I went totally weenie once on my mtb when I was riding at an OK level.I also worked in a motorbike shop so could get a lot of mtb parts at cost thru one of the importers...got my Gary Fisher ProCaliber to 10.5kgs at the time which was light especially for the frame size and the year (2000)...but it only lasted 1 race
.Ok it was 24 race but I only rode it for 6 hours...forks were trashed and so were the wheels.Since then I have tried to find a good middle round on all my bikes...obviously I fail quite often!.My biggest problem is the amount of flex I get out of bikes...always have to back the brakes way off to stop wheels rubbing when the power goes down.I expect that is because I am heavy but also tall so get a lot of leverage.
One day I would like to go totally weenie on a road bike to...over here it is a very common past time...most bikes ridden by the guys in my club are sub 6.5kgs.One is sub 5.5kgs....but I still used to beat 80% of them
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Edit...being a weight weenie is much easier when you don't break every thing you touch!....including your body!!!.