The problem with the rear sway bar kit

[jtschug]

So I bought the combo kit of a heavy duty front and rear sway bars. I installed the front one years ago and it made a huge improvement to the handling of the car. I tucked the rear one away as a project for another time. That time has finally arrived now that I'm doing major work on the rear axle and suspension.

I've read complaints that the rear sway bar adds a lot of squeaking and banging noise and doesn't make any noticeable improvement in the handling. Now that I've got the kit out and mocked up on the axle, I can see the problem(s)

The bar is attached to the rear axle with some flat iron perches bent into a "U" shape, then a flat piece with two holes goes on top of the perch and below the rubber pivot bushing the bar goes through. A bracket wraps over the rubber pivot bushing/bar and lines up with the two holes in the flat piece, then this is all secured with a big U-bolt that wraps around the axle tube with the threaded ends sticking through the holes in the flat pieces and the brackets

On the body side the kit provides two 90° brackets and a large bolt. I guess you are supposed to drill through the subframe and bolt this bracket to the inside surface.

The bar itself is a big torsion spring. When the rear axle goes up and down relative to the body, the bar pivots in the rubber bushings and the bar basically does nothing. When the body leans relative to the axle, this twists the sway bar, as the sway bar exerts a force to untwist, it pushes up the side of the car that is low, or pulls down the side of the car that is high, thus reducing body sway. Just like the other suspension springs, it takes hundreds of pounds of force to twist the sway bar even one inch. That force must be transmitted to the bar by the brackets on the body and the brackets on the axle.

On the body side, the bolt holding the 90° bracket to the subframe is perpendicular to the force, meaning the bolt is clamping the bracket to the body, then friction between the body and the bracket is what twists the sway bar. There is a limit to how much force this friction can hold before the bracket on the body moves, squeaks, and/or bangs.

On the axle side the flat iron "U" perches are loaded by the bar in the center of the middle span. There is some additional structure from the flat part with the two holes, but there isn't much structure preventing that span from sagging under load. Even worse, the full load is transmitted to the axle at the ends of the U. There is absolutely no structure preventing the U from spreading under load causing the perches to "squish" when loaded up with hundreds of pounds of force.

The attachments of the sway bar must be solid and firm in order to twist the bar and get benefit from it. If brackets can slip and squish, the bar won't be loaded with enough twisting force to act as a torsion spring and reduce body roll during turns, and this could lead to all kinds of unpleasant noises.

To fix this problem I am going to weld gussets inside the perch to brace the sides of the U and prevent the ends from spreading. I am also going to weld the flat piece to the top of the perch, then I'm going to weld the whole thing to the axle tube. This will give the bar a solid structural mount to the axle housing. On the body side, I'm not sure the subframe is thick enough to weld the 90° directly on. I think that will need to be welded to a plate, and then that plate will be welded to the subframe. I haven't decided if the bolt provides any structure. I may not use it.

[RedBird64]

I ended up using some thick angle iron for the mounts. They were very solid and didn't move at all but all that didn't help.
Also bought some clamps to limit how much the bar could move side to side and that helped a little but still had banging, especially when traversing curbs. The real problem was that the bar simply wasn't bent correctly. I suspect it's just a universal, one-size fits all for 9" Fords that "universally" doesn't fit anything perfectly.
On top of all that, I couldn't tell any difference over just having the front bar.

I finally got under there with my cut-off tool and sliced it into pieces to get it out . Didn't notice any difference in handling after removal either.

Hope you have better luck with yours. Maybe they finally figured out how to shape them right.
I suspect any handling improvement isn't realized without much wider, stickier tires and then really throwing it into a corner.

Scott

[jtschug]

That is very good information!

I've looked around and aftermarket rear sway bars shifting side-to-side is definitely a thing, and this will definitely prevent the sway bar from reducing body roll. Corvette and Miata owners who race their cars with aftermarket sway bars use a shaft collar by the axle bracket to prevent the bar from shifting. I'm going to add a bearing surface to the perch, put a shaft collar around the bar and then sandwich a large nylon washer in between with a little bit of grease.

[RedBird64]

I look forward to seeing how it works out for you!

S

[jtschug]

I've made some more progress on the anti-thump sway bar. I've welded a fork plate to each mounting perch and I've welded each perch to the axle.

There is a collar on the bar at each perch that prevents the bar from moving side to side by touching the fork plate. See pictures attached.

[JEDI BASS]

Do you feel any benefit?

[jtschug]

I've made a bunch of changes over this winter and I'm doing a series of drive tests and shakedowns in preparation for an extended drive on Saturday. The changes are:

[*]Changed rear axle from 28-spline 9" 3:1 open diff to 31-spline 9" 3.25:1 TruTrac
[*]Replaced rear drums with disc brakes in the rear
[*]New rear shocks and leaf springs
[*]Rear sway bar
[*]Subframe reinforcement/rust repair
[*]"Bedliner" undercoating in rear
[*]Replaced single master cylinder with dual unit
[*]Replaced stock prop valve with adjustable

I first did some drive tests last week and found the brakes dragging, so I had to pull the master cylinder back off and shorten the push rod. That fixed the dragging problem.

I am not getting a lot of banging and noise out of the rear sway bar, and I am noticing a lot less body roll. I've only tested it up to 40MPH so far. I think I'm going to tighten the sway bar links front and rear a bit more, there is a little rattle on pavement transitions.

One positive side benefit I am hoping for is elimination of tire rubbing on turns. I have 7" wide wheels and fender skirts, last year on hard turns I would get very slight rubbing. My testing so far has been without skirts. I hope to do some tests today with skirts and see if that is fixed.

[jtschug]

I put a couple hundred miles on it yesterday, and it is definitely a huge improvement in handling. Body sway is now minimal and steering is much more stable and predictable. No annoying noise, although earlier shakedowns had some noise I tighened up the links and it quieted right down. I also did shocks and leaf springs and all the associated bushings, so those also probably contributed to the huge improvement. I'm even happier with the brakes now :-)

The bar is mounted at about 10 o'clock on the axle when looking from the driver's side and the body "L" brackets about 7 1/2" above the hem flange on the subframe. The axle perches are equal distant from the axle tube ends, so the differential is not centered between the mounts. None of these dimensions were given in the instructions, and I spent a lot of time mocking it up to find the optimal positions. The bar is bent to fit in a pretty narrow gap between the shocks and the subframe, if the bar shifts from side to side it would definitely make knocking noise.

[jtschug]

Here are pictures of the brackets and reinforcements welded to the subframe.