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
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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