A full set of components with electronic gears will cost about $1,250 more than the newest mechanical version, which sells for about $2,750. Upgrading an existing Shimano system is expected to cost about $2,200. The system will fit onto nearly all racing bicycles.

Later this year, Giant, the largest bicycle manufacturer in the world, will offer a bike designed to use only electronic parts for about $14,000, which includes the cost of Di2. If consumers fancy the device, it will likely follow the pattern of other new electronics and drop significantly in price over time.

Electronic gear-shifting technology has spent a long time in development. Prototypes of Mavic’s first system, the Zap, made a cameo appearance at the 1992 Tour de France and the company introduced its second attempt, the Mektronic, in 1999.

For much of this decade, both Shimano, which dominates bicycle parts the way Microsoft dominates computer software, and its venerable Italian competitor Campagnolo occasionally tested prototype systems on the bikes of pro riders. More often than not, the prototypes were devoid of trademarks, presumably to limit embarrassment if results proved as unfortunate as the Zap.

The Campagnolo and Shimano systems share the basic design of current mechanical derailleurs. That is, a parallelogram that moves the chain back and forth and, in the rear, two spring-loaded wheels to keep the chain taut.

Two paddle-shaped electronic switches that sit behind the brake lever allow riders to shift gears. Tapping either paddle lightly in Di2 sends an electronic signal through a wire to a small motor inside the derailleur, moving the body and thus the chain by turning a worm gear. Even Devin Walton, a spokesman for Shimano, acknowledges that when it comes to the rear derailleur, there is little or no difference in shifting between the electronic and comparable mechanical offerings from the company.

The gains are more obvious, however, with the front derailleur, which moves the chain between the two large, toothed rings on the bicycle’s crank.