Tire pressures

[smarkamyth]

I found a neat link: http://son.pca.org/Technical_Articles/autocross_tire_pressure_calc.htm

I'm wondering, though, why there wasn't any attention paid to things like ambient temperature and/or surface type (asphalt, concrete, or some coeffiecient of friction), etc.

It came in pretty close to what I use, but has inspired me to try it's suggestion. The changes seem like they'd make sense towards the qualities I'd like to adjust.

On that note . . . I know that when you change tire pressure in EITHER direction from the "ideal" you get a similar effect, but I keep hearing from various people (Thank You, Mr. Wenzel) and finding these charts that suggest the opposite of what others say. For example, most people seem to want to RAISE rear tire pressures to induce more oversteer, where Koni and these folks: http://www.roversd1.nl/sd1web/suspension.html (in the chart at the very bottom) suggest that LOWERING the rear tire pressures will induce oversteer. What gives, man? Opinions? Experiences?

cheers,
Smark A Myth
FrankenSlead

[thirdgenrx7r1]

That's pretty crazy Smark, it says I should be running 43 lbs. front and rear!!?? Thats pretty high, I run right around 30. But it does get you thinnkin'!

Mark Mauro JR

[smarkamyth]

yeah. it says I should run ~70F and ~50R. I'm running something like 56F - 48R.

I think I'm going to try it . . . maybe drop ~5 psi on the 710s.

I bet the car will really rotate like that!!

[Captain_Solo]

That's really cool. How do you guesstimate your lateral g's? I suppose it doesn't matter, since front and rear pressures adjust proportionally when I mess with it. I think I'm gonna try it's suggestions at the next event.

Looks like I'm gonna be screwing with all kinds of stuff next event.:rolleyes:

[Donnie]

it was actually pretty close on the prelude i ran 68 front 45 rear on A3so4

it said 75f/50r there is a reason i dont have that car anymore

Donnie

[00 SS]

Intersting. If I put in street tire lateral g's, the numbers look reasonable. But with race rubber lateral g's the numbers are WAY off. Even the pressure ratio front to rear is way off for race rubber.

[smarkamyth]

To guesstimate cornering force, I found a test drive article at Car&Driver that listed a skidpad number . . . . ymmv

http://www.caranddriver.com/article.asp?section_id=16&article_id=8413&page_number=2

Smark

[smarkamyth]

Mike -
I tried a bunch of different G values, ranging from their default .86 all the way up to 2 and the F/R ratio stayed the same for me . . . . it was always right around .7

Are you sure it changed on you?

Smark

[lundgren]

it was actually pretty close on the prelude i ran 68 front 45 rear on A3so4

it said 75f/50r there is a reason i dont have that car anymore

Donnie

People ran more in S04(and S05) to eliminate wear not to increase grip. See the new S06 (coming soon), won't require those pressures, cause they should have fixed the corner wear issue.

[00 SS]

Mike -
I tried a bunch of different G values, ranging from their default .86 all the way up to 2 and the F/R ratio stayed the same for me . . . . it was always right around .7

Are you sure it changed on you?

Smark

No, their ratio didn't change when I changed the lateral g's, it's just way off of what is actually best on my car with the V710's. I do use a spread closer to what it shows on street tires.

[Donnie]

People ran more in S04(and S05) to eliminate wear not to increase grip. See the new S06 (coming soon), won't require those pressures, cause they should have fixed the corner wear issue.

yea thats why i ran em so high no camber no LSD i had to do something

they said they were gonna fix that for the SO5's

does anyone have an article or anything on the SO6's

[kwenzel]

On that note . . . I know that when you change tire pressure in EITHER direction from the "ideal" you get a similar effect
Well, you get different effects - a lot of people will increase pressure on the end that isn't sliding, to a) increase total spring rate at that end and b) maybe deform the contact patch a bit and make it smaller. Decreasing pressure can also make that end slip a bit more from contact patch deformation of a different sort, and also from tire carcass deformation. I've noticed that different tires (and people!) react... differently to these, so it's not something that I think can be easily modeled for any given tire/car/suspension. It's pretty easy to change at an autocross or test&tune, though!

