Magister Ludi

I run my C7 on the street with 32 psi. It seems to perform well on aggressive cornering. I signed up for my 1st autocross 2 weeks from now. The C7 manual seems to recommend 26 psi for such events (Section 9-5 "Track Events & Competitive Driving").

My question to those with experience: does this make sense to you?

Magister Ludi

Wow. 19 views and no responses. I guess this question must be dumber than I thought. Let me rephrase: Does anyone run pressures that low when they autocross? (thanks for indulging a rookie)

Solofast

The right tire pressure is a function of a bunch of things... First the tire construction, second the compound and third the alignment of the car and it's max G capability. It's going to be a function of the tires and what they like. The more negative camber you use, the lower pressure you can get away with.

Since these are new tires and a new car you are pretty much on your own. If you have a tire pyrometer you can take tire temps and use those to dial in your pressures for the alignment you have.

Folks haven't offered advice because they most likely don't know what to try.

Tire pressure is a big driver for gas mileage. GM want's the highest CAFE mileage they can get and most likely the tire pressure for the car could well be set for higher than optimum for handling.

I'd use the pressures recommended for performance as a starting point and depending on your alignment go from there. And yes, pressures down in the mid twenties are common for high performance tires.

I won't go into how to read tires, but if you see that you are using up the edges a lot then put in more air, if they aren't rolling over and the car is skittering a lot and not biting the pavement, go lower.

Magister Ludi

Thank you. That was very helpful.
Obviously I have a lot to learn. This is a good start.
Thanks again.

vette6500

Cool, are you coming to the SCCA autox in Fontana? Will be fun to see a C7 on the autox course!

I've settled on 30psi hot all around on my C6 Z51 after I got alignment to make -2 degrees camber F and -1.3~-1.4 camber R.

Solofast

Also remember that the starting cold pressure for a lapping session on track is going to be different than the final hot pressure after the tire gets hot and can be different than the starting pressure for autocross.

In an autocross you get a chance to adjust pressures after each run and most folks bleed them down after each run to keep the in the sweet spot.

On a track you often set cold pressures and don't adjust them until you come back in and see what you have.

In the end the hot pressures could be close to each other, but with an autocross car you start closer to the final pressure and bleed off after every run as the tires heat up.

For that reason the cold pressures that would be recommended for starting a track session may be a bit low for autocross.

Unless you have aligned the car and put in some negative camber you can also find yourself eating the outside edges of the tires in an autocross if you run too low.

Watch the outside rib and if you are beating that to death and seeing a lot of wear then up the pressures some. Or, if the car feels "rubbery" steering and the tires aren't gripping you can try more pressure.

Most folks on here haven't raced on "run flats" either. That's just not something that we have in our experience base. Run flats have really stiff sidewalls and it's hard to think about what pressures you will end up with on them. It does make sense that, since these are really stiff tires that the pressures would be lower than normal race tires.

DigitalWidgets

Another tip (courtesy of vette6500): Get some chalk and draw a line on your tires from the edge of the rim to well onto the tread before your first run. After your run, you'll be able to easily see if you're rolling over the edge of your tires too far by where the chalk is worn away.

Of course, I think this is good advice...but I keep forgetting to buy my own piece of chalk and end up bumming off of vette6500 to do my tires!

Good luck on your first autocross! I'm a relative newbie myself, and I'm totally hooked. A heck of a lot of fun, and some of the greatest people I've ever met! And, you'll be happy to know that the cones don't scratch up your paint when you hit them (ask me how I know!)....they typically just leave a little scuff that you can just rub off with your finger or a microfiber towel.

CONE RACER

All good inputs above. Another "tool" you have is on the sidewall of your tire. Look for small markers (many times small triangles or check marks) that are located on the side near where the tread pattern wraps over. This mark indicates the maximum tread that you should be using. Start with a moderate air pressure (30 psi) and adjust your pressure after your first run so that you are scuffing the edge of the tread all the way to the marker. Lower the pressure if you aren't reaching all the way to the mark. Raise pressure if you go past the mark.

Starting an AX run on a cold run-flat at 26 psi just sounds a little too soft too me. The tire has a lot more mass to heat up than AX specific tires, so pressures will be slower to come up. As Solofast mentioned, you need to check pressures after every run as they will increase as the tire gets hot.

Enjoy the car and the AX experience. And, be sure to let us know how you did!

Magister Ludi

No, just a local event at the Camarillo airport. I'll have to look into the Fontana one though. (May want to get my skills up a liitle first) thanks for the alignment info. I'll have to check to see how the alignment recommendations in my manual compare for tracking this z51.

Magister Ludi

Solo, Digital & Cone,

Thanks guys. Great tips.

I'm just hoping that I don't run off course. Those cones look a little hard to figure out at speed.

Getting stoked!

