I followed up on agokart's photos, especially the last photo.

The '05 Cabrio is manufactured with a unique rear subframe or "rear axle carrier" to which four long support bars are attached. The "rear axle support bars" run forward from the rear axle carrier and attach to the large chassis support tubes on each side of the car. The four support arms are visible in agokart's photo. The result is to creat rigid triangles in both the vertical and horizontal planes. These triangles extend from the rear subframe forward, under the chassis, to the front subframe mounting area. I am not exactly sure what the overall result of this stuctural addition has on the non-cabrio MINI chassis support system. These additions suggest to me a major upgrade on chassis rigidity in the rear for the cabrio at least. Something to think about.

The part numbers and prices of these components are as follows:
Rear axle carrier - #33316761716 - price $452
Rear axle support, Left - # 41117035341 - price $177
Rear axle support, Right - #41117035342 - price $177

I don't have the part numbers or prices for the anchoring hardware.
Thanks agokart, I going to think about these cabrio components as a possible retrofit.

John Petrich in Seattle

 

This post is to correct an error regarding the proper part numbers for the rear subframe or "rear axle carrier" and subframe braces called "tension bars". My last post was inaccurate about the part numbers and prices for the "tension" bars that run from the rear subframe forward to chassis anchor points. The parts guru at NW MINI in Fife, WA, Terry, accidentally identified "axle supports" instead of the correct "tension bars". He did some additional work and has corrected the part names, part numbers and prices. So, here is the corrected information:

Rear axle carrier - #33316761716 - price $452
Tension bar (2 required) - #51 61 7 070 779 - price $32.16
Tension bar (2 required) - #51 61 7 133 627 - price $27.86
Mount (bridges the pads on the cabrio rear axle carrier and provides a mounting point for the tension bars) - #51 61 7 113 473 - $72.36

Hope that this information helps. An expensive proposition with an uncertain benefit for non cabrio MINI's.

Regards,
John Petrich in Seattle

 

did anyone ever try the tension bars? do they help?

 

If anyone cares the audi TT roadster has a bolt-in reinforcement similar to (thought i believe its a beefy cast piece) the one agokart posted. Supposedly coupe owners have retrofitted it.

 

I did not see anyone mentioning the M7 USS, it is the answer for stiffening
the convertible . After extensive testing we found that the excesive flex you will find in the convertible is all but gone, giving you better turn in response
and a more predictable handling charateristics.....