Alignment or Suspension Related at 100MPH?

MaysEffect

I'm not entirely sure i can give anymore useful information when we are talking about customized setups that go far beyond the normal limitations of the oem setup, certainly one as modified as your own. MY G is bone stock apart from Hankook RS3's in oem size. My other cars are significantly more tuned just because i've had them longer and it was worth the cost. The only 370z's i've driven were also bone stock, so again its not like your setup (so i assume). I've worked on both the G37's and 370z a few times so i am aware of the design hands on. This doesn't change my opinion on the cars in stock form though, and that was my point about normal alignment setups, even aftermarket setups to some extent. I've never attempted to change the handling balance of any car based on alignment setup, i change the alignment only to chase the wear and heat patterns of the tires. Considering my G is stock, there isn't a whole lot to change other than camber in the rear and slight changes in toe in the front. So far my normal changes have been to marginally start to increase toe-in from my base setup depending on how fast the wear is. The last time i changed it, it was sitting at .2 degrees toe-in front, close to zero toe rear. I play around with tire pressures significantly more though before i decide to tweak the alignment. My Normal traffic temps is 37psi front, 35psi rear. Spirited driving, i set around 34f/35r.

I can't speak on your exact issues without knowing the particular driving situations and setup you have. Looking at all the stuff you've done, it's impossible to get an idea on how the car handles without experiencing it myself or seeing it in action. You saying you're experiencing snap oversteer from toe out just doesn't add up without other factors causing it to behave in an unstable manor. However like we previously discussed, the factory Caster angles and rear traction arm geometry already makes it so that the handling response is a bit fast and loose (nervous).

Are you saying you just turn in to the corner and immediately the tail steps out? Was it on throttle, off throttle? Trail braking, no braking? All these different variables dictate weight balance, and all of which can change the amount of load on any given tire and subsequently cause instability. I've never had a situation where the rear has just stepped out unless i provoked it through cornering speed, steering and throttle/braking inputs or simply causing load changes in some form such as hitting a bump or elevation changes through the corner.

I'm not sure we are on the same page as to the definition of snap-over steer if you are implying you have grip and then it lets go, that's just regular oversteer. From your description, it sounds like you are feeling exactly what i described previously. When you load up the rear wheels, the natural tendency is for the outside wheel to start to toe in. This transition state from toe-out to toe-in will feel a bit vague, and it will feel even more loose if your camber settings are not ideal for the tire being used and/or you have a combination of stiff and soft bushings. But saying it let go, doesn't seem accurate if you are saying there was grip through the corner. Some other factor had to cause the loss of traction. A change in throttle/braking input, bump, the tires maximum slip angle was reached and the TIRE let go. But the last point would be more of a problem with static toe in, not toe-out. Most cases it comes from a change in CG and instant roll center. The only situation where i can see that the normal dynamics won't be ideal is if you have stiff/solid bushings for every control arm except the traction arm and upper control arm in the rear. In which case the traction arm would be the only arm moving and the camber and toe arm will be limiting the ability for it to gain toe-in. In which case it may start to toe-out more under load. This doesn't seem ideal, but i can only imagine this would just make the motion of steering a bit digressive. Which i can see would make the car behave in a manor where the amount of rotation is not linear to amount of steering input, making steering with the throttle a bit inconsistent. Is the case with your car?

Saying you experienced snap-oversteer would imply a sudden loss of traction at the rear. This doesn't just happen from toe settings unless they were WAY off. Like 3-10 degrees off. If this is happening, something is broken. Given your knowledge and experience i won't assumed you'd allow this to go unnoticed.

With regards to my experience with snap-oversteer.
The clearest, most dramatic situation i have was when i was going for a "normal" night run through the canyons in Malibu in my mkv r32 a few years ago. To give you an idea of variables. My rear tire pressure was higher than normal at about 42psi rear, 38 psi front (i even checked it beforehand), i had a mismatched squared tire setup 225/45 ZII stars front, DZ101 rears, the camber was set to almost 2 degrees front and rear and toe out front, zero toe rear. The tires were already warmed up from the drive up the canyon (Topanga canyon to the locals reading). I was going back down the hill to PCH, and was evading some guy trying to keep up (not the cops). Going through the down hill section with tight S turns, i picked up the speed on a short straight, and threw the car into a down hill right turn that was slightly cambered inward at about 70mph (way too fast). The car turned in relatively well with only a bit of understeer with steady throttle, then as i straightened out, the rear started to slide out once exiting the corner onto the flat straight. I started to counter the slide with full throttle and a bit of opposite lock. The rear gripped up and i let off the throttle and braked while straightening the steering. Subsequently the car swung back around and spun on a dime and i headed backwards further down the straight before i stopped just before hitting the canyon walls. Post inspection of the tires, the pressure had increased to 41F/45R.

In this situation, i'm confident the alignment setting played a small role in the situation. The variables of tire pressure, too much speed, sudden shift in weight balance and steering input caused the drift and snap oversteer. I already noted before the instance that the car was sliding a bit and felt a bit loose, this wasn't normal for the alignment setup i had and still have today. All of my other situations of snap-oversteer was similar to this with other variables such as running over berms, bumps or inconsistent throttle input.


Hopefully this makes sense. Though i may have put some of you to sleep.