CapriRacer Wrote:
Mitch,
I was wondering what results who've obtained using the temperature method. In particular, what pressure did you determine compared to the placard?
I reply:
For the car I have done extensive work upon (Ferrari F55B) it was more of getting the suspension aligned properly to the correct operating behavior of the car. (There were several things wrong with the suspension when I got the car--and all were fixed without replacing any suspension parts). When I got the car it would oversteer (basically) all the time! Lots of fun, but you could never relax while driving it.
I found a combination of tire pressures that calmed the car to a reasonable degree. RF 33 PSI, LF 31 PSI, RR 31 PSI, LR 33-34 PSI. It still oversteered, but no longer pulled to the right under throttle and left under brakes, and was generally driveable.
But to be fair, the factory placard in my car has 3 different tire combinations and three different air pressure recommendations, everything from 29/32 to 31/31 depending upon the maker of the (1995) tires. So, I set out to find that combination of {ride heights, suspension alignments, and tire pressures} that would result in long life with sticky tires under both street and track applications.
I ended up setting up the suspension so that the proper cold tire pressures are 32 PSI (nitrogen) front and rear. Track applications with (wet) air are 29 PSI (overnight in garage) cold (31 track cool); and allways 40 PSI hot off the track.
Since this car has adjustible ride heights, and since adjusting rear ride height controls the roll couple distribution, I can adjust oversteer/understeer with rear ride height rather than tire pressure. So, I diddled with it (raised front, lowered rear) until it felt right at 32 PSI all 4 corners. At the track, camber was set for a nice temperature profile across the tread; and toe set for dead on center resonse. (coincidentally I get a nice temperature profile on the street also.) I am currently getting 15K+ miles on a set of Bridgestone S03s (and 13 track days--I only get 5 track days on a set (8) of brake pads). Many people won't think of this as good milage, but if you knew how I drive at the track you would be amazed that I get 1/2 of this milage (many slower track drivers don't)
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For drivers who are quite sensitive to their cars' feedback, there is a procedure where you drive a single road many times and can determine the correct overall tire pressures and the correct front to rear tire pressures.
The basic procedure is to fill up the tires to an obviously overly high pressure (say 40 PSI) and then drive a road over and over slowly letting air out of the tire(s) until the best results are obtained.
The first order of business is finding a road where you can drive at a consistently at speed many many times (a nice untraveled country road works great.) It is best if this procedure is delayed until you get all 4 tires replaced so you are starting on a set of tires that are not worn improperly. It is also paramount that you use a dial air pressure guage so you can read better than 0.5 PSI changes in air pressure and make adjustments at this level of accuracy.
First: make 5 passes over this (at leat 5 mile strech) recording you notes on an actual sheet of paper! How did the car feel under acceleration, braking, turning left, turning right, on-center feel?
Then if the car feels 'loose' let some air out of the front tires, if it feels 'tight', let some air out of the rear tires. Both left and right are adjusted in 2 PSI increments (downward) until the balance is about correct, Then in 1 PSI increments.
Now that balance is about correct, it is time to find the point of best traction--and here sensitivity is paramount. Drop the pressures in 1 PSI increments on all 4 corners until you feel the traction start to go down. Then find a filling station and put 2 PSI more in each tire. This will put you very close to the optimal tire pressures--ASSUMING the suspension is in order, and aligned properly.
Now go home and the next morning, measure the (now cold) tire pressures.
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But the real point to all of this is that the driver can find the proper tire pressures for his vehicle, the way he drives, in the conditions underwhich he/she drives! No one set of tire pressures is correct for all (pick random tire size) 225/50ZR16 on a V6 Toyota Camry. It will vary from brand to brand and even on batch to batch of the exact same serial number from the tire moulds! (not much but it does vary).
The factory numbers are simply a starting point (a safe starting point) but almost invariably on the soft side trading tire life and tire grip for ride. If this is not the optimal choice for you and your driving conditions, at last now you have a procedure to follow to find what is better for you under your driving conditions.