Rear end shifts under acceleration / deceleration

[t3itguy]

So my 08 GT started doing this weird thing yesterday. It's pretty hard to describe but it's very noticeable when driving. Say for example that I'm cruising down a straight road at 40 mph in 3rd gear. If I go to about 3/4 - full throttle, something happens. It's like the rear of the car slides to the right -or- the rear axle is sliding to the left. When I let off the throttle, the rear of the car slides back to the left / the axle slides back to the right. Of course when this happens, I have to correct for it with steering input. And no, this isn't me breaking the tires loose. It's like the body is sliding left/right over the axle. I put the car on jackstands last night with the axle hanging down, got under it, and tried to push the axle from side to side, but it wasn't moving. The only mod I have done to the rear suspension is I replaced the rear LCAs with SR Performance ones from AmericanMuscle. When I was installing them, I wasn't sure what grease to use so I used Mobil 1 Synthetic grease, which may have been a petroleum based grease. I think I have it narrowed down to 3 possibilities.

1. At least 1 bushing in the rear LCAs is shot, allowing one or both sides of the axle to move forward / backwards.​

2. The bushings on the factory panhard bar are shot, allowing the axle to move side to side in relation to the body.​

3. The rear limited slip differential is shot, and is causing one of the wheels to drag under acceleration, but the more I think about this one, the less I think it's a possibility.​

Does anyone have any thoughts or has anyone experienced anything similar?

 

[cbass]

Do you have a gopro? Mount it under the car and go see if you can capture suspension movement on video when it happens.

 

[Kenaizer]

No. He clearly said shits. Don't correct him you don't know him.

Lol

Haha!


Seriously though, it sounds like you are on the right track to figuring out the problem. Have you checked to make sure the LCA mounts are still square? I bent a LCA relocation bracket on my car and didn't know until I took it off to sell the thing.

 

[fdjizm]

You forgot the upper control arm broseph.
That oem rubber bushing takes a beating and when it gives up it lets shit wiggle around.

 

[BMR Tech]

This usually occurs when there is a thrust angle issue....and that usually happens from a damaged bushing.

A poorly designed LCA can also cause it (no offsets with poly bushing - or offets built incorrectly)

I would pull the LCA off and check them out.

 

[Kaylan1521]

I had the same issue and it was control arm bushing

 

[t3itguy]

I had the same issue and it was control arm bushing

Was it the upper or lower control arm bushing?

Do you have a gopro? Mount it under the car and go see if you can capture suspension movement on video when it happens.

Sadly I do not have one.

Haha!


Seriously though, it sounds like you are on the right track to figuring out the problem. Have you checked to make sure the LCA mounts are still square? I bent a LCA relocation bracket on my car and didn't know until I took it off to sell the thing.

How would I check that? I haven't hit anything recently, so I'd assume that it hasn't gotten bent.

 

[Kenaizer]

How would I check that? I haven't hit anything recently, so I'd assume that it hasn't gotten bent.

I think my case was caused by a hard launch. A visual inspection might suffice. You can just do that while you're down there checking the bushings and whatnot. just make sure everything is at the right angle and that the LCAs are flush in their respective mounts.

 

[CRICKET]

Mine did this and it was the passenger side lower control arm bushing shot. I believe it was the front mount. Replaced arms to BMR and no problems since.

 

[Kaylan1521]

Was it the upper or lower control arm bushing?

Mine was lower

 

[Devcon 7]

Had same thing happend to my old car, lca bushing tore.

 

[BMR Tech]

99.72356% of the time, if the bushing fails on the LCA....it will be the driver side, front chassis end. That end of the LCA sees more load than any other.

Looking at the pics of the SR LCA....it looks like they use a bushing design that compresses the bushing substantially, when it is mounted. It may not be the way they are, but it appears by the pics. Compressing the bushing when installing, any substantial amount, is a no-no on a Poly Bushed pivot point. Polyurethane does not do a good job at handling multiple load directions.

That said, it may not even be the LCA bushings.....but it usually is when the car tracks L/R upon load accel and decel. I would say, 100% of the time when I get a call like that, it is a bad LCA bushing.

Keep us updated!

 

[Kenaizer]

Haha. Love the statistic, Kelly.

 

[deathwishracer]

I have had multiple cars in the shop with Sr arms that have failed bushings. Not the best quality stuff. Failed bushings in the arm cause that exact sympton

 

[thump_rrr]

LCA Bushing is my bet.
In 2005 when Steeda released their billet LCA's they were equipped with grease zerks.
By attempting to grease them with a grease gun they would crack and split.
I once let off the throttle at the end of the 1/4mile and the rear end shifted almost sending me into the wall at nearly 125mph.
Since then I disassemble and lubricate with Energy Suspension Formula 5 Prelube.

 

[BMR Tech]

Yeah, definitely cant put much grease into them, or you will hydraulic them out.

If the bushings are designed to be used with grease fittings, they should be just fine. You need flutes, and you also need the proper size gap between the bushings.

 

[BEAST]

How much grease would you use?

 

[BMR Tech]

1 - 3 pulls is usually good....but it really just depends.

You'll know when it's too much. Gets harder to pull the trigger and the bushings start pushing out. lol

I dont even remember what we have in our instructions (I should probably check)

But, when I do them myself I put 1 full trigger pull into the bushings. Drive the car around for a few miles, and put 1, maybe 2 pulls in it....and go.

 

[Marble]

I had this happening and it was broken springs in my locker. I had put it on the lift and gone through it thuroughly several times, couldn't ever figure it out. Finally the locker broke and the guy working on it described the issue and said it was also probably causing the push that happened under excell/decel. Car goes stright now.

Suspension pieces are much cheaper than a new rear end. Hopefully yours is suspension.

 

[gray06gt]

My car was doing the same thing and it was the bushing on the lower control arm