Terracycle Chain Idler: 3D Printed Tire

The Terracycle (now T-cycle, for reasons presumably involving the transfer of money) chain return idlers on our Tour Easy bikes developed hardening of their urethane tires:

Urethane shouldn’t crack like that, but after more than fifteen years, stuff wears out.

The white ring is 95A TPU printed on the Makergear M2, which is definitely more flexy than the original tire, but has the redeeming feature of being both Good Enough and trivially easy to model:

include <BOSL2/std.scad>

NumSides = 4*3*2*4;
$fn=NumSides;

Thick = 3.5;
ID = 46.4;
OD = ID + 2*Thick;
Length = 11.2;

tube(Length,id=ID,od=OD,anchor=BOTTOM);

It printed with 5 mm brims on both the ID and OD, because TPU has the barest adhesion to the M2’s glass plate + hair glue. There’s a long-unopened box now on the bench with a BuildTak PEI surface (thank you: you know who you are!) that should improve the situation.

In any event, the tires fit well:

The layer-to-layer adhesion isn’t as good as I think it should be, so I’ll likely use those tires as testcases for tweaking the new build plate & settings.