Convertibles, scuttle shake and pig iron

Post Tue Apr 04, 2006 10:09 pm

Hi folks,

The other day I was tidying my boot out and I removed a 23.5kg ballast weight as I was curious about what would happen.

It seems like I get a bit more scuttle shake then usual now - although I'll have to stuff it back in for another test to be sure..

So I'm wondering if anyone else has tried it, what it is officially there for and if it really does detune the shake or it's removal is merely highlighting worn shocks or bushes I may have already had.

Ideas, opinions, tests etc?
325i 1986 'vert.

Cheers

Post Tue Apr 04, 2006 10:12 pm

Can't answer your question but where is this weight?

Post Tue Apr 04, 2006 10:19 pm

Never tried removing it, as I don't think they'd fit it for no reason (& I don't buy the weight distribution theory). Interesting to hear your findings!

I've never seen anything official, but I did a fair bit of searching around about it - found the same theory as you as to why they're there.

It's in the well in the drivers side of the boot Zayyan. Apparently later BMW convertibles have em too...

Post Tue Apr 04, 2006 10:28 pm

It's all to do with shake fequencies and wave periods, so yeah it's for detunting the shake, that's why it's in an extreme corner and low down to give it a larger moment. They could have put 23.5kg more steel in the chassis though!

Post Tue Apr 04, 2006 10:29 pm

Zayyan wrote:Can't answer your question but where is this weight?

On the left side of the boot is the jack. On the right hand side the floor at the edge is flat.
Removing the trim reveals the flat bit as a cover with 4 bolts. The bolts are holding the cover + weight via rubber bushes to the cars in the bottom of the back wing, at the back.
I was originally looking for some plastic fastners that had strayed when i came across it (and the fastners). I weighed it and it's 23.5kg of iron.
I suspect that the car had two main vibration (twist) nodes that are even and add together, the weight detunes one of them and therefore makes the shake much less noticeable as Q (the sharpness of the resonant frequency) is reduced and the energy spread over more frequencies. Sort of like a bell with a weight stuck to it.
But I'm only guessing - I might put in back in this week and see how noticeable the effect is - it does seem more skittish.

Post Tue Apr 04, 2006 10:31 pm

Which is of corse what I was trying to say.................but in a much more scientific way

Post Tue Apr 04, 2006 11:18 pm

So does it effect me putting in a battery or would I need to remove the "weight"?

Post Tue Apr 04, 2006 11:34 pm

Pop up stairs when the misses isn't watching an weigh your battery.

Post Tue Apr 04, 2006 11:45 pm

Sorry Widge was writing when you posted! We seem to agree - I'll probably pop it back in tomorrow morning and see what the effects are.

I guess if you want to put a (2nd?) battery in the car then as long as it's a secured weight in the same position (and total mass) it will have the same effect as the pig iron. This same effect will be the subject of tomorrows experiment!

A weight saving idea might be to move the battery from the front to that place, don't fancy routing the cable tho.. might be the original reason some models had it there (did they or am I dreaming?).

Post Wed Apr 05, 2006 12:02 am

Yeah some did/do, but cabs for some reason always seem to have it in the front.

Post Wed Apr 05, 2006 5:34 am

also found this some time ago when playing about with my sub. it is also on the left even on my RHD 92 cab (320). i did take it out and drive and did'nt notice personally any difference, but we have no good roads here as they are mainly straight or too rough to drive fast on. Put it back in in the end as the car is slow anyway and this didn't make it faster

i guess the engineers did quite a bit of testing and calculations to work out why this should be here. Also been told that if you put an Mtech spoiler on boot of a car without Mtech shocks it will lead to tyre wear due to the extra weight as the standard shocks/springs are only designed to take the cars weight plus occasional boot loads!

As for putting 25 kg in the chasis of a cab..... they already put at least 25kg in the chassis, have you seen those sills and the rear seat crossmember!!!

mrlee

Post Wed Apr 05, 2006 8:37 am

Yeah but the amount of scuttle shake there is they could have put a bit ore in there!!!

Post Wed Apr 05, 2006 8:44 am

Ive yet to see this wieght in the uk.

Andrew

Post Wed Apr 05, 2006 8:48 am

I'm busy today but I'll take a photo of it in the cad if you want.

Post Wed Apr 05, 2006 11:21 am

stevetigger wrote:So does it effect me putting in a battery or would I need to remove the "weight"?

What I meant was.......Is there room for a battery?? I need to put my battery in the boot because of the M50 conversion.

Post Wed Apr 05, 2006 11:26 am

Oh theres room, battery was in the boot of my coupe, and the well must be the same size, not sure if there will be a tray to put it on though.

Post Wed Apr 05, 2006 11:36 am

I had to take mine out to be able to run twin exit's. I havent really noticed any difference though

Post Wed Apr 05, 2006 11:53 am

mrLEE30 wrote: Also been told that if you put an Mtech spoiler on boot of a car without Mtech shocks it will lead to tyre wear due to the extra weight as the standard shocks/springs are only designed to take the cars weight plus occasional boot loads!

I find that hard to believe as the spoiler doesn't weigh much and doesn't create that much down force

Post Wed Apr 05, 2006 11:59 am

Widge wrote:Oh theres room, battery was in the boot of my coupe, and the well must be the same size, not sure if there will be a tray to put it on though.

I know theres room, Ive only ever owned Sports before this cab so every E30 I have had, has had the battery in the boot!

It was just the fact, Ive never really looked under the tray in the boot because I knew the battery was in the front!

Post Wed Apr 05, 2006 1:16 pm

i've never known about that before, i will check this out!

Post Wed Apr 05, 2006 3:18 pm

Here it is

Post Wed Apr 05, 2006 8:14 pm

Weight re-fitted:
Ok so travelling along the same roads at about the same speeds there
was much less scuttle shake with the weight fitted.

Without weight the effect was enough to have me looking in the
rear-view mirror at the rear window to see it it really was twisting about
like a giant spring - on the same bit of road with the weight there was
no shake at all.

So in summary without the weight it will still shake but it seems to
settle down very quickly and some roads simply cannot shake it any
more. Someone of the GB club forum said that a loose exhaust added
about 50% to the shake, I've got a pretty loose exhaust so I'll get some
more solid rubbers to secure that I think.

For flat roads I suspect you would not notice the difference, modern UK
roads however seem perfect to induce a good shake without the weight.

A long term project is to find a decent battery bracket for the boot and a
long thick cable for the battery - and investigate fitting a smaller metal
plate with the battery clamped on top, equaling the 23.5kg of the
original. I suspect 23.5kg is the minimum you can get away with as
BMW were trading perfect damping and weight. Then I've have a
genuine weight reduction without turning my car into a giant spring

Cheers and thanks for all the input