This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Date: Fri, 11 May 2001 16:35:42 PDT
aka Bicycle pulls to one side
Riders occasionally complain that their bicycle pulls to one side when
ridden no-hands. That is, the rider must lean off to one side to ride
straight ahead. This symptom can be from a wheel that is in crocked,
something that is easily checked by observing whether the tire is
centered under the brake bolt, or by just reversing the wheel to see
if the wheel is improperly centered.
Assuming the bicycle still pulls to one side, the reason is usually
that the fork is bent from a side impact. Bent from a frontal impact
this is easily seen because the blades have a rearward bend just below
the fork crown where the blades should be straight both fore and aft
and side to side. A frontal bend usually gives a side bend because
the blades are not identical and tend to skew to one side. This is
harder to fix and requires fixturing.
If the fork is only bent to the side, the correction must be to the
side to which the rider must lean when riding no-hands. This bend can
be done carefully by bending one blade at a time.
Lay the bicycle on its side, front wheel removed. Place the rubber
soled foot inside the crown of the fork and pull the upper blade until
the gap at the fork end increases by a couple of millimeters. This
should be measured. With the foot in the same place pull the other
fork blade until the original spacing is restored. Ride the bicycle
and assess the difference. Repeat if necessary. This must be done
with a strong arm and a bit of skill but it is simple.
If you have a non steel bicycle, buy a new fork.