Steering Stability

[Batmobile_Engage]

It's hard to tell exactly how long this has been happening, a few months maybe, but after
exhausting other ideas, I think I need some help...

The car doesn't seem to pull left of right when going straight down a flat smooth road, but
when I accelerate, the car pulls left and when I let off the throttle, I drift right.

The faster I'm going, the more pronounced the effect is. If I'm doing 75mph on the interstate
and step on the gas to pass someone, I have to add a significant amount of right steering wheel
input to avoid drifting into the next lane on the left.

My alignment is set to factory spec, my tires are equally inflated and have even tread wear. I'm
running the same wheels as I always have.......i'm at a loss here.

Not only is this annoying as @#$% but I'm afraid it's indicating something is broken, bent or about
to be.

The only things to change in the last few months are:

SPEC Clutch install
Group-N engine pitch stop install
Group-N transmission mount install
Rear wheel bearings replaced


Thanks in advance for the help!

 

[Grinder34]

when was the alignment last checked?

 

[Batmobile_Engage]

Last week...
...factory spec. (Though many suspension parts are aftermarket)

 

[Grinder34]

If its only under acceleration and not just at highway speeds, its gotta be some bump steer. If it always happens to one side, it sounds like that side has too much toe-out. Does it also happen for breaking. Mild and/or heavy?

What were the specs they gave you? You should've gotten a printout. I dont know about the STI in particular, but many cars have a range for "factory spec", so if one side is at the high end of the range, and the other is at the low end...that may be enough to cause your problem.

 

[Batmobile_Engage]

What exactly is bump steer? I can go have the alignment checked again but I literally
just had it done last week and everything is on point. I assumed the toe was out and
that's why I took it for an alignment.

I understand there is a range of tolerance for each. I can't remember the exact numbers
right now off the top of my head but I looked long and hard at the print out while pondering
the solution. Nothing jumps out at me, the measurements are even from side to side.

I don't seem to notice any difference in whats happening when I'm on the brakes but I'll
check again later today. It's really just as simple as, if I'm going straight down the road.
Steering wheel centered...and put my foot down, it pulls left and when I lift off the gas, it
drifts back right. It feels like really nasty torque steer, but of course there shouldn't be a bit
of that in a Subaru.

 

[Grinder34]

I edited my post above.

Does it happen when braking?

Bump steer is just the suspension geometry changing as the springs compress.

From here:

What is Bump Steer? Bump steer is the toe in and toe out of your front wheels created by the up and down movement of your suspension. Really ? bumps aren?t even needed! When the nose lifts under acceleration do you want the wheels to turn in or out on their own? What about when you are under heavy braking? Do you need the Right Front wheel to go one way and the left the other? Think about when the car transitions between compression and extension ? we want the driver to steer and not have to correct for the inconsistencies caused by improper front end settings. When the suspension oscillates over bumps the last thing we want is to have the tires turn themselves due to excessive Bump Steer.

 

[Batmobile_Engage]

edited above...

 

[Grinder34]

http://www.trackpedia.com/wiki/Vehicle_Dynamics_-_Cause_and_Effect_Guide#INSTABILITY

Straight line instability: under hard acceleration

• Malfunctioning limited slip differential
• Insufficient rear toe-in
• Deflection steer from rear chassis/suspension member or mounting point
• Rear tyre stagger (car pulls to one side)
• Dead rear shock absorber
• Wildly uneven corner weights

 

[Batmobile_Engage]

^ I'll have to check that link out when I get home. Every (except this one I guess...lol) forum is blocked in
our network so we're not screwing around at work.

 

[Batmobile_Engage]

A co-worker just gave me a couple suggestions...

He said that maybe I have a CV joint going bad and/or a tear in the boot allowed dirt to get in
and the turning resistance of that joint is higher than the others, unbalancing stability when power
is applied.

Also, he mentioned that maybe when the rear wheel bearings were replaced, one was damaged causing
a similar situation producing higher than normal turning resistance.

Does this sound like a possibility?

Before someone asks, I should mention... There are no strange noises I'm noticing at this time.

 

[Spamby]

^Unlikely.

Mine does the same thing when I have my summer wheel/tire combo on.
It disappears when I switch back to the factory wheels.

 

[Batmobile_Engage]

^ Wow, that's strange. This weekend I'm going to switch out my summer wheels and tires for
my winter set, which both the wheels and tires are brand spanking new. I'll let you guys know
if that makes any difference...
...though I can't see how it would.

