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Rear end corkscrew when lifting off, shock issue??
Slater - 14/10/21 at 03:09 PM

I have a sierra based MAC#1 with IRS, fitted with Gaz adjustable shocks. Recently I've found when travelling at 70 mph and suddenly lifting off the throttle the back end tends to "squirrel or corkscrew" a bit, it's quite unsettling. It's just one corkscrew movement as the rear end rises up a bit as I lift off. There are no clunking or metallic noises, I've just done a spanner check, so nothing loose on the suspension, I'm thinking it could be the shocks?? Both rear shock adjusters are set to 3 clicks from the softest setting, as per MAC#1 guidelines and were fine for 13,000 miles. If I adjust them to the max soft setting the car still does the similar corkscrew when lifting off, but to a lesser extent.

The shocks are from 2007 when I bought the kit and have done 13,000 miles from 2009 to date, there is no obvious oil leak but quite a bit of black dust stuck to them. So i was thinking of removing the shocks from the car to check them.

Is there home bench test I can do? Push and Pull???
Or should I send them away for proper refurb? Gaz refurb charge is £150 for rear pair and £176 for front pair, so it's fairly expensive.

Anything else I should be checking?
Any comments welcome.....

Note: Diff is 7" 3.92 with push-in shafts, diff mounts are all tight too.

nick205 - 14/10/21 at 03:37 PM

Is this something new that's happening when driving?

Have you changed the tyres recently or tyre pressures?

Have you adjusted any suspension setting - toe/camber etc?

I ask, because I found on my Sierra based IRS MK Indy that fitting around with some shim washers to give the rear wheels a very slight toe-in improved the handling noticeably. The MK Indy didn't have any means of rear toe adjustment so shim washers between the hubs and uprights was the way I adjusted the toe.

I aslo reached 3k miles before selling the car. If you've reached 13k miles I'd imagine the shocks need servicing or at the very least a proper setup.

gremlin1234 - 14/10/21 at 04:12 PM

if you put more weight in the back does it get any better, - or any worse?
try again with a full tank of fuel?

Slater - 14/10/21 at 04:17 PM

quote:
Originally posted by nick205
Is this something new that's happening when driving?

Have you changed the tyres recently or tyre pressures?

Have you adjusted any suspension setting - toe/camber etc?


It's only recently started happening, I've not changed anything at all, but I'll have a quick check on the tyre pressures.

coyoteboy - 14/10/21 at 04:19 PM

My bet would be rear toe. I wonder if your bushes are aging?

adithorp - 14/10/21 at 06:57 PM

quote:
Originally posted by coyoteboy
My bet would be rear toe. I wonder if your bushes are aging?


Sounds feasible. Might need more than just a nut and bolt check. Inspect uprights, wishbones, mounts, etc, for anything untoward. Maybe something has cracked and is allowing the rear geometry to vary as you lift of and the loading alters.
Some lightweight Fury chassis for instance, had problems with lower wishbone inner mounts tearing away from the box section. Presented as the car getting a bit tail happy.

Sarah - 14/10/21 at 07:02 PM

The bushes in trailing links on my old SD1 were so were worn you could drive at 10mph and steer the car just from movement of rear axle by power on and off?

So, they might be worth a look (I know the SD1 is live axle but...)

Slater - 15/10/21 at 12:25 PM

OK chaps, thanks for the comments, as it could be quite a serious problem, I think I'll take the rear end apart and do a full inspection of the wishbones/mounts/uprights etc, then replace the bushes and maybe get the bones re-powder coated. I was half thinking of doing this anyway due to age of car. I can also have a good look at the gaz shocks at the same time. At least it's happened at the right time of year.

Deckman001 - 15/10/21 at 01:42 PM

Hi ya, I hope you get it sorted easily, I thought I'd mention that on a recent YouTube watch of a turbo locost builder finding out that his rear trailing arm brackets had elongated over time making the rear end loose, so that's another possibility i guess.

Mr Whippy - 15/10/21 at 02:10 PM

quote:
Originally posted by Slater
OK chaps, thanks for the comments, as it could be quite a serious problem, I think I'll take the rear end apart and do a full inspection of the wishbones/mounts/uprights etc, then replace the bushes and maybe get the bones re-powder coated. I was half thinking of doing this anyway due to age of car. I can also have a good look at the gaz shocks at the same time. At least it's happened at the right time of year.


tbh with home builds it's probably a good idea to at least once every second year of use, just to take the suspension apart and check the welds and bushes. I had a tubular rear trailing arm snap at the factory weld on my Bluebird which almost sent me into a head on with a Golf who was fortunately paying attention and got out my way. I took off all the arms and got that one re-welded and then the lot blasted and dye penned at work.

nick205 - 18/10/21 at 12:46 PM

For servicing (and setup) of the shocks it may be worth talking to "procomp" on here. Believe they offfer that kind of service.