There are at least two things you adjust with changes in tire pressure: contact patch and ride rate.
If you're surprised by the second, remember that the tires are effectively pneumatic springs, so increasing the air pressure in a tire increases its stiffness. The effective stiffness of the suspension and tire springs in series is called the 'ride rate', calculated as 'ride_rate = (spring_rate * tire_rate)/(spring_rate + tire_rate)'. Therefore, any changes you make to the tire pressures will affect the handling of the car in a similar way to changing wheel rates, independently of any changes made to the contact patch. (I notice this a lot at the Nürbergring and Monaco, where the car will easily ground if I'm using 19 PSI all round, but doesn't if I'm using 20 PSI. The surprise is by how much just 1 PSI in the tires increases the ride rate.)
Tires change the amount of grip they yield with changes in their inflation pressure. Increasing the pressure increases the amount of grip - up to a point, after which it falls off again. So, each tire has an optimum pressure at which it yields optimum grip. The trick is to find this pressure and adjust the setup so that this pressure is maintained at each corner of the car while driving. That way, you'll know that you're getting the most out of your tires, and have a sporting chance of having a neutral-handling car. I'm guessing that this pressure is 24-25 PSI for both front and rear tires in GPL, as that is the (hot) pressure at which you get a flat contact patch. Any tire softer than 24-25 PSI will lose some grip, but gain a wider range of effective slip angles - making it easier to use.
Some folks will tell you to correct handling defects in the car by adjusting tire pressures, e.g. to cure understeer by adding 1-2 PSI to the fronts, and so on. This may be true for a realcar™, when no other adjustments are available or practical. But, in GPL, we have the luxury of being able to cure handling defects by instantly changing ride height, springs or roll bars. So don't fix handling problems with tire pressures.
I generally use cold pressures of 20 PSI in the fronts and 20 PSI in the rears. This gives predictable oversteer until the tires heat up at which point you get, in the words of Ian Lake, “one of the most stable cars in GPL”.
If you're feeling playful you could change these so that, after about 5 minutes of driving, the hot tire pressures read 24-25 PSI in the two fronts and 24-25 PSI in the rears. I cannot do this for you, as experience has shown that different people's driving styles produce different heats in the tires. You'll probably need to inflate the fronts asymmetrically to 19, 20 or even 21 PSI to get 24-25 PSI in them when hot. The rears will mostly be alright at 20 PSI (you may have to deflate them both to 19 PSI at Watkins Glen) and don't like to be inflated asymmetrically.
Warning To run asymmetric front tire pressures, you need to be able to lap consistently. Tire temperature is directly affected by how hard you are driving; if your lap times vary - even by just a few seconds a lap - then stick to 'safe' underinflated front tires with symmetric pressures and live with the slight loss in performance.
There is a lot of mileage in keeping the left and right side hot tire pressures the same - it makes for better braking, and gives you a much better feel for the grip at the front of the car. After all, it doesn't really matter what the cold tire pressures are, does it? You run 100% of a qualifying lap and 98% of a race with hot tires; if you are using symmetric cold tire pressures on an obviously clockwise circuit that means you are running all that time with asymmetric hot tire pressures!
I most certainly do not believe in setups where the tires are deliberately under or over inflated. The problem with these cars is lack of grip, relative to their power. Carroll Smith: “Grip is what our business is all about. Grip comes from tires, period”. You get maximum grip from a tire by inflating it to the right pressure, having it flat on the road, heating it to the right temperature and using it sensitively in the narrow range of its optimum slip angles. That is what setting up and driving a car in GPL is really all about.
| « Camber | Front brake bias » |
|
|
