Publication: Total Magazine Article 2009
Over previous issues we have been focusing on driving technique – relating our driving inputs to the dynamic loading of the vehicles platform as we drive on a circuit. Let’s take a more polarised look at the tyres we use and how we can optimise their grip with the tarmac.
As we drive a circuit at speed, our tyres will increase in pressure. The increased cornering forces generate friction with the consequence being that tyre temperature rises and the air expands inside. We will be asking the brakes for maximum retardation which generates further heat to soak throughout the mechanical components and in to the wheel, further increasing the heat generation in the tyre carcass. Radiator and cooler venting can have an adverse effect on tyre pressure if the venting is not central to the vehicle, as one tyre may experience more heat soak than its counterpart.
If you have arrived at a circuit on road tyres you may find you can leave your tyres at road pressure. The subsequent high speed laps will raise the pressure to a more suitable pressure for circuit driving. After your first 5 to 10 laps return to the pit lane and check the pressure. How much you run them over the standard manufacturers pressure settings is largely down to the vehicle you drive and the tyres you use. As rule of thumb 4 to 6 PSI is safe guide for road tyres. If you are in any doubt please seek advice from your tyre supplier or race shop.
You will need to monitor the pressures throughout the day. If you have swapped your road tyres for track day tyres the advice is to set the pressure cold and monitor and adjust after your first short session. As a rule you will experience a 4 to 6 PSI differential between a cold and a hot tyre. It is essential to take a basic tool kit with you to track days which includes a tyre pressure gauge and tread depth indicator. If you are driving home on the tyres you arrived with, you need to make sure you have enough tread left at the end of the day to drive home legally. The legal minimum requirement for a tyre is 1.6mm across the central three-quarters of the breadth of the tread and around the entire circumference.
Why do I need to raise my tyre pressures when driving on circuit?
Let’s go back to basics to answer the question. In previous articles we have discussed Slip Angles – let’s explore the subject in more depth. To generate a side or lateral force, a tyre has to be rotating and running at a slip angle. Slip Angle is defined as the angle of the plane of the wheel and the direction of motion of the contact patch across the road. In short, the tyre and the wheel through a corner, travel by a few degrees in a different direction to each other. As the cornering forces acting on the tyre build through a corner, the tyre will flex on the rim. The difference in the travelling direction of the wheel and the tyre generates friction and grip.
There is an optimum amount of flex (Slip) the tyre will accept before it will no longer be able to support the weight of the cornering vehicle and the tyre will roll onto its side wall and start to slide. Typically the optimum slip angle for a road or track day tyre is 6 to 10 degrees. It is worth considering at this point in the discussion a modern saloon car will generate in the region of 0.8g to 0.9g of grip between the tyre on the road. A modern supercar, for example the new GTR, will achieve a staggering 1.5g on its road tyres. Modern vehicles offer high levels of lateral acceleration through a corner and consequently our tyre pressures are a major part of the equation.
Choosing the correct tyre pressure is fundamental to maintaining the optimum contact patch on the road and tyre sidewall rigidity. A road tyre pressure may not support the increased cornering forces and increased weight experienced by the tyre on circuit. Through a corner a tyre has to experience slip as the inner and outer edges have to travel different distances. For the very same reason our vehicle has a differential in the transmission. The challenge is to adjust our tyre pressure to the correct pressure for the conditions.
Subtle adjustments to tyre pressures will increase and reduce understeer and oversteer as we moderate the characteristics of the tyre sidewall flex(Slip) on the rim. A development in tyre pressure consistency is to fill your tyres with Nitrogen. Compressed air contains water molecules and will affect the tyres ability to maintain a consistent pressure once heated. Dry Nitrogen offers a moisture free alternative offering lower tyre running temperatures and consistent pressure maintenance.
Keep safe and enjoy the longer summer days and evening bargain track sessions.
CAT Driver Training Ltd.
Tel: 01234 757633
COPYRIGHT CAT DRIVER TRAINING LTD. APRIL 2009