Wide single tyres get the green light

Back in April 2021 we reported that TIC had begun testing to quantify pavement wear for a select range of tyres, including Next Generation Wide Load Base Tyres (NG-WBT) in the 445/50R22.5 size.

trucksales welcomed the initiative, because the last significant move to more widely implement wide single tyres as alternatives to narrower duals was back in the 1980s. The findings were that the then-available wide single tyres caused more pavement damage than duals.

Significantly, the testing was done by a fledgling Australian Road Research Board (ARRB) and that organisation – now known as the National Transport Research Organisation (NTRO) – also conducted the 2021-2023 testing, with the support of tyre suppliers Michelin and Goodyear.

Nearly all European long-distance semi-trailers are fitted with wide single tyres and have been for many years. The USA has also been using wide singles for around 20 years, often to the drive tyres of trucks that visit mining and construction sites, where rocks trapped between duals constitute an ever-present hazard.

They were never successful on North American trailer axles, because the 36.5-tons GCM limit means that most semi-trailers have only tandem rear axles, in contrast to Europe’s and Australia’s tri-axle designs. The Yanks found that trailer stability was compromised in the event of a flat tyre, because only one trailer tyre remained active.

In conjunction with wide-track axles, single tyres can greatly improve trailer roll-over resistance, by allowing trailer makers to widen the spring base.

There are also cost savings for operators, in terms of reduced fuel consumption, initial wheel and tyre purchase price, and payload increase from tare weight reduction.

The adoption of wide single tyres in Australia has been severely affected by discriminatory legal axle weights: 14 tonnes maximum for a tandem-axle shod with wide single tyres, compared with 16.5 tonnes (17 tonnes at HML) for the same tandem fitted with dual tyres.

The 2021-2023 TIC, NHVR and NTRO study was aimed at evaluating the effects of wide single and ultra-wide single tyres on Australian roads, in particular those using sprayed seal, unbound granular pavements.

To address the knowledge gap, the large-scale testing program used the NTRO’s Accelerated Loading Facility.

Nine identically designed pavements were constructed, with each loaded repeatedly to determine the relative rate of pavement wear for each tyre.

The results showed the pavement deformation rates for both the dual tyres and single tyres were within a similar range, and the 255/70R22.5 dual tyres caused the highest deformation rate.

The comparative pavement wear of super single tyres was not as sensitive to modest variations in inflation pressure, when compared with duals.

TIC Technical Officer Paul Caus explained: “The pavement damage exhibited by the commonly used 11R22.5 dual tyre configuration was notably influenced by inflation pressure, with the highest damage observed when these tyres were over-inflated – a common occurrence in practice.

“In addition to the finding, day-to-day use of single tyres makes it easier for drivers to check tyre conditions, monitor inflation pressures, and inspect brake components reducing the risk of overheating brakes and wheel end fires.

“It can be expected that in real-world conditions, the wider adoption of wide tyres would not cause a discernible increase in road pavement wear.

“TIC’s view is that there is no justification in limiting axle masses when using appropriate wide single tyres, given the improved vehicle stability and efficiency they bring.

“They should be permitted to operate at the same mass as equivalent dual-tyred axles.”

NHVR’s Chief Safety and Productivity Officer, David Hourigan, welcomed the report’s findings.

“The findings of this report will be of great importance to equip road managers and the NHVR with the knowledge needed during consideration of vehicle load limits and the benefits provided by super single tyres,” Mr Hourigan said.

Interestingly, the report summary doesn’t suggest a revised mass limit for tri-axle groups fitted with wide-single tyres, but that may be incorporated in the final report that’s due for publication in September 2023. The current mass limit for a tri-axle group fitted with dual tyres is 20 tonnes (22.5 tonnes at HML), so a wide-single tri-axle should score the same.

The project was funded by the NHVR’s Heavy Vehicle Safety Initiative, supported by the Australian Government, and was led by TIC, the peak industry body representing truck manufacturers and importers in Australia.

TIC acknowledges project partners, the NTRO (formerly ARRB) and tyre suppliers Goodyear and Michelin for their expertise and efforts in delivering this important research.