speedjohnson

Suspension Guru's please chime in:

I'm having trouble dialing in my car to work on T-12 @ PIR (Portland). I've just gotta figure out how to get this one right as it opens into a long straight...and I feel like I'm wasting alot of time waiting for the car to hook so that I can then get going.

T-12 is a long mid speed sweeper that leads onto the front straight. It is a right hander and turns approximately 80 degrees. There is a small bump at the outlet of the corner that unsettles the suspension a bit.

The car set-up now is just a little twitchy at the apex transition. The back end will rotate easily with throttle off, but you must get right back on the gas to catch it form coming around too far.

I have to nurse the car after the apex. You can hardly add any throttle until the car is nearly straight or the rear end will pop out...especially if you hit it too hard on the corner outlet where the bump is.

My friends got tired of watching me drifting...they snuck over and taped an egg on my gas pedal...so I guess I should do something to avoid further humiliation.

I've got some adjustment options, but don't really know which one would work best. At this point I don't know if I should:

1) Alter my line? I'm taking the typical sports car line now...the largest arc possible. I think that it might be faster to just slow down, turn the corner and then get on with buisness.

2) I could adjust my sway bar back a notch, it'd help with the transition, but I'm unclear if this would help me with the power on oversteer.

3) I've got coilovers, so I could go with a bit lighter spring, however, I'd likely have to raise the rear a little to keep the tires from hitting the trailing arm brackets.

4) Suspension alignment? Currnetly the rear is 2.0 neg camber and zero toe.

5) Wider rear tires...not possible without lots of bodywork.

Help me get fast outa T-12 and I'd be very appreciative. sj

z060ntrack

If Oversteer is the problem, then look to create some understeer to get the car neutral.

XPC5R

Add some rear toe in, it will help the back end plant more quickly as you roll into the throttle. A good test would be 1/2 turn of toe in on each rear tie-rod. That's approx. 1/8" total toe in. It will also make the car a little less eager to rotate, so it's a bit of a trade off. Add or remove from there to taste... I've seen over 1/4" of rear toe in on a C5 work, especially important as your power level goes up.

The toe adjustment should improve all power on situations, so your drifts will be much smaller and short lived. To the extreme, even more toe would cause the car to push straight when you add power at the corner apex. The best part, it's free

h rocks

Big time. Try the rear toe in first. Sounds like you may have gone a bit too stiff on the rear spring rates also. Based on your short description, you may be charging into the corner too fast, lifting, getting some lift throttle oversteer, then hitting the gas to save it right before encountering the "bump", then having to lift again? I think you will notice the toe in change right off the bat.

LG Motorsports

Which coil over kit do you have?

Kill the rear toe. Zero is not the way to go. put up to 1/4" in and you will feel much better and go faster.

Corner weights should be 50% on the diagonal or you will have Left Right differences in the handling.

Check to see if your coil over shocks are bottoming at the rear. If so, then just raise the car to eliminate that.

basic details will get you where you want to be.

THanks
Lou G

mvitacco

Totally agree about putting toe-in in the rear.
My car was much as you described with 0 toe in the rear. Adding
some toe-in enables me to start rolling on the power at apex
Start with 1/4 total then try and see if you can get away with 1/8th total. I would definatly do this first before changing springs and bars.
If this is a C4 this is a know problem. Guildstrand makes a rear
tie-rod/roll steer kit but this may not be class legal.
The fix for stock class is to add more more tow-in.

Zee Freak

Craig, I'd have to watch you take the turn to judge your line.
As far as more rear toe goes. That sounds good to me. It worked on my T1 car.

h rocks

Hey Lou, thanks again for your hospitality at Mid Ohio

Did you and your fellow non CTS-V and Audi drivers figure out if and how you are going to respond to the 20% intake restriction imposed by SCCA? It would be interesting to see Speed Channel broadcast a five car race...(not that you guys haven't already thought of that) Good luck! :flag

speedjohnson

I'll get a t/t day in a few weeks and will be happy to report back on changes.

To make my t/t day more effective, I have some further Q's:

-I've got the exotic muscle sperical bearing strut rods, toe rods and trailing arms...this should limit the toe out caused by the factory bushings. Do you all still recommend 1/8 or more total toe in?

-Since the car drives neutral at mid corner, and the evils happen during transition (throttle off/throttle on), when would one consider changing the shock dampening vs. other methods? (eg. more rear rebound dampening? etc.) If so, how much adjustment is a noticable change...10%, 20%?

-This thing really squats under power. Can I adjust the trailing arm lengths for anti squat?

-Loosening the rear bar would help the oversteer at apex transition, but would it effect the power on oversteer?

Mostly, I'm finding myself with lots of adjustable parts, some direction, but little experience on what adjustment would make the most appropriate change. Thanks for the "schooling"

mvitacco

Corvettes (C4 and C5) already have anti swat and anti dive
designed in but as you mentioned converting to solid strut rods and
tie rods I think I know what is happening . What ever you do don't
start changing strut rod lenghts until you read up on this and get some
tools.

When you change the length of the trailing arms,
you rotate the rear up-right, this changes the height of the rear
tie-rods, that in turn introduces either roll oversteer or roll understeer
depending on if you raised or lowered the tie rods.

As you converted to sperical strut and tie rods (great idea)
I very strongly recommend you measure roll steer in the rear with a "bump steer gague". Else you are very likely to be chasing your tail (no pun indended). You can't tell how the arm lengths changed.
Considering how hammered out stock bushings get you can't
rely on measuring their center to center length accurately to know
what they were stock.

With coil over's in the back, your springs and shocks
can be easily disconnected. So this becomes a simple but tedious
job. However, I believe strongly this would be the
best $250 and 3 or 4 hours you will ever spend and this is vital
to ensure the gemotery in the rear returns to design intent.

You "should" find about .020 toe-in at 1.5 inches in bump per side.
But for the reasons stated above along with the possibility of the
rear uprights getting "tweeked" in service cars will vary. I talked to Dick Guilstrand about this becuase I was alarmed
at the large amounts of variance I was measuring. He told me this
is common hence the deveopment of the rear tie rod kit and solid
strut rods.

I bet, based on the description, you will find you introduced roll oversteer and that the rear is towing out in bump and that is why
you can't get on the gas until the car exited the turn.
This is actually common and the reason people find putting more toe-in in the rear improves the situation because it canceles out some of the roll oversteer.

If you want to be able to run a lot less toe in than normal the
modifications you made will allow this, but you have to set
your roll steer between 0 and .020, possibly close to .020,
however in no case should it ever get into the positive or tow- out range in bump because that is roll oversteer =slow corner
exit speed= slow car.

I set my car to .001 and .003 toe-out per side at
2 inches bump and rebound (4 inches total) suspension travel.
This is as close to 0 as I could get. My rear toe setting will
stay were I set it, other than the actual strut rods, rims/hubs
and frame flexing. Thus, I still set slight toe-in at the rear:
1/16 per side or 2/16th total (1/4 inch) works for me.

I have done your same exact mods on my C4. The results
were outstanding. The back end of the car stays planted.

IF you go to the Longacre racing web site there are
excellent technical papers that explain this proceedure
and theory in great detail.

Let us know what you find.

speedjohnson

I understand now that the recommended toe-in setting at the rear doesn't just check the compliance/stretch of the rubber bushings, it also counters the inherent toe out bumpsteer that occurs at the rear knuckle when the car sqauts under acceleration.

Thanks again. sj