Part 15 - Rear Fender and Tail Light

Once the rear wheel was centered and adjusted to the correct drive belt tension, the fender could be mounted. The fender is 13.5" wide suitable for a 300mm rear tire. The fender is made by Kustomwerks and supplied with 2 internal struts. The struts are 1/4" thick and 1" wide. The fender is "chain cut" for a right side drive. Due to the large differences between custom frames, you have to fabricate your own mounting.

Kustomwerks Fender Detail

The hidden shocks have roughly 2.5" of travel so a styrofoam spacer was made to hold the fender in place about 2.75" off the rear tire. More styrofoam was placed under the rear edge of the fender to keep the rear edge 1.25" off the tire - the tire moves roughly tangential to this edge so this clearance is mainly to allow for small rocks and other road debris to move through and to allow belt tension adjustments in future. The shocks are currently adjusted so that the bike is lower than it would normally be for riding. This will eventually result in a little more than 1/2" of clearance between the tire and the underside of the fender when the rear wheel upward travel limit is reached. At this time, there are no bumpers to stop the swingarm - these will be added at some point in the future.

Fender Spacer (click to enlarge)

A poster board template was cut out and made to fit the curvature of the fender. A carpenters profile gauge was used to simplify the process. I tried to use the edge of the fender to create the profile but the profile at the edge didn't match the profile further up the fender.

Poster board template for fender mounting plate (click to enlarge) 

The cardboard template was re-traced and an extra 3/8" added on the insides of the arms to connect with the hidden internal struts. The template was sent of to a local welder who cut the pattern out of 3/8" steel. 

Fender mounting plate rough cut (click to enlarge) 

Slots were cut into the fender to allow the arms to connect with the internal struts. The first paper template (without the 3/8" thicker arms) was used to determine the bottom edge of the slot and a parallel line 3/8" higher up determined the top edge. An angle grinder with a 0.080 cut-off wheel made quick work of creating the slots.

Slotted for mounting plate (click to enlarge)

The mounting plate was repeatedly adjusted by grinding away high spots until it matched the profile of the fender as closely as possible. Eventually, the fit was reasonably snug.

Grinding the mounting plate to fit (click to enlarge)

Mounting plate adjustment complete

The steel was badly pitted and had some scale. Both scale and pits were removed by grinding the surfaces on both sides. 

Grinding the surfaces clean

The fender and bracket were returned to the bike and tacked into place. This was done on the bike to make sure that the fender was centered and in the correct vertical orientation with respect to the tire. (Note the aluminum flashing to protect the bike from welding spatter.)

Fender tacked in place

The fender was removed from the motorcycle and the struts tacked into place on the inside of the fender. The ends of the struts were tacked to the mounting plate arms that protrude through the slots toward the inside of the fender. 

Internal struts positioned for tack welding to the mounting plate and to the fender (click to enlarge)

Tacking the internal struts in place

Checking to see if everything is centered and square

Once everything checked out okay, the mounting holes in the frame were transferred to the mounting plate using the poster board template and the holes were drilled.

Mounting Holes (click to enlarge)

Finally, the fender was mounted to the frame using 3/8-16 1.5" grade 8 bolts with grade 8 Nylock nuts and washers.
The fender is very sturdy but I doubt whether it will hold a passenger without additional reinforcement. The mounting frame arms flex somewhat with moderate downward pressure on the peak of the fender. This was confirmed with a Solidworks FEA model.

Fender bolted in place

Made some gussets to add extra support to the fender.

Cutting the gussets out of 1/4" cold rolled steel

Fender Gussets

Gussets test fit

After the test fit, used the flux core welder to weld everything in place. The result was ugly. I redid the welds with a TIG welder and got a reasonably good looking result. The TIG has a big heat affected zone compared to the flux core welder and things get very hot.

Gussets TIG Welded

The internal struts were finally welded in place. I couldn't run a weld bead down the complete length of the strut because access is difficult so I welded a number of heavy duty tacks instead.

I wanted to integrate the license plate holder into the rear fender without cutting into the fender. I think it is important to have the tail light in a highly visible area. Mounting tail light off the rear axle looks cool but is bad for visibility. I fabricated a sheet metal part to hold the license plate. The sheet metal was bent by hand and dead blow hammer using the rear tire as an anvil.

LED License plate holder

Ugly

The result doesn't look to bad from the rear but from the side it looks pretty damn ugly and was therefore eliminated as an option. Removed the LED tail light from the license plate assembly and  fabricated a sheet metal part to hold the LED assembly to the rear fender.

Some months later, this is what the fender looked like after body molding and primer

The LED taillight only draws 110mA when the brakes are applied which allows the use of 24 gauge wire. This makes it much easier to route wire through a 3/8 brake line that I welded to the underside of the fender to protect the wiring from rocks and other road debris.