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5.2 Frame Building Part 2




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

5.2 Frame Building Part 2

From: terryz@microsoft.com (Terry Zmrhal)

With the conclusion of the first class I hadn't finished cleaning up the
areas around the dropouts on the chainstays and fork blades after the
dropouts and blades/stays had been brazed. I went back to the shop on
my own time and spent another three or four hours on them. I'm prepared
to give a little more detail on this.

I'll start by explaining a little about the tools. We used two hand-
held air-powered sanders. The first is a small belt sander. The belt
is about 8 inches long and about a half inch wide. Each semi-circular
end had a radius of about a quarter inch for getting in tight spots.
This one is used for sanding the edges of the lugs as well as work on
the dropouts. The other was a small rotary sander. A small roll of
tightly wound sandpaper is fitted over a smooth bit. This one is used
for sanding the instead of tubes, finished lugs, rounding edges, and a
little work on the dropouts. The other tools for this are various
files, flat and half-round of various sizes, and fairly rough sandpaper.
Sandpaper can be backed by any of the files, by or fingers. Any
scratches left my files or sandpaper are covered up when paint is
applied or if the frame is buffed.

The first thing on the blades was to take the hand-held belt sander and
sand a smooth curve into the blade where the blade meets the dropout.
This will make more sense if you look at a traditional fork blade.
There are two purposes to this - one is aesthetics, the other is to
remove excess brass leftover from brazing. You have to be careful not
to sand into the dropout. The ideal is to just begin sanding into the
dropout and stop there. You don't want to remove very much, if any
metal from the dropout. You want to make sure the depth of the curve is
symmetrical on both sides of the blade as well as between the two
blades.

Depending on the size of the dropouts, how they fit into the slots
created on the blades, and how you want the fork to look will determine
some of how you finish the rest of the fork. I had some of the dropout
sticking up out of the top of the fork and very little sticking out of
the bottom. I used a file to finish off the part of the dropout
sticking out of the bottom to create a smooth curve. It's OK to file
away the tabs of the dropout, but not the part of the dropout remaining
outside of the blade. Once finished with the file, I used sandpaper to
smooth it out. On the top, since more of the tab was sticking out, I
created definitive edges/corners on either side of the dropout tab. I
used a file and ran the edge where I wanted to create the corner. Once
a corner was created, I used sandpaper backed by a file in that corner
to help define it more.

If you look at a drop, the actual curve around where the axle sits in is
a little bit thicker than the rest of the dropout and has a circular
edge defined around it. I used the corner of a file to carefully define
this edge a little better and to remove some of the brass in there.
The two hand-held sanders have somewhat sensitive levers which control
the speed of the sanders. These took me awhile to get used to. I was
very hesitant and careful sanding the first one with the belt-sander.
With the second I thought I had an idea of how to do it, but goofed a
little and started to cut into the edge around the actual dropout a
little. I later went back with a file and created an edge again. By
the time I finished the third and fourth I felt pretty comfortable with
it and somewhat in control of it, but still not perfect.

There's a little bit more after this. Depending on how much flux or
bits of brass are left, you may need to sand the dropout itself and some
of the edges to clean it off.
With the stays, it is much the same. The one big difference is that on
the inner sides of the stays, we used a large belt sander versus the
hand-held sanders. With the large belt sander we created a more gradual
curve necessary for cog and chain clearance. The cog and chain
clearance is obviously only necessary on the drivetrain side of the
bike, but for symmetry it's done on both sides. The outside curves are
created with the hand-held belt sander. Again definition I smoothed the
bottom tab of the dropout on the stay to create a smooth curve on the
underside. In most cases, you will want to define the top of the
dropout on the stay since there is more dropout to match the seatstay.
Again I used a file to define corners and edges and finished out with
sandpaper.

This can be very tedious work and requires some patience and some
control. With a file, it is very easy to create edges very quickly.
Once they are there, it is hard to get rid of them. Getting a file to
go exactly where you want it takes some control. I was very careful
about sanding and filing checking every couple strokes or so. Once the
metal is gone, it's gone. You can put some brass back on, but then
there's cleanup and time consumed again.

Now for the second class.

For this second class, we did prep work for the main triangle and fork
and then brazed the main triangle and fork. I finished with a little
bit of cleanup on the main triangle, but not much. I was able to do
some of the work this class, but not nearly as much as last time, so I
watched Dave braze for much of the class. The brazing of the main
triangle and fork is a much more delicate, detailed, and careful process
which I'll explain below.


 

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