Autocross tire pressures, Going to my first autocross on Saturday Options

[ken914]

I am going to run my 914/6 in my first autocross this saturday. Everyone in racing seems obsessed with tire pressures. I have no idea where I should set them or if they should be set hot or cold.

Can anyone please explain the pressure obsession to me so that I can be equally neurotic.

Thanks!

[jdogg]

What are you running for tires? My Kumho Victoracers seemed to like 28-30 (hot) in the front and 30-32 (hot) in the rear. Saturday will be my first outing on fresh Hoosiers, which I understand like a little more pressure than the Kumhos.

I am not sure what to tell you #wise for street tires, but I'd recommend adding some air over your normal settings, and then mark the sidewalls with chalk/shoe polish to monitor how much your tire is rolling over. If it is rolling over, add air, if not leave it be/subtract some air.

Don't get too caught up with changing air pressures and adjustments at your first AX, there will be plenty of other things there to confuse you.

Enjoy and Good luck!!

[xsboost90]

now i havent experienced much w/ the teener yet, but w/ my 944's and slicks more so than street tires, adjusting tire pressures can really help keep control of the car, which makes for better times. A tire w/ less pressure has more traction than one w/ more pressure, due to more tire making contact w/ the pavement. If the car is understeering, or pushing the front end into the corner, taking some pressure out of the front or adding to the rear can correct this. If the tail is whipping out alot-oversteer- then taking some from the rear or adding to the front can make a difference. THis is of course w/ a semi balanced car. I usually start out by making sure all the tires are equal amt pressures the first run. Say, 35psi-now this is just me here...make your first run, just try to stay smooth, but you can test you boundaries alittle in some tight corners and really stick it into the apex. See if she pushes or slides out. Adjust from there. You have to find your own happy place though. (IMG:http://www.914world.com/bbs2/html/emoticons/beerchug.gif)

[Joe Ricard]

Dude what kina tiars?
What size a Tiar
How wide the rim be?
What sorta suspension ya got
Street tires need more pressure just to stay in shape. To low PSI and the side wall rolls over

My Kumho V700 Ecsta likes 35 PSI starting out. If one end is loose I add a pound or 2 to that end
Watch the tread wear from inside to outside. If the tire is rolling you will see more wear o the very outside edge.
Get a bottle of white shoe polish put alittle at the edge of the tire face don't get it on the side wall (it looks stupid) and don't help.

Oh most of all have a blast this AX stuff is addicting. 9 event this year going for 10th on Sunday.

[anthony]

Ken, I'm going to assume you are talking about street tires. One pushes the car a lot harder on an AX course than on the street. As previously mentioned, you want them hard enough that they aren't rolling over on the sidewalls during hard cornering. You want the contact patch as flat as possible.

FWIW, I have 205-50-15s street tires on my 914 and 33/36 seemed to work well at the last AX I did.

[jdogg]

I see you are in Mount Pleasant, SC. I am geographically challenged in SC, where is that? Where are you going to run the AX?

[Dave-O]

I'll ask the same question people asked me a year ago when I had this same question..."what brand/size of tires are you running?"

I started with old, worn out, hard 205/60's and had to put almost 60lbs in them to keep them from "rolling over". Now with 205/50 Falkens I run around 40lbs.

My suggestion for street tires (based on some of the comments on the club lately...it is just a suggestion (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif) ) is to add 10 or 15 lbs to your tires, then mark the edges with chalk or shoe polish (although shoe polish is a b*tch to get off) and check the sides for scrubbing. Add pressure until there is no significant scrubbing on the side.

Or probabally the best idea is to just put 15 extra lbs. in. Go to the autocross and have a great time and get plenty of driving practice.

Hopefully a few of the more experienced autocrossers will chime in (IMG:http://www.914world.com/bbs2/html/emoticons/pray.gif)

Have fun!!

David

[ken914]

Thanks for all the input.

The car is a /6 with a 3.2, GT flares and chassis stiffened and dog ears reinforced.

Tires are old, hard and way too big for this car.

Pirrelli P7's

Front 225/45R15

Rear 285/40R15


I am due for new tires. Probably going to Yokos or Michelins. Will be 205/50/15 and 225/50/15.

