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Shock Absorber Snapped


AndyW

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Well, could have been a nasty moment but fortunately happened at low speed. Pulled away from the lights and turned into a left curve and 'bang' followed by rattling and grinding!! Pulled over to find my left front suspension collapsed and the car grounded on the sump.

 

The Gaz shock absorber shaft has snapped under the top mounting bush.

 

IMG_2353_zpsmc956qzj.jpg

 

Anyone had this before, or any thoughts on why it might have happened?

 

Looking from the side of the car at the vertical alignment of the two broken ends of the shaft there is probably about 1cm front/back displacement between them. Also the lower spring hanger was wound up 16 turns (probably by previous owner to give more ride height).

 

The spring was an 8" 350lb. When I replace should I go for the same size? I do need more ground clearance (see my previous posting on a sump guard for a Super Spec), so should I go for a 9" spring which won't need winding up as much? Any recommendations on a supplier?

Edited by AndyW
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I suspect an issue with the top mounting bush being held incorrectly. The bracket in the chassis should clamp up against the inner part of the shock bush, and not able to touch the outer. I can see some washers used as spacers on the top mount - my guess is they are clamping the outer part of the bush, or contact it under loading.

 

Gaz shocks usually use metallastic rubber bushes (or rose joints). If the bush cannot deform, then the shock shaft has to bend, which would likely cause a fatigue failure.

 

Now i see your edit: misalignment of the top and bottom mounts would have the same effect, the bushes can only take so much . Ideally you would have them as inline as possible.

If the coilover was wound up too high, you could be in a situation where it became fully extended frequently. This causes a knock in the suspension, and would cause cyclic stresses on the shaft possibly leading to fatigue. In this case a longer shock (with spring) would be beneficial.

 

Andy

Edited by Grim
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Grim - I suspect you may be right. Although I can move the top bush of the shocker by hand on its pivot bolt, so it's not that tight or binding. I think the top and bottom bush pivot points are very slightly misaligned. That side of the suspension was rebuilt with new wishbones after the previous owner bent it. Not sure how much I can realign the pivot points except by playing around with the penny washer spacers.

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I've also discovered that the aux drive belt on the engine has snapped at the same time (it never rains but it pours!!) I found the top spring cap inside the front chassis cross member so I suspect that it went flying as the shocker snapped and cut the belt. The remains of the belt are wrapped around the inside of the crankshaft aux pulley and a nightmare trying to get out. Going to have to remove the bottom pulley I think.

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You say that the suspension was rebuilt by the previous owner after he damaged it? Could be that the shock was damaged but not noticed and finally snapped? Certainly the bushes should be in line or rose joint ends used.

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cb750 - yes it's a definite possibility that the shock was damaged previously but not noticed, but could equally be due to the slight misalignment of the axes of the top and bottom bush mountings. Neither side of the car seems to line up properly which must be a "feature" of the design. I'll need to see if I can "ease" the holes in the top U shape cross member to make them align better. I did consider spherical joint ends rather than rubber bushes but was dissuaded by Dave at Dampertech.

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No. On the Super Spec the lower shock mount is a plate in the V of the lower wishbone with 4 bolts.

When the new shocker is fitted on the top bolt, this is the misalignment I get at the lower bush - about 10mm. I don't think I can remove the packing shims on the top bush mount as the spring hat will then foul the chassis U channel.

 

IMG_2355_zpsf43rxllz.jpg

 

How do I get the top and bottom bushes to align without forcing the shock absorber to fit? That presumably is what stressed it to break in the first place - the weakest part being the end of the thread by the nut on the rod under the top bush.

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Just to point out that the bracket shown is not the standard Superspec fitting, it is the GBS modification to allow the use of Gaz shocks to replace the original Zimmers. Having said that, I think the same misalignment is there in the original design as my Zimmers are definitely not vertical but also seem to be out by a similar amount. But the original fitment is a bit more flexible to allow for the misalignment.

 

Doesn't help you I'm afraid. Maybe you could get some new brackets fabricated with the U-bracket slightly offset ?

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Hi Al, yes I forgot to mention that the V plate with the bracket fits over the original lower wishbone mounting points for the Zimmers.

 

The rubber bushings will take up a small amount of the offset but maybe I do need to get the U brackets taken off and welded more to the front of the plate (L of photo).

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Guest 2b cruising

I would mark the point where the center, or both outside edges need to be.

Remove the wishbone.

Cut off the bracket.

Fettle the bracket to allow a better fitment then reeled the bracket back onto its mount.

There is no real need to space the bolts out that far if you need to cut the corner of the bracket to get it back in.

My 2b mounting bracketmis the u bracket straight onto the wishbone with a triangle plate under the triangle to spread the load. 3mm mild steel.

The u bracket has also been strengthened where the bolts go through it.3mm flat bar along the inside edge.

There is no deformity on mine whatsoever.

Edited by 2b cruising
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Guest 2b cruising

If there are no capable member local to you, any engineering place would do it for you.

Usually the smaller the business the better and more human like.

The ones I have used for various work have been very fair with the costs.

Edited by 2b cruising
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