Really vintage mountain bike are kind of..... obsolete.
Now, I LOVE those old mountain bikes from the early 80's. I bought my first in... 82 or 83 and rode the hell out of it. But today... those old bikes dont cut it for serious mt biking; not compared to a contemporary mtb. Its the geometry. They are only good for getting groceries pretty much, or a nostalgia ride.
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(Road bikes on the other hand: their ideal form was figured out long before the 80's, and you can ride a bike from that era without much compromise at all.)
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I'm pretty sure this holds for almost all mtbs from that early era. Are there any exceptions?
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