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Faults, Fixes and DIY Please share your experience and knowledge with other members by contributing your own DIY, or by helping another member find the elusive fix!


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Old Thu, Mar-25-2010, 11:33:54 AM   #31
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Agreed!
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Old Thu, Mar-25-2010, 04:40:07 PM   #32
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Originally Posted by DigitalPunk View Post
Also, how do you feel about using epoxy to fix already cracked sub frames? With welding you fill the cracks, then grind them down and it looks as if there was never even a crack and is flush as the pieces are melted and fused back together, where epoxy would just be coating the crack and not welding them into one piece. This would concern me a bit.

Whats your feelings on this?

One reason I ask is a lot of you will start do the reinforcement kit and realize after you drop the sub frame that you do have cracks all over, specially under the bushings where you couldn't see them (Like I did).
I knew this argument between welding vs epoxy would come up and I was trying to avoid it in my post.

I do my own welding and also work with structural composites so I'm pretty familiar with epoxies, their application etc, and I'm speaking from personal experience (other race cars etc).

There are pros and cons either way - this has been discussed in manufacturing circles extensively and it's been a war within itself. I'm not going to say which is better, but let you figure it out yourself.

In most cases when you weld onto thin sheet metal (e.g. the subframe mount points, the heat causes localized changes in the body metal along the weld line which in the long run may results in the edges of the material along the heat affected zone cracking due to repeated flexing due to vibrations etc (since this would be the weakest point)

However, as we all know, welding fuses the material together and it becomes one - I won't argue with this. It's tried and tested and we know this works.

Modern epoxies are stronger than you give them credit for. If you look for the correct aerospace structural epoxies the performance is pretty darn good. They are used for composite-composite, composite-metal or metal-metal bonds in aircraft, and in a lot of high performance cars. A lot of these aircraft include 30 year old commercial jets (which were never intended for a 30 year life in the first place). Since epoxy is also a little flexible

When applied correctly, the adhesion of epoxy is over the complete surface of the subframe reinforcement, vs the edges (i.e. welded at the edges). When you consider the entire surface area, the strength works out to be the same or higher. You'll have to ensure the metal surfaces are prepped correctly (clean with 10% Oakite 164 alkaline cleaner), you will get the max strength from the epoxy bond.

Downsides to epoxy is long-term weathering and heat. Water is a plasticizer for most epoxy based adhesives. Over time, water infusion (rain etc) will soften the adhesive and will make it weaker. The edges of the reinforcement will need to be waterproofed properly to reduce this effect. Heat also reduces the strength of epoxies, however due to the location of the subframe, this is a non-issue.

Anyway, I'm not saying that one method is better than another - I just prefer to use the epoxy method of attaching the reinforcement plates since it means less cutting (for the "other side" weld).

It sounds like a great idea to weld the cracks first before applying the epoxy/reinforcement. Probably something to look further into.

-A
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Old Thu, Mar-25-2010, 04:51:46 PM   #33
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I knew this argument between welding vs epoxy would come up and I was trying to avoid it in my post.

I do my own welding and also work with structural composites so I'm pretty familiar with epoxies, their application etc, and I'm speaking from personal experience (other race cars etc).

There are pros and cons either way - this has been discussed in manufacturing circles extensively and it's been a war within itself. I'm not going to say which is better, but let you figure it out yourself.

In most cases when you weld onto thin sheet metal (e.g. the subframe mount points, the heat causes localized changes in the body metal along the weld line which in the long run may results in the edges of the material along the heat affected zone cracking due to repeated flexing due to vibrations etc (since this would be the weakest point)

However, as we all know, welding fuses the material together and it becomes one - I won't argue with this. It's tried and tested and we know this works.

Modern epoxies are stronger than you give them credit for. If you look for the correct aerospace structural epoxies the performance is pretty darn good. They are used for composite-composite, composite-metal or metal-metal bonds in aircraft, and in a lot of high performance cars. A lot of these aircraft include 30 year old commercial jets (which were never intended for a 30 year life in the first place). Since epoxy is also a little flexible

When applied correctly, the adhesion of epoxy is over the complete surface of the subframe reinforcement, vs the edges (i.e. welded at the edges). When you consider the entire surface area, the strength works out to be the same or higher. You'll have to ensure the metal surfaces are prepped correctly (clean with 10% Oakite 164 alkaline cleaner), you will get the max strength from the epoxy bond.

Downsides to epoxy is long-term weathering and heat. Water is a plasticizer for most epoxy based adhesives. Over time, water infusion (rain etc) will soften the adhesive and will make it weaker. The edges of the reinforcement will need to be waterproofed properly to reduce this effect. Heat also reduces the strength of epoxies, however due to the location of the subframe, this is a non-issue.

Anyway, I'm not saying that one method is better than another - I just prefer to use the epoxy method of attaching the reinforcement plates since it means less cutting (for the "other side" weld).

It sounds like a great idea to weld the cracks first before applying the epoxy/reinforcement. Probably something to look further into.

-A
Great response and good info.

