Thursday, June 2, 2011

Part 11 - Front Fender

Rumor has it that the front tire diameter grows as much as 0.25" at speed due to heat and centrifugal forces. The front fender needs to be mounted higher off the front tire to prevent it from rubbing. Apologies for stating the obvious but when this motorcycle build started, the fact that the tire grew wasn't something that I knew. Then there is also the issue of small rocks and road debris being picked up by the tire which will need some clearance to move through underneath the fender. Add to all of this the radius of the fender - ideally the fender should be mounted at a distance from the tire so that the radius of the tire and the fender are concentric. So there you go, some considerations for mounting a fender on a custom chopper.

3/8" ID Vinyl tube spacer
A 3/8" ID vinyl tube was taped to the center of the tire as a spacer for the fender. The outer diameter of the tube is about 0.44" but collapses a bit with the weight of the fender. At the front and rear edges, a 3/8" OD plastic tube was used to reinforce the vinyl tube to make sure that compression of the vinyl tube under the weight of the fender didn't leave insufficient clearance between the tire and the fender. To properly align the fender with the left and right of the tire tire, extra pieces of roughly 2" long tubing was taped to the edges of the road surface of the tire.


The fender was placed in position on the tire and a fender spacer was taped in place on the left and right side and the outline transferred to the fender with a pencil.

Fender Spacer

Fender spacer taped in place and outline traced

Outlines traced on left and right sides
The fender was then removed and the fender spacer was taped to the fender within the pencil outlines and the positions of the mounting holes were transferred from the fender spacer to the fender by running a pencil down through the mounting hole in the spacer and scribing a circle. 

Fender spacer taped to position indicated by the pencil outline

After removing the spacer, the center of the circles was eyeballed and a dot punch used to mark the centers for drilling. Holes were drilled with a 11/32 drill bit - pilot holes were first drilled with a 1/8" bit. 

One side drilled for mounting

Finally after deburring the holes, the fender was test mounted. The fender spacers measure 1.25" in width so the mounting bolts need to be at least 1.75". The rule of thumb for thread engagement I use is 1.5x the screw major diameter for light loads to 2.5x for heavy loads. The screws are 5/16-18 so thread engagement is close to my rule of thumb for light loads at 0.44" (1.75 Screw length - 1.25 Fender Spacer - 0.060 Fender Thickness). It is probably wise to bump this screw up to 2" just to make sure since the roads are bumpy out here.

Fender mounted using 5/16-18 screws

Side view of mounted front fender





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