Worried with those subframe stories

[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.

[Alx007]

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.

[Alx007]

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.

[The Gorilla]

Hi,

Yes, Bending and Shear are two different
load / Stress factors, but the 'bolt head' that holds
the subframe to its fixing point is approx 60mm
down from where the bolt is secured [depth of subframe bush]

The original bolt is not held at the top but only
by a threaded section in the tube just above the
surface of the lower skin where the bush/subframe is clamped
up to.

Hence when the subframe starts moving around [vibration initially]
via lateral forces due to the elongation in the
bush alloy insert, this then side ways motion is what is
causing the bolt/fixing to start tearing via a pivoting pendulum
motion at the sheet metal, which over time induces the
stress cracks.

Hence, one force shear, when transmitted via
the pivoting penduliam actions of the bolt is then causing
tear, ie the sheet fatigue.

If the main subframe fixing Bolts were fixed at the top also
and the bolt had a alloy over tube acting as the dowel, then the
shear load on its own is unable move the bolt which is then
not only secured in two places, top and bottom, to prevent
pivotal pendulum type movement but it also has a much
larger surface of clamp which also assist in deflecting load
forces via the alloy spacer around the bolt, being absorbed
correctly into the bush.

The combnation of these additional factors would
mean that the M12 H/T bolt is then performing in
true shear loads only, and any load deflection would
then be loaded into the alloy 'dowel' and absorbed
by the bush as it was originally designed to do.

Its almost be almost identical to what Mercedes did on the
190 rear subframe and while many Merc 190 rear
subframe bushes have been replaced [proves it works well]
I have never herad of or seen a Merc 190 subframe ripping
out its subframe chassis mounts or cause any metal
fatigue or tears other than age corrosion.

Regards,

The Gorilla.

[AlexGTT]

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.

Shane, it does appear to be totally random. Mine has done upwards of 15 trips to Ring and average 5-6 TD's a year since I've owned it. With me it's gone from mid 20k miles to high 70k. No cracks and it's constantly inspected.

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!

That's the spirit Alx. :smokin:

Thanks Gorilla, good pics.

[NZ_M3]

Personally I feel the official epoxy fix is sound. It has been used in the past by OEM manufacturers for frame strengthening (Ford, Audi to name a few), in addition it was an official fix for F3 for suspension pickup points. The product is designed specifically just for this purpose (cavity filling, bonding of two plane surfaces and structural strengthening for increased crash rating).

I have the epoxy injected in my car as a preventative (no cracks) and I can tell you for a fact that the difference in rigidity was immediate and noticeable post epoxy.

Even if it isn't a full fix it doesn't bother me ... nothing lasts forever. Plus so long as you don't do the front points you can still put a cage in with subframe pick up (so when mine cracks I'll either cage it or part it out).

The epoxy used has the following technical specs for those more technically minded:

Compressive strength: 15–20 MPa

E-Modulus: 700

±50 MPa

Poisson’s ratio: 0,22–0,26

Volume change: 0–30 % (depending on the curing conditions/volume/geometry)

Water absorption: < 0,5 % weight gain

Test conditions: 24 hours at 98 % humidity and 40

°C

In service temperature rang -40

°C to 90°C

Short exposure (up to 1h): 110°C


And if you are in further doubt about the epoxy ... consider this. We have a factory E46 320i WTCC car here locally that was raced extensively (and did many sprint and 6 hour endurance races regularly). The floor finally gave up (and believe it or not only when the car was in NZ not when it raced in Germany) as the cage in those never tied the rear subframe into the roll cage ... a new floor was welded in and the official epoxy injected. It still races today and as far as I am aware the new floor is sound with no cracks.

As for talks about moisture and rust from the epoxy ... seriously learn a bit of chemistry people. If that was the case then expect all our carbon roof ridges to all rust to nothingness soon - the roof is bonded to bare metal with guess what ... Epoxy!!!

[The Gorilla]

Hi,

Quote[ ''As for talks about moisture and rust from the epoxy ... seriously learn a bit of chemistry people. If that was the case then expect all our carbon roof ridges to all rust to nothingness soon - the roof is bonded to bare metal with guess what ... Epoxy!!!

