Rear derailleur A complex system of springs that are balanced together means that the action of the control, transmitted by the cable, is matched by perfect shifting of the rearderailleur parallelogram and positions the chain rapidly and silently on the selected sprocket.

SRAM Rear Derailleurs use Sram’s ESP 1:1 ratio, this means that for each millimeter of cable moved in the shifter, an equal millimeter will be moved in the derailleur. Shimano uses a 2:1 ratio for it's shifting systems, this means that for every one millimeter of movement in the cable there will be two millimeters of corresponding movement in the rear derailleur. The ratios are not the same; therefore, you cannot use an ESP shifter and a Shimano derailleur, or vice versa.

Shimano Rear Derailleurs Low to Normal rear derailleurs use the spring to push the chain across to the large cassette sprocket (formally known as Rapid Rise). Top to Normal is more the traditional method where the spring pulls the chain down to the smaller cassette sprocket