Semi C&V Related, I went to the first napa bike festival today (gotta say it was nice and relaxed/fun), it was my girlfriend my friend and myself. My friend wanted to upgrade his rear derailer for his proflex, after we'd already hit most of the booths with bins of goodies etc.. so we went back through and didn't find much, I think the best was a 300lx RD before we stumbled across a vendor selling these

It's plastic.. like 90% it has a spring, the backplate of the arm itself is also metal (and small), there's pop rivets it looks like and a mounting bolt, and a nut built in for the cable tension screw.. and of course the screws themselves.. but otherwise not a drop of metal... WTF were they thinking? the vendor claims they last surprisingly long, I found this doubtul but I bought one as a conversation piece not for use. My friend ended up going back later after I bought mine to get one to find out just how long they actually last.. I'm praying for his bike.

So why did they do this? I know simplex liked plastic too, but this much? what's the reasoning besides CHEAP?

you've answered your own question

Well, CHEAP certainly leads the list, in my opinion. But LIGHT also points to the logic, surrounding the issue. Sadly, plastic is not as strong as steel nor, apparently, as resistant to the passing of time(that last comment is my opinion only). However, size for size, it was lighter back then.

Dupont's Delrin plastic, used in some Simplex products, is a prime example of a good, but poorly engineered, idea. Perhaps the derailleur, presented by the OP will suffer from the same, or similar, woes, as did the Simplex chain jumpers, which brings to mind a question that is all but guaranteed to start up flames all over the place.

Just exactly what is Carbon Fibre? Some sort of magical carbon stuff, held together in a plastic resin. So, if we look at today, and think back fifty years or what ever, Delrin plastic might well have been the wonder stuff of its time, not just a cheap alternative to steel, or brass or aluminum.

Finally, although I am not a fan of the plastic used by Simplex, I am a fan of the transmission, itself. They offer very smooth shifts and I like them, when they are not broken in half...

I think the plastics in that SunRace are lightyears better thanthe plasics in those old Simplx derailleurs.

SRAM makes a mostly plastic RD very like the Sun Race.

Top

Look at any SRAM 5.0 or less derailler made in the last 5 years, or even some of the lower end (big box-level) Shimano's. Plastic. SunRace used to be on alot of the Big Box stuff, but has seemed to move up in class a bit, and now it is all Chinese or Shimano.

As usual, the French were ahead of their time.

i am running plastic derailleurs and spokes no death or failure yet:

Just compared my sram x4 with the sunrace.. x4 is much sturdier and has a lot more metal to boot!

Good luck with that. Plastics don't age well, in my opinion. I'm in a discipline that uses lots of them. I expect that when today's plastics are 40, they'll look and perform way worse than their manufacturers and mfgr's flacks want us to believe; sorta like Keith Richards or Steven Tyler.

In addition to aging badly, plastics have pretty bad strength-to-weight ratio, and coefficient of thermal expansion that's crazy -- way higher than metals (even aluminum) -- which makes them poor partners for metals IMO.