Tyre pressures for track

[Guest]

Sorry if this subject has been discussed before and I've missed it!!! [&:]
Am taking my car out on the track shortly and wondered about tyre pressures. I have a 1989 Turbo (250bhp) with the original drop forged alloys and 225/50 x 16"s on the front and 245/45 x 16"s on the rear. The drivers handbook states 36 psi, cold, for both front and rear, which I have always thought was too hard, with the car feeling a bit "fidgety" as a result. I generally run, on the road, at about 33psi front and rear.
Should I leave as is?? [8|]

 

[Diver944]

Derek, I asked the same question to Andrew Sweetenham who raced a 944 Turbo in the Porsche Cup, and attends many track days every year, here is his reply:

On a circuit, a car is cornering faster and braking harder and hence putting a lot more heat into the tyres. Hot tyres means higher pressures. Higher pressures means the contact patch of the tyre becomes more oval and is hence reduced.

This is why lowering pressures at the circuit gives you more grip as it increases the contact patch of the tyre (compared to when over inflated).

I would suggest getting to 32-34 psi hot on a road tyre. I always run my tyres at reduced pressure on the circuit, regardless of brand or type. If you leave the pressures as is or increase them, they will wear considerably faster in the middle and will slide much more.

So do your first session and then check your pressures, they will have increased, so reduce them back down to 33psi HOT

 

[Guest]

Many thanks for the promptness and detail of your reply. [] Excellent!!
On a very slightly different tack, is my guess-timate of 33 psi, cold, for the road, about right?
Cheers mate!

 

[Robbiemeister]

Derek

I use 34psi HOT on my 944T on the track. Check them after EVERY run though as the will get hotter throughout the day. I also use 34psi on the road as well but that is cold.

Don't forget to pump the tyres up after the tack day because they will go down to 25ish when cold.

Have fun!

 

[Guest]

OK - Thanks for that!!!
Cheers guys. []

 

[John Sims]

The only exact way to establish the correct tyre pressure is to measure the temperature across the tyre immediately after a quick lap. The temperature in the middle of the tread should be at the average of that of the inner and outer edge; hotter than average and the tyre is over inflated, cooler under inflated (obviously).

With reference to Fred Puhn's "How To Make Your Car Handle" it is noted that a tyres grip will be less significantly affected by the tyre being a few pounds over its optimum pressure than it being the same under its optimum pressure. This being the case one should err on the side of caution and avoid for going with significantly under inflated tyres.

When I was racing, many in the paddock advocated testing tyre pressures immediately on return to the pits, and dropping all the tyres to match the lowest pressure. This struck me as very good sense as, noting that tyre pressures increase with temperature, you should then (at least) establish a balance across the four tyres so that they all achieve the same level of grip. I have experienced in excess of 6 lb variance between the highest/hottest (usually left front) and lowest/coolest (right rear).

Assuming you don't want to dash out and buy a pyrometer (£105 from Demon Tweeks) I would opt for about 34 lbs cold and balance the pressures after the first run.

 

[Guest]

Wow!!! That's an exceptionally technical look at the subject. Will start at my "normal" 34 psi, cold, and adjust it down the way from there.
Cheers John! []

 

[Gary Bullen]

Also note that the tyres, especially the more exotic types such as Michelin Cup tyres and Pirelli Corsa have an optimum range of operating temperatures. Most of us have watched motor racing where the comment is made that the tyes have gone off. Nothing to do with sell by dates by the way! If you contact the specific tyre company they will normally give you guidance. If however you are getting serious about track work contact people like Mark Hutchison at Hutchison Motorsport or Silverline (See ads in Porsche Post and magazines such as 911 & Porsche World). Both of these companies have a wealth of knowledge and attend lots of track days. They often take very accurate tyre pressure guages and the previously mentioned pyrometer.

If your car is set up correctly, you are driving it smoothly and running the correct tyre pressure you will go a lot quicker without increasing the risk of an accidental off!

If you go out on track with normal road settings come in after about three laps, check the tyre pressure and reduce them back to the original settings. Carry on but keep checking until they stabilise.

However make sure that when you leave the circuit to drive home the tyre pressures are raised back to normal levels asap. If you try driving on the road with low pressure there is a risk of rolling the tyre off the rim at the first tight corner. This is not good! Regards GARY[]

 

[Melv]

At Combe some years ago, John Miles and I did some comparisons of tyre pressures before and after each morning session. On average, we were getting tyre pressures some 8-10psi above the cold setting. After a few sessions of constantly lowering the pressures, the tyres seem to stabilize.

