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-   E36/7 Z3 (1996-2002) (http://www.bimmerfest.com/forums/forumdisplay.php?f=7)
-   -   z3 Rear Subframe Vibration Absorbers (http://www.bimmerfest.com/forums/showthread.php?t=606256)

jiminmaine 03-08-2012 10:56 AM

z3 Rear Subframe Vibration Absorbers
 
I have a 1997 Z3 2.8l that I have removed the rear bumper for repainting. Has anybody removed the heavy vibration absorbers? If so, have you noticed any adverse effects ?. Thanks

SUNZOUT 03-08-2012 01:45 PM

Yes - removed. Never noticed any difference.

but I installed a tow bar at the same time so did replace a lot of the remove weight!!

tohbi 03-08-2012 02:33 PM

huh? rear vibration absorbers? never heard of such a thing. is it just some weight on the tail end?

jiminmaine 03-08-2012 02:40 PM

Yeah 2 big wieghts are mounted on each rear corner behind the aluminum bumper. They have a single rubber bushed bolt mounting them to the sub frame. They look to be pretty heavy, I"ll weigh them when off. I guess they are for rear end vibration ?

Monolith 03-08-2012 02:47 PM

Part 5 (there's two):

05 Vibration absorber left 1 51128399319 $123.35
05 Vibration absorber right 1 51128399320 $123.35

http://www.realoem.com/bmw/diagrams/r/n/137.png

tohbi 03-08-2012 02:49 PM

never heard of such a thing. i remember racers drilling the centers out of bolts and replacing parts with titanium so i'm very surprised about ADDING weight to a sports car.

any more info about this is appreciated. thx

Randy Forbes 03-08-2012 03:45 PM

Old news; I took mine off about eleven (>11) years ago. If memory serves me correct, they weigh about 13 pounds each.

I did not notice any difference in noise, vibration or harshness.

tohbi 03-08-2012 03:51 PM

thanks for the info. i've read what i could find in the archives but couldn't find an explanation for the problem.

i don't find this car comparatively lightweight and fail to see what adding these weights accomplish. i mean, i have more weight in the trunk than these things weigh.

and, in a time when manufacturers are trying to reduce weight to improve gas mileage, who woulda thunk bmw would add dead weight?

Pinecone 03-08-2012 03:56 PM

The provide a 50/50 weight distribution with the 6 cylinder engine.

Purely for marketing purposes.

tohbi 03-08-2012 04:00 PM

Quote:

Originally Posted by Pinecone (Post 6687974)
The provide a 50/50 weight distribution with the 6 cylinder engine.

Purely for marketing purposes.

so, "vibration dampening" is just a misnomer?

jiminmaine 03-08-2012 04:12 PM

Well, mine will make great boat anchors. Thanks for all replies.

zooz3 03-08-2012 06:01 PM

coupes
 
are they found in coupes?

SUNZOUT 03-08-2012 07:56 PM

Coupes don't have them

Pima Roadster 03-08-2012 08:48 PM

Interesting that only the Roadster model has them & only the 6 cylinder models to boot. In the very first years of the Porsche 911, I have heard that the bumpers had weights in the ends to affect the polar moment of inertia. I think I have that phrase correct. Perhaps it is the same here?

russpe 01-13-2013 01:53 PM

The above illustration was really helpful since my repair manual didn't include it (bummer). I am getting a bad clunking rattle from this general area, could these Subframe Vibration Absorbers be the culprit? I have these in my '98 Z3 and they move to the touch, should they be securely mounted? Do they pivot or rotate? How do I remove them if they're really not needed? Should I remove them?

BeemerMikeTX 01-13-2013 04:51 PM

Quote:

Originally Posted by Pinecone (Post 6687974)
The provide a 50/50 weight distribution with the 6 cylinder engine.

Purely for marketing purposes.

Interesting. I'll be taking my rear bumper cover off soon for repainting, so will have to decide whether to remove this "balancing weight". Difficult to see any problem it would cause in a front-engine care.