The steel lip that the roof is bonded to is not only
painted but it has been sprayed with hot galv prior to paint.

Further when the roof is bonded there is
no void between the steel lip and the carbon
edge, hence no moisture can form via condensation.

That is why the factory Carbon roofs have 4 large fixing bolts
that are tightened after the resin is applied to the
lip, so as the pull the roof down evenly and remove any
voids, and also as a secondary fixing.

If you have ever taken one off then you would know this,
its not a bare unpainted lip as you suggest, its exactly
the same paint process as the rest of the A/B/C pillars
and shell for that fact..

However if you belive that an external resin injection
into a void that does not fully seal up that void [boot floor skins] and
has been previously welded will not cause condensation
on the inner sections which will cause roost then you've
not seen inside many chassis sections.

Its nothing to do with chemistry, its the plan fact that your
unable to correctly treat welded surfaces inside voids
such as chassis or multi layer skin areas.

Regards,

The Gorilla.

[NZ_M3]

The steel lip that the roof is bonded to is not only
painted but it has been sprayed with hot galv prior to paint.

Further when the roof is bonded there is
no void between the steel lip and the carbon
edge, hence no moisture can form via condensation.

The surface is in primer I know, but it isn't anything special - but there are tiny little spacers bonded on the bottom of the carbon roof to space the roof correctly for the epoxy to cure and bond at its optimal engineering strength thickness - but you'd know that if you've taken one off before.

That is why the factory Carbon roofs have 4 large fixing bolts
that are tightened after the resin is applied to the
lip, so as the pull the roof down evenly and remove any
voids, and also as a secondary fixing.

If you've seen one you'd know that the factory roof jig has 4 suction cups to hold the carbon roof skin and "six" clamping points (2 on front windscreen and 2 on rear and one on each side) - not sure where you are getting the fixing bolts from. Your roof must be special.

However if you belive that an external resin injection
into a void that does not fully seal up that void [boot floor skins] and
has been previously welded will not cause condensation
on the inner sections which will cause roost then you've
not seen inside many chassis sections.

Right and an unrepaired M3 CSL that lives in good old UK and used during winter has no rust on it at all .... please give me a break ... all cars made during that period by BMW will rust regardless of the treatment. I've seen plenty of UK imported low mileage CSLs (and E46s) to know you live in fairy land where no rust will ever form on BMW cars.

You'd also know that the two rear mounting points has sufficient access points from both the bottom and top for rust proofing to the applied after welding. (I concur much harder with the front points - hence why no epoxy is recommended to be injected in those areas)

I will however concede that when you weld plates to such thin gauge sheet metal, you've sufficiently changed the property of it that no matter how many layers of sealant or primer you slap on there it'll rust eventually.

I however don't agree that the epoxy somehow accelerates that process any quicker or how a car without any repair welds but with just epoxy injection will now rust faster than a car without the epoxy injection.

[JBird]

I also stand by the epoxy fill as the most sensible solution. I've seen profesional FEM studies and crash tests done on dual skin B pillars with and without structural foam bridging the skins. The epoxy structure was nothing short of amazing, and retained adhesion, keeping the skins at an even gap from one and other as the structure was loaded to failure. The OEM in question ended up using this to get 5 star NCAP rating in side impact. (it was some time ago, when this was a big deal to have). This gives me confidence that the epoxy injection in out floors will remain strongly bonded for the life of the car, genlty distrubuting all those axial and sheer lug loads evenly into the two skins over a much larger area than any mechanical bolt / dowel solution ever will.

My only concern is corrosion, but only from the point of view that we have to rely on a BMW bodyshop taking due car in sealing all bare metal from welds etc adequatly, which as Tom said, is in theory possible. If they havn't, well that sux but there is a lifetime warranty on workmanship.

[monkeycsl]

I can't think where I saw the thread but someone I think in America was using a bonding non weld technique to fix reinforced plates to boot floor so no issue with rust on inner skin of cavity.
This seems a very good idea as these epoxy glue chemical weld systems are really strong,what do you think guys.