What is the gear ratio of my rear derailleur and what role does it play? The gear ratio indicates how far the rear derailleur moves when the shift lever moves the shift cable by one millimetre. Example: when Shimano introduced the world's first click derailleur in 1984 with Dura-Ace S.I.S., this derailleur had a gear ratio of approx. 1.9. For every millimetre of cable movement on the shift lever, the derailleur moved by 1.9 millimetres.
Shimano soon realised that it was technically advantageous to change the gear ratio.
This means that all 6-, 7-, 8- and 9-speed Shimano drivetrains that followed had a ratio of 1.7. This is where the ratio becomes important: anyone who tried to replace their Shimano 105 8-speed drivetrain or shifters with Dura-Ace 8-speed as an upgrade was shipwrecked - they were not compatible. Only the combination of new shifters and new gears would have worked, and that would have been correspondingly expensive.
In 1997, a veritable paradise broke out in terms of standardisation: the new 9-speed Dura-Ace 7700 groupset also had a ratio of 1.7 and was compatible with all other Shimano rear derailleurs, whether for road bikes or mountain bikes (MTB). Campagnolo derailleurs, on the other hand, have a different gear ratio and are not compatible. SRAM initially offered "2:1" derailleurs and shifters that were Shimano-compatible, before developing their own "1:1" and then "Exact-Actuation/EA".
Up to and including the 10-speed road bike groupsets, Shimano stuck to its standard. However, when the 10-speed "Dyna-Sys" MTB groupsets appeared in 2010, their gear ratio was changed to around 1.2 - these rear derailleurs are not compatible with the previous ones. Because Shimano also changed the cable on the shift levers for Dyna-Sys, the 10-speed MTB system, consisting of shift lever and derailleur, still shifted the same way as the 10-speed road bike system.
This was important because it meant that the sprocket spacing of the MTB and road bike cassettes could remain the same. Just like the 8- or 9-speed cassettes before, all Shimano 10-speed cassettes are compatible with each other and fit on the same freehub. SRAM 8-, 9- and 10-speed cassettes are also compatible with their Shimano counterparts. Only old 7-speed Shimano and SRAM drivetrains have a 4.5 mm narrower freehub and cassette.
The complete article was published in Trekkingbike issue 1/2017. You can download the issue in the Trekkingbike app (iTunes and Google Play ) or in the DK-Shop order.
