I’m interested how you checked your tyres for wear using the chalking method you described redwagon, and did you use wax or chalk?[/quote]OK, a quick tire pressure 101:
People ask all the time which suspension units, springs, swaybars etc. they should fit to their cars. Truth is, proper setup of what is already on the car is going to be the best bang-per-buck improvement you can make. A performance alignment and proper tire pressure setup should be the first step even if you are going to replace the whole suspension later.
I tend to buy tires with softer sidewalls than the typical high performance models that you would use on the track or in autocross. I like sticky tires but I’m averse to loud tire noise, so I pick ones with a sticky compound and medium stiff sidewalls, then tune out as much rollover as possible with air pressure.
Note here that tire pressures shown in your (car) owners manual are the minimum values, not the optimum. The manufacturers provide these values based on what is safe in all conditions and for the average driver, and biased toward quiet and comfort. Tires may be run at any pressure you like up to the maximum given by the manufacturer.
Rollover is the tendency of a tire to stick on the road but allow the lateral load from cornering forces to try and roll the tire off the rim. Rollover is deformation of the sidewall where the section of the tire and rim changes from a rectangle into a trapezoid. Such deformation upsets steering geometry, adds undamped spring forces into the steering system and reduces overall weight on the contact patch which reduces total grip. Reducing rollover will reduce these effects and increase the lateral G’s the car can manage in a controlled manner and so increase cornering speeds.
How do you find the optimum pressure for a street car when it could lay anywhere between (say) 28 and 50psi? First you have to see how bad the rollover problem is by seeing how much of the sidewall is coming into the contact patch. With an ordinary stick of chalk you make lines from the contact patch up the sidewall at 5cm intervals around the outside of the tires. Go to a closed course and do hard corners or continous circles with a mind to creating the highest lateral G forces possible. Check and see how far the chalk has been rubbed off the tire from the corner radius up the sidewall.
Next make lines straight across each tire from sidewall to sidewall, again at 5cm intervals around the tire. What we want to see is how flat the tire is approaching the road. We want to see even pressure across the whole contact patch in order to get the best grip from the tire in a straight line and also get the best wear. Underinflating the tire would see most of the load on the corner radius areas which are supported by the sidewalls. Overinflation would see the tire balloon a bit (like a motorcycle tire) and the contact patch would be biased toward a narrow strip in the middle. Again at a closed test track, make hard starts and stops in a straight line and then examine the chalk lines. If we had started testing at the car manufacturers recommended values we would probably see the tires are underinflated and the chalk lines are worn more on the corners than in the middle. Bump the pressures up 2psi at a time until the bias shifts to a center pattern, and then back off (as a rule of thum) 2psi. Running the tires too hard reduces the contact patch and will increase stopping distances. Be sure to maintain the difference in front to rear pressure that the car manufacturer recommends at this point. For example if the handbook recommends 28psi front and 26psi, then keep the front tires 2psi harder than the rears as the pressures increase.
Once the contact patches are optimised like this we can try the cornering testing again to see how much less rollover we have. On an average street performance tire you will notice that the highest lateral G’s you can pull in the car are significantly higher, though you notice more body roll as the suspension has to work harder now that the tires are rolling less. Check the chalk marks to see how much less of the sidewall has been in contact with the road surface.
Note that touring tires with very soft sidewalls will show very little improvement via these techniques. Some of them have such flimsy sidewalls you would have to dangerously overinflate the tire to get any reduction in rollover. Never exceed the manufacturer’s maximum pressure figures. Always set tire pressures cold. Running tires near to the maximum pressure may result in unbearable road noise. Be careful running street tires at continous high G loads as they can get overheated quickly and bad things then happen.
Note that changing the offset between front and rear tire pressures will change the handling behaviour of the car, shifting toward either under or oversteer. Sometimes reducing the total grip at one end of the car or the other may actually make the car quicker on tighter courses where being able to rotate the car quickly saves time. Note that such a setup may be fine for a skilled driver in an autocross event but may cause snap oversteer on wet roads with a less disciplined driver. What works for racing is not usually the best setup on the street.
You may want to experiment with that front/rear pressure offset in order to find what works best for you, but as a rule of thumb you should probably not increase the offset by more than double or decrease it to less than half. Needing to do so would suggest you have suspension or alignment issues you need to work out first.