View original topic: Radius arm centering rings (and other ways to stop the wobbles)
raygreenwood Tue Jun 16, 2020 11:09 am

Ok…so this is something 100% of all 411/412 owners require. It has been too long coming.

These are the radius arm centering rings. Some people call these track control arms. They have a pair of donuts toward the rear.
Here is the diagram….and circled are the parts. In the crappier books….Clymer and Chilton, the printing is so bad that for YEARS…I never knew there were parts missing on my first 411.

The centering rings (called support rings in the parts book are #2 in the diagram- part #411 401 159. The track control donuts (called a damping ring) we will also be speaking of and there are 4 and they are part # 411 407 127.

NOTE: if the original donuts are not actually split from the center hole outward…just use what you have. They do not really wear out and they are thick enough that the core has not really changed over time unless they have been submerged in water or oil.

New ones are easy to make though if you need them. I will show that at a later point.

DO NOT…use urethane bushings!!!!!!!


Because the stock donuts are 45-50 durometer Buna N. They can be a little soft…and a little harder would be better. But ….using urethane will LITERALLY snap the steel suspension eye. Been there…done that.

A 50 durometer Buna N rubber bushing has the same “hardness” as a 50 durometer urethane bushing….BUT…urethane has 5X the compressive energy storage and rebound. In short, it’s a more efficient spring.
How does it break the steel socket/eye?

Because when the bushings are at rest…car level….there is very little compression between the donuts and the flange welded onto the radius arm and the thick forged washer at the back. They are “neutral”. Because the arm is at an angle in moves in “two-dimensions” when you go over a bump. It rotates and changes angle in the socket….in two dimensions.

But when you go over a bump….or even where the suspension has full extension….

You get these pinch points. The compressed, stored force of 55 durometer urethane is over 50,000 psi. It literally mangles the forged washer at the rear… but is NOT strong enough to snap off the welded flange on the radius arm. But it is strong enough to snap the socket surrounding the centering ring that is attached to the subframe.

Bushing hardness wise….if the centering ring is in place…...and its keeping the bushings centered in the recess you will see in the pictures long as the bushings are hard enough to keep them from over compressing….fore and aft…causing toe-out…they are just fine.

They do nothing else in your suspension. The stock is 50 duro approximately. Replacing these with Buna N or Neoprene…of 60-70 duro maximum….would be all you need. Urethane is the WRONG material for these bushings. Too much spring energy.

The centering rings:

Virtually all 411/412 I have worked on…no longer have this part. It’s a poorly designed part. The part has to be too thin to fit the subframe eye for the radius arm.
In order to keep it from breaking being so thin…they made of low to medium density polyethylene. With weather, dirt and grease..…it literally disintegrates. Or….if you have the control arms off for any reason, they fall out or fall apart and either get lost or are deemed insignificant.

Why are these centering rings critical?

The three COMBINED MAIN items that cause the “wobbles” or the shimmies in 411/412…are the idler arm bushing (you need the bronze bushing), the centerlink (it needs to be rebuilt with bronze thrust washers or replaced with stock every 30-50k miles)….and the radius arm centering rings being missing. Yes, slack in the strut tubes and strut bushings add a little to the wobbles..…but these three items above do much more damage.

What does the radius arm do?

Bluntly forms a triangle and keeps teh control arms stable in the fore and aft direction.
As you drive forward…road force on the tire tries to push the wheel rearward. This causes the control arms to try to pivot rearward at the control arm bushing end. Since the control arm itself is trying to keep the wheel in one place....the tires actually pivot outward
Hitting bumps does the same thing. Yes…we are peaking of “toe-out”. This spreading force is why you have "toe-in" adjusted into the system.

To control this and keep from beating the control arm bushings to death…the “radius rod” or “diagonal arm”….forms a triangle with the control arm. As the forward motion pushes the control arm backwards....the radius arm back stops it.
The radius arm pushes against a joint that has a donut bushing under load or compression on each side. That loaded rubber joint arrests rearward motion and rebounds it a little.

So….as you go over ripples and bumps at speed….this compression and rebound of the radius arm donut joint….causes an oscillating toe-in/toe-out.

This amount of flex is small...but constant but has high force levels…and is not by itself what causes the “wobbles”....until the centering ring wears out.

Because the control arm bushing is a single pivoting point….its front/back motion drives the radius arm….both up and down and from side to side in the socket. The centering ring is what keeps that motion from vibrating from side to side and from wearing out the hole in the mounting eye on the subframe.

So, the donuts and the centering ring work together to control the wobbles.

So all of that being said:

Here is all I have left of a stock centering ring:

Sorry…I have one complete one somewhere I bought NOS many years ago…when I no longer needed it. You can see that its cross section is roughly 3/32” thick all around. Its made of what appears to be low or medium density polyethylene…essentially milk carton material. It is essentially a plastic grommet.

Its only function is to stop the slight side to side movement of the arm in the hole of suspension and prevent metal to metal contact and wear. It’s the totally wrong material for that. Nylon would have been ideal.

Bluntly put, dust and grit get in between the donut and this part and it just grinds it into dust. This is why you never even find pieces of it.
In this sketch already shown…the centering ring is BLUE.

Here is a gallery of my grimy parts so you can see what I am using for centering rings. These worse superbly...better than stock by far.

Here with the back donut off you can see the nylon thick washer. You will see in a moment why this is good to have.

First glimpse…..You can just see what looks like a “packing” around the rod end right? And notice there is no flange on this side of the socket?

