Rear end rocks side to side?

Mustang5L5 · Jul 29, 2009

What's up with that? If i put my hip on the rear of car and push, i can move the car about 1/2" in each direction. In fact, under the car, you can see the exhaust move about 1" total.

I have BRAND NEW Max Motorsports LCA's (poly bushings) and NEW Ford stock UCA's with new bushings and bolts.

Should it really move that much? Driving it feels find, nothing feels out of the norm in the rear, but i'm wondering why i have so much movement now.

stang&2Birds · Jul 29, 2009

Mustang5L5 said:
What's up with that? If i put my hip on the rear of car and push, i can move the car about 1/2" in each direction. In fact, under the car, you can see the exhaust move about 1" total.

I have BRAND NEW Max Motorsports LCA's (poly bushings) and Ford stock UCA's with new bushings and bolts.

Should it really move that much? Driving it feels find, nothing feels out of the norm in the rear, but i'm wondering why i have so much movement now.

Damn Newbies and their questions!

In the back of my '86, I still have the orig LCA and bushings. The car has ~160K miles on it now. The only movement that I see is due to the tire sidewalls. If it wasn't so dark, I'd take a video. Maybe on Thursday or Friday.

Mustang5L5 · Jul 29, 2009

That;s what I thought, but this is def more movement than that.

I only have 100 miles since reassembling the entire rear suspension. Everything is new and torqued down.

Guess i'll have to throw the car back up on jackstands and have a look around and push and pull on stuff.

None of the torque boxes or mounting points are damaged. I made sure to inspect these before I POR-15ed the entire underside of the car.

What i'm seeing is the body sways side to side while the axle remains still on the ground. There's enough movement to cause me some alarm and want to check into this.

347HO · Jul 30, 2009

The upper control arms assist in setting pinion angle AND... keep the chassis centered over the rear axle fairly well without a Panhard or Watts link.
If you have noticeable movement, you definitely have an issue with the UCA's and/or their supports.

I just have to ask... do you have the correct "grip length" on your bolts?
Factory or performance replacement hardware?

Mustang5L5 · Jul 30, 2009

The UCA's are new Ford HD arms, with Factory hardware (new)

I'll go under and retorque them and see if that fixes the issue.

GDTrumbo · Jul 30, 2009

2 words..... Panhard Rod. Should have been factory equipment on every Mustang since '79. They are sloppy back there with OEM or aftermarket LCA's.

Hell, even an old 307 Chevy Malibu had one off the assembly line.

GT

67coupe · Jul 30, 2009

Is this movement accompanied with a clunking? If so, most likely worn clutches in the LSD.

To test, jack up rear end, remove rear tire and push/pull on each axle. Mine was really bad (1/4" per side) until I rebuilt and re-shimmed the clutches.

Now when pushing the rear end side to side, the only movement is tire flex. FYI, I have MM lowers and Steeda uppers.

Mustang5L5 · Jul 30, 2009

It's not the axles or anything inside. The movement is in relation to the axle. In other words if i have someone push the car side to side, the axle stays where it is on the ground and doesn't move due to the tires. The body is actually what is moving side to side. You can actually see the mufflers and exhaust move with the body around the arms and such.

Everything's new. It was dark so i couldn't see much so i'll need to investigate. Something definitely has laterally movement under there. Even without a PHB, it shouldn't be this much.

Car feels good on the highway though. I only noticed this when i was checking how much roll the car had by pushing the roof

67coupe · Jul 31, 2009

Okay, sounds like it's not axle movement.

At 130,000 miles, my clutches had the wear described in my post. It was just the first thing that came to mind.

If the UCA's were recently installed, might be worth it to check the torque of the fasteners.

Mustang5L5 · Jul 31, 2009

Well, i went by the car this AM and checked it out. It's the UCA's. They move quite a bit where the UCA's bolt to the body.

So i guess that means times to crawl under and retorque them.

I'm off on vacation for the weekend...maybe i'll tackle it monday

347HO · Jul 31, 2009

Mustang5L5 said:
Well, i went by the car this AM and checked it out. It's the UCA's. They move quite a bit where the UCA's bolt to the body.

So i guess that means times to crawl under and retorque them.

I'm off on vacation for the weekend...maybe i'll tackle it monday

I assume you used the old factory bolts?
If so, the grip length is probobably correct, but I try not to assume anything. Hell, I wouldn't even assume the hole diameter in the replacement UCAs are correct.
If you installed your own bolts, you may want to check for their grip length. If you have too much, then you're bottomed out on the threads and "think" you have it torqued when in fact you're a skosh loose. If this is the case, just install some .030" washers between the bracket and the UCA.

67coupe · Jul 31, 2009

I also re-used the factory bolts, which if memory serves, are torque prevailing fasteners. The nut has a slight oval shape to prevent loosening due to high vibrations. Typically, these fasteners are one time use and should be replaced.

I didn't replace them and since install of the upper and lower CA's, have had to periodically re-torque due to loosening of the nuts. Not loose as in "finger loose," just below the spec of ~80 lb-ft (from memory). I eventually removed all nuts and applied blue loc-tite. Haven't had to re-torque since.

FYI, I had to the same thing on a buddy's 02GT.

trbkrb · Jul 31, 2009

Good info in here guys. I'm about to take on a rear end rebuild since I have a serious clunk in my rear end everytime I start off, shift, and let off the accelerator. Like a bad u joint, but I have a new aluminum one that is perfect. I also have the same side to side movement, so I'll have to look at the UCA bolts too. I replaced them a few years ago, but maybe they are not tight enough.

Thanks for the info!

Mustang5L5 · Aug 2, 2009

347HO said:
I assume you used the old factory bolts?
If so, the grip length is probobably correct, but I try not to assume anything. Hell, I wouldn't even assume the hole diameter in the replacement UCAs are correct.
If you installed your own bolts, you may want to check for their grip length. If you have too much, then you're bottomed out on the threads and "think" you have it torqued when in fact you're a skosh loose. If this is the case, just install some .030" washers between the bracket and the UCA.

I did use the factory bolts. These are the only original bolts I used. I do have replacements though.

I'm going ot try retorquing them first and see how that works. I was planning on going under the car and rechecking every bolt i've installed on the trans and rear anyway as a shakedown after putting a couple hundred miles on the setup.

jrichker · Aug 2, 2009

Be sure to place the jackstands under the rear axle and have it leveled out as if it was sitting on the pavement, ready to drive away. If the bolts are torqued down with the rear axle sitting up or down, it puts a twist on the rubber bushings when it sets back down on the ground.

Mustang5L5 · Aug 9, 2009

Update

Just as suspected, the UCA to body bolts were loose. They were tight, but just not correctly torqued down.

I retorqued every bolt on the UCA's and LCA's to spec. When i put the car down, problem solved.

Car feels better in turns too. It feels so good when you fix a problem and see results!

stang&2Birds · Aug 9, 2009

Thanks for the update!
That's something that I'll now double check when I re-do the rear of my car. One of these days...

Neat, I'm glad you fixed it.

Shaolin Crane · Aug 9, 2009

you probably did torque them originally but if memory services me with my old car after about 200 miles i had to re-torque on the car since the weight of the vehicle normally loosens things a bit

Mustang5L5 · Aug 10, 2009

You are right, but i prob did really forget to put the torque wrench on them.

They required a full turn to reach proper torque, while every other bolt only needed 1/4 turn or so.

67coupe · Aug 10, 2009

FYI for everyone reading the thread.

I had to re-torque the LCA and UCA bolts multiple times as they loosened/relaxed over time. I eventually pulled all the nuts off and applied blue loc-tite. Have not had to re-torque since.

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