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Thanks to bartcatz (Andrew Semple), who sent me some high-res photos he took of me at the SCCA autocross event I ran at Phoenix Int'l Raceway last Sunday, I've spent about an hour this morning analyzing them carefully to get a better idea of what my car is doing on course. By using the line tool in Photoshop, I was able to measure the various angles as shown.

At this point on course, the car was doing about 50mph in 2nd gear, the corner is off-camber by 3.2 degrees, the car's rolling over ~3.6 degrees (6.8 - 3.2), the outside front wheel has 2.9 degrees of positive camber (6.1 - 3.2), and outside rear wheel has 4.3 degrees of positive camber (7.5 - 3.2) ... you can also see the inside rear wheel is off the ground just slightly, but since I've addressed this in another thread, I'll leave that alone.

Static camber at the front is set at appox. -.9 degrees and at the rear, approx. -.5 degrees. I haven't yet changed the alignment (waiting to get the car on the scales and corner-balanced first) so these are the stock settings. Based upon my times around the skidpad that was setup at the practice event the day before, I know the car is capable of pulling almost 1g (.98g was the average over 6 sessions) at the point the inside rear wheel lifts and since the inside rear wheel in this photo is hovering just above the ground, I'm guessing the cornering force to be approx. 1g here.

I realize that perspective issues mean the angles I've measured may not be exactly correct, but assuming they're in the ballpark, the following are a few of my observations:

1) With 380# springs front and rear, and H-Sport a/r bars front and rear, the car rolls approx. 3.6 degrees per g. (Off the top of my head, I know this is about the same as the 4th generation F-Body cars from GM and more than a Corvette. FWIW, my old '81 4000 autocross car rolled almost _7 degrees per g_ with the stock springs and front a/r bar!)

2) Based upon my guesstimate of the fender heights, the springs &amp; bars are stiff enough that there's still a bit of bump travel (.5"?) left before the shocks hit the bumpstops.

3) Despite the camber gain designed into the front and rear suspension, the car is still losing camber as it rolls (i.e., 2.9 degrees positive camber after starting with .9 degrees of negative camber means the camber changed 3.8 degrees while the car rolled only 3.6 degrees). Adding static negative camber isn't a total solution as this will increase the negative camber on the inside wheels and as you can see, their contact patch is pretty small already. Stiff springs and/or bars will help counteract roll, but will also reduce compliance and since autocrosses are typically held in bumpy parking lots, I'm reluctant to stiffen things up too much.

On the other hand, modifying the control arm pickup points to modify the suspension geometry so as to increase camber gain will have the effect of adding negative camber on the loaded side and positive camber on the unloaded side, which (IMO) is definitely a win-win.

4) Based upon the tire temps, I'm not sure there's much benefit to running tires wider than 245 as I never measured a temp over 160 degrees despite the course being nearly a mile long (~90 seconds) and fairly quick (~45mph average). Wider wheels, on the other hand, might prove useful.

Well, that's it for now. I've got a chest cold and am going back to bed for a while, but any thoughts or comments on any or all of the above will be welcomed. :^)

 

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From what I know, the positive camber is created from the weight of the car pushing on the bushings in the lower control arms, eh?

Although there's no a/m versions of the lowers available, the uppers can be replaced with CPP's, and the bushings there are replaced with ball/socket type joints. That should reduce the amount of + camber that's created. VonK's got this setup on his S4, and would have plenty of light to shed on this topic. Hope he sees it.

Here's my (not so great) shot of rear wheel lift, VIR, turn 2, no idea on the speed, maybe 40-60 mph?

Oh, and get better.

 

I know almost nothing abt. suspension, but 380# front springs is lowest I have heard on a perf setup

 

<center><img src="http://www.customperf.com/images/front_lowerarm_L.jpg"></center><p>CPP's also. i don't know of a retailer. also powerflex makes polyurithain bushings for the lower rear's. you can get these at forge.<ul><li><a href="http://www.customperf.com/a4.htm">http://www.customperf.com/a4.htm</a</li></ul>

 

Nice!

 

perhaps you might want to change all your suspension linkages to heim joints instead of rubber grommets. And the next logical step is much stiffer springs up front and rear.

Solid motor mounts and solid drivetrain mounts can't hurt either.

 

are your fenders rolled?

 

i've always run azenis or pilots sports (got a sweet deal) because i couldn't justify $1000 for quick wearing race tires. the price on the dunlops is amazing for the reviews i've read.

can i get your impressions on them? my setup will be the same size tires as yours with kw v1 coilovers at about .5" lower than stock sport suspension.

also, what wheel offset and are you running spacers for clearance?

-jim

 

for cold and/or rainy days i can run my pilot street tires and save the tire changing for a nicer day. i'll start checking into wheels that will work. finding info on the correct wheels is what has prevented me from ordering the tires. i appreciate your help.

fwiw, i will be rolling the fenders and will massage them as needed.

 

40 et with 5mm spacer would be 35 et not 45.