The front subframe on my E30 325i failed, not sure there's a rear subframe. Pretty sure it was aluminum. I got lucky and found a low miles convertible that had been rear ended hard at the boneyard for a replacement. Hard ass install. Lot of parts are connected to it. Somehow I got it in.

I discovered it one day, I was having some overheating, I popped the hood for a looksee, I noticed the fan wasn't spinning at idle which it normally would do, even with the clutch thing, spins slowly. Turns out the bottom of the fan was leaning on the shroud, halfway supporting the front of the engine, as the subframe was cracked. Hard to believe but true.

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Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K