I have an issue I've only just started noticing this spring - on upshifts or lift/load throttle, the car has a slight but disconcerting yaw, more noticeable on truck-rutted roads. I haven't made any suspension mods over the winter, other than replacing crappy 205/55/16 Eagle RSA's with 225/50/16 Pirelli PZero Nero's - lightly used, and the pair that went on the rear had significant inner shoulder wear due to coming off a car with significant negative camber, and some toe in. The car tramlines a lot, which isn't surprising with relatively wide, sticky rubber and alignment settings biased to track use, but my setup hasn't changed since last year, and the twitch is new, or at least noticeable now where it wasn't before. Is it possible the rear subframe is moving under load/unload conditions? And where would I look for signs of this. I was under the car a couple weeks ago checking the alignment, and didn't notice anything, but I was mostly concerned with the alignment, and didn't look for other possible problems. Or would other rear suspension bushings cause this yaw problem? I haven't hear any clunks or rattles I would typically associate with worn bushings. The car is not winter driven, has 111,000kms on it, and is well-maintained.

Here's what I have at the moment - 91 NA FC, used extensively for autocrossing, and on the street, lowered roughly 1", on GC coilovers, AGX struts, Eibach constant rate springs, 350lb/in fr, 275in/lb rear. DTSS eliminator bushings. Front alignment 1/16" total toe out, -0.5deg camber, rear -2.5 deg camber, 0 toe (probably should be slightly toed in, but ran out of time aligning it, and haven't got back to it).

 

There's your problem...
With any kind of toe out, the steering is going to be touchy.
Although this kinda alignment setting is good for autocross types of set-up's, it's bad for highway driving.

If you really want to get rid of that twitchiness, try shoot for 1/16" total toe in up front...


-Ted

 

The front toe hasn't changed over the winter - or in years, in fact. Toe in simply isn't an option - the handling sucks. The yaw is new, and always the same direction, depending on whether the drivetrain is unloaded (such as a shift, or drop throttle), and the opposite when it is loaded (getting into the gas). There's no pull at the wheel, and you literally can see the car rotate slightly around the nose- as if the rear were stepping out, rotating the car around the front contact patches, as opposed to the normal sensation one gets when the front wheels change direction, and the rear lags slightly before following.

 

No one experienced this? It's definitely a sense of something moving in the rear, but no noises or clunks. Happens when I'm on/off the gas even at 70-80km in 5th - so low revs, low power (it is na - lol), it's not like major compression braking or high torque upsetting the chassis.

 

Dead bushing or try and run some DTSS eliminators?


-Ted

 

Does the yaw feel like a rear steering effect, such as dogtracking? or from the front? One thing to try is to put the front or rear of the car on jack stands (at the jacking points on the body, not the suspension parts). Use a prybar to try and move suspension components at or near each bushing. If you pry hard enough, you could make any bushing move, what we're looking for here is an unreasonable amount of movement or perhaps more movement from one side than the other, especially subframe bushings. If you had the car on a quality alignment machine, there should be measurements for the rear that include thrust angle, offset angle and setback angle in addition to camber and toe. These measurements are used to determine if there has been a shift in the subframe or bent parts, but if the shift is only ocurring onder accel or decel, you might not see it on an aligner, hence the need to check bushings.I align cars every day of the week where I work, and when it comes to front toe, excessive toe-out tends to cause steering "wander", especially with negative camber,where as excessive toe in tends to cause over-sensitivity, or "darting".

 

The "throttle" steering definitely seems to be coming from the rear. I already have the DTSS eliminators, so I guess I just need to find some time to get under the car and pry around to see if I can get something to deflect where it shouldn't - and under fairly light load, I would guess, as the effect is noticeable when driving even in top gear at fairly low speeds.

 

Well, I got under the car last night and started checking out the various rear suspension linkages, and it wasn't too hard to find the problem: the bolt for the toe-control/DTSS eliminator on the right hub (the lower front bolt) was backed out a couple turns. I re-torqued it, and voila, no throttle induced yaw. Good thing, to, as today was an autocross event for me. Scary to think what might happen if a hub bolt had come out.

 

Fantastic.Glad you found it. Yep that would have sucked having a surprise like that at an event.

 

I didn't have time to re-check my alignment after retorquing the bolt - unfortunately, it appears it actually made a difference - I went from zero toe, which is already a bit marginal, to 1/16th toe out. No wonder the car seemed tail happy at yesterday's autocross - it was like before I put in the dtss eliminators. I thought it was just my well used and abused Kumho 710s, so I'm actually glad to find the loose rear still has a relatively easy fix (tires are easy to, just expensive!). It still has the track tires and rims on it, so I'll set it to a slightly more sane 0-1/16th toe in while I'm swapping the tires back tomorrow.