jmans093

Sorry for the long-winded post. I would really appreciate any thoughts, info, or help.

I bought an 05 C6 this spring. It's my DD, but I use it at SCCA Solo/Autox events. The car seems to have wicked, wicked oversteer. The first several times I ran, I only ran into severe oversteer if I throttled too hard out of a turn.

This past Sunday was different. First off, I've been running the car in the "Competition Mode." Secondly, it has a tire pressure sensor that intermittently reads 0 psi for awhile and then goes normal again. When it does this, Compeition Mode is disabled and there is no Active Handling/Stability Control (it does have ABS and TCS). In the third of six runs, I was in the middle of some esses Sunday, and the sensor frizzed out. There was a simultaneous "ding ding ding" chime and the back end of the car letting go sending me into a spin. The sensor stayed inoperative the remaining three runs. I spun two of those runs and barely held on the other one. Even after I'd spun and was driving back to finish at a modest pace, the back end would slide around. It wanted to go around whether I was in the throttle or not. It's like it was on ice skates (albeit 285mm wide ones ). I had no idea the Competition Mode was keeping the balance more neutral all this time. Unlike the Active Handling, I could never feel the comp mode intervening. Though it was zero fun yesterday posting nearly the slowest times of the day in a C6 vette, at least I learned the actual handling characteristics of the car without computer intervention.

The car is an F55 car. I have Z51 poly bushing front and rear sway bars on it. I'm thinking about putting the factory rear bar back on it and seeing if that helps.

The tires are Michelin Pilot Sport A/S. I hate these tires, but they were what was on the car when I bought it. Mayeb the tires are all to blame, but I'm hesitant to do so. I know the car will have more grip when these are trashed and I get a performance tire, but I'm unsure whether that will affect balance. I'm running cold pressure 29 rear, 32 front. It seems to oversteer a little less with a few less psi in the rear.



I aligned the car today. I wanted to throw 1.5 degrees negative camber all the way around, just for better overall handling. I also wanted to toe-in the rear a little in attempt to help the oversteer. Here's specs from today:



I unfortunately couldn't toe the rear in. It was too much so already on both wheels and I actually had to toe it out. That has me worried I may have made my problem worse. Adjusting camber, I found there is only -1.1 degrees of camber on the left rear with the eccentric maxed out. I could get -1.5 maxed out on the right, but I put it back to -1.2 so it was not so far from the left. I also left my front camber at -1.4, which was the max I could get without also increasing the caster (I figure 8 degrees is plenty) as the forward eccentric was maxed out.

I'm a noob at this racing thing, so once again, any comment on any part of what I posted is appreciated.



P.S.

Oh and I have a question not really related to my particular car I'd like an answer to. How does SAI/Kingpin decrease when the negative camber increases? If the lower ball joint is pushed outward like I did today, shouldn't that always increase the angle? I was looking at my alignment sheet and noticed that even though both front wheels were pushed out the same amount, the SAI increased a lot on the left and decreased some on the right. How is that?

Bill Dearborn

First off, if the tire pressure sensor reads 0 the car should turn force you out of Comp Mode and into full AH mode with traction off. If you were on a high speed track it would also apply the front brakes to bring the car down to 55 mph in every turn.

The rule of thumb for handling corrections is to stiffen the end of the car that you want to be loose. To counteract understeer you stiffen the rear or soften the front. To counteract oversteer you stiffen the front or soften the rear. Do you know if other things have been done to the car?

Not sure what you mean that you can't get any more rear toe in. Are the tie rod adjusters maxed out leaving you with toe out? To get toe in at the rear you have to unscrew the tie rods from the ends (turn counterclockwise). This pushes the rear of the wheel outward. Your measurements indicate you have a small of toe in. Toe in is usually accepted as a positive number while toe out is a negative number although there are some machines and people who reverse the polarities. Is toe in considered negative or positive on the machine you used? You have to be careful with this as it is easy to get confused since the standard really isn't a standard.

First thing I would do based on your statements is to check the wheel bearings to see if they are loose, followed by the tie rods. Usually increasing camber at both the front and rear of the Lower Control Arm will not affect caster that much. To get max positive caster you would need to have max camber on the rear of the control arm and minimum camber on the front of the control arm. On the Upper Control Arm you would need to install the maximum number of shims or spacers on the two front bolts between the control arm dog bone and the frame and nothing between the rear dog bone and the frame.

To answer your question if you had caster set at a certain angle and then increased camber on the front of the lower control arm the caster would reduce. Caster is basically a measurement of how much the ball joints are moved forward or backward along the length of the car relative to each other. If you move the front lower in and the rear lower out you are moving the ball joint forward which increases caster. If you did the same thing with the upper control arm that ball joint would move forward as well and cancel out the increase in caster.

The difference in camber adjustments on the cams is due to the cradles not being centered on the frame. GM leaves a little slop in the fitting so the two parts go together without issues. The cams are used to fine tune slight shifts of the body on the cradles. For instance I know the rear cradle on my car is located to the left side of the frame (off center by about .1 in) so I need a smaller camber setting on the right rear cam than I do on the left rear cam.

Bill

davidfarmer

I've never seen a car that didn't have enough threads to set the toe properly.

