chetcpo Posted June 10, 2008 Posted June 10, 2008 Hi guys. I'm not new to autocross, but I am new to autocrossing a 7. (locost) I am nearing completion of my scratch built 1.8 Miata based Locost. I've done about everything but paint and hang the bodywork. I thought I would give it a go at a local autocross this coming weekend. In the Locost spirit, I refuse to spend a fortune on race wheels and tires. (at this point anyway) I bought a set of VW GTI rims for $50 and some discontinued Avon Tech Ra Tires from Tire Rack for $80 each. I took them out for some testing on the local backroads. (hooliganism) and discovered that the contact patch was very narrow on the tires. I assumed this meant too much tire pressure so I let it down from 35F&R to 25F and 30R and it didn't change much of anything. The sidewalls on these tires are super stiff and about 1/2 thick and I wonder how low I can go with the pressures. I'm estimating the car to weigh around 1200lbs FWIW. Any advice? I included a pic so you can see what I'm talking about with the contact patch. http://www.locostusa.com/forums/files/kineticglass_157.jpg
WestTexasS2K Posted June 10, 2008 Posted June 10, 2008 I run my Hoosier R6 tires at 18-19 front 20-21 rear really stiff sidewall you might be able to go down some from that.
11Budlite Posted June 10, 2008 Posted June 10, 2008 I still haven't done any track events with my Birkin, but I think most people run 16-20 psi so I think you can definitely drop it further. I run 20 psi in mine and will be experimenting with lower pressures soon. The super stiff sidewalls don't help either. I'm sure some of the more experienced autox guys can give you some better info. What size wheels and tires are you running? You don't want too wide a tire on a narrow wheel. Bruce :7drive:
yellowss7 Posted June 10, 2008 Posted June 10, 2008 I run 13 inch Avon CR500's and run 18 psi cold all around on the street. Autoxing I run Hoosier slicks that I run at 12-13 psi cold. I'd definately drop them down to 20 or so to start. Tom
chetcpo Posted June 10, 2008 Author Posted June 10, 2008 I still haven't done any track events with my Birkin, but I think most people run 16-20 psi so I think you can definitely drop it further. I run 20 psi in mine and will be experimenting with lower pressures soon. The super stiff sidewalls don't help either. I'm sure some of the more experienced autox guys can give you some better info. What size wheels and tires are you running? You don't want too wide a tire on a narrow wheel. Bruce :7drive: Thanks for the replies. Sorry I don't know the rim width, but the tires are 205s. And I thought stiff sidewalls were a good thing for autocross. What am I missing?
scannon Posted June 10, 2008 Posted June 10, 2008 And I thought stiff sidewalls were a good thing for autocross. What am I missing? About 2,000 lb of car weight.:hurray: :hurray: :hurray: :hurray: :hurray: :hurray: :hurray: :hurray: :hurray: :hurray: :hurray: :hurray: :jonautox:
sporqster Posted June 10, 2008 Posted June 10, 2008 Speaking of stiff sidewalls - one of my Hoosier A3S06's was completely flat (no measurable psi at all) and I didn't even notice driving it to the event! I usually run 18psi.
slngsht Posted June 10, 2008 Posted June 10, 2008 hey Chet, good to see you on the dark side :yesnod: , and even better to see your build is on the road :thumbs: As others said, drop the pressure even more and give it a shot. Good luck at the autox.
WestTexasS2K Posted June 10, 2008 Posted June 10, 2008 depending on the car weight you might be able to go as low as 15 lbs. The only true way to know is to check the tire temp with a temp probe as soon as you exit the course. Three readings from the center and each edge will let you know the pressure to run. It will be a little harder on an Auto X than road coarse because the temp will not come up as much. You can also look at the scrub line on the tire. Most radial tires have a ridge or line around the tire sidewall near the tire contact patch. The goal is to get the tire to scrub close to that line. You can keep dropping pressure until you get close to the line about 1/4 inch from it you are in the ball park. There will be factors like the amount of camber which will have an affect on this as well. It is more like a rule of tumb than science.
haiku88 Posted June 10, 2008 Posted June 10, 2008 FWIW high tire pressure can be downright dangerous in a 7....when I bought mine it had about 30 lbs in the tires and the extreme bouncing this caused scared the crap out of me on some rough stretches of I5 on the drive home, dropped it to 19 lbs after some research and all is well
Davemk1 Posted June 10, 2008 Posted June 10, 2008 I autocross my Birkin a lot and it weighs about the same as your car. I'm running Hoosier slicks and I run 12.5 lbs. in front and 13.5 lbs. in the rear and it works very well. Dave
chetcpo Posted June 10, 2008 Author Posted June 10, 2008 Thanks guys. Tonight just for goofs I let all of the air out of my front tire and to look at it you couldn't even tell. All I was worried about was the tire coming off the bead when exposed to a high lateral load. I feel better hearing you guys say you run these low pressures with no problems.
chetcpo Posted June 16, 2008 Author Posted June 16, 2008 Well the car did OK, but the tires were less than impressive. (Avon Tech RA) I don't know if it was because they hadn't been heat cycled or properly scuffed or what, but the grip you expect from R compounds just wasn't there. It was hotter than blazes and I had to let air out throughout the day and ended up settling on 13 PSI rear and 11 front. That's what seemed to work best. Here's a couple vids. One of me (in-car) and one of my codriver. My brother in law driving it:
slngsht Posted June 16, 2008 Posted June 16, 2008 you made it back from first outing under your own power... that's a good day
Boxologist Posted June 16, 2008 Posted June 16, 2008 tyres could just have been old. find out how AVON stamps the build date on the tyre and check teh tyres out.
yellowss7 Posted June 16, 2008 Posted June 16, 2008 Nice runs, car sounds good. I see what you mean about the tires not griping. A new set of slicks will fix that. Paint it yellow, and it will be faster too. Tom
Davemk1 Posted June 17, 2008 Posted June 17, 2008 I have a few friends that have tried those tires on non-sevens for autocross and they were even less impressed than you were. Switching to Hoosiers will change your life and make you more of a man............... or at least get you around the cones quicker. dave
chetcpo Posted June 17, 2008 Author Posted June 17, 2008 I have a few friends that have tried those tires on non-sevens for autocross and they were even less impressed than you were. Switching to Hoosiers will change your life and make you more of a man............... or at least get you around the cones quicker. dave More of a man? How can I say no to that? I have actually already listed the tires and wheels for sale. I designed my car to work best with 20" tall racing slicks anyway, so that's what I'll get. Has anyone here tried any of the tires I have cirlced in the screen captures below? If not I'd like to hear any other suggestions. http://www.locostusa.com/forums/files/hoosiertires_168.jpg http://www.locostusa.com/forums/files/tires_135.jpg
Jim F. Posted June 17, 2008 Posted June 17, 2008 Jim, moved your post here to keep the topics seperate. Thanks.
yellowss7 Posted June 17, 2008 Posted June 17, 2008 By Oversteering you mean that the rear is rotating,correct? If that's the case then you may want to check if you have a rear antiroll bar, what setting it it on, and if there is room soften it a bit, by moving the setting back one position. What toe setting are you running? You may want to dial in a little toe in to reduce tramlining. No experience on the particular tires. Tom
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