Making Sure You Don’t Get Derailed

Anyone who has ridden a bike up a windy mountain pass or into a stiff head­wind appreciates the ease of shifting into the appropriate gear with a click or twist of a shifter. The mechanism that makes shifting a nearly effortless activ­ity is the derailleur.

To look at a derailleur, you’d think that it’s the most high-tech part of a bike. In reality, derailleurs are simple devices designed to move the chain between the different gears. Both the front and rear derailleurs are designed with a

Whenever possible, avoid gears that cause the chain to cross from the front to the back at an angle. For example, avoid locating the chain on the smallest chainring in the front and smallest cog in the back. This causes the chain to stretch and wear out and is bad for the sprockets.

cage through which the chain runs. When you change gears with your shifter, the derailleurs force the chain to one side or the other until the chain falls or is lifted onto the chainring or sprocket next to it.

The front derailleur consists of a cage that, in a side-to-side motion, moves the upper part of the chain — the part that transmits power to the rear wheel. Because the upper part of the chain is under the force of your pedal­ing, it wants to stay in place. It’s more difficult to shift when you’re applying a lot of power and moving slowly.

Its more advanced partner, the rear derailleur, both pushes the chain from side to side and pulls it tight. The rear derailleur is designed to serve two main roles: moving the chain between the sprockets and keeping the chain under tension.

The mechanism controlling the rear derailleur is a hardened spring hidden inside. This spring is constantly pushing the derailleur away from the bike and toward the smallest sprocket. The cable attached to the bike’s shifter opposes the force of the spring. When the shifter pulls the cable, the cable overcomes the spring and moves the chain toward the bike and onto the next sprocket. When the shifter releases the cable, the derailleur again moves away from the bike. Tension of a second spring provides the resistance, which takes up any chain slack.

Rear derailleurs have a common design (see Figure 2-6). They have a cage that holds two pulleys in the familiar S-shape. The top pulley is the jockey pulley (also known as the upper jockey wheel); it guides the chain into what’s called the cage. The bottom pulley is the tension pulley (or lower jockey wheel); it’s designed to keep tension on the chain and take up slack.

Derailleurs are designed to maintain the appropriate amount of space between the jockey pulley and the rear sprockets as the derailleur moves back and forth. The derailleur also move its arm back and forth with each shift to keep its cage centered under the sprocket on which the chain sits. Usually, one or more screws are utilized to control the amount of lateral movement and spring tension.

The rear derailleur cage that connects the jockey pulley and the tension pulley has a length that varies depending on the amount of chain slack needed to be taken up. Models with longer cages are designed to support larger, lower-geared cogs and longer chains; they’re typically found on bikes with three chainrings. Rear derailleurs also have a maximum tooth capacity, which specifies the largest cog onto which it can shift a chain. Short cage derailleurs are found on racing bikes; they offer quicker shifting and higher ground clearance, the latter of which is important when corner­ing in tight curves.

Making Sure You Don’t Get Derailed