[Elizabeth]

From my previous cars, especially the Contour SVT, I found using different tire pressures front and back made the car handle better. The SVT Contour The rear at 37 and front at 45psi was greatOn the Focus SE i use 44 front and 39 rear psi

So any ST drivers change the equally balanced 39psi in the ST to a more balanced (relative to weight on wheel) pressure?

I do not know 40 series tires.. So I would tend to lessen the differential and make the front at 42psi and rear 37psi for starters.

in particular how would this affect the tendency for the rear to oversteer?

(my guess is it would lessen the tendency to oversteer as much.. But that is a wild guess)

Anyone?

Added:
I looked up tire pressure and oversteer. And found a set of charts on tireRack which show I am thinking the reverse of what will happen.
link http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=58

Another idea I found is to use chalk across the tire width. then drive a short distance and see if the chalk is uniformly worn. Center chalk strongest, too soft, center gone first too high air pressure..I would think a really flat concrets surface would be best for this test.


Other folks discuss lowering and all that.. this is what i like to work on...

 

[jsr72]

I do not know 40 series tires.. So I would tend to lessen the differential and make the front at 42psi and rear 37psi for starters.

in particular how would this affect the tendency for the rear to oversteer?

Lower the pressure on the axle you want to develop more tire slip angle. More tire slip angle on the rear axle = less understeer. More tire slip angle on the front axle = more understeer.

The ST isn't tail happy enough?

 

[felixthecat]

Lower the pressure on the axle you want to develop more tire slip angle. More tire slip angle on the rear axle = less understeer. More tire slip angle on the front axle = more understeer.

The ST isn't tail happy enough?

Other way around^^^^ More pressure= less grip. Adding more air pressure adds more spring rate. More spring rate= less traction. That'd be like adding stiffer rear springs which would give more front grip & less in the rear. Same as adding a stiffer rear bar adds more rear roll stiffness. Can anyone guess how much spring pressure, 1 lbs of air adds?

 

[sailor]

I'd disagree Felix, at least until you get to very high pressures.

 

[jsr72]

Other way around^^^^ More pressure= less grip. Adding more air pressure adds more spring rate. More spring rate= less traction. That'd be like adding stiffer rear springs which would give more front grip & less in the rear. Same as adding a stiffer rear bar adds more rear roll stiffness. Can anyone guess how much spring pressure, 1 lbs of air adds?

Increasing roll stiffness (via springs or bars) transfers more load laterally and results in increased tire slip angle to maintain the lateral force. In the front axle that creates an understeer effect. In the rear, it's an oversteer effect.

Lowering tire pressure reduces tire cornering stiffness which also increases the tire slip angle for the same lateral force. This gives a similar type of effect. Front axle: understeer; Rear axle oversteer.

This matches what's listed on that tire rack page, as well as what I've always done (run rear pressures lower than the front) in my autocross car to improve turn-in.

 

[sir__hobbit]

I seem to like 38 front and 34 rear in mine, cold. It works good for my daily driving.

 

[Ti/12]

Elizabeth, I think you don't a have to increase front tire pressure that much.
I had a 99 Contour Sport with the V6, 5 speed and a BAT sports suspension. Terry Haines recommended just 1 or 2 more lbs in the fronts. I thought it definitely improved the Contour's turn in. So I run 41 in front 39 in back of 12 Focus Ti with the handling pkg and 18" wheels. It works very well for me!