lotus

previous page: 5.11 Frame Building Part 3: Frame Alignment
  
page up: Bicycles Misc
  
next page: 5.13 Frame Building Part 3: Fillet brazing

5.12 Frame Building Part 3: The Rear Triangle




Description

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

5.12 Frame Building Part 3: The Rear Triangle

The chainstays and dropouts have already been brazed together and
cleaned up. The only thing left on this part was mitering the
chainstays to match the bottom bracket shell which was again done on the
drill press. Once the ends were mitered, the chainstays were sanded
inside using the hand-held air-powered rotary sander and cross-hatch
sanded on the outside.

The seatstays will require a little more explanation. Somehow there
needs to be a seat post binder bolt. The style I've chosen Dave calls a
fast-back. The seatstays are attached behind the seat tube, then
drilled through so that the binder bolt is hidden within the seatstays.
But the seatstays are hollow tapered tubes (3/4" to 1/2") - how can you
drill through it and thread it for a bolt? I've seen this style before,
but never thought about it. What Dave does is uses a set of solid plugs
at the end of the seat stay which can be drilled through and threaded.
The plugs are cut from a solid tube of the same diameter as the top of
the seatstay. These plugs will need to be brazed to the seatstays. To
do this, Dave uses the lathe to shave the plug to the inner diameter of
the seatstay. Then he lathes a deeper section. Around this deeper
section he wraps a small piece of brass wire. The plug, with the brass
section, is inserted into the end of seatstay, which has been sanded
inside and outside and squared off. To braze these together, all that
needs to happen is to heat up the plug and seatstay, the brass in the
plug will melt and fill the area between the plug and seatstay. You can
stop heating when brass starts to flow out the junction between the plug
and seatstay. Quite ingenious I thought. The cleanup on these is
pretty simple, just sand and file off the brass that flowed out. (This
is just one option for the binder bolt.)

The frame is put back into the frame jig. The chainstays are fitted
into the bottom bracket and around brackets designed for the dropouts.
With these in place, the seatstays can be measured and the angle of the
miter at the top of the seat tube junction can be determined. Before
the process of attaching the plugs is done, a rough cut is done to an
approximate length measured in the jig. Now that the plugs have been
attached, the ends of the seatstay which will put onto the dropout are
cut so that the seatstay fits squarely on the outside of the dropout
with very little on the inside. In contrast to the fork blades and
chainstays, these are not centered or off-centered but completely on the
side. There needs to be as much room as possible for the chain. The
plug needs to be mitered to fit behind the seat tube. Dave roughly
measures an angle, which the drill press is set at.

Dave has another custom tool he uses to cut chainstays like this. This
custom tool fits onto the angular table of the drill press. One end is
adjustable to the length of the chainstay and adjustable in width to the
width of the rear dropouts. In this case, it's 130mm. The other end
has angular plates which the stays run along and all in held in place by
a couple clamping plates. With both seatstays in place at once, Dave
drills a cut miter onto the plugs. He then rechecks the angle of the
cut in the jig and does a final miter.

Before the brazing is done, both ends of the seatstays need to be
cleaned off of burrs, sanded on the inside, and outside. The frame is
then tacked in place in the jig. A silver tack (vs. a brass tack) is
used at the junction of the chainstay and bottom bracket. A TIG tack
(using the TIG welder) is done at the junction of seatstay and dropout.
No tack is done at the top of the seat stay. A TIG tack is used at the
junction of the seatstay and dropout since it is a more solid tack. The
chainstays are unlikely to move, but there is more chance of the tack at
the junction of the seatstay and dropout of breaking loose. A final
checked of the alignment of the rear triangle is done using a tool
similar to checking the centering of the fork before brazing is started.

The chainstays are silver brazed into the bottom bracket shell and the
top of the seatstays are silver brazed to the seat tube. The bottom of
the seatstays are brass brazed to the dropouts and I was able to do one
of these. The same techniques of brazing are used here as before.

I was able to get a little better explanation of silver brazing. Silver
melts at a much lower temperature than brass and has a much narrow
range of temperatures where it will flow before burning. Once silver is
melted it flows much quicker having less surface tension. It is a
trickier process than brass.

Why brass brazing vs. silver brazing or vica versa? Brass brazing is
economical, particularly for areas which require a lot of additional
material, for example around the dropouts. Silver is obviously more
expensive that brass. It is much harder to fill a large area with
silver since it flows so readily and quickly than brass. Silver brazing
leaves a frame stronger since the temperature required is much lower and
thus doesn't take as much from the yield strength of the frame.
Theoretically a silver brazed frame is stronger than a brass brazed
frame.

The frame is now completely together. It needs some more cleanup now
and then the addition of the braze-ons.


 

Continue to:













TOP
previous page: 5.11 Frame Building Part 3: Frame Alignment
  
page up: Bicycles Misc
  
next page: 5.13 Frame Building Part 3: Fillet brazing