Our first auto-x event of the year (test and tune) was this last weekend, and though I did very well for the first time out (better time each run) in the Evo. I had problems trying to correct the understeer I was getting on the tighter corners of the course. I tried messing with tire pressures to try to at least strike a bit more of a balance but did not really correct things to my liking.



I know to free up the back end and getting it to rotate properly will most likely take a RSTB and sway bar, but I am trying to run a couple of events in AS rather than pushing to ESP (Though ESP has a PAX Advantage of .824 to .828 for AS).

Any pointers on tire pressures to help with balance to try to induce a more neutral or even power modulatable oversteer would be appreciated. And before you ask, no I am not trying to drift the crap out of it, I am just havin to pinch the nose in a lot harder than I want to and scrubbing off time in doing so. BTW, I am running stock Yokohama tires.



In case you were curious. FSTD, second quick overall, and fastest PAX'd time. Though two of the top three finishers overall from last year were not in attendance, and the number one local guy was having engine misfire problems in his SVO.

Thanks in advance for your help. I intended to drive this car as it was designed to be, AROUND A CORNER....

and a last word of warning... Fear the Psychotic Smiley

 

Well what were the tire pressures that you were running? I found on my car with the stock tires that about a 38F/34R PSI seems to help reduce understeer and promote tire life. Has the car been properly aligned?

 

With the recommended alignment from an ealier thread, I was running 41-43 all the way around on the stock tires. With the rear sway, I am running around 42 /41 f/r. I haven't done anything else to the handling, yet

 

It was aligned to stock spec on friday (though before the next event I think I am going to have them put in more negative front camber).

The tire pressures that yielded the best time (HOT pressures) were 40 front/ 38 rear. I tried less than 40 front but started to notice sidewall roll on the chalking i had done, so I bumped back up to 40. I had the rears down to 38 on the last run, but did not have more time to try dropping to 36 or possibly 35.

I guess I was looking for confimration from other who have tested that lowering the rear presures will help, so I do not waste time changing the wrong part of the equation in trying to correct this issue.

The alignement sheet shows:
FRONT
L -1.15 Camber
R -1.10 Camber
Toe in of 1/16 inch (.0625")

REAR
L -1.05 Camber
R -1.00 Camber
Toe in at 1/8 inch (.125)

 

Here's what I had good success with last year with the stock tires...

42 psi front / 40 psi rear

Maximum achievable front camber (-1.6 to -1.8 degrees).
About -1.5 degrees rear camber.
Zero toe front.
Rear toe in of 1mm.

 

did you run that alignment all the time on the street, or just get it dialed into that for events, and then switched back to stock for regular street driving.

I just do not want to get too agressive with alignment and have it shread the tires on normal street driving.

 

What are hot tire pressures? you mean after you drive hard and the tires are warmed up? What is the cool pressure you are running when your putting about 40psi in hot?

 

I've run those same alignment settings since August of last year -- almost eight months now -- for both competition and street driving (even with snow tires). I have no problems with handling or tire wear -- well, actually I would like even more negative camber because my Hoosiers and my stock A046 tires both wear more heavily on the outside shoulder.

I'm not going to get aggressive with toe, because big toe settings will wear tires out really quick. But with camber, and my typical tire usage, I'm actually not being aggressive enough.

I do keep the tires inflated well above the Mitsu-recommended 32f/29r. I run the A046s on the street at about 40/40, and the snow tires I ran most of the winter at around 36/36.

 

I don't run in an Evo, so I won't bother talking about my tire pressure, but I do believe you would want to try running that as your cold pressure (40 front, 38 rear). As for the understeer in the sharp corners, if you like the way it corners otherwise, are you using throttle steer for the sharp ones? If not, that would be a technique to learn for a winning combination. Part of the trick is the car, the other part is the driver, and if there's one thing I've learned in autocross it's that 90% of the time it's the driver.

