What kind of tubing is used for Choppers or Custom Motorcycle frames? Probably the first asked and second most important question regarding building a frame. The most important question is what are you planning to build? Things like tube length, diameter, wall thickness and type of material all need to be considered in your design. Rather than make this a symposium of engineering, let’s not waste time reinventing the wheel. Lots of big twin frames have been made with 1” x .125” wall mild steel tubing. These small diameter tube frames were often connected by forged components making for very strong connections. Relatively low horsepower engines put less stress on these frames than the rough roads.
Today, modern engines put out considerable horsepower. Frames are stretched beyond stock design and joints are just tube welded to tube. With that, 1-1/4” tube with a .125” wall helps the strength of the frame. It is not unusual to see big engine Chop frames built with 1–3/8” .125” wall tubing. More on the overkill side and usually done for appearance than engineering are frames made from 1-1/2” tubing.
Typical tube used for custom frames is ERW (electric resistance welded) and DOM (drawn over mandrel). ERW starts as a strip of flat steel and is rolled into a tube. The seam is electric welded leaving a ridge of weld inside and outside the tubing. A cutter removes the outside weld leaving a relatively smooth round tube.
In the picture above, the pencil points to the weld seam inside an ERW tube (note that black pipe is an ERW tube made from hot rolled steel flat steel and although not recommended for building bikes, it often pops up on some commercial made custom frames. The way to tell is that the surface texture is rough, and doesn’t have an “even measure like tube since pipe measurements are “nominal”).
The picture above is DOM tubing. DOM can be made a number of different ways but is normally just ERW that is cold rolled with a mandrel inside the tube during forming. This process smoothes the weld seam and produces a very round even wall tubing. Compression of the metal during rolling tends to strengthen the metal somewhat making it a bit better to use than ERW.
Steel is normally provided in two standard types, Hot Rolled Steel (HRS) and Cold Rolled Steel (CRS). The picture above shows HRS bar on the left. Note the rough black scale on the outside (the far left corner of the bar has been sanded to remove some of the scale). HRS is formed while the bar is red hot and oxidation naturally forms on the outside of the bar. The surface of HRS is not always flat or square and may have pits and roller lines. CRS on the other hand is formed cold and normally has a smooth clean surface. It is not unusual to find CRS within a couple thousands inch of it’s named dimensions. CRS tends to be stronger and also harder to form than HRS.
Basically, 1-1/4” DOM steel tube with .125” wall is a good choice for Chopper frames. Axle plates of 3/8” HRS or CRS will hold up along with motor mounts made from 3/8” through ˝”. Of course this is all a very basic rule of thumb and does not replace proper engineering and plain common sense.