This is not the worst, it's just the most recent I've seen and fairly typical of the sort of stuff some people post.
http://www.singletrackworld.com/forum/topic/whats-your-fasted-lap-around-cwmcarn-and-what-bike-kit?replies=15#post-2907756
You don't get that sort of attitude on running and weightlifting forums, why is it so common here ?
Does it happen on other sport's forums ?
I'm guessing it's because runners and weightlifters are generally competition focussed, so there is a lot of mutual support and encouragement.
The majority of mountain bikers on the other hand don't enter competitions.
Nobody likes to admit they've spent 6 months disposable income on a bike and can't ride it properly. I suppose for some people, mocking those who can saves their own embarrassment.
It doesn't bother me. If I was easily offended, I wouldn't mention what I eat and what sort of events I enter.
It's just that I've met a few people who could make a contribution to STW, but don't post here, because of the constant personal attacks.