DigitalWidgets

If it's your first event (or your 100th)...make sure you get there early and do the course walk (multiple times if you can). The SCCA group has a "novice coordinator" who does a group course walk. It very helpful to get someone to show you the ropes and will teach you some of the cone "symbols" and meaning. The course walk is the most important thing you can do to prepare for the day.

At every event I've been to, eventually you'll see someone who has no experience and didn't bother to do the course walk. They'll skip slaloms and cut sections of the course. It creates a dangerous situation for the folks working out on the course ******** cones.

Also, grab an instructor for your first run or two if you can. A full day of instruction probably isn't practical on your first outing, but a few runs with an instructor will help you tremendously. Most of the good guys are happy to help you out a bit.

And finally, don't be shy out there....everybody is there to have a good time, and you'll meet some great people!

Have fun out there and let us know how it goes!

avizandum

I've run a number of street tires and have found the sweet spot to generally be 34-36 psi hot depending on the tire. Start out no lower than 32 on your first run and pressure down accordingly.

vette6500

Do come to the SCCA autox at Fontana or El Toro, I want to see the C7 run close up.
You can find info on solo2.com
Plus you can learn from corvette hotshoes like mountainbiker (search for and watch his videos, it was and still is my continuing education in autox). Digitalwidgets and I walk the course behind him to figure out the lines so we don't get lost out there. Def good to run with the big boys to see what/how they do it.

And Digital, please don't buy more chaulk, my pack has like 6 or 8 colors and I am still on 1/2 piece of the yellow.
I will reserve the red color chaulk for your use, so it can match your car better.

Magister Ludi

I was there today (Saturday). Thanks for the tip. I had a blast!

I took the advice here and started at normal tire pressures and then adjusted down. Sweet spot seemed to be about 32 psi warm.

Anyway, it was a great event. I learned a lot. Seat time is definitely the best teacher. Everyone there was very friendly & very helpful. I definitely plan on making it to more of these. Thanks to each of you!

vette6500

SCCA event is there next weekend. Maybe you were able to run the Speed Ventures autox. I ran their events about half dozen times before trying SCCA. The SCCA setups are on the much larger parking lot.

Magister Ludi

That's right. It was put on by Speed Ventures. Low key and a great introduction. Got in about 20 runs. If I can, I'll go to the SCCA event. Thanks again.

froggy47

The C7 manual is likely giving you a suggestion on pressure for a track day 25 min or so session, where pressures come up and stay up xxx number of psi.

Autox is different, you set the pressure that you want (based on experience) and bleed off psi gain every lap (unless it's bmw or pca autox which usually don't allow for a place to bleed off.

My friend had some new run flats on his grand sport which are the ones that come new on c6z these days (G2 - generation 2) f1 supercar and they were really pretty damn good, not an a6 maybe 2 secs off an a6 on a 60 sec course.

I forget what he ran for psi,

I would start at 32f 28r and READ the tires each lap (use chalk or pyrometer or both)

With my a6's I run 29f 24r give or take one psi for other variables.
I have never seen a vette use the same psi all around, always lower in the rear. For autox.

Magister Ludi

Very interesting, I'll give this psi pattern a try. It was pretty easy to read the tires yesterday, based on their new "melted" texture. Next time will bring chalk.

formula80_ca

Just did my first autox in my 2015 C7 Z51. It was a fairly warm day at just over 75 degrees and I started at 35 lbs front / 32 rear, then dropped 1 lb to 34 / 31. Tires were rolling over just about perfect and the car performed very well after I smoothed out my driving. I was attacking the corners too hard at first, more like I would on a race track with more exit room, but soon learned that took me too wide for the next cone. Started working on my precision, cleaning things up a section at a time. Finally, I strung together very solid run on my second to last one that launched me up to fastest time of the day in my group of 27 competitors.

Pumped, I wanted to get closer to my friend in his C7. He is a seasoned autoxer who had put sticky tires on and moved up to the super street prepared class. But I only improved by .02 so was still 8 tenths away. I had been playing around with different driving modes and I think it would have been better if I had gone without traction control for the last run like I had done the run before, it kicked it hard once and really slowed my acceleration onto a fast part of the course.

Anyway, it was a ton of fun!

If you are running a Z51 with the stock run-flats I'd try 34 / 31 to start and as others have said read the tires after each run, adjusting until the roll over is at the far outside edge of the tread.

AUTO_X_AL

Staggering the pressures in my c7 was the wrong thing to do. Myself and A pretty competent competitor settled on 28-29 hot for an auto x. Both cars were on stock ps2's. One car was aligned and corner weighted. Both cars lowered about 90mm. My car retained the stock alignment.

This equated to about 24 lbs cold on stock tires. Next time out I'll start with 26 cold in the re71r's with a competition alignment and new brakes.

The car seemed to rotate pretty well. We were both quite fast overall.