Suspension #2: Rear axle locating triangle

The differential and axle assembly has to be able move around freely as the rear suspension absorbs bumps and loading changes caused during cornering and braking.  This travel in the axle has to be limited so that the wheels can’t come into contact with the body in any loading condition.  To accomplish this the axle assembly is attached to the body by a rear triangle at its center and a pair of trailing arms at either end.   These pivot together as a sort of 3 dimensional four-bar linkage through the center of the massive ball joint that connects the triangle to a boss nearly equidistant from the two rear wheels on the axle. The ball joint is the fulcrum of a teeter-totter between the two rear wheels.  With respect to each other and this point when one goes up, the other goes down.  This motion is softened with respect to the body by springs that are dampened by shock absorbers and the travel at either end is limited by canvas straps.  The entire assembly is isolated from the rest of the car by rubber bushings.

The ball joint pin is a taper fit into the boss it goes in on the axle housing.  Removing the triangle from the rear axle required heat and a ball joint separating tool commonly called a pickle fork.  The pickle fork works by acting as a wedge between the axle housing boss and the triangle.  I first scraped as much grease as I could off the boss to avoid a fire and to minimize stinky fumes, then heated the boss up, once it was hot I used a big hammer on the end of the pickle fork and the ball joint popped out of the taper. 

A lot of road grime attaches itself to everything on the underside of the car and the axle locating triangle is no exception.

I spoke with a few of my local experts about taking the ball joint on the rear triangle apart to clean it up lubricate it because I thought it felt a little stiff.  Everyone told me to leave that ball joint alone.  Its range of motion is very small and the loads acting on it are large, so what would seem stiff to my hands trying to move the ball around in its socket would be trivial to the weight of the body shifting on the rear axle as it pivots through this ball joint.  I decided all I would do is clean the triangle up, paint it and grease the ball joint. 

I clean the triangle the same way I did the wishbone and ball joint in Suspension #1.  Scrape the big stuff off with a knife and remove any grease.  Mask off the ball joint to keep any media out of the joint then blast it to remove the easiest stuff.  After the first pass in the blaster I scrape any stubborn bits off with a razor blade then blast it again.

The area where the ball and socket is open needs to be protected from blast media. 

Here is the ring clip groove after the first pass in the blaster.  The ring clip end is pointing into the hole it goes in to lock the threads.  I’m not sure if that bump in the casting near the hole is supposed to be there and serves a purpose, or it’s just a nicely symmetrical spot of weld.

Here it is after a second pass in the blasters.  I left the clip in place so media wouldn’t fill up the lock hole.

The bushes look like new after a good blasting.

Once the entire assembly is clean and washed down with POR 15 metal prep it gets painted with POR 15 paint.  I try not to get any POR 15 paint in the ball joint threads because it would make them much harder to get apart in the future.  When it’s assembled to the body I will smear some grease on the threads to protect them from road grime.  I may paint over the POR 15 with epoxy based appliance paint before setting this part aside until it’s time to install it.  I still haven’t settled on the best finish for parts like this.

The cleaned part looks very nice, too bad it’s never going to be seen once it’s installed.

The exposed opening in the ball joint seen above is protected by grease, a metal cup shaped washer that slips over the ball joint pin and a rubber dust cover.  I still need to buy the washer and dust cover as both were destroyed by the ball joint separating tool.



6 thoughts on “Suspension #2: Rear axle locating triangle

  1. “I still need to buy the washer and dust cover as both were destroyed by the ball joint separating tool”
    I regrettably destroyed the iron ring too. Have you find a solution?
    Do you have a draw off it?

    Thank you

    • I haven’t gotten to the point of worrying about it. When I do I’ll post a follow up. I imagine Alfastop in the UK could help you.


  2. Hi Matt,

    I’m not sure if you’re still responding to comments on this old post. I’d like to know:
    – What media you are using for the blasting of these components?
    -How do you like the Caswell zinc plating kit, and are you doing any black oxide plating?
    -Are you only plating the fasteners for one component at a time, to avoid getting ’em all mixed up, or how are you dealing with the organization of plating?



    • Tony,

      I was using a fairly fine oxide media – basically what came in the blast cabinet I was using.

      Caswell kit works okay – but is very sensitive to contamination/surface prep.

      I was plating as I went to keep things straight, but at one point I got enthusiastic and plated a few hundred strays.


      • Thanks. Did you do any black oxide coating/plating- if so how? Great resource- keep up the valuable work, and good luck with the new venture!


      • Tony,

        Caswell has Black oxide and yellow zinc kits in addition to the zinc.

        Thanks for the encouragement on the new venture. I’m now working on a tie in to that will probably make my readers smile.


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