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.