Always torn...

[cyberman]

Periodically, especially when I'm bored, I toy with the idea for removing the vibration dampening weights bolted to the body behind the rear bumper. The job involves removing the bumper cover - and the weight savings is about 30 lbs. But when I read old threads, it's 50/50 on whether it's a good idea or not. Going back to 2002, some said they tried it and found too much shake, especially on rough roads, so they went through all the work of reinstalling them. Others said they found no difference.

Benefits are less weight (obviously, and an undetectable change in acceleration), and lower polar moment of inertia (easier turning), as compared with some possibility of a worse ride and a small change in the weight distribution (not 50/50). In my case, with 150 Hp, I like less weight and have already removed the spare tire and carrier with no ill effect.

I'm definitely bored...

Here's one early (2002) thread: https://www.bimmerforums.com/forum/s...Weights-in-out

Interesting quote from a later (2011) thread: https://www.bimmerforums.com/forum/s...Bumper-Weights

The Coupe does not have these weights. These weights are placed on the outer rear corners as the roadster body structure is not rigid like the Coupe. They are there to help cancel vibration inherent with an open chassis structure.

Classic British sportscars of the '50s through '70s didn't use them and were well known for "cowl shake" going down the road, as I am sure Randy can attest to with his Healey.
In the mid 1960's, I know GM used "shaker cans" which look like large coffee cans filled with a jelly-like substance in the outer corners of their convertibles like the Corvair, Chevelle, and Camero. They weighed almost 20 lbs each and there were 4 of them!. My convertible Triumph TR8 (and TR7) use cast-iron weights on the front bumper and is the only British car I am aware of that tried to control the inherent vibration of an open chassis layout.

If you were to remove them from your roadster, I wouldn't be surprised if the view in the rear view mirror at speed was fuzzy from vibration.

[BimmerBreaker]

Remove them and drive it before reinstalling the bumper cover. If you don't like it with them removed then reinstall them before reinstalling the bumper cover

[WB-MZ]

Kind of expanding on what BimmerBreaker said, if you really want to know what it does, take the bumper cover off and join an autocross, drive to the Autocross with the weights on and make a run or 2 and then remove the weights and make a few more runs, That will let you know exactly what the difference is when pushing the car. The time slip will let you know the difference or if you spin it out with snap over steer. Also find a nice rough section of road for a before and after test.
I doubt the BMW Engineers went to all the trouble of putting the weights on our cars just to confuse us?

Good Luck,
Wayne B.

[CoconutPete]

At this point I may take mine off just so i can see what the damn things look like. Usually the threads I read about them are so old the links to pictures have expired. The ones linked here, no difference.

[s8ilver]

At this point I may take mine off just so i can see what the damn things look like. Usually the threads I read about them are so old the links to pictures have expired. The ones linked here, no difference.

There you go. I replaced my Coupe shocks with Roadster shocks, and ditched the weights since Coupe never had them.

s-l1600.jpg20190809_141143.jpg


Coupe shocks just don't have the ears that hold the weights:

20190721_112312.jpg

[cyberman]

Thanks, Bimmerbreaker is, as usual, brilliant. If I can get over driving an ugly car that is.

The autocross experiment: One of the posters on the early threads did just that an reported a small improvement. Autocross is low speed, at least the ones we have around here and I'm more concerned about medium speed on rough roads and "vibration" on the highway. So, Bimmerbreaker's suggestion makes the most sense. Unfortunately winter came early here in Iowa, so I'll have to work in a cold garage and burn some propane to warm it, and then drive around on my summer tires which get very hard in the cold.

Still, I'm bored and have some time on my hands. Also, note everyone's experience may be different. I'm running stock suspension parts with a lightened rear (no spare/carrier), and an early model with the battery in the front. Now, if I could put the battery in the trunk... nah, too much work.

[bdiefAZ]

They will be removed at the next opportunity on my car!

[Hoyt Clagwell]

'looking forward to hearing your impressions of the weight delete. I'm wondering if it would increase the amount of under steer on sharp turns although 30 lbs. doesn't sound like a whole lot. I'm probably gonna' remove my 23 year old spare which I'm guessing is about the same weight but not as far back.

[IndianaRoadster]

51128399319 VIBRATION ABSORBER LEFT
51128399320 VIBRATION ABSORBER RIGHT
www.realoem.com

I think this at least explains why BMW put them there. And I might add that other car manufacturers have done likewise.

