Tyre pressures - Again ??

[Charlie_McLaughlin]

Hi Guys,

In April I am going on my first track day with my 944 Turbo (85MY). I have 17" turbo wheels and 215/45/17 and 255/40/17 tyres. I recently checked the tyre pressures and they were all different. Can anyone give me the recommended normal road use pressures for these tyre sizes.

I have just fitted the "boost enhancer" what a difference !!

 

[944Turbo]

I have 17 225 and 17 255 and run 32 all round the official figs for 1990 are 36F and 44R but I beleive for earlier cars they were 36F 36R obviously this is for 16" tyres. From experience these pressures seem far to high (wear in the centres etc.)

Those figures are all cold for the road, on dry track days I usually settle on 32 F&R Hot so have to let them down a bit (approx 6PSI ), (dont forget to pump them up before you go home)

Enjoy

Tony

Forgot to add cue everyone else with a completely different set of figures

 

[Guest]

I am running 36f 40r but again thats on 16" rims. I thought if anything you might have to run a bit higher pressure in lower profile tyres?

 

[Fen]

I'm trying 34 all round in my 18's. Both previous sets of rears have gone around the middle at 38 and 36 respectively.

Edited to get the pressures right!

 

[Guest]

You mean 36? not 26 surely.

i will have to keep my eye on the tyres, will take some readings for depth at the weekend across the section and keep a record for a few months see what happens.

 

[Fen]

Yes, just edited to reflect that...[&:]

You could try the chalk across the tread trick. I keep meaning to do it but never get round to it.

 

[Guest]

ORIGINAL: Fen

Yes, just edited to reflect that...[&:]

You could try the chalk across the tread trick. I keep meaning to do it but never get round to it.

That would assume a nice flat level road.. Not seen many of them around Scotland. Pot hole anyone?

 

[Chris Goodman]

Now - I want to preface this by saying that I absolutely accept that I probably don't know what I'm talking about but the attached pic is of my car last saturday and as you may be able to see, the inside front and rear tyres appear to have lifted off the track about 50% of their width. The outside tyres appear to be 100% in contact with the track and don't appear to have deformed too much. Pressures were set at 36 all round cold but when I checked the rears shortly, but not immediately, after coming off the track they were at 46.

Does the 'lift' on the inside tyres suggest that the pressures were too high? Or is it just that the suspension has reached the limit of its travel and is lifting the inside wheels?

Everything I read says that you should INCREASE tyre pressures for track use but I discovered with my 911SC (standard non-sport profile tyres) that the pressures rose on the track by up to 15psi due to heat and then grip certainly suffered.

Chris

 

[Diver944]

Hi Chris, There is lots of conflicting advice re tyre pressures on track. [8|] I think the raising pressure advice is more aimed at 'hot hatch' type cars that do not have such wide or low profile tyres and it stops the edges rolling under in extreme cornering.

When I first started tracking I took advice from Andrew Sweetenham the Porsche Open 944 racer and he certainly advocates lowering road tyres to around the 32-34 psi mark when HOT If you allow them to go as high as 46 HOT then things will get a bit loose as the tyre will only be gripping in the centre of the tread. Check the pressure as soon as you come in, they do cool quite quickly.

Your /40 and /45 low profile tyres will have sufficiently stiff sidewalls to cope with these supposedly low pressures. I think your picture just shows a standard 944 Turbo which is a pretty heavy car leaning to its max on a fairly soft road suspension so nothing to worry about. The only way to get rid of that lean would be rock hard springs [] and stiffer anti roll bars

 

[944Turbo]

possibly a slippery slope to credit card overload [] - see

http://www.porscheclubgbforum.com/tm.asp?m=32756&mpage=2&key=anti%2Croll%2Cbar&#35976

Tony

 

[Guest]

Problem is if you lower the pressures too much then you get the tyre moving around alot, and this creats high temps on the tyre itself. Tyres should have a safe max operating temp, and its that you need to look at rather then what the actual pressude is when warm. If the tyres is over heating then add more air. digi temp gauge and probe anyone??

 

[Chris Goodman]

Thanks Paul. Next time out (Donington) I'll try dropping the pressures as advised and see what happens. Presumably there must be an optimum balance somewhere between 'less pressure = more heat generated = increased pressure' and maybe I can find it by discovering how much the Hot pressure changes if I gradually increase Cold pressure from a low start. My photo certainly seems to bear out the supposition that 46ps1+ lifts the outside edge of the tyre off the track. Perhaps spinning wasn't just down to bad driving! Mind you - in which case, why doesn't the inside edge of the outside tyres also lift?? Front and rear offside tyres look nicely planted in the pic.

Chris

 

[944Turbo]

The lifting of the inside wheels is down to roll (weight transfer) created by grip on the outside wheels, they have little weight left so they lift. there is very little you can do with tyre pressures to help.
Looking at you pics again I would say the outside tyres (the ones doing most of the work) look reasonable.

IMO you want tyre pressures that maximise the contact patch, too hard and you will run on the centres of the tread - reducing the contact patch, too low and you will have tyres that roll under and wear on the outside edges (and the sidewalls dont have much grip). Having said that it was impossible to stop more wear on the outsides of the fronts until I increased the negative camber over standard (the previous owner had already increased the negative camber at the rear).

I arrived at my figures (above) as they gave a relatively even wear (and temperature - only by touch) over the whole tread. On a dry track day the tyres can still get very hot and the tread blocks move around and feather at the edges, especially on newish tyres.

At the Nurburgring I didn't bother reducing the pressures, and by the end of 2 fairly fast laps with only a short break in between the car was sliding all over the place - entertaining but not quick.

My solution for trackdays (well others did it first and I couldn't keep up with them) was to invest in a second set of wheels with slicks as they give a megga amount of grip and just fit in the boot in 9 and 7.5inch widths.

Tony

 

[Guest]

ORIGINAL: 944Turbo

At the Nurburgring
Tony

Name dropper.

 

[944Turbo]

46PSI was probably causing reduced grip as the tyres would have been running on the centre of the treads giving less predictable handling. So thats perfect as an excuse for spinning [] If you get the pressures spot on then the outside wheel should lift more (with no changes to suspension).

Tony

 

[937carrera]

ORIGINAL: Diver944

When I first started tracking I took advice from Andrew Sweetenham the Porsche Open 944 racer and he certainly advocates lowering road tyres to around the 32-34 psi mark when HOT If you allow them to go as high as 46 HOT then things will get a bit loose as the tyre will only be gripping in the centre of the tread. Check the pressure as soon as you come in, they do cool quite quickly.

Best advice I've seen in this thread []