TYRE TOPICS

Our trusty D-Max gains a new set of rubber

Accessories-Tyres_1 Accessories-Tyres_2 Accessories-Tyres_7

If you find it hard to get excited about the prospect of buying new tyres, then, in the terms of the tyre industry, you are not alone. Most buyers approach the prospect of replacing tyres as a grudge purchase. That means that they know they need them, but they don’t want to spend money getting the best. For some buyers at least, it’s sufficient that whichever tyre is fitted keeps each corner of the vehicle upright.

Having worked with the tyre industry at one stage of my life I find the technology incorporated in tyre design to be really interesting, in some cases able to rival that of engines.

As I was to learn from tyre technicians that had gained their expertise working with Formula One racing, the tyre is the window into evaluating vehicle performance. In racing terms, an incorrectly set-up racecar suspension can be identified by measuring a variation in temperature across the tread area. If all is well, the tyre tread temperature is consistent. If all is not well, the tyre takes the pain.

A good tyre technician can substantially affect the maintenance costs of a vehicle fleet. Tyre pressure checking is the first stop, followed by regular rotation to minimise wear in specific areas. There are also further gains to be made by matching the tyre to the type of use of the vehicle.

If the vehicle spends all its life on the highway, then you fit a tyre with an on-road construction and tread pattern. It will cope with high speed cornering, sudden braking, rain water dispersal and high temperatures without failing.

Not surprisingly, the same tyre will not work well off-road. Its sidewall thickness is much less than that of an off-road tyre in order to help heat dispersal at high speeds on the freeway. When off-road the thinner sidewall is more susceptible to sidewall staking and puncture, plus it is not going to be able to handle high loads, such as those of a fully laden ute carrying a country tonne of firewood out of a forest.

The close and compact tread pattern that provides directional stability without lots of road noise for an on-highway tyre will quickly clog up in muddy conditions. It’s here you need large, aggressive tread patterns that literally fling mud out of the grooves as the tyre revolves. Thicker sidewall construction also reduces the risk of staking the tyre.

If you are heading for the beach and travelling on sand you will no doubt know all about reducing tyre pressures in order to allow the tyre to bag out and gain grip for better traction. But did you also know that as you lower your tyre pressures you need to lower your speed?

It’s at this point we have to look at what part of the tyre actually holds the vehicle off the ground. It’s not the rubber; it’s actually the air inside the tyre.

Think of the tyre as an air spring. To support a set load there has to be sufficient pressure inside the tyre. If the pressure is set low the speed must be also kept to a minimum in order to avoid the tyre overheating, or rotating on the rim and losing its airtight seal. If you keep the load on the tyre without raising the pressure, and then increase the vehicle speed, the tyre will probably fail through overheating.

If you have a high load on the tyre and are not able to re-inflate the tyre to recommended pressures, then keep the speed down to prevent overheating. At least that way you will get to your campsite, even though you may miss the first beer.

Hopefully, you are by now thinking more about your next tyre purchase and not so much about minimising the cost.

That brings me to the purpose of this feature, the decision of which tyre to choose when replacing the original fitment on our trusty Isuzu D-Max ute.

After just 34,000 km the tread depth of the original fitment Bridgestone Dueller A/T in its 245/70R16 sizing had reduced, with all tyres evenly showing between 4.5 mm and 5.0 mm across the face of each tyre. We could have left them on a little longer before replacement, but with the ute running a regular commute from Mittagong to Scone, a round trip each week of 750 km, we felt it was time for an upgrade.

As we head into winterAccessories-Tyres_3, living in the country can mean wallowing through mud, and with an off-road 4×4 we decided to adopt a more aggressive tread pattern that would improve the off-road ability of the D-Max. At the same time, a deeper tread pattern might also extend the tyre life before the next replacement.

The solution came in the form of a set of General Grabber AT2 tyres designed for a mixture of on and off-highway work, with a more aggressive tread. The maximum load permitted for each tyre is 1380 kg in single tyre application (rather than dual-tyre fitment), and at this top weight range the maximum pressure is 80 psi (550 kpi) when cold.

In a world where so many components are these days sourced from China, the sidewall information showed these had been produced in Mexico, a country rapidly becoming popular with the major truck manufacturers as an alternative to production in North America.

At a size of LT 245/75R16 there was a slight difference in rolling circumference by comparison with the previous OE fitment, but not enough to cause concern when it came to contributing to an inaccurate reading from the speedometer or odometer.

In order to minimise the risk of vehicles exceeding the speed limit, vehicle manufacturer usually errs on the side of caution with its gearing. This often results in an indicated 100 km/h on the speedo actually showing up to six km faster than its real speed. We checked the accuracy of the D-Max speedo against a Sat/Nav unit after the tyre swap and found that tolerances were actually tighter than with the previous tyres, resulting in an indicated reading of 100 km/h being just 2.0 km above the actual speed.

The load and speed index of the 245/75R16 is 120/116S with a diameter of 30.5 mm and a weight of 22.1 kg. The tread depth of 12.7 mm is greater than that of the original equipment Bridgestones, prompting the expectation that unless the wear rate occurs faster, as can be the case with a softer tyre rubber compound, these tyres should exceed the life of their predecessors.

For those interested in heritage, the General Tyre company was purchased by Continental Tyres back around the 1980s, and it remains today a separately branded range under the Continental banner.

The Grabber AT2 is described as having an innovative tread design that promotes high levels of road traction, meets severe snow service requirements and provides exceptional traction over all terrains. The aggressive self-cleaning tread pattern enhances off-road and wet traction capabilities but manages to remain relatively quiet during on-road highway driving.

The full description of the tread design refers to an extra deep, five-row pattern with multiple traction edges, featuring a highly engineered acoustic tread pattern. A further attribute for those heading for the snow country is that the tyre is snowflake approved and has a studable design.Accessories-Tyres_P2

Our local tyre experts, Tyrepower at Bowral, looked after the tyre swap – balancing each tyre at the time of fitment. The Grabber AT2s did get the thumbs up for the manufacturing integrity for being obviously round, something that apparently not all tyre manufacturers manage to achieve. Perhaps this is another reason to go for a well-known manufacturer rather than an unknown brand from China.

Our first on-road impression was of slight tread noise at low speeds around 30-40 km/h, but as the speed increased the road noise seemed to reduce. At maximum highway speeds it was not noticeable at all.

We’ll end on a note of how to select the right tyre pressure. Given that, in its current application, the D-Max does not generally carry a significant load on the highway but weighs in at a kerbweight of 2,130 kg, our pressure selection all round in this application is for 40 psi. If we were running payloads of one tonne in the rear tray, we would suggest raising pressures on the rear tyres to 55 psi and at the front to 45 psi.

If in any doubt as to what pressure you should select we recommend seeking advice from your local tyre specialist. Remember also to rotate tyres every 10,000 km and to regularly inspect them for signs of uneven wear and correct pressure.

As we said at the beginning of this feature, not everyone likes tyre technology. Your local expert possesses enormous knowledge about a subject that perhaps you’ve never given a second thought, so make sure you use his expertise to keep your family safe on the road.

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