Many so-called tyre tests, including comparative ones run in motoring publications from time to time, are entirely subjective. Not only does the driver’s opinion play a role in assessing the tyres, but the results are analysed in such a haphazard manner that all the painstakingly collected data is largely wasted for the sake of a “satisfying” result. One tyre company actually made the mistake of sending us a copy of such a test, because the results favoured them. It took CAR’s staff about five minutes to analyse the data accurately and conclude that the “worst” tyre was logically the best.
How do the major tyre companies test and compare tyres? After being invited to visit Goodyear’s Mireval proving ground in the south of France, I made that question a priority. Predictably, I was offered drives in various cars fitted with Goodyear’s new HydraGrip tyres as well as competitive brands. Later, I visited the company’s research facility in Luxembourg, to see how tyres are developed and tested. What I saw confirmed what I already knew: a modern tyre has a great deal of very serious science behind it.
Mireval
This circuit was initially built for Formula One racing, but has been changed into a dedicated track equipped to test the various wet and dry capa-bilities commonly expected from tyres. For example, water depth can be controlled on most of the level portions of the track.
The sides of the road are lined with small metal lips that are as high as the required water depth, and sprinklers ensure that water always overflows the lips. The road is not crowned, but completely flat, so the water level is uniform.
Curved aquaplaning
The first comparison I witnessed was designed to show the difference between a new Goodyear tyre and two competitors, in dealing with aquaplaning in a corner. I imagined it would be difficult to measure the onset of aquaplaning and was fascinated by the relative ease with which this was done. Several cars were fitted with horizontally sensitive accelerometers that broadcast readings to a central point. The cars were then driven at a constant speed around a large constant-radius circle so that the accelerometer showed an unvarying reading. A portion of the track was flooded to a depth of eight millimetres and, when the speed was high enough, the tyres started to aquaplane. This resulted in the cars sliding for a short distance, which showed up on the accelerometer as a decrease in acceleration because the radius of the corner changed. Goodyear testers were quick to point out that a graph of the change in acceleration showed that their new tyres exhibited less aquaplaning than two other well-known competitive brands.
Straight aquaplaning
This comparison was just as interesting. The same car was accelerated in fourth gear over 60 metres with both wheels on a dry surface, or the left-hand wheels on a surface covered by seven millimetres of water. The change in vehicle and engine speed was noted, between wet and dry, and this was taken as a measure of the amount of wheelspin caused by aquaplaning.
Wet and dry handling
This was the only subjective part of the comparison, but was nevertheless interesting. I drove five different cars, fitted with either the new Goodyear or one of two competitive tyres, over different parts of the circuit in wet and dry conditions. It was very difficult to feel much difference between the tyres in the dry, but on a wet road the new tyres exhibited slightly more grip. In fact, after some experience, it was possible to pick out the cars shod with the new tyres without looking at the tag on the car.Wet and dry braking
This was a real eye-opener, because of the way the cars were fitted out to eliminate as many variables as possible. Each car had a pressurised piston that applied a set pressure on the brake pedal, so each test was conducted with the same pressure. All I had to do was drive in a straight line at a speed of just over 85 km/h over the selected stretch of road, and automatic instrumentation would apply the brakes and measure the stopping distance as soon as the speed dropped to 80 km/h. The water depth was one millimetre.
Wet and dry slalom
For this test I drove Fiat Stilo JTDs for 150 metres between cones placed every 15 metres. Each driver’s times were taken under wet and dry conditions, and then subtracted to give a difference, which is a measure of the tyre qualities because driver skill has been mostly eliminated. For the better drivers the difference was less than two seconds on the new tyre, but up to a second longer on the other tyres. For less experienced drivers, the times were longer, but the percentage differences were similar.
Technical Centre – Luxembourg
This is Goodyear’s major research and development centre outside the US, so the visit was bound to be very interesting. The site, at Colmar-Berg, contains research and endurance laboratories as well as another test track, and employs 900 engineers, scientists and technicians. They develop car, light truck, truck and farm tyres. They also maintain test fleets that travel on public roads, and the combined mileage covered every day on test machines, on the track, and on roads is the equivalent of 10 trips around the world.
The ability to measure what a tyre is actually doing while performing is an important part of modern research, because it enables the construction of a mathematical model. This, in turn, makes it easier to predict how a tyre will behave just by looking at the properties of the constituents. This saves time when developing a new tyre, and highlights the parts of a tyre that have a major influence on the behaviour of the final product.
The technical centre contains instruments that measure a tyre’s footprint under various loads and on various surfaces, as well as the overall flexing and wheel movement when a tyre hits a bump. They also have a special truck that measures the sideways and longitudinal forces on a tyre under various conditions of load and speed. There are machines that subject tyres to vibrations, measure the noise they make, and run them at very high speeds, to record their behaviour.
I witnessed a high-speed test on a tyre rated at 200 km/h. The tyre was run against a drum up to 350 km/h, and then examined under a stroboscope. This works like a powerful timing light, but the time between flashes is adjustable, so that any rotating object can appear to be stationary, or rotating slowly, or even backwards. At 350 km/h, the tyre exhibited a wave on its outer circumference that started at the contact patch and then slowly decreased in amplitude. The rubber also appeared to pull away from the rim at some point and later a rubber bubble formed at the side.
When the tyre was stopped the bubble slowly disappeared as the tyre cooled down. The bubble was due to separation of the layers, and the fact that it could not be seen at normal temperatures illustrated how dangerous it is to operate a tyre at speeds greater than its rated speed.
Comparative tyre testing
The visit confirmed our view that scientifically valid tyre testing requires a vast investment in equipment and specialised personnel. Subjective tests might generate the controversial opinions so beloved of the popular press, but CAR maintains a quantitative approach to testing.
Another important point to consider is that virtually all original-equipment tyres are specially designed for each model. Thus, to test nine aftermarket tyres on the same car would be a waste of time. You could not be sure that the “best” tyre was in fact the best on all the other models in the park. On top of that, the popular tests tend to emphasize small differences in performance
above all else.
