It can be a tricky business, selecting the proper gear combinations on a dual-derailleur bike. You always want to avoid "cross-chaining," a situation in which the chain is stressed by being placed at too much of a lateral angle (such as if it were running from the outermost chainring to the innermost sprocket, or vice versa). French inventor Rolland Norbert is attempting to address the situation, with his Twin Shift.
The grip-style shifter mounts on the end of a flat handlebar. With a series of single click-twists, users can move up and down through the gears, as the Twin Shift automatically moves both the front and rear derailleurs accordingly. It does so entirely by mechanical means – no batteries are required.
Not only does it keep riders from cross-chaining, but it also means that they only have to shift once for each gear-change, as opposed to having to shift both the front and rear derailleurs separately.
Plans call for the system to be made in versions suited to 3x6, 3x7 and 3x8 drivetrains.
It is claimed to be compatible with most existing frames and components, and is currently the subject of a Kickstarter campaign, where a pledge of €70 (about US$82) is required to get one. Assuming it reaches production, delivery is estimated for next June.
Prospective backers might additionally want to check out the Synchrobox system. And if they're willing to spend a bit more money and don't mind periodically recharging batteries, Shimano's XTR Di2 electronic shifting system also automatically shifts both derailleurs together.