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http://en.wikipedia.org/wiki/Electronic_Gear-Shifting_System

Cycling Electronic Gear-Shifting System

Electronic gear-shifting on a bicycle is a new method of changing gears by enabling riders to shift with a light touch of two electronic switches, compared to the conventional method of using control levers. The switches are electronically connected to a battery pack, which is connected by wires to a small motor that drives the electric derailleurs, switching the chain from gear to gear. The electronic system switches gears faster, and because the system doesn't use a chain and pulley, the chance of dirt build-up is less likely to occur, requiring less maintenance.

Design
While the traditional method of gear shifting involves using solid-state control levers that pull and release cables, electronic bicycle gear-shifting is controlled by mechanical switches located near the handlebars that automatically shift from gear to gear. The near effortless touch sends a message through a wiring harness to a battery pack, usually placed near the front derailleur, which is what moves the chain from gear to gear. The rechargeable lithium-ion battery pack supplies power to the derailleurs, which have motors, rather than using the cable/pulley system.

The rear derailleur, which features a carbon pulley plate for increased weight savings, has similar shift times compared to a traditional bicycle. The front derailleur, however, switches gear almost 30% faster than regular mechanical derailleurs. On traditional bikes, the front derailleur is problematic because it has to make a large vertical jump between each chainring to switch from gear to gear, requiring more effort. An electronic system's controlled motion would therefore eliminate these problems. It is able to automatically adjust itself so chain slipping and chain rub isn't a problem. Also, its integrated CPU automatically syncs the chain so riders don't have to worry about adjusting the chain tension, especially after long rides and wear and tear. Because the system is protected through a wiring harness, it completely prevents the chance of clog up, which causes the chain to wear down and result in less smooth gear-shifts. The motor derailleur reduces the need for maintenance and leaves the rider with a smoother, better performing bicycle.

Specifications
The current parts used for electronic gear-shifting include dual electronic shifters, front and back electric derailleurs, and a battery pack. Most current models boast a rechargeable lithium-ion battery that lasts very long. Shimano estimates that their Di2 road bike with electronic gear shifting, which uses a 7.4 volt battery pack, can last up to 1000 kilometers per charge. The only drawback to using a battery pack is that the technology for manual override when the battery dies doesn't exist yet, which could pose a problem for riders, depending on the type of terrain that they're riding on. Besides that, the entire electronic system weighs approximately 113 g less than its mechanical counterparts, which is very beneficial since weight plays a major role in bicycle races. A bicycle with less weight is able to perform better because compared to heavier bikes going at the same speed, it has been tested to cover the same amount of distance in a shorter time.

Advantages

 * the electronic system switches gears almost 30% faster than a regular mechanical bicycle
 * eliminates the need for the rider to switch hand positions in order to shift
 * allows for an accurate and effortless switch, even in difficult circumstances, such as if the rider has cold hands or is completely exhausted
 * shifting performance will never be affected by contaminated, stretch, or worn down cables because signals are all received electronically
 * automatic trim function: eliminates the problem of chain rub because the chain will automatically adjust
 * the smoothness of electronics would reduce the shock on drivetrain components

Current or Prototype Models
Three companies have tested out the new electronic gear-shifting concept. A couple systems have been released by the company Mavic, while the company Campagnolo has been race testing systems since 2002, and is rumored to launch its first system in 2010. A third company, Shimano, recently came out with its version of the electronic gear-shifting system, called the Di2.


 * Shimano Dura-Ace Di2 7970 (2009)
 * Campagnolo prototype
 * Mavic Mektronic (1999)
 * Mavic Zap (1993)