That’s because it’s a flange bushing…and not a “grommet” like the stock one.

In order to be able to manufacture this part out of a stiffer/stronger material…it needed a thicker inner ID wall section. And….being made of Delrin instead of polyethylene…it would not flex to be able to be squeezed in.

So….how does it fit in the eye if its got an inner wall section that is larger than stock?

Because I enlarged the bore.
Notice that the flange/cuff is spot-welded on. Its two pieces. As long as you do not cut any of those spot welds you can enlarge the holes almost 1/8”.

I did the enlarging with a die grinder and a 120 grit sanding drum a little at a time in about five minutes until the centering ring snapped in firmly. I think I enlarged it roughly 1/16".
Make your centering rings first and then shape the hole until they fit.

So, what did I make and how does it fit and what is it made of?

Some snap shots. By the way these look a little grubby…because they have close to 70K miles on them.

Here are the dimensions:

I made these on a drill press with a file and a dremel tool. I have also had a set machined that are virtually identical. You can use the material below. Black Delrin 3/4” ID tube. The OD is slightly smaller than what I started with. Mine was 1- 3/8”. This is 1.250”. Its 1/16: inch smaller on both sides.

I just did this by putting a rubber chuck in the center of the tube and using the edge of a flat bastard file to make a smooth notch, checking with caliper every few seconds. Then I gut it off while spinning with a hacksaw and filed it flat on both sides.

You can also make these from 6/6 Nylon and probably UHMW or polypropylene with no issues.

greggearhead Tue Jun 16, 2020 12:36 pm

Thanks Ray. I wonder what percentage of 411/412's have these anymore?

Hawker Tue Jun 16, 2020 12:56 pm

Excellent work, again Ray 👍🏻👍🏻👍🏻


raygreenwood Tue Jun 16, 2020 2:20 pm

greggearhead wrote: Thanks Ray. I wonder what percentage of 411/412's have these anymore?

I have only found them on maybe two in my life......and they were wrecks that had been sitting for years. Not especially high miles. The few rolling cars I have seen them on.....they were just pieces of the part. Not functional.

But they are definately necessary. Ray

Lars S Wed Jun 17, 2020 7:08 am

Thanks for sharing your knowledge Ray - we learned a lot today!

/Lars S

raygreenwood Thu Jun 18, 2020 11:17 am


I left out a bit of history on this part. I did not immediately come to the form and material that I showed here. I tried three methods total. All of them "performed" very well.

However, method #2 that I show below needs to be set up carefully. the first method I showed here (the flanged bushing).....just like the original stock part....the flange bushing stays in contact with the bore or "eye" in the subframe and any movement fore, aft or radially of the radius sliding against the stationary low friction bushing ring....instead of teh bushing itself sliding in and out.

With method # 2.... below...a simple tube.....the tube is butted up against the donut flange on the radius as forward motion pushes the tires outward (toe-out)....the radius arm is pushed backwards slightly compressing the forward donut and sliding the radius arm rearward.

The tube style bushing slides with the radius arm through the bore in the arm So....the bushing is going to wear a little faster. But will still work perfectly and get better mileage by far than the stock centering ring.

The tube length should be about 0.050" shorter than the length of the hole in the mounting ear. That way when it slides rearward its not crushed up against the rear bushing.

The third method works very well. Its a short tube the exact length of the hole with a thrust washer on each side. Its really a three-piece version of the stock bushing.

Its only draw back is that with the stack of parts its a little tricky to install. Its easiest to install with the control arm bushing out of its yoke....slide the forward donut , thrust washer and tube onto the radius arm, place in bore, put rear thrust washer, donut, dished washer on....and then install and make up the bolt only enough to keep the stack of parts from falling out of place....then install the control arm bushing in its yoke...then tighten the rear bolt.

I changed the name of this thread. Its going to be about all of the issues that cause "the wobbles" ....and why...and how to fix each one. Ray

raygreenwood Tue Jan 16, 2024 6:36 pm

As a second supplement:

The method in my last post shown below

....will work just good as anything in my original post.

NOTE: for the new method listed DO NOT need the thrust washers in the diagram above in gold (its good to have them but not 100% necessary). You only need to make the sleeve/ring in yellow

If you go back to my original post you can see the dimensions of the original flanged centering ring I made here:

You can make these from either nylon tubes or flange bushings from bearing and plastic supply houses.The critical dimension of the nylon tube to fit the rod is the ID....which is 0.798" or ~20.2mm.

You can find a 20mm tube or flange bushing and sand the ID until it fits or even if its slightly loose on the ID it should work

As I noted you will find that the hole in the ar on the suspension really only gives you enough for about a 1/16" wall section so the whole OD of the tube should be ~0.923" or ~ 23.4mm. So if you can find a tube or flange bushing that is 24-25mm can sand it down....slip it into the space with only the rear donut removed....then scribe the side of the tube...pull it out and slice it off whitch a hacksaw. Slip it back in and tighten down the rubber bushing , washer and bolt. Lube it with synthetic grease first. Done.

So....if you wanted to buy a piece of nylon tube that needs only a small amount of sanding on the ID and a small amount on the is a part # from the McMaster Carr catalog.

It is 20mm ID (you need ~20.2mm) and its 24mm OD (You need ~23.4mm)...and its 20mm long so you can slice off two approximately 1/4" wide slices once you have sanded it and you have your parts for about $12 not including shipping. If you are not in the should be able to find this size plastic bearing sleeve in a lot of places. Ray

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