aligment change with down force gen 2

[GTS Bruce]

After adding an autoform lemans wing and nose the car feels much more planted but tends to get squirley at higher speeds. Wing neutral and car stays level. Currently 3/4 deg neg camber front and 1/2 rear with minimal rear toe. Before aero ft tires warm only to the outside but now same temp across the face. Before rear tires same temp across face but now warmer to inside. Under rear squat it seems like camber is going more neg and toe is gone to toe out. Thoughts? Plan to dial in more rear toe and less rear neg camber. GTS Bruce

 

[GTS Dean]

It seems odd to me that your front OUTSIDES are warm. With negative camber, the inboard shoulders should be worked more. The rears would appear to be working as expected, but both ends should be gaining negative camber with jounce (compression). With a big wing, your car will be much more sensitive to wind gusts at speed and that could be part of the nervousness you're feeling.

Toe patterns do change with vertical travel, that's why there are charts in the manual for reference and shims available for adjustment. If your shock spring perches are adjustable, start by confirming your suspension settings at curb ride height. Assemble a stack of blocks to put under the frame rails near the jack points that you can increment in 3/4" to 1" steps, above and below curb height. Loosen the spring perches, pull one set of blocks, drop car onto blocks and measure camber and toe in jounce. Pull another set and remeasure. Repeat until you hit full bump. While you're at it, go back to curb and start *adding* blocks to measure camber and toe change in droop. Fire up the computer and make a chart in Excel that plots camber and toe curves. You'll then have a baseline for understanding and adjusting your camber and toe curves. If your spring preload is not adjustable, then the easiest way to measure at jounce is to pull the shocks. If you do this, be sure and ballast the car properly before re-torquing the attachment bolts.

 

[GTS Bruce]

Duh I should have been more specific. Before downforce the outside front was warm and the rear even at a track day. (3/4 deg neg ft and 1/2 deg neg rear) Now with downforce the fronts are even across and the rears are picking up more heat on the inside at a track day. I will have to check it out on a more normal street drive but I suspect the ft neg camber is now perfect while the rear is too much with downforce at the track. Aahh I also have eibach spring and stiffened oem shocks. Bruce

 

[GTS Dean]

What you're noticing is the effects of Camber Gain and dynamic toe. As the suspension compresses on the outside during cornering, the unequal length wishbone geometry increases negative camber, while the inside wheels move into positive camber. Both tend to minimize the effects of body roll on the suspension in corners.

Aero loading pushes both ends of the car into more negative camber. While you are not very specific about temperatures, I would infer that you're probably not running quite enough negative in front. In general, I run between 25-50% more camber in the front than the rear because there's less tire width working for your and you have to get the most out of it. If you run a lot of angle of attack on the wing, you can squash your springs and run out of travel. This pitches the nose high and you can get the front wheels into the wrong side of the camber curves (more positive). This is effectively a droop condition and front outboard shoulders get hot. A high nose with zero or positive camber makes the car more twitchy. The answer is less wing or more rear spring.

GTS Dean - online race engineer.

 

[GTS Bruce]

good thoughts.

 

[Viper X]

You don't mention your spring rates, but you will likely want to add stiffer springs front and rear. FYI, my 01 GTS ACR came OE with 800 front, 1100 rear springs without aero. Made for a rough ride on the street but pretty good at speed and on the track with Hoosiers. Even better when I added the Autoform wing & splitter and had a proper alignment done.

If you don't stiffen the springs, you will be (and it sounds like you already are) a victim of "dynamic toe", i.e. as GTS Dean correctly states, the car is moving up and down enough to change the toe (and camber) so that you can feel it and it's unsettling.

You also don't mention your shocks absorbers and the condition that they are in. I ran Moton double adjustables on my GTS and found them to work very well.

On the "squirelly-ness", I'd add the springs first if possible, then check the toe as it really needs to be at least neutral up front and "in" at least 1/16 per side rear for a road course.

On the camber, I always ran at least 1.5 negative up front (usually 2.0 negative), 3.4 to 1 negative at the rear in my 01 GTS ACR.

Good luck,

Dan