You'll be faster and better with AH/TC off anyway. Just shut it down and focus on throttle control and smooth inputs

Jason

I second the "bolt check." Check every single nut and bolt you can reach with a wrench. I have found crazy things loose from the factory (axle nuts!) You may not find something (you will,there is always something), but at least that will be one less variable to worry about.

TMyers

With the sensor problem you have who knows what the car is really doing. Nothing wrong with running in comp mode while you are learning the car. You will be faster though running with everything off.

I had the same setup you have, F55 with Z51 sways. With this set up you will understeer if you push the entry to hard, Understeer if you pick the throttle up to early and oversteer if you pick it up to much on exit. On exit that oversteer will be snappy and hard to control with the tire you are running.

The first thing to do is get your sensor problem fixed. Your alignment looks ok for a dual purpose car. Tires will make a huge improvement. Instead of playing with your sway bar play with air pressure. You can only drop so much air pressure before you start to roll over the tire. If you are loose, add pressure to the front tire to balance the car.

Last is seat time and is probably the most important improvement you can make. With the right tire, say Kuhmo XS's, Brigestone RE-11 you can control that over steer rather nicely. Stay away from runflats. It takes a light touch on throttle but once mastered it is fun being able to drift the car out of the corner.

PS: Check your shocks, I blew my right front and never knew it, no codes or anything. Really screwed the handling up.

Jim_Harrison

Jesse;
Give me a call and we can look at your car one night this week of over the weekend. We can check the rear and front toe.
316-204-1076

el es tu

Last alignment I had them center the frame (they already had to loosen it up for some other work) and get the shims equal just because mine was several mm off.

It would be nice if we could get collars for the subframe centering like this: http://www.rigidcollar.jp/
http://www.speedhunters.com/2011/06/...s_to_the_gt_r/

jmans093

It definitely didn't have full AH. Judging by my 3 spins, I'd say it was full off. If it's on, it shuts the car down hard at the slightest twitch. The message center stated traction was full on, although it would let me turn it off if I had wanted. It only locked out Comp Mode. I suppose maybe the EBCM or whatever it's called could be doing weird things with the brakes / ESC if it thinks a tire is flat. So with the sensor fixed, maybe I should just try it again with the AH/traction off and see if it behaves any differently.

It's all stock F55 outside of my Z51 bars, and the ****** dry/wet/snow runflats. So do you think the stock bar back on the rear sounds like a good idea to soften the rear if it still oversteers too much for me?

Sorry for the confusing post. I was trying to say I ran out of camber adjustment in the rear, not toe. I could have given it whatever toe spec I wanted, but total toe in was already almost 1/4 inch, which is getting kind of high and GM spec if 0.05 max. Toe in is positive. I definitely know how to align. That's one of the things I'm best at at work. A lot of the guys just "move it to the green" and hardly have a clue what the numbers mean. I'm over there helping a lot.

The caster definitely changed quite a bit moving the front eccentrics in or out. I'd move them and resweep the caster to update the spec and any change was pretty substantial. Like I said, I could get a ton more negative camber out of the front, but I didn't want to increase caster. Eight degrees is quite a bit, and maybe more would be better for all I know, but I knew I was safe leaving it at factory setting. And I came within .1 degree of where I was going to put camber anyways.

I kind of figured that. Even though it probably wouldn't have taken long, I didn't bother trying to cradle shift the rear in the time I had. Hopefully -1.1/-1.2 will do for now. I should be able to get it to -1.3 both sides if I moved it.



Like I said, sorry for the confusing post. I ran out of camber adjustment in the rear, not toe



Reading these posts, I'm thinking I should run again and see how it makes a difference with everything turned off and a WORKING sensor. Hopefully the EBCM or whatever they name it nowadays was doing unwanted things if it thought a tire was flat.

F55 shocks won't set a code even if blown out? Mine aren't leaking fluid or otherwise obviously blown, but I do have a slight clunking from the right rear I hope isn't a bad shock.

Hey thanks, and cool to see another member here! Will do, although I mainly wanna see your 72 I've heard the other SCCA members talk about! My dad had a C3 that drove terrible. I'm not sure how it ended up this way, maybe it was just age, but it had one of the worst alignments I'd ever seen. It was a dangerous handful just going down the highway. I aligned it and replaced some worn front end components and made it a million times better. It was a fun car to drive after that. He was impressed. It still wasn't quite perfect, but he didn't have the budget to quite replace everything that was worn. It was unfortunately sold last February though.

Jim_Harrison

The 72 was sold a couple of years ago and now in Denmark. I really miss it but the Z06 requires a lot less maintenance.

randomus

Those Pilot Sport A/S are also really hard, something like 500 treadwear rating. They slip and slide all over the place on dry pavement.

I couldn't wait to get them off my car after I made the mistake of thinking they were PS2's for an amazing price. They last forever too so good luck wearing them out

froggy47

I guess I missed this thread earlier.

First get the sensor situation fixed, none of the other stuff will matter much if your car goes flakey in the middle of a run.

The tires are junk for autox. Even for learning autox they are junk.

Fix those 2 things then let an instructor drive your car & listen to what he says.

Take it from there.