Editors Note: Science Behind the Magic is a reoccurring column by Art’s Cyclery web content editor Brett Murphy, who uses his mechanical engineering background to explain the latest industry advances, and breakdown the inner workings of common components. The original article can be found here. Please note that the original article contains tables that you can manipulate with data for cable pull, derailleur shift ratio and cog pitch of SRAM, Shimano and Campagnolo drivetrains. This data can be used to show areas of cross compatibility for mixing and matching groups.
Can you mix SRAM, Shimano and Campagnolo shifters, derailleurs and cassettes? This article will answer that question and more. I’ve commented previously on the lack of standards in the cycling industry. It can be, at times, rather frustrating. Some new part comes out that I really want, but lo and behold, my seat post is the wrong diameter, or this tapered fork is incompatible with my straight head tube. It is a constant battle to keep up with the ever-changing times.
While the industry has advanced in leaps and bounds in the past few years, certain things have stayed relatively the same over the decades. Di2 and EPS excluded, shifting is one thing that is basically the same. Yes we’ve advanced from five cogs to 11, and derailleurs now have changed to account for a varying number of forward gears, but we are still talking about yanking on a cable a certain amount to move a chain up and down a line of gears.
Also, I’ll discuss the differences amongst drivetrain competitors and the reasoning behind some of their decisions. Before we get into it all, lets define a few terms so that we are all on the same page.
The shifter determines cable pull. Every time you click your shift lever, the shifter pulls in or releases a certain amount of cable. Different brands and different drivetrain speeds (e.g. 9, 10, 11spd) pull different amounts of cable. For the most part, all the cable pulls are uniform for every shift, with the exception of some of the Campagnolo shifters. For example, a Campagnolo 10-speed shifter pulls 2.5mm of cable five times, 3mm twice and 3.5mm twice.
Derailleur shift ratios, also referred to as actuation, are the amount of movement from side to side of the derailleur relative to the amount of cable pulled. Older Shimano derailleurs all have a shift ratio of 1.7. This means that for every millimeter of cable pulled by the shifter, the derailleur will move 1.7 millimeters.
Cog pitch is the distance from the center of one cog on the cassette to the center of the next. Cog pitch changes between major brands and as more gears are added, usually cog pitch shrinks to fit more gears into the same width freehub body.
These three numbers are related by the following equation:
Cable pull x Derailleur shift ratio = Cog pitch
Looking at this equation, you can imagine the thought process behind some of the designs. If an engineer a few years ago wanted to design 10-speed drivetrains, but they wanted them to fit on the current 9-speed bikes, then the 10 cogs must fit in the same amount of space on the freehub body as the 9 previous cogs. To do this the engineer will reduce the width of the spacers between each cog. So now all the cogs are closer together. To save time and money, lets leave the derailleur design virtually the same; we will just slap a nice shiny 10-speed graphic on it.
But if the cogs are closer together, we need to change the shifter design so that it only pulls the cable just enough to get to the next cog. So the shifter is redesigned with an extra “click” and now each shift pulls 0.2mm less of cable than before. Obviously the design process is not this simple, but hopefully this over-simplified example helps to explain how things work. Tables that can be seen on the original article contain measurements for cable pull, derailleur shift ratio and cog pitch for varying drivetrains.
While the market is large and there are many different varieties of derailleurs and drivetrains, I would guess that 90% of people reading this article are using Shimano, SRAM or Campagnolo 8/9/10 or 11-Speed systems so we will focus on those combinations.