Proactive rear subframe repair

[J.Dizzle]

I've been looking for a Miata to autocross with my teenage kids but came across a very nice 2001 Z3 3.0i with 100,000 miles in the process. It's clearly been very well maintained and the spot welds the seller has been able to send me pictures of are all intact. I plan to inspect more thoroughly in person but it's almost 3 hrs away so I'll have a tentative deal before I head that way.

Anyway, I see that it's major surgery to restore and improve the structure once it starts to tear apart, but... if you're reinforcing as a preventative measure with everything still intact would it not be possible and sufficient to just overlay a steel plate inside the trunk and narrower strips below? I'm drill through to catch the spot-welded flanges below, drill some narrower steel stock for the bottom side to go on each side of the crossmember and then through bolt it or steel pop rivet it every few inches sandwiching the whole thing together? I know this wouldn't be as robust as the whole welded in Randy Forbes kit bit it seems like it might offer enough additional reinforcement to prevent failure. Or, once one does the work to remove and reinstall the rear end to do the work is it not worth doing anything less than the full Randy Forbes deal and 2 ear diff cover conversion?

Finally, how successful have people been with just putting in all new poly bushings and letting it ride? We will be auto crossing so we'll be putting a lot of lateral stress.

Thanks.

[Badger56]

You may get more replies by posting in the Z3 E36/7 sub forum.
Randy Forbes is the acknowledged guru of subframe repair/reinforcement on this sub forum.
When you search for this, you will learn that if you have thoroughly inspected and found no failed welds, it’s sufficient to simply keep an eye on the subframe. Many, probably most, find they never need reinforcement. My Z3 has 130+ k miles and is clean.
However, once you find problems beginning to appear, it’s wise to reinforce before it gets worse.