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-   -   Alignment and Tire Pressure for Auto-X (http://www.focusfanatics.com/forum/showthread.php?t=128398)

LazarusZX3 07-16-2007 09:00 PM

Alignment and Tire Pressure for Auto-X
 
So I have the Eibach Sportline drop (2'' in front 2.25'' in back), Pro Dampers, Anti Roll bars, front camber adjustment plates, rear camber adjustment bolts and polyurethane bushings.


I just had the whole thing re-aligned to as close to stock as possible. I just read an article about a Mini that got better times running -3 degrees of camber. That seems insane for a street car! I want to be awesome on the Auto cross course, get the most use out of my camber adjustment equipment, yet still be ok on the street. I have fairly expendable tires for daily driving, and Faulken Azenis for racing.


Also, what tire pressures does everyone run in Auto-X? I ran 42 PSI on all four, but I'm thinking about running 50 on the rears and 38 on the front for next time. Kinda random numbers, any advice?

mutant 07-16-2007 10:39 PM

if you keep the toe out to a minimum,you can run -2/-3 degrees camber on the street...after the camber i would run a little pressure f/r imo...ii run 225/45-15 hankoo z212`s and they get unhappy after 36psi...rear camber should be at -1/-1.2 imo...goood luck!

SVT Robzor 07-17-2007 09:10 AM

How is the car behaving currently? Is it pushing, is it too loose, or is it pointable?

I'm still fresh to autocross, but it seems the fastest FWD cars will be those that are loose in corner entry for initial rotation, but not so loose that you can't nail the tail down immediately after with the throttle. I'm not sure what the front and rear rates are for Sportlines, but conventional wisdom here is that the stiffest axle-pair will slide first. If you are running stiff front springs, weak rear springs and equal sway bars, your car will tend even more to push, which is typically slower.

If you're pushing (understeering) too much, you can add front negative camber, or try dropping your rear tire pressure a few PSI. It sounds contradictory, but you will gain grip by adding pressure, until the point that the contact patch starts to round out. Cornering work is done by the tire's shoulder, lower pressure lets the contact patch roll onto the sidewall, which is not made to grip.

Lately I've been running 44psi front/34psi rear in my Eagle F1 DS-G3s, which have such insanely squishy sidewalls that I NEED that much air to keep them from flobbing around like cookie dough. The Azenis have much, much stiffer sidewalls, you can probably drop your pressure a little, until you see the scrub marks using up the entire shoulder. I've always had trouble with the SVT being too pushy in transitions, so I typically run a lower PSI in the rear to mitigate some of the grip there.

I've read before that you shouldn't adjust your pressure more than 2PSI at a time, which makes sense to me. I probably wouldnt slide all the way to 34/50 at once, but I'd nudge it in that direction gradually and see how the car starts to act differently, or try going the other way. On some cars, like DS Integra Type-R's, I've read a lot of people will pump TONS of air, like 60PSI, into the rears because the chassis is so pushy in the first place, they need to shrug a ton of rear grip to get the car to change directions - with our stiff rear suspension though, you might not need to do the same.

LazarusZX3 07-17-2007 04:18 PM

As far as anti-roll bars go, I have the 22mm up front and the 25mm in the rear.

The spring rates, according to FJ, for the Sportlines are

Eibach Sportline: 2”
F: 131~262 lb/in
R: 102~171 lb/in

That's a huge gap. So I have no idea what the true spring rates are. "The Sportlines are progressive rate springs. Meaning that the rates vary depending on how much force is exerted under various driving scenarios." So, I'm not quite sure how to tune for camber and tire pressure. If only I could get camber and alignments for free instead of $100 each.

WeeAsp 07-17-2007 09:55 PM

Unfortunately, there is no real answer to your question.

Everyone has a different driving style. Some are naturally faster than others - regardless of car set-up.

So, I would recommend the following:

1. Inflate all 4 tires to 10 PSI over recommended.

2. Set the car to a neutral alignment setting. Namely as close to ZERO as possible. This will give you an idea of what adjustments need to be made.

3. Run the AX. Determine where the car is understeering/oversteering and where it felt the most comfortable. Also, look to where you experience wheelspin.

TAKE NOTES !!!

You are not going to win this AX, but it will give you a good idea of how to set the car up for the next event.

You mentioned camber. The thing to remember is that while camber helps in corners, it is DETRIMENTAL in straights because you do not have as much contact between the tire surface and the asphalt.

So, I guess what I am trying to say is establish your baseline and make SMALL adjustments. It might take you 1/2 a season, but you will be better prepared for the last half and the next year.

Hope this helps.

[thumb]

LazarusZX3 07-18-2007 01:14 PM

Thanks WeeAsp, I just got everything aligned and the camber as close to stock as possible. That cost $150 or so, so I'll just keep that setting for this season and take notes. I'll also have some other FWD guys ride with me and give me pointers.

It would be cool to have the equipment to change camber in the field, like GRM does.


Is there any good reason to have the rears' tire pressures higher than the fronts?

WD40 07-18-2007 03:15 PM

I'm usually at 40-45 Front, and 34-36 Rear depending on conditions and type of course.
^^^The above pressures are for Azenis RT-615.
I've never had more pressure in the rear than the front.
Always around 5-8psi more in the fronts.

stivo 07-18-2007 06:29 PM

Mr. WD40, what is your suspension set up?

WD40 07-18-2007 06:30 PM

^^^KYB-AGX dampers, Progress 22mm rear bar, stock springs.

stivo 07-18-2007 06:34 PM

and how does this compare with other SVTFs on the auto x circuit? There are so many options... I'm thinking about doing sway bars, so I am relieved to see you only added a rear and not gone with a more expensive set up.

Are the AGX dampers adjustable?


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