A Brief Analysis of Bicycle Derailleur Precision Paul Gallagher, XShifter There are many factors that affect the precision of gear shifting on bikes. Mechanical detent type shifters are essentially an “open loop” design. They do a pretty good job of positioning the derailleur, but are far from precise. Manufacturing tolerances. The shifter itself has a pawl arrangement to position the cable. This is usually not a particularly precise component. In most cases it is a stamping. The rear cassette is comprised of a sandwich of components. Each of these components has error. If you assume even 0.05mm error on an 11-speed cassette, that is potentially 0.5mm total error. The shifter detent mechanism The detent shifter is by its nature a linear device. Each step is the same. However, a derailleur is not linear. It sweeps through an arc. It requires more cable pull at the end of the stroke to obtain the same derailleur movement as the beginning. Cable stretch and housing compression As force is applied, the cable stretches, and the housing tends to buckle under the force. This creates a non-linear cable pull. Frame and cable movement: Especially on full suspension bikes, the cable housing movement significantly affects the position of the derailleur. If you hold a straight section of cable and then fold it into a loop, the cable length changes by several millimeters. So imagine how much your suspension and steering action affects the effective cable length. The cable stretch, and non-linear derailleur action do not cancel out, but are in fact additive. They work against each other. Mechanical shifting systems use the adjusting screws on the derailleur to reduce the error. So, on a 9 speed, you actually only need to get 7 gears adjusted correctly with the cable, but it’s still a challenge. Modern high-density cassettes, like 11 speeds and 12 speeds exasperate these problems. It requires a very high level of precision to make the systems work correctly. That is one of the main reasons for their excessive cost. The best solution is a computer controlled system that allows each gear to be precisely positioned. This chart shows the typical error of a mechanical system vs. optimum position. The Optimum values are from actual measured data using an Xshifter. The values from mechanical are just a straight linear equation. But in fact the mechanical derailleurs are not even this good due to manufacturing variation. The difference between a simple linear mechanism and optimum is up to 1mm difference. Therefore, it’s often necessary to find a “balance” in adjusting derailleurs, but never get it perfect. Computer control allows perfect adjustment in every situation.