Fabricating Offset Fender Struts

Originally Published In The August 2012 Issue Of Cycle Source Magazine

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My long term project is a BSA rigid bobber. This article describes fabricating rear fender struts. Home shop tools needed are: angle grinder, drill press, drill press vise, vise, drill bits, trusty Lincoln welder, safety glasses, welding helmet and gloves.

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The struts for my rear fender could have been 3/16 x 1” flat stock, but I wanted round bar with an offset to clear the chain and sprocket and line up with the skinny rear tire. The ends of the struts needed attachments for the fender and axle plates. Welding bungs (thick walled spacers) will work. Setting up a bung to butt weld on the end of a rod and getting it straight requires making a jig for accuracy. Shaft collars were easy for me and available in hardware or big box stores. They have set screws in tapped holes. When the threaded holes are drilled out, they become pilot holes for accurate welding on the end of strut rods. Some stores sell the collars made of “pot metal,” an alloy of tin and “mystery metal.” You can’t weld that stuff. Collars that appear “machined and plated” are most likely steel. I’d test with a magnet to be sure.

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Here are some examples of collars. To make the struts, you will need: (4) ½” or 5/8”shaft collars, 3/8” x 36”steel rod cut into two pieces, a 3/8” drill bit, and a 3/16” drill bit. 3/8” inch rod is easy to bend but stiff enough for struts. One half inch rod may require heating with a torch to bend.

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The collars must be drilled out to allow 3/8” rod to slide in. Set a drill press vise on the drill press table and put a 3/16” drill bit in the chuck align with the threaded hole. The drill press vise is slid over and clamped on the collar. Then you bolt the drill press vise to the drill press table. This will hold the clamped collar in drilling position. Change the 3/16” drill to a 3/8” drill, and then drill out the threaded hole.

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Grind a slot through one side of the collar to expose the 3/8” rod. That slot is to be welded fully, locking rod and collar together. When cooled down, grind this weld where it will bolt against the axle plate. That spot will need a hole drilled. For extra strength, weld and grind the other side of the collar.

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You must have the rear wheel, tire and fender in place to determine strut length and fit. The fender should be attached at the front to the frame, at least temporarily, but in your final position. The clearance between tire and fender can be accurately set by taping two ½” to 5/8” rubber vacuum hoses front to back, 2 inches apart on top of the tire. Placing the fender on the tire, with taped hoses provides a uniform wheel-to-fender clearance front to back.

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This mock-up will help determine the strut length. One or both struts may be bent for the sprocket offset now. The start and finish of offset must be determined and marked with masking tape, chalk or a Sharpie pen. A bench vise holds the rod for bending. There will be two bends. A length of ½” pipe helps make the bend. Be sure to make the bend near the collar first.

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There’s different ways for attaching struts to the fender: flat steel plate flange or shaft collar same as the bottom. In my case, the top attachment is a shaft collar again. One additional bend was made at the top of the strut so the collar would lie flat. The attachment bolt should be pushed through the fender from the inside, giving clearance for the tire. Also include a fender washer on the inside. A small piece of rubber vacuum line can center the bolt in the collar. Minimum bolt size is ¼”-20. The excess bolt sticking past the nut can be cut off with an angle grinder and hammered over to prevent the nut from loosening. Since my bike hasn’t been on the road, only time will tell how the old fender holds up.

 

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