I had a friend, now long deceased, that was a pioneer in the plastics industry. He used to say that plastics become brittle because they "lose their polymers" over time.

The fronts break at the clamp when they get old and brittle.

Simplex was designed to go through an unthreaded hole held to the dropout with a nut on the inside.

I've seen literally hundreds of Simplex RDs in the past two years at the bike shop, and I think I've only seen one with a broken body. The pulleys, of course, do chip and break; but when they're complete the work very nicely. I've never tried to replace them, but surely there's another brand with tougher plastic that would fit and work.

Yes, actually some older Suntour jockey wheels are a direct replacement. I've done it and the old Suntour stuff works fine. I forget the exact model of Suntour I used, but it's an older model from the 70's and it was the only Suntour rear DR where the wheels were compatible. All my newer, late 70's and all of the 80's stuff did not fit.

Edit: The Simplex derailer that I so carefully repaired went on an all original, 70s Raleigh Grand Prix. I cleaned that bike up, made it look really nice and fully functional then sold it on CL. After I sold the bike, the new owner tells me "Yeah, I'm going to make it a fixie." Arrrggg!.

Yeah , I know, that's why I'm wondering how this derailleur was attached to a Zeus dropout. OP says in posy #23 that the dropout is threaded. Curious how they attached the Simplex Prestige to it.

The mounting bolt is smaller than the threaded hole. Goes through the hole and is secured with a nut. Threads in the dropout aren't engaged.

It's called "outgassing" The plasticizers pretty much get emitted out of the plastic as a gas or vapor or simetimes even ooze out as an oily substance, usually accelerated when the plastic item is exposed to heat and sunlight (UV).
You mgiht have noticed this on your car, if you park it outdoors for long periods of time under the sun and not regularly clean the inner surface of the windshield, an oily substance would build up on the glass. That's the plasticizers from you plastic dashboard parts and finishes.

Chombi

I see a T-Shirt design:

"The Fixie: Mankind's best application of the Simplex Derailler"

Maybe "new" Delrin works better because of UV inhibitors (which they might not even have used on the old Simplex derailleurs) and plasticizers??

I'll try to take a picture tonight if my explanation is not clear enough, but there is a bolt that goes through the inside of the dropout that threads into a hole in the back of the RD. The diameter of the bolt is small enough to fit through the hole in the dropout without using the threads.

Hmmm...
Apart from the pulleys, the evidence is mounting that Simplex RD's rarely break, and so it seems they are better than is generally acknowledged.

I get the feeling that the "Simplex sucks" sentiment is largely due to the agreed upon worthlessness of the FD, the RD simply becomes guilty by association.

dbakl's explanation is quite accurate, no further explanation necessary. Your original description was that "a bolt went through to a bolt on the back of the dropout", or something similar. What you meant was a bolt from the upper knuckle of the derailleur went through the dropout hole to a special nut that fits on the inside of the dropout.

As I recall, that's how all the Simplex Prestige units worked, though they typically attached to a claw instead of a dropout hanger. The flange of that special nut was thin enough that it didn't interfere with the small cog, and it had a flat side that went to the front to allow more room for the axle to come back. The only way it would have worked with a dropout with integrated hanger is for the nut itself to fit into the threads, or else it didn't protrude into the threads at all. In any case, if your dropout has good "Campy" threads you can just screw the Suntour into it.

Simplex 'works' just like an Allvit 'works', but you ain't gonna love it! Its not a thing of working beauty like 'others'.

I also suspect the RD's did not break as much as the FD's did, but there must have been a good enough reason why Simplex decided to reinforce their Delrin RD arms with some sheet metal covering on later RDs like the SX610s which are now known as great shifting and totally reliable RDs. So at least, Simplex did eventually wake up to do these improvements.
Also note that they also did not use any totally plastic main pivots on their later RDs.

Chombi

This may be a matter of semantics, but I did indeed mean that a bolt goes through the hole from the inside of the dropout and threads directly into the back of the RD. There is nothing protuding from the back of the RD that goes through the dropout, it is flat with a threaded hole that accepts the bolt I'm referring to.

Not quite all of them Jim. My Simplex derailleur is threaded female on the upper pivot and is attached to the dropout using a shouldered bolt, M6 x .1, that is driven by a 5 mm hex wrench. I had assumed the OP's was the same.
edit: as I see above, it is.

You could be right. Honestly, Simplex are really not my thing, though I've had a few.

Since this bike has campy style drops I would revise my opinion to sugest that vintage Shimano 600's would work and look good on this bike exspecially the Arabesque ones.

sorry didn't mean to doublr post.

I don't mind them , but yeah...that's why I put the Nuovo Record on my Gitane.

Just to clarify:

I have a bunch of higher end Simplex derailers an all that are intended for attachment directly to a dropout use a bolt through the back of the hanger. In other words, the pivot bolt is drilled and tapped for a bolt. Those that are intended for use with a claw have a pivot bolt that takes a nut. The bolted derailers also have a tabbed washer that is not present on the nutted version, so they are not interchangeable.

I replace my Simplex pulleys with Bullseyes. I use two of ithe supplied 6mm washers on each side of the pulleys.

As soon as it gets warmer, I'm going to go out in the garage and put some Bullseyes on the SX610 on my Jeunet. The came in a bag of parts I bought for $17. There was also two sets of Stronglight 93 dust caps in the bag,so it was a bargain. I made an arbor for the pulleys and chucked them in my drill press so I could polish them. They're much too pretty for my beater Jeunet.

I vote to stay Simplex, but...

I had the original Prestige (plastic) version DR's on my GP when I got it last year. The rear DR had a shipping incident, which caused a couple of broken pulley teeth. Other than that...it shifted fine.

During my resto on the bike, I decided to up-grade the DR's to Simplex Super LJ components and LOVE them for precise shifting and lack of major (potentially troublesome) plastic components. And who could argue with the better looking LJ's?

https://velo-orange.blogspot.com/2006...erailleur.html

Hey, there ya go. If you can find an SLJ, that would be spectacular (but expensive). The SX610 is the SLJ's poorer, chubbier cousin and will be both period-correct and as functional as anyone could want.

Love Simplex for what it is; don't hate it for what it is not.