| After you decide on
your frame design, the next thing to consider is frame material. If
you will be welding or brazing, you will probably want to use chrome
moly steel. This tubing is available from several online sources
such as Wicks Aircraft.
Steel is very easy and forgiving to work with. I like brazing, but
TIG is better if you can afford one. Stick or wire welders and even
composite techniques have been use successfully as well.
Steel tubing sizes such as 1.5"x.049"
and 2.0"x.032" have been used and are stiff enough both for Z-frame
and straight tube frames. 2" muffler tubing (muffler moly) can be
used, but it does not have the same fatigue resistance and
resilience that chrome moly has. Plus it's heavy...
Aluminum tubing can be used as well,
but welding aluminum requires a lot of skill. Alternatively aluminum
tubing can be joined with composite methods. When joining aluminum
parts, be careful to carefully clean the aluminum and roughen it in
the area to be joined. This removes the oxides and gives the epoxy
more surface area to adhere to. If you are using carbon fiber, use a
layer of fiberglass next to the aluminum, as the carbon fiber has a
galvanic (bad) reaction with the aluminum. Because composites tend
to fail abruptly, it's a good idea to overbuild the joints. After
the lay-up is made, it can be wrapped in electrical tape to smash
the layers of composite together and squeeze out some of the excess
epoxy. This will strengthen the joint.
If you really want to make a high
zoot bike, titanium tubing can be used to make a lightweight but
strong frame. Titanium has the strength of steel, but the weight of
aluminum. In addition, titanium is very fatigue resistant. Titanium
is difficult to weld. It requires TIG welding in an inert gas
atmosphere. Composite techniques can be used to join the parts, and
it's fine to have carbon fiber in direct contact with titanium.
Here
are some things to consider when you are building your frame:
- Whether you
are brazing or welding, the best joints structurally and visually
are made when there are no gaps to fill between the tubes.
- Butt joints have poor strength
compared to fish mouth joints. This is because a fish mouth
joint has a much higher surface area.
- Fish mouth joints can be made
with a file, but that is laborious process.
- If you are planning on making
lots of frames, fish mouths are best made with hole saws and a
tube notching jig. Inexpensive tube notch jigs are available
from retailers such as Harbor Freight Tools.
- Even with fish mouth joints,
it's a good idea to use frame gussets to add extra strength.
| Main frame joints
should be reinforced with some type of gusset. A gusset can
be as simple as a chunk of 1/8" thick sheet metal that
spans the inner angle of the joint, or as complex as sheet
metal formed into a U shape and cut to fit the inner
angle of the frame. This example shows a frame built with
fish mouth joints, and simple gussets (gussets in orange). |
 |
As an alternative to specialized
tools or laborious hand filing, fish mouth joints can be made
easily with a hacksaw, using the technique illustrated below
by Dana Barlow |
