8 replies to this topic

[DomCr250]

Just rebuilding a 1980's Mk4 City that's been sitting outside uncovered for 12 years.

 

The rear subframe was in almost perfect condition - interestingly whoever fitted it years ago has filled all the cavities with expanding foam ...never seen it done before.

 

Now I know Austin / Morris played with foam filled sills back in the '60's - but has anyone else seen the expanding foam in the subframe trick before ...it's never occurred to me in all my years of mini ownership?

 

 

 

 

[nicklouse]

mmm great place for holding water next to the frame.

 

some people tried it as insulation for camper vans. and found that it just caused rust.

 

and I wonder why BMC never took it further.

[A-Cell]

The foam filling of the original BMC mini was done to prevent water ingress. Eliminating the water actually improved the corrosion resistance of the early mini sill sections. With 1960 cars outlasting 1964 cars in the sill replacement stakes.
Expanding closed cell polyurethane foam was used in the Rover 200 A posts at launch to reduce transmitted road/tyre noise.
Providing the metal (steel) is suitably sealed from water and air then there can be no corrosion. In the case of the original mini whilst the metal was mild steel and the internal treatment was achieved through the rotodip process, essentially a phosphate bonderising coating followed by a primer coat.
In the Rover 200 organic coated steels were then electrocoated.

I have used polyurethane foam in wax oiled box sections to good effect. Keeping water out.
The subframe you have is good evidence of the effectiveness.

[tiger99]

I concur. If done correctly, foam filling is good. It improves stiffness by a surprising amount.

[Carlos W]

But is awful when it burns

[minicooperr]

I had heard before that expanding foam is not great for rust and actually causes it to hold the water

[Steve220]

I blocked a lot of areas and joins on my subframe with tiger seal. But in general, the rear subframe is a poorly designed unit and the new ones have a terribly rubbish finish on them.

[tiger99]

Stainless steel is the answer but it would need some redesign especially where joints are concerned to avoid fatigue cracking. I am surprised that no spares manufacturer has them already. They would not be cheap, but we hugely expensive upgrades for other classic cars, so why not?

In the meantime I would seriously suggest galvanising, plus the appropriate primer to get a bond to the zinc, followed by at least epoxy primer and stone chip. Should be good for upwards of 50 years.

[Cooperman]

I once had a rear sub-frame sand blasted then hot dip galvanised. Then I primed it with red-oxide primer followed by 2 coats of 'chassis black'. I don't have the car now but it is still going and I believe the sub-frame is still on it 27 years later.
However it was expensive to do and since then I have simply made sure that all the rear frames I have fitted have been properly painted and then rusy-proofed.