Well now, that depends on what kind of mtn bike. The lightest singlespeed mtn bike that I've ever seen weighed 13 lbs.
Cannondale The brand new Cannondale is about 16.5 pounds. Which is the lightest fully geared mountain bike. Flash: Cannondale's 16.6-pound cross-country bike - VeloNews
what kind of mountain biking. If your downhilling then dont worry, if your doing anything else, i bet a 575 would be one of the lightest and nicest
Lightest MTB? Currently, and I may be wrong, but the lightest hardtail, and standard full suspension bikes are made by Scott. I am talking about the frames only not the complete bike. The HT is 1.98 lbs., or 899 grams. The FS is 3.94 lbs., or 1790 grams. Both work with Disk breaks only.
Lightest moutain bike I have a Scott Spark Limited. This is the lightest full suspension mountain bike (20.6 lbs). The frame alone weighs 3.4 lbs.!
575 yeti Ok, can't rave enough about Yeti bikes. I have been a big fan for many years. The 575 is a great trail bike that can handle many downhill situations, but even according to Yeti's website, it is a burly 7lb. frame. For people that are looking for a one and done do everything bike that you can ride trails on and race some cross county, I would recomend the super lite ASR5 Carbon (4.75 lbs) with 5 inches of travel. For those downhillers who are looking for the one and done trail bike that they can do some downhill racing, look at the ASR7, (7.5 lbs), who cares about the weight, 7 inches of travel. Both bikes are still trail focused and less race specific, but that is the way of things. If you want a race bike, know that it will be awesome for racing, and little else. My GF finally understands why I have 4 mountain bikes. They each have a specific job.
The lightest MTB weighs a couple of ounces. They're found in the toy section of WalMart next to the tiny 1/10 scale skateboards. You didn't say "lightest USABLE mountain bike"....
Niner , Air 9, Full carbon rigid single speed . 19lbs . A friend of mine just built his and it is scary light .
Anyone go on the weightweenies website or light bikes? Lotta cool stuff on there. I've still got a stash of parts for a light mtb someday.