 

[HolyCrapItsFast]

I'm thinking a sticky caliper maybe

I'm moving this into the suspension thread... Not really an urgent issue according to the rules. :tup:

 

[Spamby]

Here's the deal with mine with the summer combo. Under any degree of throttle at highway like speeds, the car wants to veer to the left under throttle. Off of the throttle, the car drifts back to the right. Steering wheel is still dead nuts center. Basically, loading or unloading the suspension, at speed, cause the gradual left or right drift. Slight counter steering i s needed when working the throttle while loading or unloading the suspension. At constant speed, like just cruising, only consistently maintained throttle input, the car moves straight.

My theory is this:
I put a high degree of work into the front end, changing all of the bushings to urethane, ALK, RCA, rack, etc. The front bar has Kartboy endlinks.
The rear is left alone, still utilizing all of the soft squishy bushings, sans a bigger bar but only mildly upgraded endlinks. (never put the Kartboys in the rear.)
Camber is about -1.7 front and around -2.3 ish in the rear IIRC. Toe is zeroed f/r.
Summer combo is a 265 with an 18x9.5 +38mm. Winter is the factory BBS at +55mm. (Why does this play into it? I will explain)

I have essentially created an unbalanced suspension. I knew this and know this. I can feel it everyday on the summer setup. The summer combo is heavier and with a more negative offset. This scenario exasperates the hard vs. soft suspension. The more negative and heavier combo plays on those soft bushings, links, etc. creating more leverage and thus causing the suspension, in the rear, to dynamically change faster or more so than the "locked" down hard front end.
Essentially, the suspension in the rear is steering the vehicle. I have experienced this many of times while doing Jeep work, adding soft rubber bushings, loosening links, softer shock and spring combos, widening the track, purposely doing so all while trying to create a super soft, flexy, stable and compliant off-road suspension. Now I digress, back to the subie.

At speed, or greater speeds, physics, energy, downforce, lift, drag, leverage of wider and heavier wheels on the springs and other components, play a more dynamic roll on your suspension. These energies and what not, get more extreme and cause the undesirables to play out more aggressively in an unbalanced situation and thus at greater speeds.

My solution is obvious, make the rear match the front. Lock it down and balance the situation. Then add some positive camber back into the rear to help in stability with a broader tire contact patch.

On stock suspension, with summer combo, I didn't have this effect. Now that I have a half-assed setup, this scenario is a reality.

Also, watch out for alignment shops and how they do things. They have an acceptable tolerance they go by. When dealing with a more sensitive suspension setup, a few 10ths of a degree can add some wackiness to the drive. One side may be at 1.7* camber and the other at 1.5* camber. This may be enough to add some instability to the mix. This is why it is always best to try and find an outfit that will work to get you even at all corners. Otherwise, most shops say fug it, we are within tolerance, give me money and move on to the old ladies Crown Vic. There goal is to get you close within limits not dead nuts.

Also, tire growth can play a role or be the role. The tire can grow at higher speeds. A tire that may have a few broken belts or plies, or even worn, especially one or two, may cause the tire to grow excessively compared to the others. Or at least more so than the others. Storing tires over the winter, especially stacked on top of one another (guilty!!!) can cause stress on the sidewalls, fatigue and/or deformation. This will cause instability, as well.

I don't know if this pertains to you but your scenario sounded similar. Sorry for the long wind but hopefully this is food for thought.

 

[Grinder34]

Spamby, I'm not sure you're right about the front/rear theory, but i'm with you on the alignment shop.

The suspension is symmetrical L/R. Firming up the front shouldnt effect side-to-side stability. If the weight transfers to the rear, it should transfer to the LR and RR equally, compressing each spring equally. The only thing would be if one spring has some binding, a shock has blown, or the alignment is a-symmetrical.

Actually, thats an interesting thought, could a shock have blown?

The only other thing that plays into unequal L/R loading in a straight line is the torque from the engine due to the longitudinal moutning of the engine. I suppose that could be significant enough to load one side of the suspension more than the other.

If it were me, i'd ask for a printout from the alignment shop and see if things actually look close, or theres some discrepancy L/R.

 

[Batmobile_Engage]

You guys have me thinking...
I have Tokico D-spec (6-way adjustable) gas shocks on all the way around.
What if one of my rears is softer (or stiffer) than the other one? That could
do it, right? To clarify, the 6-way adjust is on the rebound damping.
Maybe I need to zero them all out and re-dial in equal settings from side to side
especially...
...thoughts?

 

[Batmobile_Engage]

Fyi...

Tokico D-spec gas shocks
RCE Yellow springs
Whiteline sway bars f/r
Whiteline sway bar mounts f/r
Whiteline endlinks
Whiteline polyurethane sway bar and endlink bushings

 

[Batmobile_Engage]

I'm thinking a sticky caliper maybe

I considered that, but I'm sure I'd notice noise, vibration, etc. if that was the
case, don't you think?

 

[Grinder34]

Absolutely, re-set your dampers:make sure they're all the same setting and re-test.

 

[finallymysti]

have you hit anything lately?