[cyberman]

I did it. Took about two hours since I was cautious (and fat and old) following Pelican's Brief on the subject (https://www.pelicanparts.com/BMW/tec...eplacement.htm). Maybe because mine is a first year model, I found a few differences. 1) My big bolts (Figs 5 and 6) that come through the bottom are T50, not T45 and I have two 8mm fasteners on each bottom corner connecting the bumper cover to the fender liner, not one (figs 1 and 2). Otherwise, there are a small plastic nut centered on the fender liners that had to be removed to get the liners disengaged enough to access the plastic "christmas tree" rivets and it's not mentioned. I wasn't able to get those rivets back in (fig 4 and note the head is facing down, so get your tool under the head and pry down). I think one would have to remove the wheels and really jack the fender liner out to reinstall them. But it looks like their function is mostly covered by other fasteners.

Now, the immediate impressions. It's cold here and I have summer tires so handling is not great right now. So, I reserve judgement on driving impressions other than say nothing immediately jumped out at me on a short test drive. Maybe a bit more shake over bumps, but really hard to gauge.

Otherwise, I wasn't surprised that my back end lifted enough to be noticeable and the negative camber increased a bit. I have sort of calibrated eyeballs on that since I've been adjusting camber with various thickness spring pads. Aside, Haggerty's Hack Mechanic has a good article on measuring camber here: https://www.hagerty.com/media/opinio...-camber-gauge/ So, I've got to make some measurements to see if camber is more negative than it should be.

I've had a bitch of a time with that since,

A) one spring is slightly more compressed than the other and

B) it seems my car is extremely sensitive to small changes in springing and/or suspension. Some of may recall the nightmare I had with some new springs that were supposed to fit, but ended up jacking up my rear. Perhaps because I removed that spare and carrier and as a first year car, the battery is in front it really reacts to rear weight changes. Those new springs are gathering dust now.

But, I wasn't surprised since before I did anything I lifted the car standing behind the bumper with light force (maybe around 30 lbs.) and noticed it lifted a good 1/4 - 1/2". It's funny, but I can and have loaded 50 lbs. in the truck (as part of a standard ride-height procedure) and didn't notice it settling much. I'll cut some square stock per the article and measure the camber soon.

Suspension is mostly stock except poly subframe bushings and Monroe struts and shocks.

And yea, it's really satisfying to yank those weights - one can't know how heavy they seem until they're out. I'll save them as they sell for over $150 each on eBay and, who knows, I may want them back in at some point. 30 lbs. is about 1% of the curb weight, so I doubt I'll feel any difference in acceleration. Now, If I could swap out those boat anchor seats...

[s8ilver]

I would assume your negative camber decreased, not increased.

[IndianaRoadster]

I would assume your negative camber decreased, not increased.

That's what I was thinking, but I know he switched his springs and had some issues.

[cyberman]

I would assume your negative camber decreased, not increased.

The ride height increased (apparently) and the apparent negative camber increased. And you're right that it went the opposite way than expected.

Not sure why, but the wheels splay out more at the bottom now than they did before. Like going from -1.5 to -2.0 as an example. It should have gone the other way, but it did not. And, I'm definite on the camber change, I won't say it's night and day, but it's noticeable enough that I know I'm not mistaken. The right (weaker) spring has almost 15mm of spring pads and the left has 10mm. I recently purchased and installed this set of spring pads to make adjustments (below).

I'll stop by Home Depot to get some square steel tube stock so I can measure the camber.

pads.png

[cyberman]

So, I take it all back. Basically, can't see any change at all now. After I took it off the jack stands, I drove it about 20' out of the garage and then back in. Can't explain this one.

[BimmerBreaker]

Not sure how raising it would ever increase camber. The rear suspension follows a pretty simple, predictable path. Lowering increases camber and raising it decreases it

[cyberman]

Not sure how raising it would ever increase camber. The rear suspension follows a pretty simple, predictable path. Lowering increases camber and raising it decreases it

I know, just shoot me.

[zellamay]

So, I take it all back. Basically, can't see any change at all now. After I took it off the jack stands, I drove it about 20' out of the garage and then back in. Can't explain this one.

You hit upon a big reason these conversations keep going: Lack of accurate, reliable, repeatable, measuring.

[Pima Roadster]

Interesting that BMW added the weights to the rear of the Roadster & not the Coupe. It does not appear likely that their addition is to reduce NVH but possibly to improve the handling. The early Porsche 911 had weights added to the outer ends of the front bumpers to improve handling. (moment of inertia or polar moment of inertia?)
https://www.roadandtrack.com/car-cul...the-early-911/ The early SWB 911 was very light in the front as opposed to the rear so perhaps the weight was added to the rear of the Roadster to compensate for the weight towards the front?

