BOB Yak Fender Fracture: Fixed

We agreed that repairing the failed flag ferrule made the trailer much quieter, but it still seemed far more rattly than we remembered. It just had to be the fender, somehow, and eventually this appeared:

BOB Yak Fender Mount - fractures

BOB Yak Fender Mount – fractures

The obviously missing piece of the fender fell out in my hand; the similar chunk just beyond the wire arch fell out after I took the pictures. Yes, the wire has indented the fender.

The arch supports the aluminum fender, with a pair of (flat) steel plates clamping the wire to the fender:

BOB Yak Fender Mount - screw plates and pads

BOB Yak Fender Mount – screw plates and pads

The cardboard scraps show I fixed a rattle in the distant past.

Being aluminum, the fender can’t have a replacement piece brazed in place and, given the compound curves, I wasn’t up for the requisite fancy sheet metal work.

Instead, a bit of math produces a pair of shapes:

BOB Yak Fender Mount - solid model

BOB Yak Fender Mount – solid model

In this case, we know the curve radii, so the chord equation gives the depth of the curve across the (known) width & length of the plates; the maximum of those values sets the additional thickness required for the plates. The curves turn out to be rather steep, given the usual layer thickness and plate sizes, which gives them a weird angular look that absolutely doesn’t matter when pressed firmly against the fender:

BOB Yak Fender Mount - Slic3r preview

BOB Yak Fender Mount – Slic3r preview

The computations required to fit Hilbert Curve surface infill into those small exposed areas took basically forever; given that nobody will ever see them, I used the traditional linear infill pattern. A 15% 3D Honeycomb interior infill turned them into rigid parts.

The notch in the outer plate (top left, seen notch-side-down) accommodates the support wire:

BOB Yak Fender Mount - outer

BOB Yak Fender Mount – outer

The upper surface would look better with chamfered edges, but that’s in the nature of fine tuning. That part must print with its top surface downward: an unsupported (shallow) chamfer would produce horrible surface finish and life is too short for fussing with support. Given the surrounding rust & dings, worrying about aesthetics seems bootless.

The original screws weren’t quite long enough to reach through the plastic plates, so I dipped into my shiny-new assortment of stainless steel socket head cap screws. Although the (uncut) M5x16 screws seem to protrude dangerously far from the inner plate, there’s another inch of air between those screws and the tire tread:

BOB Yak Fender Mount - inner

BOB Yak Fender Mount – inner

Given the increase in bearing area, that part of the fender shouldn’t fracture for another decade or two.

I loves me my M2 3D printer …

The OpenSCAD source code as a GitHub Gist:

The original dimension measurement and design doodle:

BOB Yak Fender Mount - doodles

BOB Yak Fender Mount – doodles



  1. #1 by madbodger on 2016-08-10 - 09:24

    If there wasn’t plenty of clearance below the fender, you could always have put the screw heads inward, I suppose. I totally agree on omitting a chamfered edge.

    • #2 by Ed on 2016-08-10 - 09:45

      The inner plate has M5 threads for the OEM screws that were exactly the right length to match the plate. I briefly considered gaudy Knurled Brass Inserts™ inside the fender, then came to my senses… [grin]

  2. #3 by Jack S Bakeman on 2016-08-10 - 10:37

    Why not shorten those screws with your shiny new lathe?

    • #4 by Ed on 2016-08-10 - 10:53

      In this case, Good Enough fought Perfection to a standstill, then stuffed it under the workbench.

      I really, really must fix / improve the cutoff saw vise, because that thing makes short work of screwcutting. Well, it would if the vise actually worked as I expected. [grumble]

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