SIMPLEX SPIDEL SUPER LJ SLJ PX PY 10 rear derailleur Mercier

This Simplex Spidel Super LJ rear derailleur weighs 185 grams and is built from aluminium. It carries the SLJ designation associated with PX and PY 10 drivetrains, the kind of component that belonged on serious French road machinery of its era. The Mercier connection in the naming points to a period when this derailleur family was fitted to high-level production racers.

Why this derailleur matters for a rebuild

Aluminium keeps the mass down while providing the rigidity a rear mech needs under chain tension. At 185 g, it sits in the same weight class as many later competition units. The Simplex LJ architecture was designed to move across a tight cluster with short, accurate throws. For anyone assembling a period-correct drivetrain, this is a component that belongs rather than one that merely fits.

The manufacturer part number is recorded as 0000, a placeholder rather than a true reference — the physical markings on the derailleur body are the useful identifier here. Used stock of this type often shows storage wear on the surface; the aluminium finish is no different. Each photograph forms part of the description, so the condition is documented visually rather than guessed.

Key specifications

• Country of origin: FRANCE
• Material: Aluminium
• Mass: 185 g
• Manufacturer part number: 0000

A matching Simplex shifter set sits elsewhere in the catalogue, along with other period drivetrain pieces such as the SIMPLEX gear shifters for 1970 PX/PY 10. For the broader build context, the derailleur category gathers related rear and front mechs.