Efficient performance- a reality check

Tyre performance is often predicted by carrying out laboratory-scale rubber compounding studies. Traction, tread wear and rolling resistance are then evaluated. Tyres, however, are complex, and tread compound is only one of the factors influencing tyre behaviour – there is no perfect correlation between lab studies and tyre tests.

Numerous lab studies carried out by Nynas, a global supplier of tyre oils, indicate that naphthenic oils can reduce rolling resistance. “Most common tyre polymers, such as NR, BR and SBR, have a medium polarity and therefore show a very high compatibility with naphthenic tyre oils. This, combined with the somewhat lower glass transition temperature (T_g ) of naphthenic tyre oils, makes for an excellent balance of properties, especially for winter tyres.

To verify the correlation between laboratory and field tests, Nynas launched an extensive tyre testing programme. FSSBR/silica-based tread compounds were prepared using four different tyre oils: naphthenic black oil (NYTEX® 4700), heavy naphthenic oil (NYTEX®840) and Group I refining sidestream derivatives TDAE and RAE. These were then used in retreading: carefully selected tyre carcasses were retreaded in collaboration with Kraiburg Austria. Performance was then evaluated in tyre tests.

Wet traction

Wet traction and rolling resistance tests were carried out by Prüflabor Nord. They followed UNECE Regulation No. 117, which is used in determining European tyre label ratings. Essentially the same regulation is used in other countries with tyre labels such as Japan, Korea, Brazil and soon also China.

In a typical dynamic-mechanical analysis (DMA) of tyre compounds, the glass transition temperature (T_g) of oil affects rubber behaviour. Higher oil T_g often leads to higher tan delta at 0°C. This in turn is the most common wet traction indicator: the higher the tan delta, allegedly the better the traction. Aromatic oil (DAE) and RAE with higher T_g typically yield 5-15% higher tan delta than other tyre oils such as TDAE and naphthenic oils. Correlation of tan delta with traction, however, is not perfect.

Wet traction results are presented in the chart below. Tyres were compared with a 16” Uniroyal reference tyre (SRTT), and breaking results were indexed. It was found that the four tyre specifications tested yielded very similar indexes. Considering the deviation in the test, wet traction of each tyre specification was equal. Although the RAE compound had 10% higher tan delta, it did not result in better traction.

Rolling resistance

Reducing rolling resistance leads to an improvement in a vehicle’s fuel efficiency. In rubber compounding, the property that best correlates with rolling resistance is tan delta at 50…70°C. Compared with wet traction, the correlation of tan delta with rolling resistance is typically a lot better.

Again, rolling resistance was determined following the ECE R.117. It was found that all tyres tested yielded the label rating C. There was, however, a difference between different specifications. The best oil in the test, the naphthenic black oil NYTEX® 4700, yielded 5% lower rolling resistance than TDAE. As in the lab studies mentioned earlier, naphthenic oil improved fuel economy without affecting traction.

One can naturally discuss whether the 5% improvement is significant. An experienced rubber compounder will surely acknowledge that to achieve the best tyre label ratings, several compounds and several raw materials need to be developed. Naphthenic oils offer an excellent additional tool for rolling resistance reduction.

Tyre testing in winter conditions

Winter traction of tread rubber can be estimated by looking at a few commonly accepted parameters. Requirements differ remarkably from summer tyres: compound needs to remain soft and flexible in the cold. The most common indicator is tan delta at -25°C; yet its correlation with tyre test results is not good. Far better correlation is found between dynamic stiffness and traction. The lower the stiffness in the cold, the better the traction. Many of the world’s leading winter tyres are thus augmented with naphthenic oils, which keep the rubber soft in cold weather.

The braking distance of tyres was tested on snow and ice by Test World. Braking performance is indexed using NYTEX® 4700 as a reference tyre. As expected, high stiffness of the RAE compound led to longer braking distances on ice and snow. Three other oils exhibited the same traction on snow. In the ice braking test, however, there were larger differences: naphthenic oils and especially NYTEX® 840 performed very well. RAE was 6% worse than NYTEX® 840 – braking distance (20 km/h to 5 km/h) was 70 cm shorter with NYTEX® 840. If the speed, however, was e.g. 50…60 km/h, the difference would be several metres, and better performance might save lives.

Testing of wear resistance

The four tyre specifications were also tested for mileage: they were mounted on taxis in Austria and Germany, and tread wear was monitored. As the tyres were mounted on different types of cars, the testing can’t provide perfect accuracy, but it should reveal any serious issues with wear resistance. When the average mileage was extrapolated to 3 mm tread depth, all tyres yielded 35000 km +/- 2000 km.