Simplex plastic derailleurs were fitted to many brands of bicycles.  That said, this problem does not target French bicycles only.  Any, bike fitted with the Simplex plastic chain jumpers, is at risk.

Considerable evidence would suggest that the plastic, used to manufacture many of the transmissions used on Old School French bicycles, was not capable of doing its intended job.  Put even more simply, the infamous Dupont Delrin Plastic, of which many Simplex derailleurs were made, tended to fail catastrophically.

Fail catastrophically?  Consider the seven mid-seventies Peugeots presented.  The bicycle all, at one time or another, randomly found their way into The Old Shed.  Of the seven bikes, one is a five speed, sporting no front derailleur.  The other six do have front derailleurs, all Simplex, installed.  Out of the six front derailleur equipped bicycles, two of the front derailleurs are cracked or broken.  That represents a 30% failure rate in this instance.

A broken front derailleur can feed itself into the crank rings, while a broken rear can result in spoke destruction, or bent drops, or bent chain.  What ever the situation, the result was, usually, a bicycle that could not be ridden.

Why Simplex chose to use plastic to build derailleurs, and shifters for that matter, is a question for time's historians to answer.  Chances are the decision was driven on two fronts.  Cost and weight.

Generally, a piece of plastic will weigh less than a equal sized piece of metal.  And, a piece of plastic is much easier to manipulate (work, reshape, machine) than is a piece of steel, or aluminum, or brass.  Additionally, more often than not, plastic is less expensive than metal.  The point is, by building a lighter bicycle and for less, the bicycle, itself, should become more competitive, both on the show room floor (lower price) and the race track (lighter weight).  Admittedly, that is an assumption, but might well be bang on the money, for what ever it is worth.

The problem is, the plastic used, tended to fracture under use, abuse and even during installation.  With that in mind, the Simplex derailleur has earned a poor reputation, amongst vintage bicycle enthusiasts.  This is a sad thing since the transmission tended to work very well, when not broken.

The first place to look, when considering the structural integrity of a Simplex derailleur, is the front chain hopper.  The Simplex plastic front derailleur is the one most prone to failure.  The piece tends to crack and/or then break, clean through, at the clamp assembly.  Uneven or over tightening this clamp tends to load the piece excessively and the result is material failure.

With this in mind, when ever considering purchasing a bicycle fitted with a Simplex transmission, look VERY closely at the front derailleur.  All failure tends to be in the same place - next to the derailleur cage on the drive side.  Cast an eye over the spot, clean it up and cast an eye again.  It is even wise to have a look, under a good light and using a magnifying glass.  Doing so just might allow the user to see the beginning of what will, someday, become the final demise of the component.

The rear derailleurs are not nearly as subject to failure as are the front ones.  In fact, failure of the rear mechanism is rare.  And, failure of the Dupont Delrin shifters is all but non-existent.

Another, minor, issue with the Simplex derailleurs is the sticker applied to the rear mechanism.  Put as simply as possible, be careful with any cleaning procedure.  Do not immerse the derailleur in any cleaning solution, be it hot soapy water or any kind of solvent.  Doing so will cause the sticker to un-stick, and peel off.  Needless to say, this advice does not apply to sticker-less Simplex rear derailleurs.

In addition to the mechanical failure aspect of the derailleurs, there was a stigma attached to the plastic parts - cheap!  Though more or a novelty than an issue in their day, collectors tend to shy away from the plastic chain jumpers, both because of their inherent design flaw, coupled with the poor aesthetics.

In all fairness, later Simplex offerings had eliminated the use of plastic and were much better products for that decision.  The new derailleurs did not shift any better than their non-metal predecessors.  But that has little to do with the reputation, or stigma, that plagues all Simplex derailleurs.  And, even more sad is that it just might be that the problem was not completely of French origin, to begin with.

NEXT - FRENCH BICYCLE TUBING SIZES