What I tend to do now is start off with lower pressures, probably aout 25-26psi and let the tyres warm up to about 32psi, which seems to suit my Cup Car. 2 psi over on the rears and the car gets wobbly!!

The optimum hot pressure I found on my old C4 was about 34-36psi, on a variety of different tyres in different amounts of wear.

Spending the day at Donington with Mark Hutchinson, I watched him check the surface temps of a couple of GT3's running Michelin Sport Cups, and to everyone's surprise, the optimum hot temp was found to be 36psi, a lot higher than expected.

Melv

 

[John Sims]

Overheating of tyres reducing grip is a very valid point.

I ran a set of Yokohama A021R tyres on my racing car as wet/intermediates. They were great in the dry, but only for 3 laps. After 3 laps the heat in the tread was so high that the outside of the tyre would overheat and start to melt. The tyres then became slippery and you could see feathers of rubber rucked off the surface on return to the pits.

Overheating a tyre can also effect the tyre subsequently by case hardening reducing the suppelness of the tyre - changing it effectively from (for example) a medium to hard compound tyre.

Age will also effect a tyre and induce case hardening. If someone offers you a set of top line tyres second hand, establish how old the are and what they've been used for before clinching a deal. If the main use has been half a dozen track days over four years (assuming there is any tread left) the tyres could be next to usless.

 

[Diver944]

Hmmmmm, that's got me thinking now John.

I changed to full slick tyres on a spare set of wheels nearly two years ago purely for economic reasons. On average I seem to be doing 4-5 track days a year and I know I have done exactly 8 since the tyres were new. They have tread depth indicators which were 4mm when new and now they are down to 3mm

On this basis I have another 24 days or 5 years to go until I will be down to the canvas. Now obviously these tyres are going to be useless due to their age long before this, but I wonder how I will know. Maybe when you see me pirouetting down the main straight you could point this out to me, until then I am always very careful on the first session of the day until the tyres have warmed up.

Have we sent you to sleep yet Derek?

Another Sweetenham tip regarding correct tyre pressure was to draw a thick chalk line across your tread. Go out for a drive and see which bits you have worn off, adjust accordingly. I currently have the standard 16" tyres at 34 f and 36 r for road use.

 

[Guest]

I'd no idea that the subject could be quite so complex!!! [&:]
Think I'll invest in a digital tyre pressure gauge and take it from there.
If the bug bites, which I suspect it will, I can involve myself in the more technical side of things later.

 

[944Turbo]

If the bug does bite, I can highly reccomend going for a second set of wheels with slicks, these conveniently (just - watch the wiper motor and put them into black sacks) fit in the back of a 944. Your speed through the corners will increase and your road tyres won't suffer.
If it rains you can keep the road tyres on - wear should not be an issue in the wet.

Enjoy,

Tony

 

[Guest]

Cheers Tony!
Probably a difficult/impossible question to answer, but approximately how much should a set of second hand wheels and slicks cost me?

 

[Melv]

Do bare in mind that if you run slicks you will be putting the brakes, drive line and suspension under additional stresses.

It's the way to go though, if you are serious about track days.............

Melv

 

[Guest]

Hmmmm - not sure if that's something I want to do to a near 15 year old car with 114,000 miles under it!! []

 

[Melv]

A good set of road tyres, i.e. Bridgestone S02 would be fine.

Melv

 

[Guest]

I currently have a set of Fulda Extremos on the car. I only bought these in May of this year and they have probably only covered about 3000 miles. Presumably these will be OK? [8|]

 

[Diver944]

Derek, if it was any car other than a Porsche I would agree with you. I have read lots of posts on other trackday forums about excess wear and tear, dry sumped engines, oil starvation etc and obviously you need to make sure your car is up to the task. However Porsche are built to take this (and touch wood) so far I have only had to renew brakes more often than I normally would, and I can do this myself

A spare set of Design 90's can be bought for around £200, they will probably come with a worn set of tyres that you can use on track. Keep an eye on Ebay and the Porsche magazines. Mark Hutchison can obtain slicks (he's listed on the trackday pages of PP) but I got mine from Silverline tyres for £85 each, brand new. Economically it makes sense - unless you detonate your engine due to excess G forces []

 

[Guest]

Now, there's a sobering thought! []