maybe a dumb question - tire pressure sensitivity

[PopsRacer]

I always try to run at 29/36 on my 3.2 carrera because nearly any other setting makes the car unstable/twitchy. My tires got hot at an event last w/e and I forgot to check the pressures before going on course, and let me say that the John Wick stunt team would have been impressed I didn’t go off course backwards. (I got a round of applause from my fellow auto-xers) The pressures were about 5 high in the rear and 3 or 4? in front. The grip was gone and it was twitchy & tail-happy as all heck. Went through one gate sideways.

With the right pressures, it’s flawless. But why is it so sensitive to any changes? I don’t think I have ever driven another car that was so sensitive to this one thing.

[Locker537]

What tires?

What alignment?

[shoooo32]

Are those readings hot or cold? A street tire will gain roughly 5-7psi from cold pressure when driven hard. With 16x7/9 wheels on my SC I normally set them between 28-30 cold for autoX (OEM recommendations for rear pressure are too high IMO).

If you want to get deep into it, buy a pyrometer. Tire temps tell a much better story than pressure alone.

[Tobra]

I have noted that some tires are much more sensitive to pressure. Had some Falkens on the Miata that were magic at the right pressure, 38 hot maybe, do not have the notebook. +/- like 3 degrees and they were pigs.

[PopsRacer]

Originally Posted by Locker537

What tires?

What alignment?

'88 carrera- the heavy one (G50 & factory air)

Continental Extreme Contact DWS 06 Plus (my second set. I love them in general) on 7 & 8” Fuchs.
EU Ride height, stock TBs, matched set of Eibach sways.
Alignment is pretty ‘up the middle’ for a dual use car (auto-x on saturday, go out to dinner on sunday)

Fr: -1 Deg, 5 degrees caster & slightly less toe in than stock
Rr: -1.5 Deg, slightly less than stock toe.

[David Inc.]

You have all seasons and they got too hot.

Edit: In terms of tire pressures--I'm usually aiming for 37-38psi all around after a cool-down lap coming off track, so being in the middle/high 30s isn't the problem, it's the tire and the temperature.

[shoooo32]

560 treadwear rating and stockish suspension/alignment means you're going to find the limit really quickly, especially on a hot day. How much rake is in the ride height? That will give you great turn-in but also big slides on exit

[safe]

That is a ***** tire for most everything, specially auto-x or anything high performance driving. I would say its not the car that is sensitive to tire pressure change its the tires or at least the car and tire combo. With good tires its not sensitive like that.

So get some proper ultra high performance summer tires.
What you felt in the Auto-x is what you are going to experience on the streets if you get in a hairy situation. Bad tires on a 911 is not something fun!

[shoooo32]

I give this advice to novices - if you're not a good driver on all seasons, you're really in trouble when you make a mistake with sticky rubber. They're less forgiving at the limit and you'll be going much faster before they lose grip. There's nothing wrong with a lower threshold of grip. Some would argue it's more fun to have a controlled slide than a car with too much grip.

Anecdotally, I've autoX'd on DWS/06. I was setting the car up for a rally where snow in the mountains is always a possibility. Sure, the car was moving around on the tires but it was absolutely hilarious to drive. Minor tweaks to air pressure and driving style were necessary, but otherwise was no different that the 300tw tire I normally run on the targa. And there wasn't a massive hit to performance - I think I was 2nd out of ten cars in the vintage class that day.

[Otter74]

I'm curious what anyone reading this who runs Michelin Pilot Exaltos runs for pressures.

Agree with Tony about all-season tires. Sure, they'll feel diferent, but if you're on the street lower limits and more predictable loss of traction aren't a bad thing. I'm putting minivan tires on my Fiat for this reason. Don't need 200TW gumballs on a car like that, and normal tires are more fun.

[PopsRacer]

Originally Posted by safe

That is a ***** tire for most everything, specially auto-x or anything high performance driving. I would say its not the car that is sensitive to tire pressure change its the tires or at least the car and tire combo. With good tires its not sensitive like that.

So get some proper ultra high performance summer tires.
What you felt in the Auto-x is what you are going to experience on the streets if you get in a hairy situation. Bad tires on a 911 is not something fun!

Litereally what they are. Did I write the name wrong or do you have them confused with another model? They are hp summer tires. I do run all seasons in the winter for winter events (see below). Been driving a very long time, auto-x on and off since the mid 80s and on my second set of these thing bc I *really* like them otherwise. Very predictable, a great dance partner. Great in the wet too.
Just thought it was curious how nuts things got with the pressures off.
Thanks

[Otter74]

DWS06 is the all-season tire. I can’t remember the name of the summer version.

[Quicksilver]

An all season tire has many small tread blocks so it is easy for them to lay over and give you a higher slip angle. This means the whole car can yaw more from side to side before the rubber really starts sliding. More 'monkey motion' means you can get the mass of the car rotating. And once the mass of the car starts rotating something must be done to stop that rotation.

An all season tire makes it easier for that rotation to get away from you. The thing on your side is that their lower grip level means it will take a bit longer for all this to happen so it gives you more time to deal with it. A higher grip tire will be more stable but you have less time to react to it because it distorts less so it reacts faster.

The big lesson to really drive these cars hard isn't just to feel and control oversteer when it's induced.
You need to drive to control the rate of rotation of the whole car's mass.
The rotation of the car starts before the tires think about slipping or the car's suspension even takes a set. People think about using the tires to supply the force to make the car corner but you also have to have enough grip left to stop the car's mass from rotating. If you think about this you will easily be able to react earlier and it will be harder for the car to get away from you. (When you see someone lose a car to oversteer almost every time you can see that their hand speed is too slow and too late in their counter steering inputs.)

Drive the rotation of the car. You do that and most everything else falls into place.

[silverlock]

Originally Posted by Otter74

DWS06 is the all-season tire. I can’t remember the name of the summer version.

ExtremeContact Sport 02 (ECS02) is the extreme summer version, comparable to a PS4S. DWS06 definitely a (good!) all season, but still a noticeable delta between the two.