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Frame Tube Techniques

Warren Beauchamp 2/2008

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

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