Depends on the races.... Hi Brian,
Unfortunately there's no simple answer for what type of mountain bike to buy to compete in mountain bike races. If you're referring to cross-country mountain bike races (vs. downhill), and if you're an average rider who enjoys getting out on weekends and who wants to occasionally race, I'd say get a "cross country full suspenstion" mountain bike. Trek, Specialized, and many others make great bikes with around 3" or 3.5" of suspension, front & rear which will make for a good climb and a fun descent. Ideally you want to get as light of a full suspension bike as possible (under 27lbs), which is generally dictated by how much you want to spend on a bike. You can buy race ready bikes for $2000 on up. I suggest spending $3000 or $4000 if you're really going to do a bunch of racing. There may even be some $1500 race worthy bikes, but I'm not certain about that. I'd suggest something with Shimano LX, XT, or XTR components (or the equalivant SRAM components) and light wheels if possible. I suggest full suspension since they're a lot more comfortable for day to day training as well as racing. But if you're a hard core racer and purely concerned with speed and not with comfort, you can get a race worthy hard-tail for less money, and they weigh less (~23lbs and under?). But as someone who owns 4 mountain bikes, I can tell you that my hard-tail sits on the wall and ALL of my riding & racing is done on my full suspension bikes. My back simply can't take the abuse of a hard-tail. I happen to race on a Trek Top Fuel 110 and LOVE IT. It's relatively light, ~25lbs, has 3.5" of travel, great Shimano components, and both climbs & descends well. If you go with one of the big name brands, you almost can't go wrong: Trek, Specialized, Giant, Rocky Mountain, Canondale, etc.
I hope that helps!
David's advice is great! Traditional thinking is that hardtails are faster, but some recent tests have shown most full susp bikes to actually be faster... you just may have a bike that weighs more, thus you need to be stronger... but the suspension keeps you in contact with the trail more.
Also, don't be afraid to look @ 29ers as an option (for xc or endurance races). The key though is WHAT TYPE of race/discipline, your price range, and FIT!!! Doesn't matter how cool or light... if the fit is bad, you'll hate it down the road!
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