Before the tests on super singles began, the team at the NTRO made a falling weight deflectometer which can measure the deflection of the pavement in response to the weight.

During the tests there were nine geophones which were measuring seismic activity in the road surface. They also measured the profile of the road’s texture and the size of the rocks with a laser measurement scanner which scans backwards and forwards to create a 3D model. The surface was measured every 10,000 cycles.

10 tests were done with four tyres at different inflation pressures. Each of the different tyres were tested with proper inflation, uneven inflation pressures, then over inflated. The testing took a two year period to complete.

“The real life measurements aren’t as perfect and precise as a calculated formula,” said Anthony Germanchev, Data Analytics Portfolio Lead at the ARRB.

“We presented the results to the pavement engineers at Austroads. They had never looked at a profile like this before because they don’t test for tyres, they test for pavement construction methods. They were very interested in this. All of these questions were raised during the testing.”

The results were then used to calculate a complicated formula. The idea behind this formula is to apply an adjustment factor for the deformation rate of the road when different tyres are travelling on it.

This rate was calculated from 9000 to 52,000 cycles with road deformation rising steadily for every cycle. For each type of tyre the total deformation, deformation rate and adjusted deformation rate were calculated.

“The first thing to note is the total deformation was only four millimetres, which isn’t much,” said Anthony.

“We’re looking at really small amounts of deformation. If you were to have 100,000 cycles, you’d get one mm of deformation. You don’t want deformation rates higher than that.”

The most damaging, the 255 had a much higher deformation rate, twice as much as the 11R22.5, which had the lowest deformation rate. On the final leaderboard for the results of the testing, the 255 is by far the worst, but the 11R22.5 was not as bad as the researchers expected.

“There’s a decision that needs to be made if we’re going to push this forward to road managers,” said Anthony.

“They’re currently allowing the 255 the same loads as the 11R22.5. If that’s allowed, then why aren’t the other tyre types allowed? If you go to a deformation rate that’s not adjusted, and we look at total deformation just to get a feel for these results, it’s very clear that the 255 comes up the worst in all cases. The other thing to point out from our conclusions is that we’re looking at very small margins.”

The bullet points to take forward to road managers include the fact that the deformation rates for both duals and single tyres were in a similar range. The deformation rate for 255 duals was clearly the highest. When inflated to the recommended inflation pressures, the differences were small, but statistically significant.

The deformation rate ranked the tyres, from worst to best, 255, 445, 385 and then 11R22.5. If you look at total deformation, the order worst to best is 255, 11R22.5, 445 then 385.

“The 385 has always been criticised as the most damaging type,” said Anthony.

“Another argument we could take forward is that the duals traverse a wider path and the singles have a narrow path, so together they share the wear over a larger area of pavement. It’s expected traffic over the same pavement section of both duals and wide tyres will result in more evenly dispersed wear pattern.”