1st Autocross...need tire pressure tips

[TrackAire]

Good Day,

Planning on doing my first autocross with the car this weekend. Stock 2009 coupe with the stock Michelin tires. Any advice for a starting pressure front and rear? It's an ashpalt surface. Should be in the 90 degree temp range.

Does the Michelin sidewall "squish" outward under load creating more negative camber or do they roll under?

Any input from members that have autocrossed on these tires would be appreciated.

Last question...those with 08/09 cars, when autocrossing, did you ever get out of 1st gear with the stock rear gearing?

Thanks,
George

 

[Marv S]

Hello George,

gearing: it depends on the layout. I have seen setups where you can run the entire run in 1st and never bounce off the rev limiter and some where you have to shift to 2nd.

tire pressure advice depands on: is this YOUR first auto-x?

 

[Tom F&L GoR]

Take a wax pencil or shoe polish with you. Make a mark three places on the tire shoulder around the tire. After each run look at the mark and see how far down the sidewall the mark was worn off. This will tell you if the tire is rolling under (too low pressure) or not.

There are tiny 1/4" Michelin men on every third or fourth tread block. They should get rubbed off, but no lower onto the sidewall.

A Viper has more than enough torque to rip the treads off at almost any speed in a parking lot. You might try shifting to second ASAP and running the rest of the course that way. It will avoid spinouts, burnouts, and the other drama and let you concentrate on driving.

Good luck.

 

[TrackAire]

Thanks,

I used to autocross recreationally 15 years ago. Sold the toys to start a business and raise a family. Soooo yes I've done my share of autocrosses, but I'm sure I'll be very rusty.

I just don't know how these Michelins behave under cornering loads. I'll mark the tires as suggested so I can get a look at whats going on. I plan on taking out all my "Michelin Men"...

Back when I was running the BFG R1 autocross tires, their sidewall would actually squish outwards and not roll under like traditional tires. This was a great way to get artificial negative camber. But those were 40 and 45 series tires...good amount of sidewall do work with.

This weekend's autocross is the American Autocross Series...its course are a little faster and designed for the big Corvettes, Camaros, etc. It will be fun to finally feel the break away of the car in a safe environment.

Cheers,
George

 

[j-rho]

Hi George,
I would recommend mid 30's front pressure and low 30's rear. The tires should behave fine. If you're going to be doing a lot of autocross or track stuff, I'd recommend an alignment with some negative camber, especially up front. The car will work a lot better around the corners and the tires will last longer.

On street tires I would recommend using 2nd gear most of the time because it'll be tough to put the power down in 1st. AAS events are generally pretty fast - 1st is good for 60mph but you will probably need 2nd a lot of the time anyway.

 

[Tom F&L GoR]

I used the BFG R1 on my replica Cobra. The inner sidewall was stiff and the outer sidewall was soft. Maybe that's why I liked the old, old XGTZ, which seemed soft on both sides and prone to break away.

 

[TrackAire]

Hi j-rho,

How is your autocross season going...have you got the diff issues worked out?

Thanks for the starting pressures, we'll give it a go.

Back in the day, those old BFG R1's were cutting edge.

Tom, are you saying the old XGTZ's broke away easy or communicated well that they were going to let go?

Cheers,
George

 

[Viperless]

Good Day,

Planning on doing my first autocross with the car this weekend. Stock 2009 coupe with the stock Michelin tires. Any advice for a starting pressure front and rear? It's an ashpalt surface. Should be in the 90 degree temp range.

Does the Michelin sidewall "squish" outward under load creating more negative camber or do they roll under?

Any input from members that have autocrossed on these tires would be appreciated.

Last question...those with 08/09 cars, when autocrossing, did you ever get out of 1st gear with the stock rear gearing?

Thanks,
George

I've autocrossed my '09 twice. Both were pretty short courses. Never got out of 1st gear. The latest was this past weekend at the Car Craft Summer Nationals. I was in the Real Street Eliminator competition. I finished second behind a pro-built Vette race car running race slicks. I ran my best time with the fronts at 35psi and the rears at 34. I also took the blue/white stripe ACR out for a few laps but the rear was very tail happy with the same air pressures. I tried letting air out of the rears but it didn't make any difference. Ran out of time and nearly gas so I didn't get to play with it anymore.

The Michelins do not squish outward. Like Tom said, if you're rolling over too far on the sidewall, increase air pressure.

More event pics:

Impermanence Photography: Auto Art » Car Craft 2009 » _emm4884.jpg

Impermanence Photography: Auto Art » Car Craft 2009 » _emm5381.jpg

 

[j-rho]

Hi George,
This autocross season is going well, things are really starting to come together for the Viper. I do believe I finally have the diff situation worked out to the point things are workable. Planning my trip to Nebraska for the national champsionships, this will be the first time I've gone since 2006. Darn life thing kept getting in the way.

I ran my car several times on street tires (crappy Gen3 OE runflats) and it wasn't totally awful. You'll just be limited in mid-corner and corner exit traction, and have to drive accordingly. It'll often be easier to drive in 2nd gear even if you have the revs in first, as it dampens inputs and lessens engine braking.

Have fun man, AAS events are a blast!

 

[TrackAire]

Viperless, although an ACR is to die for, I've got to admit that graphite colored coupe looks awesome. I think it is my favorite color on an SRT coupe. Thanks for the input on the psi's.

j-rho, good luck at Nationals...as you know, a great diff makes all the difference in the world on a 40 second run. When I was campaigning the Camaro, I found a guy here in town that did national level competitors diffs for the Camaro. He did mine and we've become good friends ever since. And it shaved at least .5 second off my time. But then again, the Gov Lok those cars use to come with couldn't have been worse when stock.

Cheers,
George

 

[Tom F&L GoR]

Hi j-rho,

How is your autocross season going...have you got the diff issues worked out?

Thanks for the starting pressures, we'll give it a go.

Back in the day, those old BFG R1's were cutting edge.

Tom, are you saying the old XGTZ's broke away easy or communicated well that they were going to let go?

Cheers,
George

Yes, they were good to me and good for me. Kept them on year round.

The old XGTZs had floppy sidewalls that (to me, with my tire pressures, whatever they used to be) seemed throwable - get the car rotating and then it would take a set as if the sidewalls had been pulled over enough. Once in that state they communicated OK but not easily catchable either. If it were a steering rack, I would say not good on-center feel, but much better when turning. That was actually good for me in AX because I could be quite aggressive entering/rotating and if I could keep the tires loaded I was good.

It is laughably apparent when you have an XGTZ off the rim compared to an MXX3 or newer off the rim.

 

[escapedan]

George - I'll be out there this weekend with a 2006 Blue/White Coupe. I'll look for you. Dan

 

[TrackAire]

Hi Dan,

I'm bummed that I can't do the American AutoX series this weekend. We've got a work project that's going through the weekend and I have to be here.

The weather looks like it will be nice down in Santa Clara. Have a great time.

Cheers,
George