Rear End Movement on Abrubt Driveline Loads

[CaliforniaSpecial]

Thought I'd try to pick your guys' brains a bit on an issue I'm having. 2008 Mustang GT with mods in my sig (doesn't include my poly/heim LCAs/panhard). Vehicle has seen a handful of autox and drag days with one road course event and has 70,000 miles on it.

This spring i noticed a weird "shimmy" in the back end of my car when down shifting aggressively or letting off the gas at a high rpm. It is very consistent in that it will shift towards the driver side when throttle is applied and shift towards the passenger side when throttle is released.

I first thought my issue might lied in the suspension geometry/weight transfer but the car will do it in cruise around 45mph in 5 gear blipping the throttle which makes me believe it is more driveline related.

The LSD still locks as i can do a burnout so I don't know if it's clutch pack related. To make things more interesting there is a fairly loud "howling" noise that becomes apparent around 30 mph that i've determined is not the tires through a wheel swap.

Sorry for the long post guys but has anyone seen something like this before? Going to tear into it tonight and could use some words of encouragement/direction! Thanks!

 

[weather man]

Check your panhard bar and brace.

 

[CaliforniaSpecial]

I checked my panhard bar and torqued both ends to spec though I didn't check the brace (not sure if it has one?). The movement is much more pronounced during throttle changes at high rpms.

 

[skwerl]

Not just the bar, it sounds like a bushing failure. I would check bushings on the panhard bar as well as the upper and lower control arms. The big rubber bushing on top of the differential (upper control arm bolts to it) could be shot. That one is easier to address by removing the axle completely so you can get it out where you can see what you're doing.

 

[CaliforniaSpecial]

I imagine a failed bushing is probably easy to spot but what exactly should I look for? Completely remove whatever control arm I'm inspecting and look for any cracks/deterioration?

 

[skwerl]

I'd get the car on a lift (or jack stands), take a rag (to keep from getting crud all over your hand) and go through grabbing and shaking various components until you find whatever is moving. If it's that bad then you should be able to have some visible movement pushing on it with your hand.

 

[TGR96]

A friend of mine had almost the exact same thing going on in his Nissan Pathfinder. We put in new LCAs with new bushings, and that cured it.

And I don't have any personal experience with heim joints, but don't you have to keep an eye on the torque specs fasteners? It seems that I heard or read somewhere that they are more prone to backing out than those with rubber or urathane bushing. I dunno. At least I think it would be worth checking the torque specs while you are down there.

 

[CaliforniaSpecial]

A friend of mine had almost the exact same thing going on in his Nissan Pathfinder. We put in new LCAs with new bushings, and that cured it.

And I don't have any personal experience with heim joints, but don't you have to keep an eye on the torque specs fasteners? It seems that I heard or read somewhere that they are more prone to backing out than those with rubber or urathane bushing. I dunno. At least I think it would be worth checking the torque specs while you are down there.

Yeah I've recently torqued the fasteners on my lca's and panhard. I've tried shaking things around with my hand before and didn't find anything loose but will try again.

Thanks for the responses.

 

[5.0 Probie]

Great advice in this one... May I add...

The use of your hands with the car sitting on its weight is simply no match for the weight of your vehicle, and the force of torque under use.

I am leaning towards a failing part and or bushings. Heck it might be as simple a a bolt broken in half, but stuck in one of the bushings giving the illusion that everything is bolted up.

I would get the weight of the car off the rear-end and start grabbing parts to see what moves.

Keeping in mind movement by our hands is nothing compared to the vehicles own power...

 

[07 procharger]

Bushings. Mine did the exact same thing. I had to remove my LCA to even see where the bushing was split.

 

[KoolerSL2]

This /\. mine was doing same thing.
ordered a rebuild kit and all is well.

 

[CaliforniaSpecial]

LCA/Panhard bar bushings. Look okay to me.




Some deformation on outboard surface of body-side LCA bushings...




Body-side UCA bushing (1).



Body-side UCA bushing (2).





Blurry axle side UCA bushing shots.

I don't think I'm seeing a smoking gun here in these bushings. Do others agree? I feel like the movement that I was seeing in my rear end is a lot worse than what these bushings show. It could damn near send me into a lane change at times...

 

[CaliforniaSpecial]

One of the heim joints was fairly seized up that I coaxed into moving more freely. Wouldn't think that would do much but idk.

 

[Sharad]

I agree with the LCA bushing assessment, even though the OP didn't find any bushing issues. But he described what happens to a T.

LCA bushing design is critical, and many manufacturers get it wrong.

 

[CaliforniaSpecial]

I agree with the LCA bushing assessment, even though the OP didn't find any bushing issues. But he described what happens to a T.

LCA bushing design is critical, and many manufacturers get it wrong.

So you would agree that deformation I'm seeing shouldn't be an issue? Can you tell from the pics if the UCA bushing is shot or not? Thinking i'll end up taking it off anyway to check it out but i'm not gonna hold my breath considering the main job of the UCA is pinion angle right?

In order for my rear end to shift left on quick accel my passenger rear would have to move forward and/or my driver rear would have to move backward. It almost felt like my body was moving in parallel with my axle so maybe that contradicts the previous statement. Any help to un-fluster my situation is greatly appreciated.

 

[CaliforniaSpecial]

Update: Axle is off and looking into the differential. The axle play (distance I can move the rear axle inboard/outboard) on the driver side is around 1/4" and on the passenger side closer to 1/2".

Did a little searching around and it looks like most people say this is a little excessive and is the result of worn clutch packs? Can anyone concur?

If this is the case I think this would make sense as the possible root cause to my problem.

 

[skwerl]

Good move on continuing to search it out instead of ignoring it. I don't know but it sounds like maybe the C clip may be damaged or missing and the brakes may be the only thing holding the axle in?

 

[AbdullaGT500]

Do you have LCA relocation brackets? If so, did you make sure they are tight?

 

[CaliforniaSpecial]

Do you have LCA relocation brackets? If so, did you make sure they are tight?

Yeah I do and I had that thought as well. They were/are tight and there are no witness marks anywhere that would indicate they were moving.

 

[5.0 Probie]

OK... Yes, all parts have a tolerance, but a 1/4" difference (If accurate) is excessive to me. It does sound like, after your research so far, that something is internally broken or bent or beyond its productive life in the rear end.

Questions... Is this car lowered and if so, is there an adjustable PHB? That could be the 1/4" difference if you have lowered the car and have not changed to an adjustable PHD. But from what you have said, it sounds like the bushings are OK and should not be the sloppiness factor...

Can discs allow what you are describing? Boy, I have seen some crazy stuff, but I wonder if skwrel is closer to the issue than a few worn discs is.

Just get the car in the air, pull off the calipers and see how much slop you have in each side. You should find out dang fast if the pig or discs or clip(s) are the issue. Unlike Bushings, those parts are either working or not...