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5.3 Frame Building Part 2: Fork Prep




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This article is from the Misc Bicycles FAQs, by various authors.

5.3 Frame Building Part 2: Fork Prep

Prep work on the fork involved finishing the rake on the fork, cutting
the fork to the right length, cleaning the lugs and prepping the steerer
tube. To finish the rake on the fork involved this lever mechanism
again which we used to start the rake. I needed 520mm of fork rake
(I'll explain this number and a few others in another section on the
choice of frame measurements). It was at about 200mm so I had a ways to
go. Getting the fork rake up to about 480mm was done in about 3 steps,
each time checking how far I needed to go. Not knowing the properties
of the metal and having not done this before I went slowly. On the
third step I was nearly hanging my entire body weight (150 lb.) on it
and barely moving it. Now for the tough part. I had about 40mm to go.
For four successive tries I hung my entire weight on the blade and moved
it maybe 10mm. Ron, the other instructors tried and he didn't have much
success either. So he put his bodyweight into it and moved it another
10. My turn again. This time I put my entire weight on it and gave one
good bounce. That put it to 600mm! It was a good bounce. We simply
bent the fork blade back to where it should be by reversing the blade in
the lever. The other blade was a little easier to get to 520. Finally
they matched at 515mm - close enough.

How to measure fork rake without having a bike to check with? They had
a 90 degree ruler - on one side was a metal block with a triangular cut
out which the for blade would rest in. On the other side was another
block which had a bolt around which the dropout would fit and slid along
a ruler defined to the mm. The trick was to get the fork blade laying
flat in the triangular cut-out and find out where the rake was. The
usual problem was figuring out whether the blade was sitting squarely in
the triangular cut or not.

Now to cut the fork blade. The length of the fork is a function of
wheel size, whether there could be fenders, brake type, and crown height
among other things. There is a figure they called E-length which is the
distance from the center of the axle to the top of the crown. My E-
length should be 380mm. From this they subtracted 29mm which was the
distance from the top of the crown to where the fork blade would sit
against the crown lug itself. So the fork blade should be 351mm. They
used another self-made tool to measure this length. It was a wooden
board with a peg for the dropout and a cross-hatching on it. One cross
hatch measure rake, the other fork length. Once designed it worked
well, how they came up with it I don't know. Cutting the fork blade was
just a matter of using the jig saw to rough cut it. Only a rough cut
was necessary since it would be hidden next to the lug.

The ends of the fork blade were squared off using the belt sander and
then sanded inside to smooth it. The outside was cross-hatched with
sandpaper. This means taking sandpaper across the top of the blade at
an angle one way and then at a perpendicular angle. The idea was to
create a cross-hatch of scratches on the fork blade where brass could
flow when brazing was done.

Prepping the lug involved using the hand-held belt sander to square the
edges and then the round sander to smooth the inside corners. Squaring
the edges can be tricky as you move around the lug, trying to maintain a
grip on the lug, stay perpendicular to it, and not getting the sander
moving to fast. The inside surfaces where brass would adhere were
sanded using the round hand-held sander. The blade were fit into the
lug to make sure they fit at all - being snug.

The top of the fork was machined by Ron. It involved cutting the
steerer tube to the right length, threading it and machining off some of
the bottom so it would fit into the fork crown. They had these done by
the time we arrived so we didn't get to see what they did or how they
did it. Cutting to the right length was probably done with the jig-saw
and the threading and machining of the bottom were probably done with
the lathe they had.

Finally the fork was prepped in the fork jig. First four vent holes
were drilled - two on each blade, one towards the top, one towards the
bottom on the rear inside. These were to allow gas to expand and move
about once heated. The steerer tube and crown are held in place upside
down and then the fork blades are hung from clamps mounted on top. The
checking here is that the dropouts are aligned. If they aren't, the
dropout is clamped into a vice and the blade pushed/bent the appropriate
direction. Once the dropouts are aligned, they are inserted into the
crown. Next the crown is checked to make sure it is square. The final
step is tacking the blades to the crown. Tacking is done to keep the
pieces together while the brazing is being done. Tacking involves using
small amounts of brass to hold together the blades and crown. The fork
is now ready for brazing.


 

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