A vehicle’s motive and cornering forces are generated at the tyres’ contact patches. We attended the Goodyear Eagle F1 SuperSport launch at Spain’s Ascari Circuit to learn more...
In the tyre industry, UHP stands for “ultra-high performance”. It accurately describes the grip generated by the Goodyear Eagle F1 SuperSport R tyres fitted to the Porsche 911 GT3 RS I piloted around Ascari Racetrack in Spain.
I went hard on the brakes, turned in and felt the lateral force building to maximum at the apex. I opened the steering slowly upon exit and the instructor requested I feed in the power. My foot was already flat at this stage … slightly naughty, I know. With 383 kW channelled to those grippy 325-section rear gumballs, the lizard green GT3 catapulted down the straight with not so much as a squeal or wiggle from the rear-end.
Earlier that day, I got to spend time with Goodyear’s tyre engineers to learn about the science of grip.
To simplify, grip is generated at the contact patch between the tyres and the road surface as a function of the normal force (the weight perpendicular to the road) between the tyre and road surface, tyre-slip percentage and the friction coefficient between the rubber and asphalt. The normal force (perpendicular to the road) on each tyre varies while driving, as the vehicle’s centre of gravity results in weight transfer during longitudinal and lateral accelerations. In a nutshell, when a vehicle brakes, the normal force on the front tyres increases while the force on the rear wheels decreases. The opposite happens during acceleration. During cornering, the weight is transferred to the outside wheels while the inside wheels experience a decrease in normal force. In the absence of aerodynamic downforce or road gradient, the sum of the normal forces under each of the four wheels always equals the mass of the vehicle.
Engineering theory shows tyre width plays no role in the possible grip, as contact-patch area is not used in the basic grip calculations. A narrow tyre results in the same normal force carried on a smaller area. On a molecular level, though, the stress in the rubber is much higher, with a small contact patch resulting in earlier breaking and shearing of rubber bonds, and therefore less grip. With a larger contact patch, the stresses are dispersed over a wider area and the rubber has an easier time, resulting in higher ultimate grip levels.
Where Flateral is the lateral force (N), µ is the coefficient of friction, m is the vehicle mass (kg) and g the gravitational acceleration (m/s2).
A vehicle model is used to calculate the theoretical maximum speed a vehicle can negotiate a certain radius bend. The most basic model is a single wheel carrying the mass of the entire vehicle. This simplistic model gives a ballpark answer, although the suspension setup, steering geometry and weight transfer are ignored. A slightly more advanced model is the bicycle with a wheel in the front and another in the rear. This one takes longitudinal weight transfer and steering angle into account, but not lateral weight transfer and suspension geometry. Modern computer software is able to simulate the entire vehicle, including the suspension settings and track layout, to form the most accurate picture. This is the type of analysis used by Goodyear to evaluate the performance of a tyre long before raw materials reach the tyre moulds.
Testing and scaling
Goodyear employs a smaller control size – for example, 235/35 R19 – when testing a new tyre (construction, compound or tread pattern). After the data parameters have been established under the test conditions, a computer model can scale the tyre to the desired size for a specific application while keeping the performance constant. This is important when a tyre is to be used on a wide variety of vehicles.
According to Goodyear lead engineer, Helmut Fehl, the contact patch is critical in the performance of the tyres. Goodyear introduced several technologies in its SuperSport tyres to improve the grip generated at the contact patch.
The more rubber in contact with the road, the higher the performance. This is the reason slicks are used on racecars. The problem with road tyres is they must be road legal with at least two circumferential grooves (and a minimum of 1,6 mm depth). Wet performance is severely impacted if there are fewer grooves, as standing water cannot be channelled away, leading to the risk of aquaplaning. As is evident in the comparison table, the Asymmetric 5 tyre with 8 mm tread is best suited for everyday use, while the SuperSport RS tyre is barely street legal and intended for racetracks.
The rubber compound determines the performance in wet and dry conditions and impacts the durability of the tyre (they’re usually contrasting characteristics). A dual-compound tyre employs two compounds in different locations of the tread to lessen the compromise. In the SuperSport tyres, the central tread blocks are optimised for wet-road performance while the outer edges are reserved for a compound excelling in dry-handling capability. Only dry conditions will allow enough weight transfer for the outer edges to come into play while cornering.
The compound determines the optimum temperature-operating range of the tyres. A more road-biased unit such as the Asymmetric 5 from Goodyear can generate peak grip at only 40 degrees Celsius while the SuperSport RS version needs at least 80 degrees before it produces the levels of grip expected. This explains why care must be taken when a performance vehicle is driven on cold UHP tyres.
For predictable handling, it’s important the contact patch remains stable during extreme dynamic events. The tyre carcass and sidewalls of performance tyres are extremely stiff to prevent deformation (to the detriment of comfort). Interestingly, in the SuperSport tyres from Goodyear, its PowerlineCover is used which consists of circumferential fibres to counter tyre bulging at high speeds owing to the acting centrifugal forces. Without this band, the contact areas of the tyres become narrower as the speed increases, which is not ideal when driving a supercar.
Tread disperses standing water but results in less rubber on the road at the contact patch. Goodyear uses an asymmetrical design on its UHP tyres with fewer grooves on the outer part of the tyre. Weight transfer of the vehicle pushes the outside of the tyres into the tar, resulting in more grip. The SuperSport RS tyre can be seen as a semi-slick with no tread grooves on the outer edge of the tyre.
Another innovation is the bridge strips linking the outer tread block across the central groove. The idea is they keep the contact stable by resisting the tendency of tread blocks to separate under extreme cornering forces. The bridge intervals are located at spaces around the tyre that guarantee at least one is present at all times in the contact patch – without impacting wet-weather performance – while ensuring the tyre remains legal.