As someone said before, Portes du soleil is the best area to go DH riding if you are in the neighbourhood. Morzine is a good starting points, with a lot of shops, guides and hotels that accomodate cyclists. If you are interested www.club-giraffe.com are specialising in tours in the area with local guides
Chamonix can be a great place for DH if you follow a few simple rules. First up, don't bother with July & August, as stated elsewhere on the board. Next, try to get a good guide to show you around. Quite often the good stuff is hard to find. Finally, bring (or rent) a big bike. Chamonix eats lightweight freeride / enduro bikes as it's rocky, droppy and bumpy rather than smooth and fast (PDS style). Incidentally, for that reason, it's better in the wet.
Guides - without a doubt MBMB. In fact, ten50, formerly the only other bike guiding agency / operator catering to the English, aren't there this summer. MBMB have a pretty good website.
Rental - There are three bike shops in town...Legend, Grand Bi and Zero G. Most people end up at the latter - central location, decent current-year Specialized rental park and English-speaking staff - Chamonix bike hire @ Zero G.
Trails - see Guides section! Le Tour and Les Houches are the bike-friendly areas. Check the CMB website for more. Bear in mind that you can't take your bike on the Aiguille du Midi, Montenvers, Tramway du Mont Blanc or Index lifts. Everything else that is open is game.
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