 

To help loosen the back on a bigger road course we always go to 2-4 lbs more in the rear depending on how loose we want the back end to be...with lower rear pressure your increasing traction at the back.

 

Apparently this is the case for some people/car/tire combos, but with the exception of the old BFG R1s, I have always used lower rear pressures to loosen up the rear. I believe this to be most common situation, but it is clearly not the only way. One way to look at it is that there is some pressure at which the rear tire will get maximum grip; going higher or lower than that will result in less traction at the rear. Going higher will result in a higher effective spring rate, which will also tend to loosen up the rear. Still, I tend to take the low road.

Having said that, I usually ran 5-8 psi lower in the rear relative to the front last year and that was both with the stock tire and R compound V700s...

JW

 

In accordance with jwtodd, I used to believe adding tire pressure always reduced traction, but after buying some decent tires, I realized what he said, that there is an optimum tire pressure. My Falkens got better grip all the way up to about 50 psi cold.

But generally, reducing pressure increases traction to a certain point, and at that point you are weakening the sidewall, thus losing traction again and also risking rolling your tires.

 

Here is the procedure we use for tire pressures as each tire brand and model requires a different setup:

- Find the front tire pressure using the chalk method within a given camber and toe. This pressure should yield maximum roll without going over the sidewall marker. Once you get to that pressure, there isn't more you can do with the front for better grip. (at the same time note JBrennen's comments about his tire pressures with the Advans)

- For the rears: if the vehicle is understeering, increase rear pressures until it does not; generally, this means increasing the rear pressures to same or above to front pressures. This can also be achieve by lowering the rear pressures to the point that the front/rear psi difference is greater than ~3-4 psi (depends on tires) but lower pressures will yield more tire wear and less effectiveness of alignment settings. I'd suggest going the other way and increasing rear pressures; however, overall front/rear pressures should be within 2-3 psi to one another. Of course one has to be able to properly judge wheather the understeer is due to driver, alignment, or tire pressures first

- If the car is oversteering, decrease the tire pressures on the rear. The bad kind of oversteer is the one that "snaps" the rear. The good kind is the one that allows you to brake late and swing the rear out progressively or again progressively slides the rear when throttle is applied. To get the right balance, reduce rear pressures but stay with 2-3 psi to the fronts. If still there is a problem, then you have to look else where for the solution.

The increase and decrease in pressures should be done by 1 psi until close to desired levels, then by 0.5 psi. Some people truly get amazed how much of a difference 1/2 psi makes on the Advans, but they do.

 

Kyperion sounds like we’re at the same point; we had the same observations with understeer after our rookie autox event. Car felt good except in the tightest of turns, mainly corners that turn back on themselves so to speak.

I run 32F/32R in the nearby mountains so I knew how that felt but wanted to bring out the stock suspensions tendencies so I ran 35F/35R in the first event, all five runs. After racing midgets it felt good to drive a car that would honestly show me its problems and it did. The understeer was distinct in the slowest turns but I could control the rear end with the throttle and bring it around when needed in the faster sections.

Next event we’ll run the stock suspension one more time but use tire pressures of 39F/36R. I expect to still have a slight push in the tightest turns, more interested in learning how the car responds to change first, need to feel it for myself. It’ll more than likely be better but not 100% until I make alignment changes, those I’ll save for the third event.

Actually I think the majority of drivers in autox are headed the same general direction in regard to tire pressure, camber and toe in adjustments, you can actually start off in a good ballpark with what others have already experienced in these areas.

We also had an absolute blast, can‘t wait for the next event!

Rick

 

Alrighty,
Car is in as I write this for an alignment
F -1.75 degrees camber and 1/32" toe in
R -1.25 degrees camber and 1/16" toe in

Our first local points event is Sunday, I will see how the alignment combined with the Rear strut tower bar do to help balance in the car. Thank you to everyone for theri input, and I will report back monday on how things went. Depending on the results, the rear sway bar may be the next change to happen.