I personally don't believe in the under/overbiked thing at all. If the same rider does X.XX time on a course on a rigid/CX bike, they will quite likely annihilate that time on some fancy uber-MTB. But for every guy obsessed with speed and/or marginal gains, there's a guy that doesn't wanna pay for it.
If some guy is toodling around at 14mph on a Cannondale Black Edition, it's not like he's wasting it. Ridin' is ridin'. Plenty of guys buy supercars to get them mired in LA traffic. Wants > needs. By the same token, if someone wants to go full-bore around the local crit course on '80s gas-pipe steel, nothing wrong with that either.
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