I know epoxies can be as strong as steel but will they bond to steel with the same strength as a weld? What about bearing weight and dealing with friction and flexing? This is stuff that epoxy will have to deal with under the sub frame.

The heat issue with welding can be avoided by getting a skilled welder to use butt welds instead of just laying down a bead. This is how I weld sheet mettle and reinforcement plates so they don't warp.

Epoxy is some amazing stuff and something I have considered using many times, this is why I have so many questions about it.
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Old Thu, Mar-25-2010, 05:01:46 PM   #34
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Also, one thing I just realized. If you follow the steeps above to use the epoxy method your skipping a very important steep of welding the top of the sub frame in the trunk to the mounts below. This is why you make the two square cuts, to access those two points to weld. this ties everything together.

epoxy could not be used here as it requires penetration into the mettle to accomplish.
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Old Thu, Mar-25-2010, 06:35:32 PM   #35
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Originally Posted by DigitalPunk View Post
Also, one thing I just realized. If you follow the steeps above to use the epoxy method your skipping a very important steep of welding the top of the sub frame in the trunk to the mounts below. This is why you make the two square cuts, to access those two points to weld. this ties everything together.

epoxy could not be used here as it requires penetration into the mettle to accomplish.
Actually, if you want to avoid the cuts, you could inject structural foam inside the cavity -- per BMW's fix.

I've done this on the door sills, A/B/C pillars on race cars and it does help a lot with stiffening up the car/chassis, and is pretty common to see racers do this.

Anyway, the point of my initial post was to stop others from making the same mistake when using the epoxy method by leaving in a layer of epoxy that is too thick.

[OFF TOPIC]

BTW, there are very different epoxies with different adhesion (how well it sticks) and peel characteristics when used with different materials. Some like 3M 8115 are designed to work better with certain metals (e.g. steel or aluminum) while others are designed to bond composites or different combinations. In many tests of epoxies specifically designed for metal, the metal test strips deform even before the epoxy bonds break off the substrate (the material being glued together).

That's why these material/adhesive datasheets are important. We use a lot of this type of info when working on composite/hybrid bonded parts to form a statistical model to predict failure in aerospace applications (i.e. new applications that have been otherwise untested). The results from the models are then passed on to the FAA for approval. Check out the attached datasheet if you are interested in learning more.

-A
Attached Files
File Type: pdf 3m8115.pdf (96.0 KB, 81 views)
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Old Thu, Mar-25-2010, 10:26:43 PM   #36
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Awesome DIY, Does anyone know if I can do this for my Vert ? Ive heard that the verts didnt really have a problem with the subframe cracking? Thanks
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Old Thu, Mar-25-2010, 10:44:44 PM   #37
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Oh and i am going to be doing the BMW foam method along with the TMS kit! Dont wana have issues EVER! LOL!
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Old Fri, Mar-26-2010, 02:01:26 AM   #38
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Oh and i am going to be doing the BMW foam method along with the TMS kit! Dont wana have issues EVER! LOL!
Same here. better safe then sorry

Also hoping the foam will kill some of the road and differential noise as an added bonus too.

Last edited by DigitalPunk; Fri, Mar-26-2010 at 02:21:42 AM.
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Old Sun, Apr-04-2010, 11:23:20 AM   #39
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Speaking from my experience in aerospace composite manufacturing, I felt that I should correct this error/misconception before someone else makes the same mistake when they install their reinforcement plates. (more below)



Almost all (of not all) are designed to work best with a thinnest bondline (thinnest layer) as possibe. I looked up the datasheet for 3M 8115 and found the following:

Adhesion to Steel at Varying Bondline Thickness

*all adhesion values in psi
Bondline Thickness 0.036" thick steel 0.057" thick steel
10 mils 2690 3935
20 mils 2638 3863
32 mils 2653 3693
41 mils 2601 3510
47 mils 2432 3268


You'll notice that thicker bondlines result in poorer adhesion. Jeppe mentions using a 0.5-1mm thick layer (that's 20-40 mils) while typical adhesives are designed for a 5-12 mil bond line. For 3M 8115, the recommended thickness is 10-12 mil. To make sure this is correct 3M 8115 even goes one step further - there are small glass beads mixed into it that ensures proper bondline thickness.

Going thicker than the design specs also results in different curing characteristics and this may result in internal stresses within the thick epoxy layer that eventually reduces the strength.

Anyway, if you choose to use epoxy to attach the reinforcement plates (yes, modern epoxies can be stronger than welds - seen it first hand) remember to squeeze out as much of the epoxy from between the two bonded surfaces as possible. Clamp it down, apply pressure, and let it sit and cure.

Still haven't found cracks on my car but if it does become an issue, this will be my preferred repair methodology, however I'd probably look at different epoxies too.

-A
I did a edit on the DIY, thanks.
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Old Thu, Nov-04-2010, 11:16:20 PM   #40
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Any updates? What kind of foam did everyone end up going with?
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Discussing DIY: Subframe reinforcement epoxy/foam method. e46 in the Faults, Fixes and DIY Forum - Please share your experience and knowledge with other members by contributing your own DIY, or by helping another member find the elusive fix! at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)