What Frame Materials Are Needed To Build A Rigid Bobber or Chopper Chassis?

We received a question from a subscriber, Treyton Brown. It is a common question so we thought we would provide an answer that might help a lot of our customers.

Question: What materials will I need for building a rigid bobber frame. I am planing plan to use 300mm tires on the rear, and on the front have a rake of 45 degrees, and a trail of 12 inches?

For building the frame, you will need some steel tubing and some steel plates for gussets and axle plates.  You also will need the right welding tools as well as a nice frame building Jig.  Having a good jig will really help you a lot in making sure the rails are properly aligned and the neck and various angles are correct.  A tube bender is also necessary.

Regarding material, you can go with a variety of materials.  ERW, DOM and Chrom-Moly are the common materials available for making frames.  Below is a brief description of each.

What's The Strongest Material For A Chopper or Bobber Frame?

ERW or Electric Resistance Welded tubing is not as strong as DOM or Drawn Over Mandrel tubing.  Usually when tube is made, it starts off as a flat sheet and this is rolled up into a tube and welded.  DOM tubing is usually made the same way but there is an extra process where this tube is drawn through a die along with a mandrel to control the outer and inner diameters of the tube.  This extra process strengthens the steel since it is cold formed by up to 20%.  Another type of common tubing is Chrome-Moly also known as Chromium / Molybdnem Steel.  This type of steel also uses the same process but has these additional elements within the steel alloy which increase strength even more.  Chrome-Moly is usually used in tubing for cars and other applications over 3500 lbs. 
Motorcycle frames are made with both ERW and DOM tubing.  For small engines with not a tremendous amount of horse power a 1" x 1/8" mild steel ERW tube should be fine.  Modern engines with a lot more horsepower as well as stretched frames require at least 1 1/4" x 1/8" wall thickness at a minimum.  Bigger choppers even use 1 3/8" x 1/8" walls.  More on the overkill side done for design or aesthetics is 1 1/2" tube with 1/8" wall.  All applications under 3500 lbs pretty much can get away with using ERW tubing.  Roll cages and dune buggies, car frames are almost always made with DOM tubing.
All three, Chrome-Moly, DOM and ERW weigh about the same but it is important to note that Chrome-Moly is the strongest, followed by DOM and finally ERW being the weakest of the three.  You can take advantage of this by replacing a larger diameter tube with a smaller diameter tube of stronger material.  For example, you can replace a 1 1/2" ERW tube with a 1 1/8" DOM or Chrome-Moly tube.  The advantage here is that you can reduce your frame weight by making such a change.  It is very critical to confirm that it is ok to make this kind of change on your frame.  Each frame has a different geometry and different loads that will bend and twist the steel in various ways.  Finite Element Analysis (FEA) conducted by a qualified engineer can provide you with correct tube diameters and wall thickness that can be replaced for your specific frame design.

Below are some photos showing DOM and ERW tubing.  Note that the ERW has the weld seam on the inside of the tube.  DOM although made the same way has the additional process of being drawing and the mandrel removes this seam and provides a nice smooth surface on the inside of the tube.  Please let us know with any additional questions or concerns.

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