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Originally Posted by blue2000s
Technically, this isn't correct.
Assuming all pressures are within +/- 5 or so psi of the level that the factory recommends, increasing pressure reduces grip, increasing slip at that tire. If the poster is experiencing understeer, there is insufficient grip at the front axle. This calls for a decrease in pressure at the front or an increase in pressure at the rear.
Check out this web page, specifically, the table near the end:
http://en.wikipedia.org/wiki/Car_handling |
Both of you are correct and neither of you are correct. Here's the deal - for a given tire, car setup, track surface and temperature there is an ideal tire pressure for maximum grip. Raising or lowering the pressure away from that ideal will produce less grip. There is no easy answer like, "if the car understeers, reduce front tire pressure"
A driver must find an ideal pressure through experimentation. Once found both for the front and rear you can ADJUST the balance slightly if you find the car unsersteers more for a given surface on a given day. But you CANNOT cure handling imbalances with gross pressure adjustments.
I will agree that 25psi is far too low for at-the-limit cornering with any street tire.
Here is the order I would attack the problem:
1) If you are relatively new to this then get an experienced driver to drive the car at the limit. Ask them what they feel the overall balance is like. You may be surprised that they do not think the car pushes like a pig but is relatively neutral. If that's the case then fix the driver, not the car.
If the experienced driver confirms that the car unsdersteers a lot then:
2) Set all the pressures at 35psi. This is a good safe starting pressure
3) Get an alignment!!!! You need maximum front negative camber and maximum camber. Also try to reduce rear camber to -1.5. If the rules of your race series permit then lenghten the slots on the front top strut mounts which can allow you to get 2 degrees negative. This will really help at the expense of increased tire wear. For more agressive turn in go with toe out in the front. Also make sure rear toe is 0. (if you like to live dangerously think about 1/8" toe out in the rear - but watch out for snap oversteer!)
4) Even out tire widths - why run a wide rear tire and smaller front? Lots of track Boxster drivers use a balanced setup of 245s all around.
5) Install a GT3 front swaybar. While it goes against most of the general guides you see on the web, a big front bar will significantly reduce body roll. Since our cars have McPherson strut design suspensions, body roll has a drastic effect on camber (this is unlike most racecar suspensions which have much more friendly camber curves). By reducing body roll you increase front grip more than the increase front roll stiffness reduces it. An added benefit of a bigger front bar is it allows you to put down power better on corner exit which can be problematic without a LSD.