Shimano Positron FH (PF20)

Shimano Positron FH derailleur (PF20) main image

For 1982 Shimano released the Shimano Positron FH derailleur, in many ways the definitive Positron derailleur. On the face of it the Positron FH was simply a Positron II - except adapted for six speeds, with longer parallelogram plates and with six notches instead of five.

But, as the crazed Ann Widdicombe might say, there was a ‘something of the night’ about the innocent looking Shimano Positron FH. It was only supposed to be used with either Shimano’s proprietary cassette hub or Shimano’s proprietary Front Freewheel system (which had the freewheel mechanism between the cranks and the chainrings) and, naturally, it was only to be used with a Shimano’s proprietary Uniglide chain and Shimano’s proprietary Positron levers. No third party components were allowed. It was the first real instance of Shimano’s policy of Total Integration/Total World Domination. Nobody mentioned a 1,000 year Reich, but there was a touch of totalitarianism about the whole affair - and that remains part of the Shimano psyche to this day.

Of course there were good rational reasons for all this. A front freewheel mechanism keeps the the chain moving - allowing the gear to change when freewheeling, cassette hub systems allow the placing of the cogs to be properly defined, the Uniglide chain had built in shifting bulges that genuinely helped shifting, having the whole transmission built by one manufacturer guarantees compatibility and keeps quality consistent etc. etc..

But despite all this beguiling rationality, the peasants revolted and the whole fancy confection was a commercial failure - except possibly in Germany and Japan. There’s probably an ironic political joke in there somewhere - but I am too polite to even explore the idea.

This example is the ‘Medium Cage’ PF20 version - which has an unusual (for Shimano) 67mm long pulley cage.

  • Derailleur brands: Shimano
  • Categories: Shimano - Positron indexing insanity
  • Country: Japan
  • Date of introduction: 1982?
  • Date of this example: 1983 (two letter date code, stamped on inner parallelogram plate, is HL)
  • Model no.: PF20
  • Weight: 327g including hanger plate
  • Maximum cog: 32 teeth (34 teeth if used with a single chainwheel) (Source: Shimano)
  • Total capacity: 32 teeth (Source: Shimano)
  • Pulley centre to centre: 67mm
  • Index compatibility: switchable between 5 and 6 speed
  • Chain width: 3/32”
  • Logic: operated by piano-wire cable with no ‘normal’ position
  • B pivot: sprung
  • P pivot: sprung
  • Materials: steel
Shimano Positron FH derailleur (PF20) additional image 01

Browse associated documents.

Shimano Bicycle System Components (1982) page 94 - scan 97 of 144

Shimano Bicycle System Components (1982) page 94 - scan 97 of 144

Shimano Bicycle System Components (1982) page 94 thumbnail

Shimano PPS-FH - instructions

Shimano PPS-FH - instructions

Shimano PPS-FH - instructions scan 1 thumbnail