AX tire pressure question

[DonCostello]

I recently changed from a Boxster S to a 2008 911 4S. Love the car, but not sure if i understand the best AX tire pressures as I felt I did on the Boxster. Car handles very well, but getting some understeer on sweeping turns. Running Hankook Ventus R-S3's (seem very good) at starting pressures of 37/43 (factory pressures are 34/40), based on what I did with Boxster (add about 3 pounds to factory specs). Tire wear on all 4 looks very even and right to the edge of tread. Short of pyrometers, chalk lines, etc, any general suggestions? Maybe increase fronts closer to rear pressures? Thanks.

[AGill]

Don,

Hopefully others will chime in who have actually recorded pyrometer data on a similar 911 platform/alignment/tire package as you; however, short of that I'll give you my $.02. If the factory pressures are 34/40 then that is meant to be cold. If you run even those pressures to start with at an AX I bet you would see hot pressures of 38/46 at least. If you are really pushing the car on a hot day you could even see those rears get up close to 50. Those are all too hot and you are giving up grip by reducing the contact patch. Now, you have a heavy car so you can't lower the pressures too low but if I were to have your car with those tires and set a starting pressure I'd probably try 30/36 cold with the hopes to hit 34/40 hot. I'd then check the tire rollover at those pressures (being sure not to mistake rollover when cold as the amount of rollover when hot and the car is pushed hard). If all looked good then I'd get a bunch of laps at those consistent pressures (bleeding the pressures down if needed after each session) and see how the balance of the car was. If you still have too much understeer, then you could try lowering the front a couple pounds (depending on rollover) or raising the rear a couple pounds.

Also, as I'm sure you know...your new 911 (which is gorgeous) is never going to handle like your old boxter! Each car will shine in different areas on a track.

Hope that helps.

[ttweed]

Don,
I'm guessing your understeer is worst in mid-corner by your description? Or are you experiencing this on corner entry (car doesn't want to turn in)? My very limited experience in a C4S (instructing at the PDS) indicated to me that they have a very prominent tendency to understeer in stock configuration. I am not sure that you would want to cure this tendency entirely with tire pressures alone, as you might need to go very radical to do so, in a way that would sacrifice overall grip and increase tire wear. The ideal is to increase front grip without reducing rear grip, while maintaining acceptable wear levels. Using pressures alone to achieve a good steering balance is a makeshift solution, as most tires have a certain small range of hot pressures at which they work best and generate the most grip, so changing them outside that range (higher or lower) is a compromise that sacrifices overall grip and increases wear. Unless your balance problem is fairly slight, it may not respond well enough to pressure changes alone.

Your cold pressures of 37/43 seem high to me, but I have never used those tires or driven a car as heavy as yours regularly. After gaining 4-5 psi or so in a session, that would put you over 40 in the front and nearly 50 psi in the rear, no? The only way to tell if the tire really likes that high a pressure is by using a pyrometer, putting it on a skidpad, and seeing what pressure range generates the best grip with acceptable tread temps, but I haven't seen many performance tires that work best at pressures over 40 psi hot.

Have you had a performance alignment done on the car? What kind of camber and toe are you running? What tire sizes? I would guess that it may take a combination of adjustments to achieve the kind of handling balance you desire without sacrificing overall grip. If you can, some of the best choices might be more negative front camber, a softer front ARB, or changing to a wider front tire.

Here are some ideas from CARROLL SMITH’S ‘PROBLEM AND CAUSE’ GUIDE for race car handling, some of which won't be applicable to your street car:

Corner entry understeer: car won’t point in and gets progressively worse
• Driver braking too hard, too late
• Relatively narrow front track width
• Excessive front tire pressure
• Excessive front roll stiffness (spring or bar)
• Relative lack of front download (excessive rear download)
• Incorrectly adjusted packers or bump rubbers (car rolls onto packers)
• Insufficient front toe-in
• Insufficient Ackermann effect in steering geometry
• Front roll center too high or too low
• Insufficient front damper bump force
• Insufficient front toe-out
• Insufficient front wheel droop travel (on non droop limited cars only)
• Nose being “sucked down” due to ground effect
• Excessive Ackermann steering geometry
• Can also be caused by unloading the front tires due to rearward load
transfer under acceleration – cures include:
• Increasing front damper rebound force
• Increasing rear damper low speed damper rebound force
• Increasing rear anti-squat
• Droop limiting front suspension (will also make turn-in more positive
and will reduce overall understeer)

Mid-corner (mid-phase) understeer
• Excessive front tire pressure
• Excessive relative front roll stiffness
• Excessive front toe (in or out)
• Excessive Ackermann steering geometry
• Insufficient front dynamic camber
• Relatively narrow front track width
• Insufficient front wheel travel (car rolls onto packers or bottomed shock)
• Insufficient droop travel (on non droop limited cars)

As you can see, in both understeer cases above, "Excessive front tire pressure" is mentioned as a cause, so if that adjustment is your first (and last) resort, I would try lowering (not raising) the front pressures relative to the rear, while not allowing the rears to go over 40 psi hot.

HTH,
TT

[DonCostello]

Tom / Adam
Thanks for your very helpful replies, I appreciate it. Can't wait to try out. I'm sure I've been carrying way too much air when tire are hot.