Differences Between Top & Bottom Swing Derailleurs
- A front derailleur consists of two parts. A cassette through which the chain passes and a fixed section which is clamped to the frame contains an attachment for the cable which runs up to the handlebars and a strong return spring. As the rider tightens the cable, it pulls on the cassette, which guides the chain onto a new ring and compresses the spring. When the rider loosens the cable by shifting in the other direction, the spring pushes the cassette back to take up the slack on the cable, moving the chain back to its original ring.
- The bottom swing derailleur, also known as a "high clamp" or "traditional" derailleur is the more popular of the two styles. Essentially, the cassette hangs below the clamp and pivots to the left and right. It is called a high clamp because with the cassette hanging below the clamp, the clamp must be mounted higher up on the frame than a top swing derailleur. Bottom swing derailleurs are sometimes thought to be more reliable than top swing derailleurs.
- On a top swing derailleur, the cassette is controlled by an arm that comes out of the top of the clamp. The cassette hangs on the side, instead of underneath, and is attached to a pivot point. The chain guide of the cassette is below the pivot and the arm from the clamp is anchored above it. As the arm on top of the clamp moves left and right it pivots the cassette in the opposite direction. The top swing derailleur is more compact and often used on frames where there isn't enough space for a traditional derailleur, such as a mountain bike frame with rear suspension.
- There are a few differences between the two derailleurs aside from the way it controls the cassette. The top swing derailleur is more compact, allowing it to be mounted on a frame with space constraints. Since the derailleur is mounted on the seat tube of the frame, a bike with rear suspension may not have enough space on the seat tube for a traditional derailleur, so a top swing derailleur must be used because it can be mounted lower.
- The cable pull on a derailleur is also referred to as "top" and "bottom" and should not be confused with a top or bottom swing. The pull refers to where the cable running to the rider-controlled shifter runs when it enters the derailleur - top pull means the cable enters on the top and bottom pull enters on the bottom. The top pull is often associated with the bottom swing and the bottom pull is often associated with the top swing so the cable does not interfere with the cassette, but they are not the same thing.
- When chosing a derailleur, make sure you match it to the shifters on the bike. The shifter and the derailleur should be the same brand for the best performance. There is nothing that prevents you using one of a different brand, but you may find they do not work together as well as they should. Also make sure that both the derailleur and the shifter are adjusted carefully for the best performance. The shifter should move the derailleur cassette over the center of each ring.