Worried with those subframe stories - Page 9

The Gorilla · 07-01-2014, 11:23 AM

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** · 07-01-2014, 06:43 PM

Quote:

Originally Posted by lawsy

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.

Quote:

Originally Posted by Alx

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.

Thanks Gorilla, good pics.

NZ_M3 · 09-01-2014, 10:21 AM

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 · 09-01-2014, 12:15 PM

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 · 09-01-2014, 10:59 PM

Quote:

Originally Posted by The Gorilla

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.

Quote:

Originally Posted by The Gorilla

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.

Quote:

Originally Posted by The Gorilla

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 · 10-01-2014, 02:15 PM

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 · 10-01-2014, 03:29 PM

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.

09-01-2014, 10:21 AM	#83
NZ_M3 S5, Sport On, DSC M-track Join Date: Jun 2008 Location: Auckland, New Zealand Posts: 1,434 Casino cash: $13579	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!!! Last edited by NZ_M3; 09-01-2014 at 10:45 AM.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

07-01-2014, 11:23 AM	#81
The Gorilla S5, Sport Off, DSC M-track Join Date: Apr 2008 Posts: 740 Casino cash: $14401	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.

09-01-2014, 12:15 PM	#84
The Gorilla S5, Sport Off, DSC M-track Join Date: Apr 2008 Posts: 740 Casino cash: $14401	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.

10-01-2014, 02:15 PM	#86
JBird S5 - Full Throttle Join Date: Mar 2012 Location: Munich Posts: 280 Casino cash: $5259	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.

10-01-2014, 03:29 PM	#87
monkeycsl S5 - Full Throttle BMX Park Champion! Join Date: Jan 2008 Location: essex Posts: 444 Casino cash: $18732	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.