British GT - Tyres

Tyre technical engineer Gavin Johnson tells us how to get the best out of a car's rubber

By Oli S / Thursday, 26 June 2008 / Loading comments

The Avon Tyres British GT Championship will be at Thruxton this weekend and as with all forms of motorsport tyre technology will play a huge role. The British GTs use Avon tyres and PistonHeads has asked Gavin Johnson, the company’s tyre technical engineer, to explain what can be a mystifying subject.

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The Basic Principles

The subject of tyres used for motorsport applications is wide and varied; largely due to the fact that there are many forms of motorsport and the usage of tyres can be unique to the form in question. However, generally speaking the basic principles are the same:

The tyres are the only form of contact with the circuit, therefore all loads/forces are transmitted via the tyres.
Essentially tyres are springs and therefore have measured spring rates. Therefore it is possible to regard the tyre as a component of the suspension system.
There are operational parameters within which tyres are designed to operate most efficiently. These parameters relate specifically to the pressure and temperature experienced in use.

Most motorsport tyres are designed to fit a number of criteria:

Performance
Size
Weight (tyre as well as vehicle)
Durability
Cost

All of these criteria are directly related to the most important factor of performance. All the other factors can be adjusted to achieve the performance goal, with the possible exception of cost! The two key types of tyres used in racing can be categorised as either X-Ply or Radial construction. The main differences are detailed below:

X-Ply
The same ply components are used to support the tread as are used in the sidewall. The performance characteristics of an X-ply tend to be more forgiving to the driver and well suited to junior formulas such as Formula Ford. The tyres tend to be lighter and therefore cooler running and require less camber to allow the tyres to work at their optimum.

A radial Avon tyre construction

Radial

Additional breaker (belt) plies are used to support the tread. This form of construction offers greater consistency with respect to the size, and is therefore more convenient when setting vehicle corner weights. Radials can provide a stable platform for aerodynamics. Commonly a radial will be more camber tolerant than an x-ply and will potentially produce less drag on straights.

The Effects of Tyre Pressure

The internal pressure of the inflation gas used for a racing tyre is critical and tightly controlled to affect the spring rate of the tyre, i.e. the higher the pressure the stiffer the spring, the stiffer the spring the more the compound is worked. Any increase of temperature produces a direct increase in pressure, and as a consequence internal tyre pressure is built as a result of temperature gain from brakes and tyre temperature in use.

As it is desirable to maintain a consistent tyre pressure throughout a run, it is preferable to use more stable gases when inflating tyres. Nitrogen would be the first choice, although dry air makes a pretty good alternative and is far more practical. When checking tyre pressures, it is mostly meaningless to quote cold pressures as there are so many variables which can contribute to the final hot pressure.

Therefore hot pressures are those which tend to be recommended by the tyre engineers. In order to gain a starting point for cold pressures, it may be useful to record the hot pressure immediately as the vehicle completes a run, and then also take a pressure reading once the tyre and wheel assembly has fully cooled.

Having this information along with the ambient conditions, track temperature, vehicle setup etc will allow for a reasonably accurate cold starting pressure for future running. Additional factors such as using solid soap when fitting the tyres, a standard practice for all Avon Motorsport personnel, results in less moisture being introduced to the inside of the tyre, and therefore avoids inconsistent pressure gains.

It is also good practice to thoroughly remove any moisture from the surface of a rim before fitting a tyre to it. Typical tyre pressures vary quite a lot depending upon the chassis or vehicle that they are designed for. It would be reasonable to expect values of :

18 – 24 psi for a formula car (single seater)
30 psi for a GT car
Up to 45 psi for a saloon application.

Tyre Temperatures

As mentioned earlier, tyres are the only contact points with the circuit. As a result of this accurate tyre temperatures can provide a useful tool when attempting to analyse the handling characteristics of a vehicle and also when monitoring the effect of any changes that are made to the setup.

Tyre temperatures should be measured at the base of the tread, ideally at three points - the inside edge (the most heavily loaded side), the centre line of the tread and the outside edge (the least loaded side). The reason for stipulating the base of the tread as the point of measurement is because this will be the point at which the maximum tyre temperature will exist and will also be the area least affected by external/ambient conditions.

As rubber is an insulator its temperature transfer properties result in the base of the tread maintaining temperature for a much greater time than the tread surface which is usually bombarded by a moving airflow. Having this information for all the tyres on the vehicle can provide a good picture of the relative loads that each tyre is being subjected to during a lap.

Most critically analysing tyre temperatures can indicate imbalance within the chassis, highlighting any understeer or oversteer characteristics. In addition it is possible to see whether the setup of the vehicle has excessive roll, whether there are any undesirable wear rates as a result of setup and if the tyres are being operated within their optimum temperature range to maximise performance.

New Tyres ‘vs’ Scrubbed Tyres

The manner in which a new tyre reacts to its first heat cycle can be quite interesting and an important factor for the driver to understand. Generally speaking, from new a tyre will reach a performance peak relatively early in it’s life if it is pushed hard and worked with an aggressive style.

Following this peak the tyre would tend to go off at a steady rate of degradation, not directly linked to tread wear rates. Scrubbing tyres in is a recognised method of increasing the longevity.

A typical scrubbing in would involve taking a new set of tyres and completing a relatively short run, three or four laps depending upon lap distance, starting off at approximately 50% of the driver’s ultimate pace and gradually building this up until the final lap is at 100%. Ideally the set of tyres would then be allowed to cool in a cool, dark environment overnight before the next use.

The result of this procedure is an increase in performance consistency throughout a longer period of sustained full pace running. The trade off is that a scrubbed in set will not achieve the ultimate pace of a brand new set, but the performance longevity will be much improved. Additional benefits are that an accurate hot and cold pressure reading can be taken, more accurate corner weights can be set for a race, tyre balance is known and any initial rotation of tyres on rims can be corrected.

Wet Tyres

Tyres designed solely for wet weather use work in three ways:

Much softer compound generates more heat.
Extra mass of rubber generates and retains heat.
Tread patterns are designed to allow dispersion of water from the contact patch.

Due to the fact that wet conditions are usually experienced with accompanying lower ambient temperatures, wet tyre hot pressures are much closer to the hot operating temperatures. It is reasonable to use the slick hot pressure as a wet cold pressure in most cases.

Older or worn wets can be more suitable for drying conditions. In the absence of intermediates, a worn set of wets can offer similar performance. Wets will also usually require less camber as it is not normally possible to load the tyres to the same degree as a slick.