Electronic gear-shifting system
An electronic gear-shifting system is a method of changing gears on a bicycle, which enables riders to shift with electronic switches instead of using conventional control levers. The switches are connected by wire or wirelessly to a battery pack and to a small electric motor that drives the derailleur, switching the chain from cog to cog. An electronic system can switch gears faster, and because the system does not use Bowden cables and can calibrate itself, it may require less maintenance.
In 1990 the Japanese bike component manufacturer SunTour introduced the Browning Electronic AccuShift Transmission (SunTour BEAST) - a triple chainset system for mountain bikes in which one quarter of the circle is hinged along a radius. During shifting, this segment is pushed sideways by a relay operated mechanism like a railroad switch and picks up the chain that is currently running on the next cog.
In 1992 the French manufacturer Mavic introduced their first electronically controlled gear shift mechanism called Zap at the 1992 Tour de France. It was a prototype, but it achieved neither technical success nor commercial application. A development of this was used by Chris Boardman to win the opening time trial (prologue) of the 1997 Tour de France.
In 1999 Mavic introduced the Mektronic, its second electronic shift system, which suffered from reliability issues and was subsequently discontinued.
The first commercial electronic gear shift system for road bicycles was introduced by Shimano in 2009, the Di2. Three professional teams used the Di2 in the 2009 Tour of California: Columbia High Road, Garmin Slipstream, and Rabobank; and several teams and riders, including George Hincapie, used it during the 2009 Tour de France
As of 2009, one system was commercially available from a major parts manufacturer: Shimano's Di2 (Dura-Ace 7970) for road bicycles. While the traditional method of gear shifting uses mechanical control levers that pull and release Bowden cables and spring-loaded derailleurs, Di2 is controlled by solid-state switches located either in the integrated shift levers and/or at the end of time trial bars. The switches send signals through a wiring harness to a battery pack, placed near the bottom bracket. The rechargeable lithium-ion battery pack supplies power to the derailleur motors, which move the derailleurs via worm gears. Shimano estimates that their 7.4-volt battery pack can last up to 1,000 kilometres (620 mi) per charge. The system also has an LED light to warn when it needs a charge.
The rear derailleur has shift times similar to mechanical systems and a break-away system to protect it in case of a crash. The front derailleur, however, switches gears almost 30% faster than Dura-Ace’s mechanical counterpart, with most of the improvement in shifting the front derailleur. On traditional bikes, the front derailleur is problematic because the chain can be under tension and has to make a large vertical jump between chainrings. The electronic system's controlled motion overcomes these problems. The Di2 can also trim the front derailleur to eliminate chain rub and calibrate itself to adjust for wear and tear. Finally, the entire 7970 groupset weighs approximately 113 grams (4.0 oz) less than the 7800 it replaces but 68 grams (2.4 oz) more than the new 7900.
In 2011 Shimano introduced the Ultegra Di2 electronic gear change set, a cheaper version of the electronic Dura-Ace system. This set seemed to provide an electronic option within reach of a wider audience. Campagnolo introduced their first system of electronic shifting in the same year. By 2012 Campagnolo had three electronic shifting systems available. Cyclists began to see a growing range of electronic alternatives to traditional mechanical shifting for derailleur gears.
A wireless system has been developed by Tiso.
An electronic system can have several advantages over a comparable mechanical system:
- eliminate the need for the rider to switch hand positions in order to shift (in the case of Shimano dual control levers for time trial and triathlon)
- allow for an accurate and effortless shift, even in difficult circumstances, such as if the rider has cold hands or is completely exhausted
- shifting performance is not affected by contaminated, stretched, or worn Bowden cables
- automatic trim function can eliminate chain rub
- the smoothness of electronics can reduce the shock on drivetrain components
An electronic system may have some disadvantages when compared to a mechanical system:
- There is currently no option for manual override when the battery is exhausted. This could pose a problem for riders, depending on the type of terrain on which they are riding.
- As currently implemented, each shift requires an individual button push, while many mechanical systems can move the chain multiple cogs with a single lever movement. N.B. The new 2013 Dura Ace Di2 9070 groupset can be programmed to multishift upon holding down the shifter buttons. This is now also available on Di2 Ultegra with a firmware update.
- Reliability issues have caused previous electronic systems to be withdrawn from the market. As of Recently, all Electronic group sets are Fully Waterproof. External Connectors are completely waterproof, but Internal Connectors (internal routing) need to be shrink wrapped with shrink tubing to be waterproof. 
- Cost: the system is expensive compared to lower end mechanical derailleur systems. (The Shimano Dura Ace Di2 system costs in excess of $2500 in May, 2010).
- Weight: a Di2 system is heavier than the equivalent Dura Ace cable shifted system. N.B. the new 2013 Dura Ace Di2 9070 groupset is actually slightly lighter than the 9000 groupset
- Hub gear - another bicycle gear shifting system alternative
- List of cycling topics
- NuVinci - continuously variable planetary transmission for bicycles
- Semi-automatic transmission - electronic gear shifting for automobiles and motorcycles
- Best, Paul (08 04 2009). "Shimano's Dura-Ace Di2 electronic shifting to give road racers a time advantage". Gizmag. Retrieved 2010-02-10.
- Michael Sweatman. "Browning". Retrieved 2012-09-10.
- Ian Austen (February 13, 2009). "Cycling Enters the Electronic Age With a New Gear-Shifting System". The New York Times. Retrieved 2010-02-12.
- "SRAM History". Retrieved 2010-02-17.
- Phillips, Matt (9 Feb 2010). "The Shift to Electric". Bicycling Magazine. Retrieved 2010-02-10.[dead link]
- Lennard Zinn (September 15, 2009). "Campagnolo's Magic Lives on in Vicenza". VeloNews. Retrieved 2010-02-17.
- David Arthur (9 July 2009). "Pro riders on Shimano Dura-Ace Di2". RoadCyclingUK. Retrieved 2010-02-12.
- "TSR Advanced SL LTD". Giant Manufacturing. Retrieved 2010-02-12.
- "Madone 6 Series". Trek Bicycle Corporation. Retrieved 2010-02-12.
- Crowe, Paul. "Electronic Shift By Wire on Bicycle". The Kneeslider. Retrieved 2010-02-09.
- James Huang (Aug 2009). "Shimano Dura-Ace Di2 transmission". BikeRadar.com. Retrieved 2010-02-13.
- Hagerman, Eric (2008-07-31). "Shimano Shuns Cables for Full Electronic Shifting". Wired. Retrieved 2010-02-09.
- Cole, Matthew (01 08 2008). "Shimano unveils Dura-Ace Di2 electronic groupset". BikeRadar.com. Retrieved 2010-02-09.
- Richard Tyler (Jun 20, 2011). "Shimano Ultegra Di2 – First look". BikeRadar.com. Retrieved 2012-09-10.
- Stephen Farrand (Oct 25, 2010). "Campagnolo show off new electronic gear system". BikeRadar.com. Retrieved 2012-09-10.
- Robin Wilmott (Jul 17, 2012). "Campagnolo Athena EPS 11-speed launched". BikeRadar.com. Retrieved 2012-09-10.
- Ben Coxworth (December 18, 2012). "Tiso unveils wireless electronic gear-shifting for road bikes". GizMag. Retrieved 2013-11-27. "Shift signals are transmitted from the shifters to the control unit via Bluetooth – Shimano and Campagnolo’s systems, by contrast, use electrical wiring."
- "The Competitive Cyclist". Retrieved 2010-12-22.
- Shimano's description of their Di2 with technical specifications and exploded views
- Campagnolo's description of their EPS with technical specifications and exploded views