Worried with those subframe stories

[alexk]

Alex dont be so frustrated.

Just drive the bloody thing and enjoy it.
Life is short.

[Alx]

Originally Posted by alexk

Alex dont be so frustrated.

Just drive the bloody thing and enjoy it.
Life is short.

True...

[The Gorilla]

Hi,

Alx- I do not have pics of what I am
suggesting only the direction I have
gone, which is far away from stock/std.

There are several ways the rear subframe
/ subframe mounts can be altered, without
taking out the boot floor section which
BMW seemed to favour.

Its the rear mounting points where all the
issues first stem from, as the front [rearsubframe] mounts
are weled into the 'monocouque beam section'
fabricated into the shell.
Its the same section where the seat belts are
fixed to on the strap bottoms etc, and where
BMW welded in massive 'damping plate' for the
diff etc.

I will see if I can take a picture or two,
as the direction I have gone does now allow
better visual of the existing rear subframe mounts
which I am no longer using.

Regards,

The Gorilla

[AlexGTT]

Originally Posted by The Gorilla

Hi,

AlexGTT- I think you could achieve the
same results with less work.

On a Car that has not suffered any metal
fatigue then a 12.9 HT Bolt that has a fixing
point on the top when bolted up from underneath
[ie welded plate recessed into the boot floor]
would act as the dowel, then an alloy collar
fitted over the bolt that sits into the correct
sized recess in the subframe bush when tightened
up could act as the dowel peg.

The existing boot floor fixing point is 'spot' welded at the bottom
skin and then the metal tube extends upto the upper
skin with another flange, where its spot welded
to the upper skin.

The tube is to accommodate bolt length.

So by continuing the hole thru into the upper boot
floor skin you can weld in a fixing point below the
boot skin and cover with an access plate etc.

Nice thick plate welded at the top would allow the bolt to be fitted
from below as norma,l and then fit a large M12 flange bolt
tightened from the top, with or without access plate [if recessed]


Its basically what Merc did on the 190 etc, but there
access plates on the upper boot skin are tack
welded on, and sealed, more to stop condensation
causing corrosion.

This would give you a HT 12.9 Bolt fixed at top
and bottom with a alloy collar fitted over to
create the dowel/peg for the recess into the
alloy centre of the subframe bush.

Shear factor on a m12 12.9 bolt is way
above any lateral forces or spot loads
that the CSL will ever generate.

Could either make own subframe rear bushes with
shore 90 or 95 poly, [or delrin ?] and then have inserts made
to suit the new dowels/pegs, or if there is enough
meat on the std E46 bush insert, re-drill them and
use them for a little compliance on a Road / Track car.

I spent a long time fucking around under the
E46 CSL rear boot area cut much away and learnt that
BMW basically got it wrong and the design is
flawed regarding rear subframe mounts.

Given that the standard look was lost long ago
I have gone a different route to that above, but its a gamble,
so time will tell if the Primate's thinking outside the
box, is in the pound seats or not as the case may be.
When its finished and works will give the SP
and few pics.

Regards,

The Gorilla.

Thanks Gorilla, I'm liking the sound of that. Anything to make the fix easier! I'll post pics, whatever the solution.

Originally Posted by Alx

Hi Gorilla,

That looks interesting.

Any pictures of this modification?

I am quite frustrated at the moment.

I think I'll end up doing the foam injections myself or just selling the car...

Alx, don't just throw the foam in. I'm convinced, as others, this is a bad idea.

[Rick H]

I have to admit to not fully understanding the proposed solution - but two things to be aware of are local bearing failure (i.e. tearing of thin steel around the bolt) and block shear failure if the bolt is close to an unstiffened edge (think pages in a ring binder). These two modes of failure should be greater than the shear capacity of the M12 bolt to allow it to reach full capacity. Welded-in bosses will help this - but only locally, they may just transfer the problem elsewhere, it's difficult to judge without seeing a sketch.

[shane@mbtech]

Plate, weld, tie cage in. Should do the trick.

After that I will do probably 3k per year if that. Should suffice for me.

It's funny how some go and some don't. Alex you must have put in a fair few ring laps in yours and lots of trackdays too, but yours is ok. My old Csl was hammered around tracks, it had a track life before I got it, I could tell. But my boot floor was fine.

I like to get going fast and stay going fast.

My theory is it's the traffic light Grand Prix session launches which do the damage. That coupled with poor design obviously.

[Alx]

Originally Posted by AlexGTT

Alx, don't just throw the foam in. I'm convinced, as others, this is a bad idea.

Thanks for the advice Alex.

Some have said that the foam just postpones the problem, but I don't really know what to believe anymore.

As some suggested, I will just go and drive the car - we only live once!

[The Gorilla]

Hi,

Here are the pics taken which show
the metal tube that is welded in to
the bottom of the boot floor where the
subframe bushes sits against and the
middle section of the 3 sheet layers.

Centre picture shows it the best.

The bolt that aligns with the std subframe tube was my
''guide' bolt for welding on my plates with
dowels as I used the original subframe
bolts as my template for setting up the
way I have now done the rear subframe and
suspension etc.

As I said before its a work in progress and
a radical move away from the std set up,
but the issues reagrding the std subframe
fixings are illuustrated and can be rectified
without ripping out the boot floor even if
the sheet metal has suffered fatigue etc.

The main reason that the metal fatigues
and tears is that the subframe bolt thru the std
bush is only fixed at the bottom of the
tube it screws into, and thus when the
subframe moves around [sawing action]
due to the elongated hole in the bush for
subframe fitting, the sawing action tears
and rips at the bolt fixing which
in turn loads up and splits the surrounding sheet.

If the subframe bolt was also fixed at the top, then
the bolt would be unable to move around
like it does and hence the subframe bush
would do its job correctly, and absorb the
load and lateral forces etc.

As I mentioned by recessing the top skin
and welding in a plate, then fitting a longer
bolt and having a flange nut tightened also
from above the whole tube bolt arrangem,ent
would have to bend [shear] before any deflection
was absorbed into boot floor skin to start the
tears etc.

As you can see I have gone in a totally different
direction and now have all four subframe bushes
with 'dowels' which fit inside the subframe bush
inserts, M12 HT bolts with clamps across the
bushes, and then hold it all in place.

Ripping out the boot floor skins is counter productive
as its not the floor skins which are the issue, its
the design of the mounts and how the subframe
bolts fix a flat bush without dowel.

The foam is really a nonsense, in that its not
the floor skin sheet metal that is the issue.

Regards,

The Gorilla.

[Alx]

That's very interesting!

Thank you for taking your time to explain and to take the pictures.

[Rick H]

Hi again - bending and shear are two different failures - shear is a slicing action of two parallel planes (scissor blades shear paper) whereas bending is caused by eccentricity of the load from fixity (diving board) resulting in a tension tear/compression ripple in the bolt - assuming it is held fast enough to allow generation of full fixity that it. You dont want ANY bending on an M12 bolt (f=m/z - and z is tiny cos all the material's in the middle of the cross-section), any bending will crucify the bolt - even worse for a fully-threaded bolt (thread right to underside of head). Combined shear and tension is about 1.45 unity from memory.

There is double shear (two shear planes on the bolt) - if that can be generated up to full capacity that may help - the bolt would need to be fixed both sides of the component to generate that.

Again, I can't tell from the pics how the bolt will be loaded so the above may not be relevant.