Rear shock mount reinforcement plate placement

Springs and shocks. What keeps your car tight around corners?
EBSF
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Rear shock mount reinforcement plate placement

Postby EBSF » Sun Feb 16, 2014 1:21 am

So, upgrading shocks and springs (Bilstein HDs and H&R OE Sports, going for stock appearance in a daily driver). I got the Z3 rear shock mount reinforcement plate (51-71-8-413-359) in the kit from Pelican. The question is, where does it go?

I have seen two configurations in schematics and forums:
(a) on top of the sheet metal inside the trunk with the collar nuts directly on top of the plate (with the RSM and gasket in the wheel well); and
(b) on top of the RSM, with the gasket on top of the plate, inside the wheel well, and the collar nuts directly on top of the sheet metal inside the trunk.

Any thoughts would be most appreciated.
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Re: Rear shock mount reinforcement plate placement

Postby M20_fever » Sun Feb 16, 2014 7:31 am

Like this:

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#16 is you reinforcement plate.
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Re: Rear shock mount reinforcement plate placement

Postby 7pilot » Sun Feb 16, 2014 7:19 pm

The plate goes on top of the sheet metal and then the two lock nuts that secure the top of the damper to the body.
The plates spread the shear load in order to prevent the sheet metal from getting torn out.
The answer is (a)
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Re: Rear shock mount reinforcement plate placement

Postby EBSF » Mon Feb 17, 2014 1:17 am

Thanks. Everything went in just great. One potential problem, though.

On my test-drive, I went out to do some freeway on- and off-ramps. The thing went around as if on rails. I also had the car up to speed, say 75mph, sustained, no problem. Around mile 7, however, I suddenly had a vibration going through the entire car. I reduced speed, limped to an exit, saw nothing wrong on a quick inspection. I started up again and no problem, sustained 45-50mph. Got back and the lug nuts were perfectly snug.

I suspect a harmonic vibration from the road surface, which was grooved longitudinally (i.e., with traffic, not laterally, 90 degrees to traffic) when the vibration started. On the re-start I was on asphalt (recently resurfaced & totally smooth) and had no problem.

The question is, what else might this be? I have brand-new billies in, so I would imagine they would dampen any road vibration, my hypothesis notwithstanding. Maybe some drive train issue?
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Re: Rear shock mount reinforcement plate placement

Postby M20_fever » Mon Feb 17, 2014 4:56 pm

drive shaft flex disc.
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Re: Rear shock mount reinforcement plate placement

Postby EBSF » Mon Feb 17, 2014 6:32 pm

I was thinking something drive-train-related. I had the center bearing go out on an E28 a while back, although that was much worse.

The flex-disc (guibo) seems fairly far forward to have been affected by dropping the final drive in connection with the rear spring replacement. Any chance it's the center bearing or anything to do with the final drive mounting bushing?
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Re: Rear shock mount reinforcement plate placement

Postby M20_fever » Tue Feb 18, 2014 2:35 am

Doesn't matter how far forward it is, the damage would come from wheel hop, tire chatter, or just wear if its old. I go through a couple every season because they don't hold up to burn outs very well, lol, same symptom.
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Re: Rear shock mount reinforcement plate placement

Postby EBSF » Tue Feb 18, 2014 4:57 am

It doesn't look like a huge job, just an unexpected one and so a PITA. I've got four Billies in the garage waiting to go into the wife's Passat and I was hoping to be done with car stuff for the time being once those were in. Guess not.

Just got the H&R OE Sports into the front end and the struts into the mounts. I'll button up everything and see whether I can get in a test drive with the bad drive shaft. I'm figuring I've got a whole new car once this all is done.
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Re: Rear shock mount reinforcement plate placement

Postby M20_fever » Tue Feb 18, 2014 4:06 pm

famous last words, lol.
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Re: Rear shock mount reinforcement plate placement

Postby EBSF » Tue Feb 18, 2014 4:58 pm

I'm thinking a new set of transmission mounts wouldn't be a bad idea for the sake of prevention, while I'm at it. Any thoughts about brands? There seem to be a number of after-market options (UUC Motorwerks, etc.). Bear in mind this is a stock M20/ZF 4 HP-22 configuration in an enhanced DD, so low horses and no tracks but is driven.
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Re: Rear shock mount reinforcement plate placement

Postby M20_fever » Wed Feb 19, 2014 2:54 am

Its an auto? Just get stock mounts.
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Re: Rear shock mount reinforcement plate placement

Postby 7pilot » Wed Feb 19, 2014 8:20 pm

Did you feel the vibration thru the steering wheel?
Or via the seat?


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Re: Rear shock mount reinforcement plate placement

Postby EBSF » Wed Feb 19, 2014 9:00 pm

It was ubiquitous and more in the aft cabin. When my E28's center bearing or guibo went several years ago, it banged on the floorboards. In the present case, the vibration was higher-frequency, more of a hum than a bang.

The procedures floating around (Pelican's and maybe Bentley's) have one remove the final drive mounting bolt to drop it 1"-2" to facilitate spring replacement. No one mentions replacing that bolt's lock nut. I didn't have a new one handy over the weekend but I do now, and I plan to replace it once I get done getting the new springs into the front struts. Any play around the final drive could account for this.

Further consideration has me wondering whether the cause instead might be CV joint or maybe U joint damage from moving things around. Once I get things buttoned back up, I'm going to do some more research and see whether I can re-create the problem.
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Re: Rear shock mount reinforcement plate placement

Postby Ryki » Sun Feb 23, 2014 8:33 pm

You'd be surprised how sturdy the CV joints are, I've rebuilt quite a few and seen some major punishment go through them. Hopefully you can get everything figured out!
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Re: Rear shock mount reinforcement plate placement

Postby EBSF » Tue May 27, 2014 10:45 pm

Just to close the loop on the vibration problem, I just posted the following in another thread:

So, the problem seems to be the final drive mount lock nut. This came off to lower the final drive so as to install new rear springs. The procedures I was following (Pelican's and Bentley's) made no mention of this point, so I re-used the existing lock nut, albeit with some trepidation. I observed the vibration on the test drive and ordered a new part. I've been testing on an increasingly demanding basis for a few weeks since replacing it, and not a hint of the vibration I observed.

Interestingly, the nut that came off was a lower-grade nylock nut, while the stock replacement that I re-installed was a higher-grade self-locking nut.

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