[Hoyt Clagwell]

Could it be that it was done so that BMW could claim a 50/50 weight distribution? I saw a video somewhere where BMW engineers had a Z3 with a man in the driver's seat, up on a beam under the center of the car and it balanced perfectly. 30 lbs is not much % wise but added to the very rear of the car could make a fair amount of difference in how it balanced on a beam. I'm not sure if that would actually improve handling. 'seems to me that adding weights to the extreme ends of the car might actually make it less stable in the corners but I'm just thinking out loud.

[BimmerBreaker]

Could it be that it was done so that BMW could claim a 50/50 weight distribution? I saw a video somewhere where BMW engineers had a Z3 with a man in the driver's seat, up on a beam under the center of the car and it balanced perfectly.

I've seen an image of that for a few BMW's (E39 6 banger, E30 wagon for example) but never for the Z3...

This was, by far, the coolest implementation of the concept. And IMO one of their last great commercials



And not trying to take the thread off topic but this commercial was also amazing

[cyberman]

You hit upon a big reason these conversations keep going: Lack of accurate, reliable, repeatable, measuring.

Yep, got some square bar stock today to rig up for measuring camber.

But I still say at the time I reported it immediately after removing the weights it was clearly different than now. Big news today is that it warmed up to the 50's and I took it for a long road test all around town, including highways, secondary road, some railroad tracks, basically everything I could throw at it and neither I nor my wife could hear or feel any difference in vibration, noise, or handling.

[Hoyt Clagwell]

I've seen an image of that for a few BMW's (E39 6 banger, E30 wagon for example) but never for the Z3...

I did a quick search on YouTube but couldn't find the video I saw. As I remember it, it wasn't necessarily a commercial. It was indoors and there was a BMW engineer at each end of the balance beam. At first they were holding on to it but as the driver settled in, they both let go and the car balanced perfectly. If those rear bumper weights were indeed installed just so that they could claim a 50/50 balance, I don't think it would necessarily add up to improved handling . I would think it would be better to have the bulk of the weight between the axles but I'm just an armchair engineer. I know up here in the great white north, if I put weights (sand bags) in my pickup truck bed to improve traction in the snow, I put them over the axles. I used to put them at the very rear of the box thinking that the extra leverage would put more weight on the rear wheels. It probably does but it also makes the back end much more prone to fish tailing out of control.

[cyberman]

Hmm, had a disappearing post so I'll try it again. Bottom line on NVH is that after a 40 mile road test involving everything from highway driving to rough roads and 35 MPH smack at railroad tracks, we couldn't detect any difference, so for now, the weights stay off!

[KeysCoupe]

Those weights are there to dampen out “cowl shake” that many convertibles exhibit. The engineers went to some lengths to address this. The weights you are removing are equal to a partial tank of fuel.
Even my old ‘65 Corvair convertible has “shaker cans” in each of the corners, essentially a coffee can filled with silicone gel. They weigh about 15lb each, but without them, there is very increased shake/vibration, especially on rough pavement.
I don’t think the primary purpose of the weights added to the Roady is to approach a 50/50 weight distribution. The Coupes don’t have them because the fixed roof ties the unibody together eliminating the twist and flex of the open body Roady.
-Donny

[cyberman]

Here is the definition of Cowl Shake from the wikipedia:

Scuttle shake (sometimes called cowl shake in the US) is the term used for the phenomenon experienced in many convertible or open top automobiles where, due to lower structural rigidity caused by the lack of a roof, the middle section of the chassis flexes, causing the bulkhead in front of the passenger compartment to move and vibrate when the vehicle is subject to uneven road surfaces.^[1][2] Passengers feel it as a noticeable vibration and shudder.

It's a real phenomena. Although we had FEA to analyze this type of thing computationally back in the 90s, a fully equipped car would be extremely complex and my guess is they used a shaker table during development. A shaker table is programmed to simulate real-world driving including (and especially) extremes. The car is instrumented with accelerometers to assess the vibrations and changes made, such as adding the dampers during subsequent runs. My guess is that under some conditions, there are vibrations and that the dampers do quell them. But, and there are some huge buts here, the whole exercise really boils down to the programming of the shaker table and the engineers' assessment of the vibration data. The programming could be unrealistic, I mean, after all, who really drives for long on really rough roads in a low ground clearance sports car? When I hit railroad tracks now, perhaps a sensor could pick up increased shake, but I cannot, which leads to the engineers' assessment of vibration and my assessment, particularly since I'm sitting on springy foam.

Here's video of a shaker table in action:



In the world of racing, these things get beyond complex: https://mooregoodink.com/how-renault...d-mass-damper/

And in this second article, the author reveals there is a measurable difference in vibrations depending on whether a crash dummy (or sandbags) are used versus a human being. Note the mention of uncontrolled belly mass and that both my wife and I are amply endowed in that regard: https://mooregoodink.com/the-greates...carry-ballast/