well that all depends, for example you could be a sub 120 pound guy riding a 12 pound mountain bike and be fine, could you say the same for a 220 pound guy? then you may throw into the equation riding style, you may only be using your mountain bike on the road, in which case you will/should be allowed to ride a lighter bike
Not only should safety be a consideration; durability also is a factor. I tried to build a light full-suspension bike for mainly hardpack trail riding that would have roughly 4" on both ends but not be something requiring constant repairs. I believe I accomplished the goal. There are a few parts I did investigate and decide to not risk using. Lightweight cassette is the first one that comes to mind as I don't want to give up shift quality. I also prefer the pedals I use even though I could drop over 100g with others. You need to do some product research when building up a lightweight ride and then make choices based on budget, your weight and riding style, where you ride, and how often you plan to want to work on the bike.
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