Fun with a boroscope: installing structural foam in the rear subframe cavity

[mrgizmo04]

One thing I've heard (I hope I'm remembering incorrectly): doing foam first somehow makes it harder to weld in plates and/or do a topside reinforcement later.. is this true? It's not clear to me why it would be.

Few things - foam is flammable and can result in higher localized heat of the already thin metal you would be welding to on the subframe. Also, you have to cut into the cavity in several places when trying to prep the area and expose some of the popped spot welds and with foam there serving as an obstruction you don't have room so you would have to chisel the foam out.

[martin.oconnor]

The foam in the box - looks like it expanded quite a bit. I wonder what long term effect is if now due to expansion and curing it is actually pressing out on the racp panels.

As it expands while curing the foam will move to the empty central part of the cavity. The rubber plugs on the bottom of the cavity would be forced out by the expansion if the expanded volume exceeded the capacity of the cavity. So there should not be any pressure after curing.

Some good videos on the foam here by a UK company called Redish Motorsports:

https://www.youtube.com/watch?v=xePUafKy49A

https://www.youtube.com/watch?v=iwBR42yVZ3U

And just found this 5-year followup by the same company:

https://www.youtube.com/watch?v=ysfTHCKiESs

Car had 32k miles plus some track days since foam installation (and no plates) and has no cracks.

[ATB88]

Few things - foam is flammable and can result in higher localized heat of the already thin metal you would be welding to on the subframe. Also, you have to cut into the cavity in several places when trying to prep the area and expose some of the popped spot welds and with foam there serving as an obstruction you don't have room so you would have to chisel the foam out.

Right, thanks. A few followup questions:

1. The first concern (flammability of the foam) would be mitigated if I did an epoxy based plate/brace install, right? But then what if, god forbid, cracks develop after the foam. From what I understand of the plate installation process, existing cracks need to be drilled and then welded before plates, regardless of the method used for installing the plates, is that right? So in that case foam would still get in the way of doing a correct repair? Or is there an epoxy solution for that as well?

2. Would chiseling/drilling out this foam be extremely difficult? Is this prep only necessary if you actually have popped spot welds? If so, then assuming that there are no problems before and after the foam, would this problem be a non-issue for doing the prep for installing plates and/or a brace?

Thanks for any insight for anyone can offer -- it would be good for myself and anyone else in my situation to fully understand the risks and problems that doing foam on a car that's currently in good shape might present for doing a full-on reinforcement later down the road. It seems very attractive as an easy and cheap bandaid solution for people with lower mileage lower risk cars, but if my car's definitely going to need plates/topside later and this is going to make that significantly harder, then it's probably not worth it. I just don't want to end up with a car that's a nightmare to deal with down the road because I foamed it.

[E46m3zcp]

Thanks for the great writeup and the pdf, very useful. The foam is hard to source, where did you get it? It also has a very short shelf life, did you use one past its expiration date? I wonder why the shelf life is so short on these products.

[332]

foam bad, makes things worse.. more welding more metal

[martin.oconnor]

Thanks for the great writeup and the pdf, very useful. The foam is hard to source, where did you get it? It also has a very short shelf life, did you use one past its expiration date? I wonder why the shelf life is so short on these products.

Unfortunately, it appears to be discontinued, at least in its BMW branded incarnation

TEROSON EP 1401 appears to be the equivalent (or more likely the BMW stuff was this product rebranded):

https://www.henkel-adhesives.com/cn/en/product/structural-adhesives/teroson_ep_1401.html

But I do not know how easy it is to source that.

Despite conventional wisdom, structural foam works very well at strengthening the subframe mounting points.

There is a theoretical case that strengthening those areas just moves the failure points to the RACP-to-wheel arch connection points and that those welds would eventually fail. I never saw enough evidence to convince me of that.

[E46m3zcp]

Yeah, I'm mostly worried about rust forming in the long term if the welding job is not done perfectly with the plates.

[E46m3zcp]

I found some Teroson Terocore 1401 (BMW rebranded this product as their structural foam, but that's the original stuff BMW used to distribute) online on Klium.com and I was able to get it delivered to Canada using forward2me.com

I will be doing the structural foam reinforcement soon.

[martin.oconnor]

Interesting new video from Redish Motorsports on various changes BMW made to the reinforcement of the threaded bolt in the left rear subframe mount in the RACP:

https://www.youtube.com/watch?v=dNhI_wmFgOg

[Albino09]

I am genuinely surprised that nobody talks about this plastic block insert before Redish made this video.