Bottom brackets are the most challenging installation. In virtually all frames
the tubes drain directly into the bottom bracket. Here, at the lowest point,
crud dislodged from the sides of the seat tube joins the smaller streams of
crud from down tube and chain stays in an abrasive, oxidative attack on the
crank bearings. If the bike has an open-topped seat post and no fenders, a
portion
of the gritty ditch water which flies off the tire hits the underside of the
seat and dribbles down the seat post into the crank bearings. Additionally,
the welding gas relief holes at the rear dropouts admit sandy stream crossing
water into the chainstays until they are full. Once in the frame, this sludge
has no egress save by rusting its way through the bottom bracket cup threads
and grinding its way through the ball bearings. After my first mountain bike
race, I poured a quart of water out of my frame, and later removed a
tablespoon
of gravel from the bottom bracket.
If you don’t have a closed-top seat post, put a cork in the end of the
seat post. Also, put thread-locked fasteners in any unused water bottle
braze-ons
to plug the holes, and solder or epoxy those nasty gas relief holes shut.
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