If you’ve never encountered a Simplex Tour de France rear derailleur (first introduced in 1947), you would be among the majority.  Even experienced mechanics can be flummoxed by these plunger/push-rod designs, as they bear little resemblance to modern derailleurs.  Founded by Lucien Juy in 1928, Simplex gained prestige and market share by developing and perfecting a rear derailleur which properly tensioned the chain regardless of which gear the rider was using.  This breakthrough translated into 100’s of prestigious races won in the 1930’s with bikes equipped with the Simplex derailleurs.  In 1936, Simplex was the first company to introduce a 5 speed rear derailleur – a breakthrough which lasted until the late 70’s.

Fortunately, the technical manuals have survived, and there are internet resources to help – the most valuable one being at my favorite U.K. website – Classic Lightweights.  However, if you don’t speak British, some elements of the setup will be mysterious, at best.

One of the most puzzling aspects of this derailleur is the chain routing.  The photo at the left depicts the WRONG chain routing.  The photo at the right depicts the CORRECT chain routing.  It’s important to carefully study the drawing in the technical manual to make sure you’ve got the chain routed correctly.

There are two tensioning mechanisms in the derailleur – something which is true of all modern derailleurs.  The plunger/spring at the left photo above controls the tension on the pulleys.  It can be adjusted by moving a tab which is inserted into the notches shown on the left side of the knurled nut, shown in the above left photo.  The inside spring which attaches to the arm of the derailleur controls the arm swing, shown in the above right photo.  It’s adjustment is controlled by loosening the screw on the outside of the arm and moving the spring’s hook up or down on the arm.

The inside spring controls this movement.  There needs to be some flex here to handle the changes in the chain when the gear is changed.

One thing you will learn, eventually if not right away, is that each wing nut is of a different proportion.  The drive side wing nut arms are positioned higher so that they will clear the derailleur.  You’ll need to completely remove the wing nut in order to get the rear wheel in position for mounting.

Another thing you’ll need to know if you are working on a bike without its original parts is the cage swing capacity of the derailleur.  In this case, its swing capacity was for a 3 speed freewheel, and not the 4 speed freewheel that I wanted to use.

Once set up, my methodology for adjusting this derailleur involved trial and error – shifting rapidly among the gears and checking to see how the chain tension was affected and how the derailleur performed.  If you are working on a bike without the original chain, you will have to guess at chain length, which these derailleurs are very sensitive to.  If you have the original chain – don’t mess around with the chain length – keep it exactly the same even though this seems counter-intuitive.

The derailleur has no limit screws.  However, the whole mechanism can be moved inward and outward from the freewheel by loosening the outer bolt and turning the derailleur inward or outward, then tightening again.  There isn’t a lot of adjustment here, but this adjustment mimics the limit screw adjustment on modern derailleurs.

Having set up a few of these now, I feel I am almost getting the hang of it!