Building A Chromoly Swingarm

Published In The September 2014 Issue Of Cycle Source

Article By: Will Ramsey – Faith Forgotten Choppers www.faithforgotten.com

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Kevin Dunworth of Loaded Gun Customs is fast becoming one of the premier designers and builders in the motorcycle industry. His Retro mod designs are pushing the envelope in both performance and style. I met Kevin two years ago at Mountainfest in West Virginia while I was explaining some fabrication techniques for frame building and how I account for weld shrinkage. My science based approach to fabrication caught his ear and we began a steady stream of conversation over the last two years. Kevin reached out to me recently about designing a modular fixture and fabricating Chromoly swing arms for some his of builds. His unique and brilliant design of the Triumph based Bucephalus motorcycle define the parameters of the swing arm in this article. Once I received the plans I had to decide the best way to construct a fixture and account for weld shrinkage. All too often you will see hardtail frames and swingarms that drift inward at the axle, forcing the builder to flex them out in order to maintain a square alignment. This can be avoided if the welding K sequence is thought out rather than simply performed randomly. 4130 (Chromoly) has a few characteristics that must be understood in order to weld it properly. The higher carbon content found in 4130 makes it a heat treatable steel. Therefore, the welding process alone can have a dramatic effect on the performance of the material, and depending on the welding rod used a heat treatment process may need to be performed to avoid embrittlement along the weld seems. In the case of thin walled tubing (under .120” wall), a ductile mild steel filler rod can be used safely without the need for subsequent tempering of the material. Although 70s-2 filler rod can be used on most structures, 80SD2 filler is often preferred due to the higher strength which more closely resembles the properties of chromoly. Preheating thin walled chromoly is not necessary; however, the rapid cooling caused by the material soaking heat away from the weld can cause the weld to crack. This is best avoided by simply slowing the travel speed of your weld, thus providing an additional heat soak which will slow the rate of cooling around the weld area. If all of these factors are considered during the weld process then an extremely strong a reliable weld can be obtained.

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The 1”- 2” tubing is cut to length according to the plans for the chromoly swingarm.

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There are many ways to mark an angle cut on a piece of tubing. In this case I chose to mark the lengths on a surface plate with a height gauge.

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Using a 6” scale as a straight edge a cut line is scribed between the marks.

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The cold saw is an excellent tool for making accurate and repeatable cuts. If you really take your time and utilize the layout marks, this tool will allow you to hit the mark. An accurate square cut is critical to a perfect fit up.

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The 1” axle slot is machined on the Bridgeport. First by plunging an undersized endmill to rough out the slot; and then, by making a smooth finishing pass with a clean new onsize cutter.

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In order to minimize weld shrinkage and maintain a square part, the fit up is absolutely critical. All too often I hear the phrase, “oh that’s close enough. I’m a good welder, I can fill that gap.” A gap in the material will increase the draw between the parts during welding and amplify distortion throughout the part. Instead of being concerned with saving money by hacking junk pieces together, focus on improving your skill set and save your conscience by doing it correctly. If it does not fit tight, scrap the piece and make another cut! The pieces on this swingarm fit together so well that they hold themselves in place with pressure only.

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Before the parts can be welded all the mill scale must be removed. This can be performed in a blast cabinet with a fine medium resulting in an even finish. Or, as in this case, a scotchbrite wheel can be used on an angle grinder.

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As always, all parts must be thoroughly wiped down with a solvent like acetone, including the filler rod!

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All the parts are first tacked in four places at each joint and then completely welded using 80s-D2 filler rod. Notice that the tubing is not tacked to the bearing housings. This is done to allow weld shrinkage to occur without affecting the alignment of the swingarm. Think this through carefully and it will serve you well in many other applications. Finally the bearing housings are fully welded to the swingarm. Here I am using a #14 cup that was designed and produced by Collin Meyers of Toxicfab. The gas coverage of this cup allows the tungsten to stick out 2” to reach into difficult spots. I have two of these cups and I truly believe they are a game changer in the world of welding. I paid full price for both cups and believe they are worth every penny.

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With proper planning of weld shrinkage and paying close attention to joint fit-ups, the chromoly swing arm is finished and measures dead square. It is ready to be shipped to Loaded Gun Customs and used on the next customer build. As always please feel free to contact me with any comments, questions, or concerns.

 

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