got new tires? Best AX/ and or street pressures?(newbie)

[LEAKYSEALS951]

Okay, I've got a 78 sc and just mounted some bridgestone potenza re-11 tires on it. (today)
The car originally had 6 and 7 fuchs on it. I have 7 and 9 inch with 205/55/16 in front and 245/45/16 in back. I have no idea what the tire shop filled them to.

My question is this... Do I use the recommended setting in the car sticker?, or what exactly works best for street/ and or AX use? Is there even a difference? I think the auto store has them over inflated. I will check tomorrow. Everyone I saw at the last autocross I was at was checking/ changing tire pressures religiously. What gives?, or at least, what is a good starting point for this size combo? I'm thinking 95% street/ 5% AX? Throw me some numbers!

What is your experience?

Thanks!
Ron

[Pete R]

Not knowing anything about your set up I imagine you will want to keep them in the mid 30's psi or so. The reason people check them is the tires heat up, build pressure and lose grip. They are bleeding them back down to the pressure they want. After an event they may go down into the 20's when cold and you'll have to put more air in them for street driving.

[cary]

Start with 32 front and 35 rear.
Mark them with chalk and see how they perform.

Where do you live? If it was hot, the tires were gaining pressure.

[LEAKYSEALS951]

Thanks!
Okay, that makes sense. Mid atlantic/ va. I think it's a stock setup, but a PO to the PO to the PO (literally) might have put in some oversized torsion bars. It drives stiffer than I'd expect (more like sport sway bars) and someone rolled the fenders at some point. I'm thinking someone did some work on it and sold it it with 6 and 7's later on, but until I get into the suspension to redo bushings, I don't know.
Ron

[cary]

RE-11s on 7/8 *16's.

[Dave at Pelican Parts]

Different tires grip better at different pressures. For instance, most hard-compound "real street tires" tend to grip better with higher pressures, probably due to allowing less sidewall deformation. But competition-type tires tend to grip better at lower pressures, in part because their sidewalls are already pretty stiff and partly because their rubber is sticky enough that having more of it touching the ground makes up for any downsides.

Those are all "up to a point", and of course different tires have different ranges they are best in. And the car setup can influence what pressure gives the best grip--and even driving style can!

That said, most of the improvements from optimal tire pressures are going to be a whole lot smaller than getting more seat time, at least if you're as inexperienced as it sounds like you might be.

In the Zone 7 schools, we typically told students, "Set it to 36 PSI all the way around, and then forget about it." That's a number that is sort of reasonably mid-range, so it should be at least mediocre on anything. And it's more important for students to focus on their driving than on fiddling with setup, including tire pressures, during the event.

--DD

[Driven97]

Dave is spot on above. Put in a few extra psi over the sticker numbers, and don't obsess over it at first. You'll have enough to focus on already without worrying about your tires. Just make sure you stay below the max pressure listed on the side of the tire for safety reasons.

There are little triangles somewhere near the edges of your tread. The wear on you tires should go roughly to those triangles. If that triangle is getting chewed - more pressure. Not there? Drop a few psi. Note this is a very rough dial in, and contingent on not overdriving the tires or you'll get bad readings. Some people use chalk / shoe polish on the corner of the tread to better see how far the tire is rolling over. I got pretty lucky with my first guess when I switched brands:



Once you get more comfortable, you can tweak. Remember there is a "best" pressure for each surface on each day. Either more or less will reduce grip. But tires are also part of the suspension, easily tunable springs. More pressure acts like more spring, less pressure is like a softer spring. Sometimes a small pressure change can tweak the balance to do some good.

[Driven97]

Oh, and you are going to be at non-optimal areas of the camber curve for the tire:



Before you laugh at the poor little Hyundai, the 911 camber curves aren't any better:

[LEAKYSEALS951]

Thanks for all the feedback. I took the RE-11's out for a drive today and holy #$%#! they are awesome! I look forward to getting out to another autocross soon.
Nice pics everyone. Looks like the stig is about to brake the bead on the tire!
Ron