There is a slightly bewildering array of derailleur options available, not all of which are intercompatible. Even when you’ve decided how much money you’re going to spend, it can take some thinking to work out exactly what you need.
For example, in their XT and XTR mountain bike ranges, Shimano currently make three different versions (top normal, low normal and Shadow), with two different cage choices for each, making a total of six different options. Road bikes have fewer varieties. Most bikes are fitted with a ’top-normal’ rear derailleur. This simply means that increasing the cable tension makes the derailleur shift the chain onto a larger sprocket (lower gear), while releasing the cable tension allows the chain to drop back to a smaller sprocket (higher gear).
Less common are ’low normal’ derailleurs, which used to be called ’Rapid Rise’. These work in reverse, so that increasing the cable tension makes the derailleur pull the chain onto a smaller sprocket, while releasing the cable tension means the chain climbs onto a larger sprocket. These were supposed to have all sorts of advantages, such as better shifting under pressure, but haven’t really ever caught on. They take a bit of getting used to if you’re used
to conventional shifting responses.
Increasingly, more and more mountain bikes are coming with Shimano’s latest rear derailleur design, the Shadow. This is ’topnormal’ so works in the conventional way, but has a number of advantages. Firstly, the cable routing on the derailleur has changed, so that the final section of outer casing is much shorter and doesn’t have to curve all the way around the back of the derailleur. This reduces cable friction. Secondly, the Shadow derailleurs have a dramatically reduced profile – they don’t stick out so far from the side of the bike and so are less likely to get mangled in a crash.
Shimano have set the standards for compatibility – they are the stock derailleur for the overwhelming majority of new mountain bikes, so most other manufacturers make sure they’re dancing to Shimano’s tune.
The only significant exception is SRAM. The difference is the ’actuation ratio’ – the amount the guide jockey wheel moves sideways in response to pulling or releasing cable with the gear shifter.
- The Shimano standard is for 1mm of cable movement to move the guide jockey 2mm sideways – a 2:1 ratio. SRAM Attack shifters are compatible with this standard.
- SRAM’s x3, x5, x7, x9 and x0 mountain bike (and SRAM road bike) shifters and derailleurs all work on a 1:1 ratio, so every 1mm of cable pull moves the chain 1mm sideways. SRAM’s XX derailleurs can only be used with SRAM XX shifters, as they use their own Exact Actuation ratio, migrated from SRAM’s road groupsets.
“Increasingly, more and more mountain bikes are coming with Shimano’s latest rear derailleur”
Shadow rear derailleur
Setting the derailleur angle with the B-screw
The B-screw sits at the back of your derailleur and adjusts the angle that the derailleur sits at relative to the frame, altering the gap between the top of the guide jockey wheel and the sprockets. If the chain sits too close to the sprockets, it will clatter on the cassette teeth as you ride. If the gap is too big, you’ll get sluggish shifting because the chain will flex sideways when you try to change gear rather than meshing neatly with the next sprocket.
Use the B-screw to adjust the gap so that there are there is around 2.5cm (1 inch) of clear chain between the sprocket and the guide jockey. On Shadow derailleurs, the B-screw sits below the derailleur’s main pivot (see left). On conventional derailleurs, the B-screw sits behind the derailleur’s anchor bolt and abuts on a tab at the back of the frame. In either case, turn clockwise to increase the gap between cassette and guide jockey, anticlockwise to move them closer together.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Adjusting the end-stop screw on your rear derailleur”