I saw an suv with a bike rack strapped on the back of it when I was going to exchange some videos. And there on the rack, were propped two examples of the greatest travesty to ever befall the creation of the two-wheeled transportation known as the bicycle: LandRiders. If you have never seen one of these things, they are bikes that shift automatically. They are, of course, advertised via infomercials that prey on those who have never actually ridden a bicycle with any semblance of modern gears. "It just looked like it would be so easy" I have heard more than one customer say. Well, the fact of the matter is that the "auto" shift bike was probably first developed shortly after the first derailler.
If you know anything about the history of the modern bicycle, then you will certainly be aware that Campagnolo was the first company to introduce a mechanism to shift gears at the rear hub. It was known as the Paris-Roubaix gruppo and it did not work well. It featured several levers, one of which would loosen the hub in the dropouts (allowing it to move and thus accomodate the chain length change), while the other lever was used to pick up the chain and lift it on to the appropriate cog, all while riding, mind you. Legend has it that Fausto "il campianissimo" Coppi was the only rider to ever master it.
Well the LandRider works almost as well as the Paris-Roubaix gruppo did. It uses this centrifugal force device to sense the time to change gears. There is one slight flaw to the system: you're never in the right gear. It forces you to spin at such a rate that a track cyclist would get spun out. Those poor retirees (invariably) perched on the bike, look positively worked as their feet spin around like hamsters in a workout wheel. Please, please, please if you know someone who is contemplating getting one of these things: Do not hesitate in getting them to the nearest bike shop and showing them the "modern" wonders of indexed shifting!