To my other TR6 pages

September 1, 2014

Rear Suspension

The TR6, like some of its ancestors, has an independent rear suspension using semi-trailing arms.  The arms are largish aluminum castings that carry, in addition to the wheel hubs and brake backing plates, a recess for the coil springs, and a bracket for a link to the lever arm shock absorbers.

My trailing arms were dirty, but otherwise in good shape.

The trailing arms pivot on brackets mounted to the frame.  The factory rubber bushings can be a chore to remove, but a little shop built persuader helps a lot.  I found that running a 3/16 drill bit into the rubber through the bushing all the way around made it even easier.  There was some corrosion in the bores, but after cleaning it out they still measured within spec.

Brackets and bushings removed, ready for cleanup.  Also all the rubber bushes and plugs removed from the arms.

I'd read in multiple sources that the studs that mount the hub to the trailing arms are prone to stripping.  This wouldn't be unexpected for fairly shallow fine threads in aluminum.  A very common fix (or preventative) is to install thread inserts into these holes.  The threads in my arms all appeared to be fine, but there is no better time to install an ounce of prevention than when everything is apart anyway.  I decided to proactively upgrade the threads with Helicoil type inserts.

One often discussed challenge to installing inserts in the trailing arms is that the arms are such a large and odd shape, they are just about impossible to mount on a drill press, and a potential danger of drilling by hand is getting a crooked hole.  I really believe this threat is a little overblown.  There is a straight hole already there, and it only needs to be enlarged a little, and then tapped.  The larger drill bit will try to follow the original hole, and anyone who has handled a hand drill before will know how to let it find the path of least resistance.  Likewise with the tap--it wants to go straight down the hole, and will only go off track if there is some outside force deflecting it.

On the other hand, these trailing arms are not cheap if they need to be replaced due to some unforseen problem in drilling or tapping, so some kind of insurance seemed like a good idea.  There are some guide jigs available commercially to help with this, but they aren't cheap.  If buying the jig were the only option, I'd probably just do it by hand, and be pretty sure it would come out fine.  In the end I made my own small dril/tap guide.  Unlike at least one commercial offering, mine only guides one hole at a time, mainly because I wasn't willing to devote a larger piece of stock to it.  Ther jig consists of an aluminum carrier block and three steel guide bushes--one to go over an original stud to center the jig, one for the drill, and one for the tap.

I chose to stay with 5/16" studs, but to install inserts for course threads instead of fine.  

Here are the trailing arms, cleaned up, painted, and plugged.

The springs measured a little longer than the spec allows.  After a quick query on one of the forums, it appears that everyone's springs are longer than spec, so I didn't worry about it.  The pics show the springs after derusting, in the powder coat oven, and ready to install.

I planned on using urethane bushes in the trailing arms as opposed to the original rubber, mainly for longevity.  Since urethane bushes, unlike rubber, need lubrication, I installed grease zerks on the bottom side of the bush bore.  There is already a nice land there in the casting, as if it was an option the factory wanted to leave open.

Now comes the interesting part.  The original design of the TR6 rear suspension didn't provide any means to adjust wheel camber.  A number of enthusiasts have devised schemes to correct this omission, and there is at least one popular aftermarket product that addresses it.  A quick look at some of the geometry of the trailing arm mounting points can help illustrate the challenge of camber adjustment.

The inner and outer brackets that hold the trailing arm are different parts.  The main difference is the height of the pivot pin holes relative to the frame attachment points.  This pic shows a 7/16 rod run through the pivot bolt holes in the two stock brackets.  It is easy to see that the rod is not parallel to the plane of the chassis rails.  A close look will also reveal that both brackets are slightly cocked to the right to allow the pivot bolt holes to all be colinear.  There is just enough slop in the frame and bracket holes to allow this.

Measurig the difference in height of the pivot pins and applying a little trig, we find that the pivot angle is about 1.8 degrees relative to the plane of the chassis members.  One way to change the wheel camber is to adjust this angle.

There is a very nice analysis of camber geometry and a method for adjusting it at the Buckeye Triumph site.  The method involves swapping out brackets with different pivot heights.  While I'm sure this approach can be very effective, swapping brackets is a pretty time consuming task, and finding the right combination might take several iterations.

There is also a product available that includes brackets with pivot pins that can be adjusted vertically to give a range of angles.  The product is popular, and many users endorse it, so I assume it works adequately.  I consider it pretty pricey, though, and it does have a few quirks that give me at least a little pause.  For one, I'm not sure if the brackets can provide the colinerarity of the pivot pins through the entire range of adjustment.  That is, with one bracket pin higher than the other, will they be colinear?  If not, it appears that the pivot bolt must be cocked a little inside at least one of the bushings.  If the pins are colinear, then it looks like the pins must be cocked a little in the brackets, which could affect how well the bolt holds.  I should say that I have not examined these brackets up close, so it is possible that these things are addressed in ways I'm not aware of.

In the end, I decided to make up an experimental adjustment system that soothes my worries.

I got a couple of pieces of 3/16 x 3" steel, and cut them a little shorter than the trailing arm mounting members on the frame.  These serve as a carrier plate that the trailing arm brackets will attach to.  With the 7/16 rod on place to keep the brackets lined up, and the carrier plate drilled to allow the bracket mounting bolts through, I tack welded the brackets to the carrier.

Then I drilled, tapped the carrier, and countersunk the brackets for flat head screwas to hold the brackets on the carrier in the aligned position.  I then ground away the tack welds so the brackets and carrier could be separated.

I had to weld little extensions to the outboard brackets to allow a larger adjustment range.  It might have been easier just to make new brackets.

Then the mounting holes in the carrier plate and outboard brackets were milled to a short arc centered on the lower inboard mounting hole.  The carriers were also shaped and lightened a little.  The idea here is that the carrier, along with the brackets, can rotate a few detgrees around the lower inboard mounting hole, but since the brackets are fixed to the carrier, the alignment of the trailing arm pivot holes remains constant.  The upper inboard mounting hole also has to be elongated slightly.  There is also a little adjustment pin welded in at the far outboard end of the carrier.

Everything was then powder coated or plated.  All four of my trailing arm brackets had four shims from the factory.  The new carrier does the job of three of them, so I still needed to install one shim.  The shim also proivided a little space between the carrier and the frame member.

For an adjustment mechanism, I welded two little ears to the frame at the ends of the trailing arm mounting members.

The adjustment mechanism itself is just a long bolt, held in place at each end.  There is a little stainlesss block that can be moved up and down the bolt with a nut above and below.  The block engages the pin at the end of the carrier plate.

With the parts of the experimental gizmo finiished, I had to turn to the other pieces of the rear suspension.  The shock links are an integral part of the suspension.  They hold the rear end of the trailing arm up.  I noticed on the two links I bought that one had zinc plated washers, the other had plain washers.  The difference can just be seen in the picture, but it was a lot more obvious in person.  I get a little tired of being the suppliers' QA department.

Also the paint was pretty pathetic on the links--it would easily scratch off with a fingernail.  In a few steps that I admit might be a little anal, I stripped the paint (it didn't take much), plated the lower parts of the links, and painted with real primer and paint.

So here are most of the parts for the suspension.

Mounted the lever shocks.

A cargo strap worked OK as a spring compressor.

On the frame.  I used a nice hefty stainless washer plate to bridge the arced holes.

A closer view of the camber adjuster.  I've got it set in the middle--about what the stock brackets would give--until the suspension is fully loaded.

Getting impatient to get this thing on its own feet.

Comments to:

To my other TR6 pages