[kujo]

Being a total car junkie, 9 cars in the last 6 years would qualify right ? ;-)

I find myself always striving to make whatever car I have better.

I read alot of stuff on different forums I'm a member of and see people 'talking' about cures for Understeer.

No one ever talks about Tire PSI.
I use that to great effect on my AWD Audi's.
Also, in my last two BMWs to get the right amount of Oversteer.

Also, in my Karts, PSI is crucial to good lap times.

A little chart for you all.
Some may know ALL this already, but I find most don't.

PARAMETER: TO INCREASE UNDERSTEER :TO INCREASE OVERSTEER

Front Tire Pressure Lower Higher
Rear Tire Pressure Higher Lower
Front Tire Width Smaller Larger
Rear Tire Width Larger Smaller
Front Camber More Positive More Negative
Rear Camber More Negative More Positive
Front Springs Stiffer Softer
Rear Springs Softer Stiffer
Front Sway Bar Stiffer Softer
Rear Sway Bar Softer Stiffer

Keep the shiny side up.

[CFar]

i too am a car junkie like you. i have wondered why decreasing psi in the rear would inhibit oversteer because if you think about how drag racers reduce rear psi for more traction, its counter intuitive. oh well

[inpuressa]

I think what he meant is how the air pressure affects the side bite. Less air, the sidewalls will be softer, resulting is a loss of side traction. For forward bite it's better since you have more contact patch, like a tank tread. Me think...

[kujo]

yes, less PSI in the rear is good, IF you are only going in a straight line.
If you try to corner like that, you'll spin completely.

Find a parking lot you can do 'donuts' safely.
Bring a portable pump and GOOD tire gauge.
Start at 10% of your total car weight as each tires PSI.
So, ex 3500 lbs, gets 35 psi per corner.
Start a circle, like on a skid pad and see where the rear end finally wants to give out.
Then, put 2psi more in the front and 2psi less in the rear and repeat.
You'll lose the arse end sooner this time.

Underinflated tires make a car squirrel.
Slightly underinflated tires just lose grip sooner.

I know it's not a scientific reason, but it's what happens.

kj

[STI_SLC]

ha... you guys are whacky. unless you're talking about HUGE adjustments in psi... like to the extreme ends of the scale..
but other than, as far as psi your comparison up there seems kind of backwards.
springs are correct.

[kujo]

Originally Posted by STI_SLC

ha... you guys are whacky. unless you're talking about HUGE adjustments in psi... like to the extreme ends of the scale..
but other than, as far as psi your comparison up there seems kind of backwards.
springs are correct.

I can drop 2 seconds a lap at Streets of Willow, with a 3900 lb Audi S4, with 3-4PSI of adjustment.

Whacky ? NO.

If you have a deft enough touch with the throttle and your suspension is set up right, you can tell.

HELL, Formula 1 cars make .2, .3,.5 PSI adjustments to lose a second a lap.

[PBR600RR]

I agree, I can tell the difference with 2 psi of change. Tire pressure is the only thing i mess with at the track.

Originally Posted by kujo

I can drop 2 seconds a lap at Streets of Willow, with a 3900 lb Audi S4, with 3-4PSI of adjustment.

Whacky ? NO.

If you have a deft enough touch with the throttle and your suspension is set up right, you can tell.

HELL, Formula 1 cars make .2, .3,.5 PSI adjustments to lose a second a lap.

[STI_SLC]

trust me... im not arguing the importance AT ALL...
just those scales and figures up there are all messed up..

Originally Posted by kujo

I can drop 2 seconds a lap at Streets of Willow, with a 3900 lb Audi S4, with 3-4PSI of adjustment.

Whacky ? NO.

If you have a deft enough touch with the throttle and your suspension is set up right, you can tell.

HELL, Formula 1 cars make .2, .3,.5 PSI adjustments to lose a second a lap.

[kujo]

Originally Posted by STI_SLC

trust me... im not arguing the importance AT ALL...
just those scales and figures up there are all messed up..

IT may be hard to read, due to the fact that the forum post won't allow me to tab out the columns.

Bottom line, it's all true, on the track and on the street.

more Neg camber in the front, less Understeer
fatter rear sway, less understeer.....

it's all there, in the 'list', if you don't want to call it a chart....

[kujo]

Originally Posted by STI_SLC

trust me... im not arguing the importance AT ALL...
just those scales and figures up there are all messed up..

you agree ?
But you said you needed to make HUGE adjustments in PSI.....

So not true......

1psi in my Kart, makes HUGE differences.
3-4 in a 3300lb STI, would also make a big difference, but only to someone that has some finesse.

Anyone that's a mash and go, drop the clutch at 5K kinda driver, isn't going to notice that.

Eyes up, hit your apex and track out !!!

[cleophis t. bufflehead]

So, any suggestions for optimal front/rear pressures on an '08?

The recommendations are biased even further to understeer with 32f 33r.

[speedball3]

Originally Posted by CFar

i too am a car junkie like you. i have wondered why decreasing psi in the rear would inhibit oversteer because if you think about how drag racers reduce rear psi for more traction, its counter intuitive. oh well

Drag racers reduce rear psi...for more REAR traction, right?

More rear traction = harder to break the rear loose and hence, less oversteer, more understeer.

Of course the under/oversteer difference is not hugely noticeable on the street (I hope you don't drive like that on the street). But if you're running on a track/autox course and want to fine tune your car then yeah, playing with tire psi can make a difference.

[BLACK 05 RS]

^^for straight line you get more grip with underinflated tires. In a corner you will roll onto the sidewall easier and loose traction.

[kujo]

Originally Posted by cleophis t. bufflehead

So, any suggestions for optimal front/rear pressures on an '08?

The recommendations are biased even further to understeer with 32f 33r.

Car weighs in at 3300 lbs right ?
I weigh 200.
So, for me, I'd START with 36F/34R
See how she handles and adjust from there.

I'll also probably run the DCCD with the most power to the rear as possible, all the time, to make the car feel even more rear biased.

I loved my AWD Audis, once I did the suspension work to make then not 'push'.
I loved the RWD of my BMs, so I'd love something kinda in the middle, if possible.

Also depends on tires you run, sidewall construction makes a difference too.

10% of total weight is good starting points.

kj

[cleophis t. bufflehead]

Originally Posted by kujo

Car weighs in at 3300 lbs right ?
I weigh 200.
So, for me, I'd START with 36F/34R
See how she handles and adjust from there.

I'll also probably run the DCCD with the most power to the rear as possible, all the time, to make the car feel even more rear biased.

I loved my AWD Audis, once I did the suspension work to make then not 'push'.
I loved the RWD of my BMs, so I'd love something kinda in the middle, if possible.

Also depends on tires you run, sidewall construction makes a difference too.

10% of total weight is good starting points.

kj

Cool, that's about what I was thinking.