People are calling it “great customer service” that I went out to Madison to fix Mike’s Stokemonkey, but my trip really wasn’t an act of sacrifice. I truly didn’t understand what could have been causing Mike so much trouble, so merely refunding his money with apologies would have left us both poorer, and I’d be liable to similar failures afflicting future customers. That’s understatement: I went out to Madison to save Cleverchimp’s ass. I simply had to solve the mystery, and I’m grateful to Mike for his patience in letting me.
I’ve come away with a more nuanced understanding of how the mount I designed actually works. I learned that some installation details I hadn’t even bothered to document appear, in fact, to be critical.
Mike had been experiencing sudden losses of drive chain tension, leading to derailments and much freezing roadside cursing. Sudden losses of tension imply that something on the mount was slipping. There are three possibilities: the tension cable assembly, the lower clamp on the FreeRadical top stay, and the top clamp on the motor bracket pivot. Lacking a closer look, my early phone support had boiled down to “make things tighter.” This wasn’t helping.
I asked Mike to send a photo of his installation if he could, so I could look for anything unusual. The extremely beat up appearance reflects the history of failure and aggravated handling, the harsh winter riding conditions, and Mike’s general gonzo style, which we are honored to support. The things to notice are the attachment point of the tension cable up around the seat tube/top tube junction, the relative proximity of the motor body to the back of the seat tube and to the FreeRadical top stay, and the clamp position on the top stay (exposing the letters “xt” of the xtracycle logo).
The tension cable serves to constrain motion to an arc whose focus is the attachment point. This arc is indicated by the yellow line in the image with overlays. Note that this arc comprises points that would place the motor nearer the bottom bracket, permitting the drive chain to go slack. A point nearer the focus of the blue arc, at the seat stay bridge, would be a more effective point to attach the cable, as indeed it has been in nearly all other installs. Thus the cable attachment point was inappropriate, though I had authorized it as an option in documentation, for compatibility with bikes lacking suitable seat stays.
Moving the cable attachment point may alone have sufficed to solve the problem, but we wanted to optimize all other aspects of the install, too, so we’ll never know.
Now, slippage is a failure mode that should rightly occur in preference to permanent deformation or breakage of the attached parts, in response to overwhelming force (such as a crash). The response of the assembly to normal operating stresses should be to flex, to transfer the stresses progressively to structures large and strong enough to bear them with rigidity adequate to maintain correct chainline. Slippage can thus be understood as an overshoot of the assembly’s flexibility, or an inability to distribute the forces involved away from the clamps. Increasing the flexibility of the system, I theorized, should prevent unwanted slippage.
Mike’s mental model of the mount’s function resembled a bow and arrow, with the drive chain and tension cable acting like the lower and upper runs of the bowstring, and the motor axle pulling back on them to increase their tension. Thus, he clamped the FreeRadical top stay as far back as he could. This has two unwanted effects. The first is to inhibit torsional flex of the top stay, because the top stay tube is less subject to torsion at its welded ends than in its middle. The second is to incline the mount bracket arm downward (for a given length of drive chain), effectively lengthening the lever that gravity and the drive chain act upon in opposition to the top clamp force. Thus, pushing the stay clamp more than an inch forward, until the motor nearly touches the seat tube, increases the flexibility of the assembly and lessens the leverage of forces resisted by the top clamps. This will appear in future installation instructions.
We also discovered a fairly shocking amount of salt or galvanic corrosion on the stainless clamp bolt threads, which could only have prevented their proper tightening into the anodized aluminum bodies. Mike rides on slushy salted roads with no rear fender, subjecting Stokemonkey to a nasty spray of caustic grit in operation. When he attempted roadside repairs, he was unable to dry, clean, or re-grease the parts before re-assembly. We replaced the clamp assembly with a clean, properly greased new assembly. The forthcoming wedge-shaped controller bag should serve also as a shield to fend off the worst wheel spray, and Mike is now riding with prototype #3 of that item.
Nothing has slipped yet, nor has it on any other install to my knowledge.