up the sandy slope. The aggressive M/Ts surprised again, with a relatively easy lift-off and run up the slope.
At 110kPa (16psi) all four tyre types lifted off easily and climbed the slope without much wheel slip, using only 1500rpm in the process.
We theorised that the aggressive M/Ts worked better than expected because they dug quickly into the wet-sand layer, where the smoother patterns spun on top. That advantage wouldn’t apply in totally dry sand.
However, the overriding findings from the beach were that tyre pressure is more important than tread pattern.
Interestingly, none of these LT tyres bagged out very much: most of the increased footprint gained by dropping pressure showed as a lengthened contact patch, not a wider one. This is desirable from the rolling resistance point of view, because a longer, not wider, patch gives a smaller ‘hill’ of soft sand in front of the tyre.
Fact 5: Pressure is more significant than tread pattern in soft sand.
Myth 6: “You don’t need to drop tyre pressures on trails”
To bust this myth we took the four 4WDs onto a fire trail and found a very steep, shaly, hillside. One driver took each vehicle in turn up this loose track at varying tyre pressures, starting with 180kPa (26psi) – the pressure we’d usually use for low-speed trail driving.
The 4WD with H/Ts hardly made it into the steep section before spinning its wheels excessively. We dropped pressures to 150kPa (22psi) and the Prado made it to a gutter across the track.
At 125kPa (18psi) the vehicle made it across the gutter and up the very steep, one-in-two section, albeit with some wheelspin.
The A/T-shod 4WD traversed the first steep section at 180kPa (26psi) pressure, but baulked at the gutter. Dropped to 150kPa (22psi) the A/Ts got it halfway up the one-in-two section, but needed to be dropped to 125kPa (18psi) to complete the hill.
At 180kPa (26psi) the mild M/Ts took that 4WD to the mid point on the one-in-two section and, when dropped to 150kPa (22psi), made it to the top, but with some wheelspin.
We expected the aggressive M/Ts to outshine the other tyres on this slope, but they spun, at first. After negotiating the slope at 150kPa (22psi), with much wheelspin, the pressures were dropped to 125kPa (18psi), but there wasn’t much reduction in spin.
Then, we dropped pressures to only 82kPa (12psi). The M/Ts didn’t bag out excessively, but just enough to put the tread-patterned shoulders onto the track and the 4WD walked up without drama.
Hill section H/T ATR M/T M/T+
1st slope 150kPa 180kPa 180kPa 180kPa
Gutter 150kPa 150kPa 180kPa 180kPa
1-in-2 midway 125kPa 150kPa 180kPa 180kPa
1-in-2 top 125kPa 125kPa 150kPa 125kPa*
*Better grip at 82kPa
This test proved that dropping pressures can enhance the trail-climbing ability of different-tread tyres. It also showed that purpose-designed, stiff-sidewall rock-hoppers don’t perform at their best unless pressures are dropped to quite low levels.
There’s no way we’d have dropped pressures as low as 82kPa (12psi) with H/T tyres.
Fact 6: Lowered tyre pressures aid traction on trails.
Six myths busted in one test – we were quite happy with that. Although the initial testing was done with Cooper tyres, subsequent testing with Maxxis, Bridgestones and Kumhos showed us that the principles applied to these different LT 4WD tyres as well.
Tyre pressures for all conditions
We couldn’t believe what we were hearing: “We’re all running 50psi in our tyres.” This was from a tag-along tour leader and the convoy was doing the Canning.
It’s much the same story when we do driver training and ask people what pressures are in their tyres: “Whatever they put in them”. Who are they?
Pressure selection is easy for car drivers, because the placard tyre pressures are pretty right for all conditions, but for 4WD users the ideal tyre pressure varies with load, speed, road and off-road conditions.
An important part of 4WD ownership is knowing what to do with tyre pressures. We all know that pressures need to be dropped for soft sand, but what about mud and stony trails?
In the case of LT tyres in soft sand you can go as low as 110kPa (16psi) without risking a tyre coming off its rim, but that assumes sensible driving, not high-speed or dramatic ‘circle work’. You can drop to the same level for sloppy mud that doesn’t hide ‘stakes’ or debris.
Low-range, rocky trails are best handled with lower-than-highway pressures – usually in the 150-190kPa (22-28psi) range.
The starting point for setting the correct tyre pressures for your vehicle is the maker’s tyre placard that’s on a door pillar or inside the glove box lid.
The maximum pressures recommended by 4WD makers vary from around 180kPa (26psi) up to 240kPa (35psi). Light truck tyres used on 4WD utes can handle pressures up around 380kPa (55psi).
The pressures quoted are for maximum load at highway speeds and the recommended rear tyre pressures are often higher than for the front. It’s not necessary to have the rear tyres inflated to the maximum pressure listed on the placard for lightly-loaded, lower-speed driving.
For example, a ute that spends most of its time empty or with only a 100kg in the tray doesn’t need the high pressures that its light truck tyres are rated for. Using that much pressure in light-load conditions will give a harsh ride, poor traction on rough surfaces and will wear the tyres in the centre of the tread.
However, if you load up the ute or wagon for the Big Trip, you’ll need to up the pressures in proportion to the load. Wagons with a full load of camping gear or the trailer hooked up will probably need the maximum pressure listed on the vehicle tyre placard.
It’s easy to overload a 4WD wagon or dual-cab ute, because the real-world payload capacity is only in the 500kg-750kg range and it doesn’t take much in the way of freight and people to exceed those figures.
Tyre pressure shouldn’t be used to over-compensate for overloading
The problem facing many 4WD wagon owners is that the standard tyres don’t look right when the vehicle is loaded up for an Outback trip. With only 180-240kPa (26-35psi) inside them the rears, in particular, often bulge and it’s tempting to over-inflate them to make the sidewalls ‘stand up’ better.
It’s understandable that some owners do increase pressures above the placard limits and in many cases there’s no obvious penalty for doing so. Provided the tyres are run on smooth surfaces there’s often no distress, other than the ride quality deteriorating. The big troubles start when over-inflated tyres are run on stony roads.
Over-inflation makes the tyre too firm to deflect and casing damage or a tread puncture is likely.
Casing crack caused by hitting a rock at high speed with high pressure
The damage doesn’t always show up instantly, but a slow leak begins, dropping the pressure. The driver doesn’t feel the pressure drop, because there’s too much rough-road input happening. When the pressure gets very low the tyre overheats from excessive carcase flexing and a ‘zipper’ blow-out usually results.
It’s our experience that the worst people to trust with tyre pressures are tyre fitters. Almost without exception they’ll inflate tyres above the recommended placard figures.
Many people don’t drop their pressures off-road because it’s too inconvenient blowing them up again when it’s time to go back on-road. However, it takes less time to blow up tyres than it does to swap a flat one, and it’s much cheaper and more convenient than replacing a destroyed one.
Puncture-proof tyres are coming, but not yet…