Can You answer this? SOLVED
 

2013 2SS/RR 1LE, heavily modified.
The car has been on Pedders lowering coils for about 7 years. Just changed to BC Racing coil-overs in an effort to fix a steering issue.

Running 880 rwhp and on a hard launch, the car does not go straight. It doesn't matter if traction control is on or off. It will start to go left, then right as I correct and it truly feels like the rear wheels are doing the steering.
I have replaced the rear upper control arms, end links, and toe rods to BMR (this was done before Coilovers to try and correct the issue).
Four wheel alignment is bang on.
I've had 2 diff shops try to find the issue, whatever it is, to no avail.

I'm open to suggestions. Ask me questions and Ill supply the answers in an effort see if any of you folk might have a suggestion or maybe have experienced this yourselves and what did you find.

Thank in advance.

Have you looked at the cradle bushings and differential bushings to a lesser degree? Also did it do this with lesser power? It might just be a situation of the car is screaming oh shit with that much power... lol

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Same here and Im still on factory springs and shocks. I have upgraded toe rods, trailing arms and lower control arm bushings also. Hard launch she pulls mostly to the right and sometimes to the left..

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Factory cradle bushings will not stand up to 800+ HP. They're bad enough under
factory load. Your cradle is shifting under that torque. That is called torque-steer,
and can happen for a number of reasons.

Suspension shifting; bad or flexing bushings, loose parts.
Unbalanced power; one wheel puts down more torque than the other.

The first, and the most common type produces a noticeable pull to one side on initial
acceleration. This is when the steering wheel turns to one side, when taking off.
Releasing the throttle makes the problem far less noticeable. The vehicle may also
drive straight once speed is obtained; Causes:

Tie rod end with too much slack.
Excess ball joint movement.
Deteriorated LCA bushings(s).

Rear-wheel drive vehicles may exhibit torque steer when the rear suspension shifts,
due to worn components. Control-arm bushings and tie rods may allow the drive-
wheels to shift under acceleration. Movement changes the thrust angle and can
cause the vehicle to pull. A close examination of components in the rear should reveal
slack.

Other common causes include a tire low on air pressure or a tire with
conicity. To test for tire conicity, the tires can be temporarily moved from
side to side. If the vehicle now pulls in the other direction, a tire issue is
likely.

In your case, putting down 800+HP at the wear wheels, the cradle bushings
are the cause.

Thanks for the tips so far. I did upgrade the cradle bushings. It was the first change to the rear end but it has also been4-5 years plus lots more added hp. I have this idea in my head to get a shop to tear my rear suspension out and slowly reinstall it looking for a worn out culprit.
Great info Camarodude!!

Did you go solid or poly? I ask because I'm not having issue at 435/425 but I'm getting ready to do heads and cam I've got some parts stacked up and I'm curious if the poly ones I have will be good enough.

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I went poly. Back when I changed over, I had heard that solid is just that...SOLID. So my choice was poly. Maybe others can chime in on this and give us their feed back on solid or poly

ZL1 sway bar set on full stiff cured this issue for me.

I have one of the 32mm Pedders Bars out back. That might be part of why I have zero issues.

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I think my poly bushings and my big ass rear pedders bar might be good at mid 500s where I plan to land.

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1LEs have a ZL1 rear sway bar from factory.

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Yeah, it's 28mm rear and 27mm front. I added a Pedders 32mm rear bar years ago.

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Yeah big rear bar helps. I have bmr poly cradle bushings, toe Links, trail arms, upper arm inner bushings to eliminate any wheel hop and help my dse bar keep it all in line. And the dse rear bar is set soft with no issues

I'm reaching here. But how wide are your tires and how low are you? I had a problem with the inside of my tires (315's) hitting the wheel well and causing me to go left and right. As if the rear was steering. I had all the suspension mods that are in my sig below. I didn't realize how close the tire was to the wheel well liner. I couldn't figure it out until I pulled the wheel off. There were signs of the tires rubbing the plastic liner. I cut out part of the liner and rasied the car slightly. That fixed my problem.