Article and Photos By: Will Ramsey
Originally Published In The December 2016 Issue Of Cycle Source Magazine
There has always been disagreements surrounding the use of 4130 (Chromoly) tubing for chopper frames. Many of these discussions focus on the welding procedures which seem to be misunderstood. I have found that many of the arguments lack credible research and are full of regurgitated stories. The internet can be a blessing and a curse when it comes to information. When researching a subject, you must be able to discern between credible sources and the bar-room style brouhaha of internet forums. In order to have an intelligent discussion about a subject we cannot allow the mindset of “this is how we have always done it, and it works” to take precedence over hard science. Doing something wrong a thousand times does not make anyone an expert. I would encourage everyone to make decisions based off of scientific findings from accredited sources like the American Welding Society (AWS). It is from this view point that I will present facts to debunk some of the myths between 4130 (Chromoly) and 1018 (Mild Steel) chopper frames. The first and most often heard myth is that “Chromoly is lighter than Mild Steel.” This is terribly incorrect! For all measurable purposes 4130 and 1018 weigh the same at 491 lbs. per cubic foot. The weight saving in using Chromoly to construct a frame is obtained through the use of thinner walled tubing, therefore less volume of steel. Chromoly has a significant strength advantage over mild steel in both tensile strength and yield strength. This increased strength to weight ratio in Chromoly allows for the use of thinner walled tubing to obtain the same structural support.
The most common source of argument surrounding the construction of a Chromoly frame is the welding process. 4130 contains a higher amount of carbon than mild steel and is classified as a heat treatable steel. As a consequence, large increases in heat from welding can require that a weldment be normalized or (drawn out) after welding to avoid embrittlement of the material. However, in the case of thin walled Chromoly tubing – (.120” or less) it has been determined and published by the AWS that there is no need for post heat treatment of a weldment if the weld is made using the GTAW (TIG) process, a mild steel filler rod is used, and proper welding procedure is adhered to.
Chromoly is much less forgiving than mild steel when it comes to welding. Excessive heat can affect the temper of the steel causing embrittlement and therefore strict guidelines must be followed. Thin walled tubing does not need to be normalized when TIG welded due to the smaller amount and more concentrated heat used to weld the joint. Therefore, there can be no allowance for excessive heat input. The amperage should never grossly exceed 1 amp per .001” of material (i.e. 120 amps for .120” wall tubing), and the fit up must be near perfect.
A poor fit up requires extra heat to fill the gap and can compromise the temper of the Chromoly tubing. It is also completely inappropriate (and dangerous) to dry wash a Chromoly weld. Dry washing a weld is a novice technique some welders use to go back over a weld to create a better weld bead appearance. This generally serves no other purpose than fulfilling ones ego. Although this can be done with mild steel without severe consequences (still never recommended), when this technique is applied to Chromoly there is a good chance of hardening and embrittling the surrounding material. Clearly the weld technique must be on point; but let’s be honest here, if you are building a chopper frame to ride down the road at 70+ mph please be honest about your abilities as a welder. One of the best pieces of advice I ever heard from an experienced professional builder was when Jeremy Cupp of LC Fabrications was asked to give beginner builders some advice. He simply said “if you’re still learning to weld a motorcycle frame is probably not a good place to start.”
The filler rod selection for welding thin walled Chromoly tubing is very important. A mild steel rod like 70s-2 or 70s-6 should be used (I prefer 70s-2). The use of 4130 filler rod is incorrect and dangerous for this application. When using 4130 filler rod, the weld joint must be heat treated and tempered to avoid embrittlement. A stronger mild steel rod such as 80s-D2 can be used to weld thin walled Chromoly without post heat but 70s rod is a better choice for frames as the increased ductility will serve better at absorbing repeated impacts. In conclusion, if weight reduction is important in the design of your chopper then Chromoly tubing is an excellent choice. It is simply the responsibility of you, the builder, to be honest about your ability to competently fit and weld this material. If there is any question at all, I would encourage you to look into discussing your design with an experienced frame builder that has experience with this material. If weight is not a concern, then 1018 DOM is clearly an excellent choice and much more forgiving in the welding process. As always please feel free to contact me with any comments, questions, or concerns. email@example.com