Is the failing Simplex derailleur a fault of the derailleur design, or material design or something completely foreign?  Consider the round peg in a square hole situation.  It just doesn't work properly.  Well, if you make a clamp for one size of tubing and then try to fit it onto a larger tube, one might well conclude that it just doesn't work properly, either.  And, in the case of the Simplex plastic front derailleur, it didn't.  Perhaps the poor old Simplex chain jumper got a bum rap?  Perhaps not.

None the less...

French bicycle frame tubing is not always the same size as English bicycle tubing.  In fact, there is considerable difference in all three main tubes.

The English seat tube has an outside diameter of 28.6mm while the French one measures out at 28mm, offering a difference of 0.6mm, which is just a little more than the thickness of a single page from a phone book.  Not much more, but perhaps enough to turn, what was supposed to be a clamp, into a wedge.

If the Simplex derailleur clamp inside diameter is machined for a 28mm tube, but then forced to fit onto a 28.6mm tube, chances are a properly machined derailleur clamp will be forced to expand, rather than pull.  And the result, of course, might well be a crack.  Anyway...

The point is that French tubes are different in diameter, than the tubes of bicycles built in other countries.  The 0.6mm difference in seat tube outside diameter has already been mentioned.  The top tube of the French bicycle is 0.6mm larger than its counter parts from around the world that measure 25.4mm.  And the down tube of the French bike is 0.6mm smaller than the rest.

The smaller down tube might prevent a shifter set from properly clamping to the down tube.  This, in turn, might well permit the shifter clamp to slip, peeling off original paint and art in the process.  In fact, this problem was so common that frame builders began adding a small braze-on tab that would help to keep the shifter set where it was supposed to be.

With these, barely visible to the trained eye, differences it is little wonder that interchanging components can, and did, cause problems. For example, down tube shifter clamps might not tighten properly, thanks to a slightly smaller tubing diameter.  The clamp on rear brake cable guides might not fit properly, either, and the possible problem with the front derailleur clamp assembly has already been mentioned.

It stands to reason that, if there are outside diameter differences between French bicycle tube diameters, when compared to all others, then there must also be inside diameter differences to consider.  And there are.

NEXT - FRENCH POST AND STEM DIAMETERS