And Pima, you are almost correct. The very first 911s did NOT have these weights, but after more than few "less than F1 quality" drivers experienced car and driver damaging lift-throttle oversteer after they entered a corner too fast Porsche added cast iron weights in the front bumpers to move the weight balance a little farther forward. They also tried two small six-volt batteries (which could be placed farther forward) instead of one bigger 12-volt, and lengthened the wheelbase, before finally addressing the problem with wider rear wheels and tires.

Kornknarr 01-14-2013 09:54 AM

"but after more than few "less than F1 quality" drivers experienced car and driver damaging lift-throttle oversteer after they entered a corner too fast Porsche added cast iron weights in the front bumpers ".

Would that mean that on our cars removing the weight from the rear would lessen lift-throttle oversteer? I'd guess that wouldn't harm?

BeemerMikeTX 01-14-2013 04:31 PM

Quote:

Originally Posted by Kornknarr (Post 7312321)
"but after more than few "less than F1 quality" drivers experienced car and driver damaging lift-throttle oversteer after they entered a corner too fast Porsche added cast iron weights in the front bumpers ".

Would that mean that on our cars removing the weight from the rear would lessen lift-throttle oversteer? I'd guess that wouldn't harm?

I don't think the Z3 has a significant lift-throttle oversteer problem, since it is a front-engine car, although the semi-trailing arm rear suspension may contribute a little. Removal of the vibration absorbers might help a little, and couldn't hurt this, but would also contribute to a little more understeer.

The 911 had a different problem. With the weight of the engine/transmission in the back, if you lift the throttle after you are already in the corner, the weight would transfer to toward the front, which loads up the front tires more and unloads the rear tires some, which reduces the traction available for the rear tires while increasing the traction available for the front tires. If the rear tires were already at or near their traction limit to hold the rear of the car, the rear of the car could suddenly begin sliding out and the car would rotate around the front tires, which have plenty of extra traction, and off the road you would go . . . rear-end first. Not good.

This is not the Z3's vehicle dynamics.

Pinecone 01-15-2013 05:15 AM

All cars have Trailing Throttle Oversteer. It is just a matter of how much, a little or a LOT.

The early Porsche 911 had a LOT, as did my Fiat 850 Spider. My E46 M3 has a bit, just enough to bring the tail out when I want.

The problem with rear engine cars is once the rear started moving out, all that mass out back made it very hard to catch and stop. Also more modern cars are tuned more towards understeer, so the TTO becomes almost neutral handling.

BeemerMikeTX 01-15-2013 05:41 AM

Quote:

Originally Posted by Pinecone (Post 7314348)
All cars have Trailing Throttle Oversteer. It is just a matter of how much, a little or a LOT.

The early Porsche 911 had a LOT, as did my Fiat 850 Spider. My E46 M3 has a bit, just enough to bring the tail out when I want.

The problem with rear engine cars is once the rear started moving out, all that mass out back made it very hard to catch and stop. Also more modern cars are tuned more towards understeer, so the TTO becomes almost neutral handling.

Which I think is essentially what I said. We can quibble over semantics about whether reducing the amount of understeer is the same thing as oversteer, but I think we can all agree that abruptly getting off the throttle if you find yourself going too fast in a corner is a bad idea.

And yes, I agree about the Fiat 850. When I autocrossed one of those in college you had to be smooth with the throttle in corners.

Pinecone 01-16-2013 11:10 AM

My point was the Z3, etc, also have TTO, just not as violent as the early 911s.

ALL cars have it to some degree.

I autocrossed my 850 also. Later, when I did a Skip Barber School, they had a TTO exercise, and they kept telling em I was not inducing TTO, as they did not see the rear step out. I tied explaining to them I a had autocrossed an 850 and did not LET the tail get out far enough for them to see. They were not happy, so I just lifted hard and spun to make the happy. :)


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