On The Track

Tyre Pressures

The tyre placard in my 928 recommends pressures of 250 kPa front and 300 kPa rear (36/44 psi) for the standard 215/60VR15 tyres. The car has been fitted with 225/60R15s ever since I have owned it. Over the life of my first set of tyres, the rears wore more in the middle than the sides (no, I didn't spend all my time on the freeway!), so I have run them at 280 kPa (approx 42 psi) ever since. During previous sprints and driver training at Eastern Creek and Oran Park, the 928 understeered excessively in the hairpins. As I recall, I first stuck with the street 250 kPa in the front tyres, and later tried 260. At Wakefield Park in April, I ran 280 kPa in the fronts and the car seemed to handle the tight corners much better, so I used 280 all round again last Sprint.
Glenn Evans, '80 928 Petrolblaumetallic 'S' look 5 speed

Porsche recommends tyre pressures based on road driving: 2.5 bar (250 kPa, 36 psi) all round for 225/50R16s and for 215/60R15s fitted to the front, and 3 bar (300 kPa, 44psi) at the rear for 215/60R15s fitted to the rear. The tyre guy's statement accord's with Porsche's recommendation so, unless you have reason to vary the pressures, don't.

General info: I was told ages ago that radials suffer excessive heat stress if run routinely below 190kPa (28 psi). The upper limit is the maximum inflation pressure for the tyres you have fitted; it is on the sidewall of the tyre. Most manufacturers usually recommend the lowest pressures they can get away with, to give the softest ride.

If you over inflate a tyre for the load placed on it, the tread area will balloon and the edges will be lifted from the road in straight running. This will reduce grip in corners also!!

I consider that there is scope to increase tyre pressures for road use before reaching the point at which grip is reduced. This will vary between tyres and perhaps between cars. You may or may not find that increasing the pressures over Porsche's recommended ones gives too harsh a ride on the road ... that's personal preference.

I ran my first set of new tyres (225/60R15s) at 250 front and 300 rear, and wore out the centre of the rears (eventually; not quickly), indicating slight over inflation. Ever since then, I have run 280 kPa (41 psi) in the rears. I now run 280 kPa in my fronts as well, after this seemed to work better at Wakefield Park last year, and I still find the ride on the road OK.

PLEASE NOTE: that all these pressures are measured COLD!! Tyre pressures obviously increase with temperature, and the increase is greater with sustained hard cornering and high speeds. Reducing tyre pressures after a run on the track - just because the hot pressure is more than the COLD pressure in the owner's manual - is dabbling in the unknown, unless you evaluate the results systematically. Ideally, this should be done in conjunction with measurements of the tread surface temperature, to ensure that no part of the tyre is too hot or too cold. If the car is skatey, and reducing the pressures a little reduces both this AND your lap times (factoring out your own improvement if you are inexperienced - like nearly all of us) AND your tyres are not losing grip more quickly because they are overheating, you may have lucked on to a better pressure.

It seems to me that there are a lot of variables to get right if you want to experiment with tyre pressures during a Supersprint. I've never felt confident that I would make an improvement, so I have run the same pressures all day the handful of times I have used the car on the track.
Glenn Evans, '80 928 Petrolblaumetallic 'S' look 5 speed

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At Wakefield I started with 43 front / 46 rear and gradually let them down as the day went on, after a certain amount of skating and spinning. I now think that 34/36 is the nicest balance while not on the track. The manual says to set them at 36/43, but I prefer them softer. Mine are 225/50/16 all round.
Stuart Greaves, '84 928 S

Good point re tyre pressures.

They were probably a bit low. I started off at 40psi all round (warm) then moved 'em up to 42psi for the next session. I will actually try 44psi next time, but anything higher might cause a bit of skating. David S.

I've done a bit of experimenting with tyre pressures at three tracks now. Previously I'd had them too high, I'm sure of it. On Sat (23rd June '01) I started with 38F, 39R, and after the first session they'd gone up to 44F, 45R, up 6psi. The rights possibly a tad higher than the lefts. That's where I left them. It felt pretty good at that. I don't think increasing them further would have helped.
Stuart Greaves, '84 928 S

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Body Roll and Brakes

The body roll would certainly be reduced with firmer suspension, and some of the cars on the day were setup with suspension that was so firm that pushing down on the body panels with both hands would barely move the car at all.

But driver experience and courage would absolutely make a big difference. I know now that I needed to stomp on the brakes at the last possible moment when coming into the corner and enter the corner wide so that I had a straighter line to the apex. The risk of course is that if I ran out of brakes I would have collided with an (Oran Park) concrete wall!

Some of the guys on the day were using their ABS to the max and really braking hard just before entering the corners and this made a big difference to their lap times.
Regards, David Silverstone, '87 S4 Felsengrun (with virgin ABS)

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Benefits of tyres

I can see the benefits of tyres. I was out at Oran Park again. I resisted the temptation to go out in the 928, even though I was free to do so. I had a ride in a new R8 Clubsport 250LSi, with a good driver. Nice car. Very nice car. Muted, smooth and quite well damped. A quiet V8. I could really feel the grip of the Bridgestone S-02s was much better than my RE-711s. He was lapping in the high 56s with me aboard - similar times to the 928.

Then I had a short ride in Mal Rose's car. Whoo-hoo! I could feel the slicks were tired, but even so, I had one hot lap in, wait for it... 45 seconds!

Yee-haa!  600hp, 1320kg, 11x17" slicks!

The primary differences between the way we drive and the way he drove was this:

1. Ripple strips upset our cars. If we hit them, instead of just brushing them, (especially in the flip-flop over the crest) they'll upset the car and pitch us off-line. Mal's car was so stiff he could almost straight-line that whole section, carrying the inside wheel over the strip, even over the concrete outside the strip. It was amazing how it unfazed the balance of the car.
   
   
2. Clutch. He doesn't use it. "Nah, it's a dog-box and triple-plate clutch. You can't hurt 'em." He never uses the clutch on upchanges, and often not on down-changes. Bang bang bang!
   
   
3. Brakes. Ours are strong, but his are twice as strong, and didn't fade all day. He hit them hard enough to chirp-chirp-chirp them every time. I got harness marks on my shoulders. Very, very impressive.

Regards, Sturat '84 928 S