[URY914]

Just for a point of referance,

I run 16 pounds in mine. (IMG:http://www.914world.com/bbs2/html/emoticons/ohmy.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/lol2.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/screwy.gif)

The guys at tech always freak out.

Paul

[messix]

nothing beats tuning tire pressures with a infared thermometer. checking across the inside,middle and out side. but thats for the pros or the anal.

dont go wild with high pressures, it doesnt make for more traction, it just make the tire hard and wear out fast.

start at street pressures and try to balance from there going down in psi first might supprise you.

[messix]

p.s - use lap times as a refence not the way it feels. lower pressures will feel slow on turn in, but will have better traction.

this comes for 20 years crewing for stock cars, sprint cars and [sssshhhhh] street racing.

[SirAndy]

QUOTE (Dave-O @ May 19 2005, 01:02 PM)

and had to put almost 60lbs in them to keep them from "rolling over"

(IMG:http://www.914world.com/bbs2/html/emoticons/huh.gif)

just to show you how much tires can differ, my street tires that i ran for all of the 2003 season (Nangkang Tires, 225 x 50 x 15 on 7" rims) ran by far the best with 22lbs cold !!!

anything more than that and they would start losing grip under hard cornering once they got warmed up!
they had their best grip when cold! which meant my first run was usually the best (tire wise) which sucks because it doesn't give you time to learn the course ...

anyways, point is, the right tire pressure depends very much on brand/size/rimsize/driving style etc. ...

and i agree, your first time out, you'll have a ton of other stuff to worry about!
(IMG:http://www.914world.com/bbs2/html/emoticons/driving.gif) Andy

[xsboost90]

you said your running 285 40 15's, any idea where i can get a set of those. Im building a 924 carrera gt for a guy and i need that size for the wheels he is running.

[SirAndy]

QUOTE (xsboost90 @ May 19 2005, 05:22 PM)

you said your running 285 40 15's, any idea where i can get a set of those. Im building a 924 carrera gt for a guy and i need that size for the wheels he is running.

you're sooo out of luck. no one makes them anymore. his pirelli's must be at least 10 years old, probably older than that ...

(IMG:http://www.914world.com/bbs2/html/emoticons/sad.gif) Andy

PS: and if you *do* manage to find a set, let me know as i sure would like to run 285s on my 9" as well ...

[SirAndy]

QUOTE (ken914 @ May 19 2005, 01:42 PM)

Tires are old, hard and way too big for this car.

there is no such thing as too much tire! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

btw. those sizes (245/45 and 285/40) were factory options for the 914-6 GT race cars in the early '70s ...

assuming you have 8" wheels in the front and 9" wheels in the rear, those sizes are "correct" ...

unfortunately, pirelli stopped making those tires sizes 15 years ago ...
(IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif) Andy

[J P Stein]

There is an ideal pressure for max grip from every tire/car combo. Deviating pressure up or down from that ideal will lessen grip.

Finding that ideal is a bitch. You can't go by lap times as lap times will drop for each run (baring a screwup)....particularly those by novice. It is done by a combination of tire temps, pressure rises, wear pattern, and feel.....or ask somebody that's running the same combo...if he's fast (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

For tires the size of those concerned, I would go with your normal street pressures plus maybe a couple more pounds and forget about it. You're right, they prolly are too big to ever get any heat in em'....even if that would do em' any good. Do take a pressure gauge along and, after a couple runs, check for pressure rise and return to the original settings if required.

You'll find with a 3.2L 914 that big stickey tires are required for AX.....prolly something larger than 225s/205. There are
3 good Dot R, AX tires. Hoosier A3s, Kuhmo Victor racers(for the budget minded) and Kuhmo 710s
all the rest are pretenders....just my opinion.

[nebreitling]

my 02:

for your first AX, set the pressures where you normally do and forget 'em.


don't start fooling with sway bars and tire pressures and such unless the car is really misbehaving. focus on your driving, that's where your times will improve.

[sean_v8_914]

Falken Azenis 205 50 15. I start at 25 psi cold. at the end of my runs I check before they cool. if the tire scrubs out too much, it will get hotter and increase pressure. I end up with 28 to 30 psi hot, then tune from there based on teh track