The New Hotness

I eventually tracked a distressingly loud rattle from the BOB Yak trailer to a fender mount failure:

BOB Yak Trailer fender front mount - aluminum fatigue — BOB Yak Trailer fender front mount – aluminum fatigue

The screw clamped the round aluminum fender between two flat washers (the other of which has been touring the workbench). The hole in the aluminum started as a screw slot and eventually fretted away around the edge of the washers, leaving a trapped fragment to fall out as I loosened the screw.

Well, this mount lasted a decade longer than the wire mount at the top of the fender, so there’s that.

As before, a bit of math conjures a chunky mount from the vasty digital deep:

Fender front mount - solid model - Show view — Fender front mount – solid model – Show view

The first iteration didn’t have the hole for the threaded insert angled downward at 10°, but it’s easier to make better measurements with a “pretty close” prototype. I’m reasonably sure the angle is a glitch due to hand-brazing the frame tubes, but we’ll never know.

The inner plate angles to match the insert, thus keeping the screw & washer perpendicular to the surface:

Fender front mount - solid model - Mounts view — Fender front mount – solid model – Mounts view

A brim around that chip of plastic ensures a good grip on the platform:

BOB Yak Trailer - fender front mount - PrusaSlicer preview — BOB Yak Trailer – fender front mount – PrusaSlicer preview

I suppose rounding the corners would make it prettier:

BOB Yak Trailer fender front mount - inner plate — BOB Yak Trailer fender front mount – inner plate

The original screw was slightly too short, so that’s a shiny replacement from the Drawer o’ Random M5 Screws. If I ever have occasion to go in there again, I’ll use a button head screw, although there’s certainly enough clearance:

BOB Yak Trailer fender front mount - tire clearance — BOB Yak Trailer fender front mount – tire clearance

From the top, the gray PETG-CF looks like it grew there:

BOB Yak Trailer fender front mount - installed — BOB Yak Trailer fender front mount – installed

I figured the mount’s radius by feeding measurements into the chord equation and assuming the overall curve is circular; the radius came out slightly too large, which likely won’t make much difference.

The OpenSCAD source code as a GitHub Gist:

	// BOB Yak Trailer – fender front mount
	// Ed Nisley – KE4ZNU
	// 2026-06-15

	include <BOSL2/std.scad>

	Layout = "Show"; // [Build,Show,Frame,Fender,OuterMount,InnerMount,Mounts]

	/* [Hidden] */

	ID = 0;
	OD = 1;
	LENGTH = 2;

	HoleWindage = 0.2;
	Protrusion = 0.01;
	NumSides = 432*4;
	Gap = 5.0/2;

	$fn=NumSides;

	WallThick = 5.0;

	Washer = [6.0,16.0,1.5]; // M5 fender washer

	Rivnut = [5.0,10.3,1.5]; // M5 rivnut in frame

	FrameOD = 16.1; // trailer frame tubing
	FrameAngle = 10;

	FenderOA = [52,440,1.5]; // minor major thickness

	BlockOA = [0,40.0,1.25*Washer[OD]];


	//—–
	// Define things

	// Relevant part of the trailer frame
	// origin at center of rivnut

	module Frame() {

	yrot(FrameAngle)
	union() {
	left(Rivnut[LENGTH])
	ycyl(2*BlockOA.y,d=FrameOD,anchor=RIGHT);
	xcyl(FrameOD/2,d=Rivnut[OD],anchor=RIGHT);
	left(Protrusion)
	xcyl(10,d=Rivnut[OD],anchor=LEFT);
	}

	}

	module OuterFender() {

	torus(od=FenderOA[OD],d_min=FenderOA[ID],orient=FRONT,anchor=LEFT);


	}

	module FullFender() {

	difference() {

	OuterFender();

	right(FenderOA[LENGTH]) // make it a cup
	torus(od=FenderOA[OD] – 2FenderOA[LENGTH],d_min=FenderOA[ID] – 2FenderOA[LENGTH],
	orient=FRONT,anchor=LEFT);
	right(FenderOA[ID]/2) // remove inner half
	ycyl(2*BlockOA.y,d=FenderOA[OD] – FenderOA[ID],anchor=LEFT);
	down(2*Washer[OD]) // remove bottom part
	cuboid(2*[FenderOA[OD],FenderOA[OD],FenderOA[OD]],anchor=TOP+LEFT);
	}

	}

	module OuterMount() {

	difference() {
	right(FenderOA[ID]/2)
	cuboid([FrameOD/3 + Washer[LENGTH] + FenderOA[ID]/2,BlockOA.y,BlockOA.z],
	rounding=1.0,anchor=RIGHT);

	Frame();
	OuterFender();
	}

	}

	module InnerMount() {

	difference() {
	render()
	intersection() {
	yrot(FrameAngle)
	cuboid([WallThick + FenderOA[LENGTH],BlockOA.y,BlockOA.z],anchor=LEFT);
	right(FenderOA[LENGTH])
	OuterFender();
	}
	yrot(FrameAngle)
	xcyl(FenderOA[ID],d=Washer[ID],anchor=LEFT);
	}

	}


	//—–
	// Build it

	if (Layout == "Frame") {
	Frame();
	}

	if (Layout == "Fender") {
	FullFender();
	}

	if (Layout == "OuterMount") {
	OuterMount();
	}

	if (Layout == "InnerMount") {
	InnerMount();
	}

	if (Layout == "Mounts") {
	OuterMount();
	InnerMount();
	}


	if (Layout == "Show") {
	OuterMount();
	InnerMount();
	color("Gray",0.6) {
	Frame();
	FullFender();
	}
	}

	if (Layout == "Build") {
	up(BlockOA.z/2) left(FenderOA[ID]/4)
	OuterMount();
	up(BlockOA.z/2)
	xrot(180)
	yrot(-FrameAngle)
	InnerMount();
	}

view raw BOB Yak trailer – fender front mount.scad hosted with ❤ by GitHub

The New Hotness

BOB Yak Trailer: Fender Front Mount