FT: Frame (Twin) Rear shock absorber alignment

[davidd]

The Comet gearbox and RFM flex if worked hard. Not a serious problem, mostly. But, when racing hard there is a lot of flex.

The shocks will work fine if there is no side flex. I always made an effort to keep parts aligned.

The above was made in 2000 before Works sold a coil-over.

I redesigned it in 2014 for a lighter design based on the original mount.

David

 

[medat727]

Medat,
It looks like you made a new lower spring seat in addition to the new piston rod. I am doing the very same piston rod fix on my Brampton / Works conversion, the result of the same mode of failure. My spring seat was shot into infinity during the incident so I am making another. Is the spring seat continuous or does it have the slot milled into it as in the original? I don’t feel the slot is necessary since the lower mount is removable.

Thanks very much for the pictures and details

Mine too was ejected never to be found, I made mine without the slot and a little thicker than the original, the seat and the longer tailpiece were both made from 7075 for added strength.

David

 

[oexing]

My verdict in this matter is, when you feel the need to have a bigger thread on the damper piston you just fight symptoms but really you should better go for the cause . On any other bike that original thread will be allright . Not so on a Vincent when no give is designed into the damper for non-parallel bores in damper bodies, rfm holes and oil tank through holes for damper bolts. So nobody should be surprised about snapped rods from undetected sideloads as you can expect all sorts of motions in the geometry of rear suspension parts over full stroke. Same goes for dampers on Girdraulics and Bramptons. In some places you can do mods for Rose joints or spherical plastic bushes like from Igus so this will save you from more snapped rods and leaking seals in the damper. Any side loads onto damper rods are no-go definitely, there is no substantial bronce bush inside and in consequence you´d just have to live with short overhaul intervals due to oil leaks from shot seals and fu**d up damper piston. One end of the damper may have no space for spherical bearing properties so at least the other end will get that kind of give for misalignment.
Also when looking at TT´s photo of the AVO damper with the offset cross tube welded on - not overly brilliant I´d say, the tube should really be on centerline of the piston rod, else you get that dreaded bending load on the rod and wrecked seals in there, and bad for the piston as well.
Anyway, while having a job to be done on these items you better try best to care for these "side"effects in suspension or you will do same in short time again. As somebody said: Do it right or do it twice . . . .

Vic

 

[medat727]

My verdict in this matter is, when you feel the need to have a bigger thread on the damper piston you just fight symptoms but really you should better go for the cause . On any other bike that original thread will be allright . Not so on a Vincent when no give is designed into the damper for non-parallel bores in damper bodies, rfm holes and oil tank through holes for damper bolts. So nobody should be surprised about snapped rods from undetected sideloads as you can expect all sorts of motions in the geometry of rear suspension parts over full stroke. Same goes for dampers on Girdraulics and Bramptons. In some places you can do mods for Rose joints or spherical plastic bushes like from Igus so this will save you from more snapped rods and leaking seals in the damper. Any side loads onto damper rods are no-go definitely, there is no substantial bronce bush inside and in consequence you´d just have to live with short overhaul intervals due to oil leaks from shot seals and fu**d up damper piston. One end of the damper may have no space for spherical bearing properties so at least the other end will get that kind of give for misalignment.
Also when looking at TT´s photo of the AVO damper with the offset cross tube welded on - not overly brilliant I´d say, the tube should really be on centerline of the piston rod, else you get that dreaded bending load on the rod and wrecked seals in there, and bad for the piston as well.
Anyway, while having a job to be done on these items you better try best to care for these "side"effects in suspension or you will do same in short time again. As somebody said: Do it right or do it twice . . . .

Vic

This is straight from the horse’s mouth, I had contemplated the 1/2” thread as I was looking at the damage trackside and this just confirmed it, that and the bushings or a wide tailpiece like DD does will do the trick.

David


I would recommend making a new shaft out of 17-4 h900 or h1153 1/2” ground rod.

The threads in the eye are 3/8 x 16

The end that holds the piston on is 1 inch long and is threaded 5/16 24

Some type of binding or sideloading has caused the fracture.

By making the shaft at 17-4 stainless steel

You will immediately double the strength of the shaft additionally if you simply threaded 1/2 by 20 TPI where it screws into the aluminum eye, you can increase the section where all the stress is being put. If you do this you must run a half 20 then hex jam nut under the bump stop and then tighten the eye up against this.

If you just want it simply fixed back to stock you can call Works shox spelled with an X this is Gil‘s daughters company Works Performance is out of business of course.

If there is no binding in the system the shaft should have been plenty strong the same design is utilized on BMW mono shock motorcycles and Honda ST 1100s for many years with the Same shaft design. Due to the fact that the Vincent may experience sideload and the eye does not allow for side to side twisting without loading the shock it may be worthwhile to upgrade the shaft design.

Pierre