Grip decreases rapidly with lowering pressures - but falls off more slowly at high pressures. In my road car I typically use 35-38 pounds per square inch (versus 120-130 PSI on my road bicycle by the way) and if I was going to do a controlled circuit 1 or 2 lap dash with that car and those road tyres I'd bump the pressure up to 45-50 PSI. With the heat generated by racing, even just for a lap, the pressure would build by another 5 PSI or so, so go a bit lower if you are doing multiple laps. Go too high and you'll blow the tyre off the rim... you don't want to do that!
Adjusting handling characteristics by manipulating tyre pressures is the last resort - unless you have a $0 budget. Ideally, make changes to other variables first and then tune the tyres last. How do you do that? On a road car, like a VW Golf, of which I've been lucky enough to own a couple, you will get understeer at the limit, and power-off oversteer if you back off mid-corner. The power-off oversteer assists you in the turn, as it negates the otherwise terminal understeer. So with your Golf, like most front-wheel-drive cars, you can bump up the front tyres relative to the rears, such that the fronts may have 38 PSI and the rears just 32, to get more of that lift-off oversteer. But be careful, as overcooking it will result in a spin...
For rear-wheel drive cars like my Alfa GTV whilst bumping the pressures up a bit I'm mindful of the equal (50:50) weight distribution. Just a few more PSI at the rear is enough - then see what the effect is. Generally speaking weight transfers rearwards under acceleration, so a bit more air in the back aids traction on take off. Bear in mind that under braking weight transfers forwards, driving the front tyres into the road.
Ignoring suspension settings like camber for a moment, an underinflated tyre wears at the edges, an overinflated one in the centre. You can image that an ideal pressure will see more even wear, but it depends on load, weight distribution, engine torque, driver skill, which wheels do the driving and the actual use of the vehicle (a trip to the shops puts very different loads on a tyre than a lap around a race circuit).
Index
Get a life! Or one person's diatribe on staying alive. Skippable.
Handling 101 - What's handling good for?
Handling 102 - Let's get technical about handling
Handling 103 - Adjusting your car's handling characteristics
Tyre pressures matter
Camber plays a part
Sometimes you must Toe the line!
Caster look over 'ere, mate!
Having a Polar Moment
Roll Over Beethoven
Oversteer
Understeer
Weight transfer explained
Shuffle off this mortal Coil
Tyres and Wheels can do more than just look good
Pitching a fast ball
Other stuff people often don't understand
Heel and Toe
Double declutch
Wheel offset
Rollbars
CVT - Constantly variable transmission
Twin camshaft motors
GTV - Grand Touring Veloce
GTi - Grand Touring Injection
GTam - Grand Touring America or maybe Allegerita Modificato
RS - Rally Sport or Renn Sport
GTO - Grand Touring Omologato
Copyright 2008 Robert K. Russell; updated September 26, 2005. Many of the images on this site are for sale, including at higher resolutions
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