RSM's & rear knocking on bumpy roads.

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TerryO
Joined: Fri 23 Sep, 2011 12:37
Posts: 32

  Z3 roadster 1.9
Location: Yeovil

RSM's & rear knocking on bumpy roads.

Post by TerryO » Sun 02 Sep, 2012 08:23

Hi all, after months of trying various heavy duty rear shock top mounts to cure a bad knocking noise with no success, I finally bit the bullet and bought a pair of Rogue enginering mounts for £100. However, after a trip to watch the Winscombe hill climb yesterday, the knocing noise from the hood stowage area is now worse than ever! If it's not my rear shock upper mounts knocking, where can the sound be coming from? I have a PI shocks and springs fitted. This is 30mm lower. I bought the kit from POTN earlier this year. I can't see how the Rogue mounts can allow this metallic knocking as no metallic parts can come into contact. It's as if someone is using a hammer on the rear bulkhead every time I drive over the slightest bump in the road surface. I've read that the fuel tank can start moving if the strap cushion pads drop out, so I'll check those later. Can worn rear anti-roll bar bushes produce the same noise, I know the fronts can? This noise is really beginning to annoy me now. TerryO.
1998 Z3 1.9 (M44). Arctic Silver, red leather seats & door panels, manual roof, 18" Team Dynamics, 30mm lowered Pi suspension, generic twin outlet SS tail box.

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Gazza
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Joined: Tue 04 Oct, 2005 21:58
Posts: 9201

  M roadster S54
Location: East London/Essex

Re: RSM's & rear knocking on bumpy roads.

Post by Gazza » Sun 02 Sep, 2012 08:41

The rear ARB bushes can cause a similar noise.

I had a broken ARB mount on the rear floor which caused the ARB to knock on the top of the exhaust box.
Gazza

"Understeer is when you hit the wall with the front of the car, oversteer is when you hit the wall with the rear of the car. Horsepower is how fast you hit the wall and torque is how far you take the wall with you"

S54 M roadster Image, BMW Z1, BMW M3 CSL, Mustang Fastback

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Del
Joined: Sat 19 Nov, 2011 18:35
Posts: 2122

  Z3 roadster 1.9

Re: RSM's & rear knocking on bumpy roads.

Post by Del » Sun 02 Sep, 2012 09:04

Hi - Given what you have done already, my guess for bumpy road noises would be knocking noise = the rear anti roll bar mounting bushes, rattling noise = drop link(s).

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stu
Site Admin
Joined: Fri 10 Oct, 2003 18:49
Posts: 3540

  Z3 roadster 2.8 supe
Location: On the edge!

Re: RSM's & rear knocking on bumpy roads.

Post by stu » Sun 02 Sep, 2012 09:07

Checked fuel tank mounting straps?
someone in a minority once wrote:I know I'm in a minority
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Mike Fishwick
Joined: Fri 19 Jun, 2009 11:27
Posts: 1991

  Z3 roadster 2.8
Location: Daglan, France

Re: RSM's & rear knocking on bumpy roads.

Post by Mike Fishwick » Sun 02 Sep, 2012 12:31

Are your mounts secured with the proper nuts - with built-in washers? Normal nuts can make a lot of noise, allowing the mount to slightly tilt in response to load. If your car is a pre-September build, do you have the original steel spacers fitted above the mount? If this is OK, try fitting an extra couple of gaskets between the mount and the body.
A Z3 is not just for Christmas - it's for life!

TerryO
Joined: Fri 23 Sep, 2011 12:37
Posts: 32

  Z3 roadster 1.9
Location: Yeovil

Re: RSM's & rear knocking on bumpy roads.

Post by TerryO » Mon 03 Sep, 2012 12:01

Hi, Previously I had HD Meyle mounts fitted. These had two flange nuts fitted on top of the oval strengthening plates from inside the hood stowage area. These were tightened to 20 lb.inches. The Rogue mounts come with a similar strengthening plate, with studs facing downwards. So you have to fit the flange nuts from inside the rear wheel arch! Not easy when your car is lowered 30mm. As the Rogue mounts have a small rubber doughnut either side of an aluminium alloy body, the rubber can not be punched through like the standard BMW mounts. The Rogue metal washers are also smaller in diameter & can not contact any other metallic parts, even if the rubbers are squashed.

http://www.rogueengineering.com/rogue/S_BUSH/RSM.html

I now believe the rear anti-roll / stabiliser bar mounts must be defective. The drop links look fine, so it must be the 'U' clamp rubber bushes bolted to the rear bulkhead. I plan to have a crawl under the car tonight. I'll have to drop the spare wheel out to check the diver side bush.
TerryO.
1998 Z3 1.9 (M44). Arctic Silver, red leather seats & door panels, manual roof, 18" Team Dynamics, 30mm lowered Pi suspension, generic twin outlet SS tail box.

Del
Joined: Sat 19 Nov, 2011 18:35
Posts: 2122

  Z3 roadster 1.9

Re: RSM's & rear knocking on bumpy roads.

Post by Del » Mon 03 Sep, 2012 13:06

Hi Terry

The non-M, OE BMW rear ARB bushes are only £5.60 each and the rear drop links only £7.56 (Lemforder) at carparts4less.co.uk. Hopefully not an expensive job.

wifevanda
Joined: Mon 09 Aug, 2010 20:36
Posts: 52

  Z3 roadster 2.0
Location: dagenham

Re: RSM's & rear knocking on bumpy roads.

Post by wifevanda » Mon 03 Sep, 2012 19:49

hi i had knocking noise on pot holes.after changing rear shocks top mounts, found rear springs different heights .so change those . still the noise was there , and then had the n/s trailing bushes changed. result prefect, worth a look

TerryO
Joined: Fri 23 Sep, 2011 12:37
Posts: 32

  Z3 roadster 1.9
Location: Yeovil

Re: RSM's & rear knocking on bumpy roads.

Post by TerryO » Tue 04 Sep, 2012 13:19

Hi, Thanks for the suggestions. I had a crawl around under the back of the Z last night before it got dark. My ARB bushes look & feel fine i.e. no play there at all. Luckily while under the car I did notice that my rear exhaust hanger bracket had lost three of its four spot welds, so I've bolted that through now. Thats the thin metal bracket spot welded to the rear of the boot area on to which the rear most exhaust rubber hangs.
Looking at the top of my rear shocks, there's only 30 - 40mm of the piston rod exposed with the car stood still under its own weight, which doesn't seem much travel, even though it has a 30mm lowering kit fitted. Could the rear shocks be bottoming out internally? I would have thought the combination of bump stop on the shocks & rubber cones in the middle of the rear springs would prevent this. My rear shock bump stops are currently a 10mm thick x 40mm diameter rubber doughnut on each rear shock piston rod. Should these be thicker? I've seen these bump stops made of 50mm high polyurethane foam on previous cars.
If it's not the shock bottoming out internally, then my next check has to be the trailing arm bushes.
TerryO.
1998 Z3 1.9 (M44). Arctic Silver, red leather seats & door panels, manual roof, 18" Team Dynamics, 30mm lowered Pi suspension, generic twin outlet SS tail box.

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