Tyres

By: Pat Clarke

We are starting to see some teams form strategic relationships with tyre companies, and Clemson were at Pontiac sporting Michelin radial ply tyres and Hankook Company makes special tyres for the South Korean teams. Watch this development. The tyre companies need to source the next generation of tyre engineers from somewhere, and Formula Student is an excellent place to recruit.

Tyres should be selected and the data studied before any decisions are made regarding camber curves and Ackermann etc.

‘Reading’ tyres should become second nature. Terms like ‘Schallmach waves’, ‘Tread area deflection’, ‘Hallum model’, etc will become second nature. Believe me, the judges will inspect your tyre wear patterns in great detail, and you better understand what they are ‘reading’ when they ask you! Most importantly, understand ‘tread graining’, a symptom of a poor setup.

Any search on tyre data, research and virtual modeling will bring you to the work of Pacejka, and difficult though it may be to follow, there is real information (read ‘speed’) in the Pacejka model. Beware though, his research is mainly based on passenger car tyres where heating of the tread compound is not a factor.

At a more basic level, having chosen your tyres and put a considerable dent in your team budget by purchasing them, what do you need to understand to make the best use of your investment?
Lots of good advice is ignored by even the smartest teams! Right up to formula 1, so we are all in good company.

Slick racing tyres only work properly when they are hot, at their optimum working pressure.
The old rule of thumb was that slick tyres worked best when the tyre temperature reached 100°C, although this seems very high for FS tyres. Temperatures of 80°C to 85°C are more suitable for these tyres.

The tyre temperature is generated from within the tyre rather than simply a result of friction between tyre tread and racing surface. Those who play squash know how important it is to warm up the ball before playing a game. This warmth comes from ‘exercising’ the rubber in the ball. Tyres are no different, the temperature is generated inside the tyre, so reading tyre tread temperatures can give very misleading information on setup.  Some teams rig up IR heat sensors and test whilst logging data from these sensors. The information gained is useful but not complete. There is no substitute for measuring tyre core temperature across the tread using a needle type pyrometer.

Fig 2. Measuring tyre temperatures
This shows a standard digital thermometer with a needle type probe being used to measure tyre core temperature. There is no need to buy a special memory type tyre pyrometer for this task.

Racing tyres ‘cure’ a little every time they get hot, or are ‘heat cycled’. Cured or hard tyres are never fast, so rule one is to try control the number and intensity of heat cycles. A tyre that has gotten hot enough on the car to ‘go off’ (feels greasy and loses grip) has been destroyed and will never again offer the grip it once had, so NEVER abuse your tyres intentionally!

Tyres that have been over heated have a smooth shiny look, like rubber that has melted and re-hardened. Funny that! The softeners and plasticizers will have boiled away.

Fig 3. An overheated wet tyre caused by being used on a dry track
This picture shows a FS wet tyre that had been run in the dry. You can see how the rubber has melted and congealed, smearing into the rain grooves. Although this tyre had minimum use in the dry, and probably be recovered by rasping off all the dead rubber, this tyre has been seriously damaged.

Fig 4. This overheated tyre probably caused by using a compound that was too soft for the conditions.
This picture shows a tyre that has been overheated after very little use at Formula Student 2005. Yes, it is possible to choose tyres that are too soft!

If the first enemy of tyres is heat, the second is misalignment. Misalignment simply abrades the tyre away, probably overheating local areas of tread in the process. Sometimes misalignment is hard to see and measure. Indirect misalignment such as difference in tyre diameters, or mismatched caster or SAE angles across the car may contribute to tyre wear.
Always measure tyre diameters and fit them in matched pairs. Failure to do so makes corner weighing redundant.

Mismatched caster angles will cause the car to pull (to the side with the least caster) and the correction will impose a side load, wearing the tyre.

Teams need to be very careful with the amount of Ackermann they use. Much importance is placed on ‘Ackermann’ angles, yet the real importance of Ackermann is the difference in front wheel toe angles as the steering is turned. This can be affected by things other than Ackermann angle such as the steering rack position.  Teams should understand the change in toe across the entire range of steering angles. 

Excessive Ackermann angle will wear the tyre by causing the inner tyre to be dragged across the turn apex.
Uneven Ackermann across the car (usually caused by failing to centralize the rack before doing a wheel alignment or a bent steering arm) can cause problems that may be difficult to analyze.

Obviously, camber and toe angles have to be optimal for the tyre, otherwise local overheating will occur and the tyre will quickly be ruined. If the tyre ever goes over on the outside edge (Insufficient negative camber at some stage of operation) the tyre will be ruined in short order. Not only that, but once that tyre wear has started, the tyre cannot be easily recovered. Reversing the tyre on the rim may extend the competitive life, but the tyre will never perform as well.

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