Given the many design flaws on the 159, what makes you think the factory rear lower wishbone bush design isn't another one of them? I understand what you're saying about stresses and loads, theoretically you have a point but we're yet to see a failure of anything in the suspension or drivetrain that has been suspected to have been caused by changing the lower wishbone rear bush material. Just because Alfa spent many hours designing something, doesn't mean the did a good job. Are subframes that rust through well designed? Are plastic screws in coolant transfer pipes that pop out under pressure a good design? Are one-piece handbrake cables that you have to remove the whole interior to fit a good design? Is sub 7,000 miles on a pair of front tyres at the factory alignment settings an acceptable design to you? And I'm not talking about old bushes here either, they did it from brand new, there are threads in this lounge dating back to 2006 & 2007 about excessive front tyre inner edge wear and until we changed the bush material, there wasn't a way to overcome this wear without deviating from factory alignment settings, to the detriment of the car's handling and steering feel. Powerflex bushes might not be perfect, but do what they say on the tin.
I think you miss the point. One cannot equate the use of a sub-frame that rusts, to faulty design. It was simply an omission not to have it galvanised properly. They could have sent them to a specialist galvanizing company (Porsche perhaps), but that company would not then state they couldn't galvanize it because of design short-comings. Not perhaps a good idea to fit plastic screws where pressure is concerned. But, not a design fault, just a simple error of judgement - unless of course some over-zealous mechanic had over tightened them, and thereby weakened them? Tightened to the correct torque level, they probably would be fine. One can buy torque spanners and screw drivers for even for watch cases and inner workings down two incredibly low settings. Handbrake cable? Perhaps you have a point. But I know several engineers who think it a good idea to protect, what is in effect a fall-back safety device, by putting it inside the car. Mine has been changed, not by me, once in a 100,000 miles by the previous owner. I had four handbrake cables changed on my 156 (137kmiles) and two on my GT (133k-ish).
I have seen many 156, transverse arms which have Poly-bushes, worn oval due to their inability to flex in a "controlled manner". Essentially resisting the design criteria that Alfa had decided upon to achieve an element of "Rear Wheel Steer", something which rightly made the 156 in all it's forms stand out. Co-incidentally Auto Italia, issue 230 has an advert for a company, who now produce bushes for the 916 GTV and Spider rear lower suspension plates. Finally, they have come to understand "Alfa's" design philosophy for the, dare I say, "Legendary" GTV Rear Wheel Steer. It has been obvious to some of us for years. These are intended to flex as in a manner similar to the originals and who knows, they could be Ideal for the Job. But now, because they will flex, the "Work - Done" by them will ultimately result in them failing. So the choice then will be simply; which ones, the original or the replacement are going to maintain the suspension characteristics longer? I think I know where my money is going, not least because the Alfa arm bushes do more than just control the rear-wheel steer.
Reading the detail contained in the PDF file, the design motives for using the soft rear bush on the front and a "Relatively" hard one on the back; of the front suspension assembly, are clear. Equally, the Hydraulic bush at the front of the rear suspension, is complimented by a "relatively" hard rear Teflon coated "Ball", at the back. Again, one must refer to the text, as this arrangement on the rear suspension assembly works in conjunction with the front suspension assembly. The consequences of which are quite clear and I will not dispute the company's assertions. What I will add however is; and I haven't yet researched it enough, this lay-out also adds an element of "Rear Wheel-Steer". to the 159's driving characteristics.
Sporting performance comes at a cost and although 7000 miles on a set of tyres seems excessive, the suspension must have been well out of alignment. But if the car is properly set up, tyres are the price I would rather pay, and if it means a lighter driving style, then perhaps it's not too much for me to ask at 68. I've had my fun in earlier years, when my responses were more in keeping with these cars performance. But I do like to see them being thrown around the track, knowing I don't have to foot the bill.
Erratum; the detail in the PDF colour picture is for both front and Q4 rear suspension set up's. Sorry if I introduced any confusion. However, there is a question I have continually failed to get a response to. Why did Alfa; given the yearning for a rear wheel drive car, not simply remove the Double "C" Torsen front differential and fit a conventional gearbox and maintain the rear-wheel drive set-up, as is?
Thanks for the response.