As a side note, "The Racing and High-performance Tire (http://www.amazon.com/exec/obidos/redirect?link_code=ur2&tag=rmsolodiscuss-20&camp=1789&creative=9325&path=http%3A%2F%2Fwww.amazon.com%2Fgp%2Fproduct%2F 0768012414%2Fsr%3D1-1%2Fqid%3D1138928931%2Fref%3Dpd_bbs_1%3F%255Fencod ing%3DUTF8)" by Paul Haney is a very good read if you want to get even more confused about why tires act the way they do (you'll be in good company).

[kirstein]

It seems like this totally leaves tire construction out of the equation. I know on the same car, I've used drastically different tire pressures for different tires. I guess it could give you a starting point but I'd bet it ignores sidewall stiffness entirely, which (I would assume) would give higher numbers than what you would want.

[PaulL]

For a data point, with race rubber on my Cobra, my Geez cube consistently records between 1.20 and 1.30 sustained lateral g's.

Let's see Car and Driver evaluate THAT!

[gordon0412]

You guys know a lot more about this stuff than I do, but with my curb weight + me, 275/40 tires, 57/43 F/R distribution, and 1.0 g the numbers come back as 53 front and 39 rear. I've been doing 48-50 and 32-34 with my Hoosiers. Seems like the higher rears might help with some of the push the bigger front sway bar brought back. Given some of my results it's certainly worth a shot. :)

[smarkamyth]

As a side note, "The Racing and High-performance Tire (http://www.amazon.com/exec/obidos/redirect?link_code=ur2&tag=rmsolodiscuss-20&camp=1789&creative=9325&path=http%3A%2F%2Fwww.amazon.com%2Fgp%2Fproduct%2F 0768012414%2Fsr%3D1-1%2Fqid%3D1138928931%2Fref%3Dpd_bbs_1%3F%255Fencod ing%3DUTF8)" by Paul Haney is a very good read if you want to get even more confused about why tires act the way they do (you'll be in good company).

Thanks for the recommendation!! WOW! $60?! I don't suppose you have a copy you'd loan for a bit?

alternately, Amazon is offering the this one in combination for $160:
http://www.amazon.com/gp/product/1560915269/ref=pd_bxgy_text_b/103-0213526-5307021?%5Fencoding=UTF8

Is it worth it? There had better be some Wenzel-beating advice/data in the combination for that price. :D


Smark

[MrPickles]

I agree with Kevin that the driver is a factor in tire pressures as well. For my driving style, I run the pressures REALLY low compared to most people. But from my conversations with Gary Thomason, the 710s performed best around 25-30 lbs (gary never gave exact numbers, only ranges..but I guess top pax time 2 years in a row..he'll keep his secrets to himself :) )It all depends on what you are looking for in tire performance. Which comes in time. I didn't know anything about pressures and what effects that had until my third year of autocrossing. Once you get ...you get it! :) I wont give away what I use, but I adjust my pressures based on several things. 1) Weather 2) course design 3) # of drivers in the car 4) pressure deviation between runs.

As Kevin stated, use test n tunes (or BMW events) to test your pressures and see what works best for you. Then start to tackle shock adjustments! (new thread for that one)

[kwenzel]

Thanks for the recommendation!! WOW! $60?! I don't suppose you have a copy you'd loan for a bit?

alternately, Amazon is offering the this one in combination for $160:
http://www.amazon.com/gp/product/1560915269/ref=pd_bxgy_text_b/103-0213526-5307021?%5Fencoding=UTF8

Is it worth it?

The Milliken book (Race Car Vehicle Dynamics) (http://www.amazon.com/exec/obidos/redirect?link_code=ur2&tag=rmsolodiscuss-20&camp=1789&creative=9325&path=http%3A%2F%2Fwww.amazon.com%2Fgp%2Fproduct%2F 1560915269%2Fref%3Dpd_bxgy_text_b%3F%255Fencoding% 3DUTF8) is worth it at just about any price, IMHO. It's much more technical than the Paul Haney book on tires, but there's an amazing amount of information in it.

[Built-By-Bones]

$160 is a good deal for those books.

The Milliken book is definitely not light reading. But I refer back to it often. I have found the sections on suspension and design helpful even when designing rockcrawling stuff.

Grant