Once that the rear wheel was centered correctly, I moved on to mounting the rear fender. The rear fender has three mounting points: the lower mount off the battery box tray, the upper mount just under the seat and the rear mounts which attach to the sissy bar. Each mount had to be fabricated to match the curves of the fender and since I am using a ribbed fender there were a lot of curves…
Set the fender clearance
The first step was to temporarily position the fender so that I could start measuring for all the different mounts. I also wanted to make sure I had the clearance needed for the rear tire, so I used the tried and true method of taping a rubber hose to the top of the tire to set the clearance equally all the way around the wheel.
With a flat fender you would usually use two lengths of hose on either side of the tire, but my ribbed fender sat nicely on one single hose taped down the middle of the tire. I also used some magnets to hold the sissy bar in place just to get an idea of where it would mount to the motorcycle fender.
Painter's tape and a 3/4" piece of hose were used to set the fender clearance.
Fabricate a custom mount for the ribbed rear fender
I decided to go with the hardest mount first, which was the lower mount to the battery box tray on the frame. Paughco already has a tab for chopper rear fender mounting and normally you would just bend it to the right angle and bolt the fender to it. That is if you are running a flat fender. Having the ribbed rear fender meant I needed to fabricate a custom mount to go from the profile of the fender to the tab.
That ribbed fender is not just going to bolt up to that flat tab.
To fabricate a chopper fender mount to attach the rear fender to the frame tab, the first thing I had to figure out were the angles of the fender. I did this by tracing the profile of the fender onto a piece of graph paper. Next, I used a straight edge to extend the lines so that I could measure them with a protractor.
The fender angles measured out as 35 degrees and 5 degrees.
Using a piece of ½” thick aluminum bar, I first cut a 5 degree “V”, followed by a 35 degree “V” down the center using a milling machine.
In chopper rear fender mounting, the trick to getting these angles if you don’t want to adjust the head of the mill is to use angle blocks to set the piece at the correct angle in the vice before cutting.
Starting with an oversized part which will be trimmed to fit the fender tab.
Matching the radius of the fender using sandpaper
You’ll notice that I only accounted for the two of the angles on the fender, so what about the curved radius? To match the radius correctly I sanded it down by hand. I took a sheet of 100 grit sandpaper, divided it into 4 pieces and then adhered one of the pieces to the bottom of the fender using double stick tape.
Then I proceeded to sand down the aluminum until it matched up with the radius of the fender. This took a lot longer than I expected, but with time and many sheets of sandpaper it did work.
Matching the radius of the fender using sandpaper.
Make a solid mounting point for the bottom of the fender
Before cutting down the mount to match the fender tab, I drilled three holes into it. In the center I drilled a 3/8” hole to match the hole on the fender tab. Then I drilled two ¼” holes to use for attaching the mount to the fender.
I’ve decided to use the 3/8” hole for running the wiring to my taillights. To help with figuring out the spacing, I cut a few spacers out of 1/8” plywood that I could stack in between the mount and the fender tab.
Mount and spacers to fit between the rear fender and frame tab.
Using a transfer punch, I marked the holes for the mount on both the fender and the frame tab. Then I drilled the holes and de-burred them before bolting the fender to the frame via the new mount.
To get the perfect chopper rear fender mounting, I spent an hour bending the frame tab and adding/subtracting shims until I got the exact spacing and angle, I was looking for.
Wooden shims were used to determine the spacing.
Next, I milled a piece of aluminum to the same dimensions as the shims and coated it with Dykem layout fluid. I bolted everything back together and used a scribe to trace the outline of the frame tab onto the mount.
Frame tab scribed onto mount, ready for grinding to the correct shape.
With the outline drawn on the mount, I used a band saw to cut off the excess material and then finished it out with a belt sander. The resulting piece lined up perfectly with the frame tab on one side and matched the fender profile on the other, making for a solid mounting point for the bottom of the custom motorcycle rear fender.
Lower fender mount rigidly attaches to the frame tab with two 1/4-20 screws.
Adjusted the position of the upper fender with two bungs
For the upper Harley rear fender mount, I decided to fabricate two bungs which I could weld directly to the frame. I started out with 1” steel round bar which I drilled and tapped for a 3/8”-24 fastener. I then cut off two 1” lengths which I coped on one end to match up with the frame.
Just like with the lower fender mount, I had to consider not only the angle of the fender, but also the angle of the frame. After some careful measuring I determined that the coped portion of the mount needed to be cut at 2.5 degree angle in order for both ends to sit flush.
A pair of 3/4" mild steel bungs with a 2.5 degree cope to use for the upper fender mount.
Next, I held the fender in place with a pair of clamps while I adjusted the position of the upper fender mount bungs. Once they were laid out correctly, I marked their locations with a Sharpie before removing the fender.
Marking the upper fender mount bungs.
To make sure I had both holes positioned symmetrically on chopper rear fender mounting, I used a transfer punch to mark the position of one bung and then measured the location of the divot from the end of the fender and the side of the fender.
Then, I marked that same position on the opposite side of the fender and used a center drill to create a divot. I drilled out the holes with a 1/8” bit before reaming them out to 3/8” with a step bit. Once the holes were drilled, I attached the bungs using the 3/8”-24 bolts through the back of the fender.
Bungs mounted and ready for welding.
Getting the bungs lined up correctly on the frame took some tedious measurements using a telescoping gauge and a set of F calipers, but I finally got everything centered and clamped up.
At this stage, I decided to tack weld the bungs on so they would be easy to remove if I needed to make any changes to the fender position.
Bungs tack welded to the frame.
With both the upper and lower mounts in place, the Harley rear fender was finally rigidly mounted to the frame. Now it is on to the sissy bar.
Upper and lower fender mounts completed.