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September 19, 2020

Rear Damper Brackets

The earliest GT6 cars apparently had a simple swing axle arrangement at the rear with a very typical tube damper spanning from a stud on the vertical link to a bracket on the frame.  With the advent of the MK2 in 1969, the rear drive design was upgraded to include a rubber "Rotoflex" joint at the outboard end of each half-axle.  While this did improve performance, it also required that the top damper attachment be relocated.  The damper had to be swung outward to avoid fouling the Rotoflex, and Triumph Engineers chose to create a new top damper fixing point on the inside of the wheel well itself.  Many people feel that attaching a damper (and bump stop, by the way) essentially to body sheet metal was a bad design.

The top damper bracket was welded to a doubler plate that was in turn welded to the wheel well sheet metal.




While that view may not look so bad, the fact is that water found its way between the doubler and the wheel well, and went to work.




I managed to repair the wheel well on both sides, but couldn't bring myself to recreate that factory damper bracket.




I'm not the first one to feel this way, and there is even a commercial solution available to move the top shock fixing back to the frame, using an extension from the old MK1 mount point.  Though it's easy to order the kit to make the change, my Inner Engineer rose up and parked himself between me and my keyboard.

"You don't need to order that", he said, "It's a good solid Engineering problem to solve!  Piece of Cake!"

"I don't know", I protested.  "Seems like a lot of work."

"You really need to do more to keep your skills up, Dude", he said, rolling his eyes. "Now close that window and open up some CAD!"

So I did.  I took some measurements from the suspension and mocked up a simplified version of the mechanism.




The fixed line at the right represents the frame itself.  The upper horizontalish line is half of the transverse leaf spring.  The leftmost line is parallel to the plane of the wheel, while the verticalish line just to it's right represents the vertical link.  The two diagonals that end in mid air represent the tube shocks, the right one being attached to the original MK1 chassis fixing point.  The other one is the new shock orientation, and the new adapter bracket's job is to span between those two upper red dots.

The three red dots at the left represent, from bottom to top, the point at which the axle hits the frame, the intended lower limit of suspension travel, and the upper limit of suspension travel.  Since there are no explicit bump stops in this design, the damper itself determines the distance between the two limits.

The commercial design uses the original MK1 shocks, so I did, too.  In effect, the adapter just rotates the original MK1 shock outward just enough to clear the rotating axle parts.

Making the shock more vertical has other effects, though.  The travel of the MK1 shock itself is about 2 1/4 inches, but for an angled shock, this translates to a little more travel at the wheel.  A more vertical shock reduces the travel. 

So, I fiddled with the shock position and orientation for a while, trying to balance the requirements: stay clear of rotating axle parts, keep the axle from hitting the frame on rebound, and try to keep suspension travel as close to stock as possible.

In my final design, the model predicts about 2 3/4" of travel at the wheel. This is less than the MK1 travel, which I believe was over three inches.  It predicts about a minimum 1/4" gap between axle and frame on rebound.

Now, I couldn't really take the predictions of this model as gospel, mainly because it was so hard to get precise 2D measurements from the actual 3D suspension.  So the next step was to actually build the adapter.  From computer model measurements, I made a paper template to fit the existing bracket.




Then, cut some parts from 1/8" (11 gauge) steel.  I was feeling confident, so I cut parts for both sides.




The adapter would attach to the original bracket using the original shock mounting hole, but it seemed like a good idea to install a spacer inside so cranking down on the fastener wouldn't just distort the bracket.  Unfortunately, the hole on the forward side of the frame bracket was larger than the other one.  This was probably because a large shoulder bolt was used to fix the shock.




So, to get a spacer to work, I made these little stepped washers to adapt the hole.




At this point, I could take some measurements to see if I still agreed with the model.  So far, so good.  Next up was a top piece to connect the two side pieces.  This is essentially the same design as the commercial adapter.




Before welding, I wanted to actually mount the shock.  This is a MK1 shock.  It came with the rubber bushes, but not the internal distance pieces.  I had to make those.




Installed the shock, and everything still looked good, so welded 'er up.




Took it all apart, powder coated the adapter and plated the hardware.




And put everything back together.




Hard to see in this bad pic, but with the suspension hanging from the shock, the axle is about 1/8" from the frame rail.  I wish it were a little more.




I compared the dimensions of my final adapter with those that a fellow enthusiast sent me that were reportedly taken from one of the commercial adapters.  They were very close.  So, I consider the design good enough.  Right now, the chassis is up on its top bunk and against the wall, so I can't do the other side until I bring it down.

Even though my Inner Engineer sort of bullied me into this exercise, I really do enjoy this kind of hands-on engineering.  I'm not sure what the commercial adapters cost, but these were less than $10, just for steel and hardware.

Comments to Ed at elhollin1@yahoo.com

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