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How Rear Derailleurs Work

By Georgena Terry

rear-derailluerIf you’ve ever held a rear derailleur in your hand, you’ve probably noticed it’s kind of “curled up’ in a fetal position. You can coax it into a more open position by spreading it apart. It’s hard to do, because of the strong spring in the derailleur that has to be stretched in order to open the derailleur.

Without this spring, the derailleur wouldn’t work. The spring creates a tension that has to be overcome by the shift lever cable. Because of its spring, the rear derailleur prefers to be outboard, hanging out on the smallest cassette cog until cable tension coaxes it onto the other cogs.

When the chain is on the small cog on the cassette, the derailleur is in its “fetal position.” As you shift to larger cogs, the shift lever pulls on a cable which pulls on the derailleur, overcoming the tension in the spring. As you move the lever back, the spring tension moves the derailleur back onto the smaller cog. The derailleur is designed to move so that it stays a constant distance from the cassette throughout its range of motion.

This theme of “tension” is at work not only in the rear derailleur, but in the front derailleur and the brakes as well. Here’s a thought experiment to reinforce the concept. Imagine you’re a section of derailleur cable. You’re just hanging out, enjoying the scenery when your owner sees a hill and decides to go up it. She taps the shift lever and wow, all of a sudden, you feel much more stretched at both ends. Your head is being tugged toward the shift lever and your feet are being tugged toward the derailleur.

Indexed shifting relies on an exact relationship between the movement of the shift lever cable, the movement of the rear derailleur and the spacing between the cogs on the cassette. When the derailleur is working properly, the chain should be very quiet as it runs on the cassette (assuming the chain is clean, lubricated and not stretched). If it’s making noise, jumping around or just not responding to your commands, the rear derailleur needs to be adjusted.

Shimano has very precise instructions for setting up the rear derailleurs. In essence, the inboard and outboard position of the derailleur is set first. This assures that the chain doesn’t fall off the smallest or largest cogs. From there, the remaining adjustment focuses on setting the proper amount of tension in the cable to make sure the derailleur moves in concert with the spacing of the cogs in the cassette.

Shimano’s website offers technical documents for all of its products. They are a great way to learn more about how your bike operates. Each one is like a mini-manual that walks the user through the set-up of each part. Knowledge is power! This service document explains the adjustment of the Ultegra rear derailleur:

Once you understand the simple concept behind the rear derailleur — tension — the fog will lift, the sun will shine and you will be an informed and savvy cyclist!

 

GTA _signature

http://georgenaterry.com/

Georgena will answer your questions about bike-fit, bike-frames, saddles, and bike-maintenance. So don’t be shy. Take advantage of her knowledge and decades of experience to get answers to any questions you have about bicycles. Email me at [email protected].

When Georgena Terry first started designing bicycles, over 30 years ago, there were no women-specific bikes. Many female cyclists were forced to put up with neck, and shoulder pain, and other physical problems that came from riding bicycles that didn’t fit them. Georgena Terry changed that. Looking at anatomical differences between men and women, such as body mass and body strength, she began designing bike frames that optimized comfort and performance for female cyclists, with special attention to women under 5’2”.

Working out of her basement in Rochester, NY, Georgena first designed bikes for herself, then for her cycling friends. In 1985, she sparked a revolution in the cycling industry by launching Terry Precision Cycling, selling custom-built, high-quality steel frame bikes for women. Larger cycling companies sat up and took notice. Eventually following her lead, they developed their own brand of women-specific bicycles, clothing, saddles and accessories.

Today, Georgena continues her passion, designing and building custom steel bikes for women cyclists of all sizes. Contact her at georgenaterry.com

 

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