epoxy the subframe..

[scottkibler]

Has anyone ever just used epoxy on the subframe? Besides welding a piece of solid steel in, it seems like the best option... I got mine from a mechanical engineer and he did it in 2 spots and it has held up except for 1 weld and he only did it in a couple spots. All the rubber is about gone after 20 years, but that is too be expected. What makes any other option more speacial? Welding will not be as strong.

[Vintage42]

Has anyone ever just used epoxy on the subframe?... All the rubber is about gone after 20 years... Welding will not be as strong.

Where exactly is this on the car?

[BimmerBreaker]

Did you just say epoxy is stronger than welding?

[z3forlife]

Go to sports cars plus llc and look in the gallery photos. You’ll easily answer your question.

[raubritter]

Interestingly, if you look at TIS you'll find that BMW has switched to blind rivet+adhesive for some repairs:

https://www.newtis.info/tisv2/a/en/e...-body/8GUSlkqc

Not just for stuff like roofs or sheetmetal, note that they include the engine carrier and engine support part of the frame in this.

I've also wondered if there was some sort of resin you could fill the diff mount crossmember with to make it a lot stronger. And I read a passing mention of this in an E46 M3 restoration writeup, but no info on what the resin is but supposedly it was a BMW item.

Not saying this could substitute for Randy's fix - he reinforced my car himself and I'm really glad to have gotten it done because it really improves the structure of the car - but maybe as an addition to his fix But I think it's safe to say that it wouldn't help much with a car that's already ripping apart.

[Mr Bingley]

Epoxy panel bonding is common place in auto manufacturing today. However, it is simply a different method of bonding materials together--neither better or worse than welding, but different for different manufacturing processes and purposes. Different epoxies have different qualities, strengths, and weaknesses, and can be very specifically engineered to particular applications. Whether welded or epoxied, the question is less what method of panel bonding is used, than what panels are being bonded by that method. In that regard, an epoxied reinforcement solution could look very much like Randy's kit--only epoxied in rather than welded. I can imagine some advantages to that approach: heat welding necessarily changes metallurgy, and can promote brittleness and corrosion; while epoxy could provide a stronger bond by being applied across the entire surface area, rather than just at plugs or across a bead. Of course, any epoxied solution would necessitate removing all paint on the surfaces to be bonded--i.e. to bare metal--and roughing the metal surfaces being bonded; as such, an epoxied solution might involve greater preparation then the current welded in one. There is also the question of how the epoxy bond would hold up to temperature extremes and body flexing--but that is more a matter of polymer science than a limitation. Another advantage to an epoxied solution is that the range (and weight) of reinforcement materials is greatly increased--Kevlar, aluminum, carbon fiber, glass mat, etc. could all be used rather than steel. It would be a lot of work (read expense) to engineer and test an epoxied solution, but in the end it could well be a better approach. However, that solution, whatever it might be, does not include simply applying some off-the-shelf epoxy to the already welded together and painted surfaces of our trunk floor, cross member, and differential mount bracket, or simply applying some epoxy to broken spot welds. Those approaches seem to me to be more akin to the "put a screw in it" or "re-weld it" solutions that are occasionally debated here. But it is an interesting thought and worthy of consideration.

[Randy Forbes]

Ralph describes it above, and I will reiterate: for bonding methods to work, ADHESION is key.

Had the car been manufactured last month, with minimal exposure to surface oxidation and (dirt) contamination, it might be possible to impart enough strength into the hollow XMBR that supports the trunk floor to make a difference using an expanding foam. Destructive testing would be required.

However, there are two (2) distinct and separate areas of failure, almost universally addressed, and repaired/reinforced at the same time. While the above might address one (1) of them, it won't re-stamp the differential mounting bracket to remove the popular stress-riser that BMW graciously built into the part. The point of origin for the mount to crack (the fractured spot-welds are a separate problem).

More than one (>1) person on here__during the last fifteen (>15) years__has attempted to use my reinforcement package as the basis of a bonded repair and subsequently abandoned the idea part-way through (wasting money on materials, time and effort in the process) and then reported back to me that they ultimately welded it in. I took an interest in each of their proposals, as I too was curious of the outcome.

[JustinHalverson]

From my reading on the subject, I think the only repair that has passed the test of time on multiple vehicles is the RF reinforcement kit. Many other approaches have proven inadequate. I think that any method other than the RF kit has to be considered an experiment. Often there is enough data out there to argue that many of those experiments will fail, making them ill advised.

That being said, I remember a forum in which Randy F said he had never had a Z3 come into his shop which had poly RSF bushing and failed spot welds. So my questions to Randy are, Do you remember posting that? Is it still true? Would your subframe kit still be as effective if you installed stock bushings instead of upgrading to poly bushings?

[Randy Forbes]

I have done a few cars where the owners had requested that OE subframe bushes be retained, and that was quite a number of years ago, maybe over a decade. I haven't received a phone call from any of them, so I can only assume there're no problems since (I have never had a car come back to me to redo any portion of a trunkfloor/differential mount reinforcement. While rare due to geography, I do occasionally get repeat customers for other work (2 such cars are here now).

From the standpoint of the repair/reinforcement's strength, I don't think there is any difference with either type bush material, though the handling may not be as sharp as it could've been; the E36 differential mount (dual ear) would've reduced, if not eliminated the rear-steer effect, but if the job was to reinforce the single ear, then handling would remain pretty much as stock, except the floor wouldn't be flexing as part of the suspension.

Doing this more than fifteen (>15) years, I have had a few cars come in with urethane bushes already fitted, but I can't specifically recall which cars or their exact condition (c'mon, I remember a lot, but it's over 100 cars now). Just the fact that the cars were sent here leads me to believe there was at least some damage.

Someday, another guy tinkering in his garage may build a better mousetrap, but I expect it, by necessity, would be a variation of the groundwork I did when the cars were barely out of production (my black car's reinforcement dates back to Spring 2004, with an additional 4 cars done before the end of the year).

With each car I've done, I'm always looking for ways to improve the design, and about the only thing I could come up with__beyond streamlining the installation process with more and better tools__would be to make it out of thinner gauge material; but how thin?
I don't subscribe to the Colin Chapman method of reducing it until it breaks, then adding some back! Adding twelve pounds (12#) net, directly over the rear wheels in a nose-heavy car, its weight isn't an issue for 99.99% of the cars I've done (even Dan's ultimate track Coupe kept its a/c).

I do enjoy the discussions though, it keeps me thinking too!