What I learned in Madison

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.

mountI’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.

anteI 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).

linesThe 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.

postMike’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.

boltsWe 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.

15 thoughts on “What I learned in Madison”

  • Matt Liggett

    Have you considered using cadmium-plated hardware? They’re frequentnly used in aircraft (often mostly aluminum) to prevent galvanic corrosion. Aircraft hardware is also sometimes called AN (Army-Navy) hardware.

    Reply
  • Mike

    Gonzo style? hmmm I represent that remark. ;-> Heck, if you’ve got a 20 mile (one way) commute, you should be able to take it in an hour or less with a full load.

    Todd, I’m not sure that your explanation will make sense to many folks. I know what you’re talking about, and I still had to carefully read the explanation. The Stokemonkey cognoscenti might grasp it, but I wonder if the curious potential buyer will?

    Stoked in WI
    PS I would prefer to ride with a rear fender, but a 700c-equipped Xtracycle does not have sufficient clearance.
    PPS The wedge-shaped controller bag is very cool. Looks good, and works well.

    Reply
  • Todd

    Matt, yes I have looked into that. Overkill, toxic, expensive, not particularly attractive. But that was before this happened. Perhaps filling the gaps between the clamp plates with a sealing foam or caulk-type substance would be good for riding in salty slop. And hail fellow INTP.

    Mike, I agree that this post is dense. I felt compelled to report one way or another, and I figure dense and tedious is better than long and tedious. Glad the bag is working for you; I’m moving ahead with it.

    Reply
  • tricky cotote

    The FreeRadical should have clearance for a fender and 700c tire. You need to make sure you installed the V-rack risers that come with the 700c kit. Call Xtracycle HQ 888 537 1401 if you need some. They screw into the bottoms of the racks and raise them about an inch. Also critical for 700c passengering with some tires.

    Reply
  • Sean Hickey

    Mike, did I see your Xtracycle parked outside the swap meet today? I meant to check it out, to see if it really was the Stokemonkey, but didn’t get around to looking before it left.

    I’d love to put together a Stokemonkeyed cargo hauler, but I think the upcoming Kogswell porteur is calling my name, and I’m not sure how they’d work together, and I’d also prefer to leave my Atlantis as a standard frame.

    Ah well, I’ll add that to the list of reasons to buy a new bicycle. I think I’m on Volume 7 of that list. I wonder how many I can buy before I hit the price range of a decent new car?

    Reply
  • Mike

    Hi Sean,

    Yes, that was my Stokemonkey at the Swap Meet. I was hoping people would at least get a look at it. Don’t know how many noticed it. There were sure a lot of bikes there.

    I love the Stokemonkey. I can haul loads of stuff, and also maintain relatively high rates of speed.

    Reply
  • Mike

    Thanks Kipchoge! I didn’t see the extension anywhere, and the folks here in Madison evidently hadn’t heard of it either, as the wrench I bought the Xtracycle from had just shredded a fender carrying someone on his 700c Xtracycle.

    For a variety of reasons, I’ve decided to convert to 559s. So clearance isn’t any issue anymore.

    Reply
  • Todd

    so you’re going to go from 700c to 559s on the same bike, with discs as we discussed? keep us posted on how the ride changes, pedal clearance etc!

    Reply
  • tricky coyote

    I was just cleaning our office and I found some little packets of stuff called Ti-Prep Anti-Seize compound. Just dab a little on the bolts to prevent galvanic corrosion. It’s made by Finish Line. These little packs are 6.5 cc and could perhaps be optionally supplied to your customers in wet environments. I know what I’m gonna do with mine!

    Reply
  • Mike

    The reconfigured Kona Dew Deluxe with 559s is a real pleasure to ride. I much prefer the fatter tires (Continental Town and Country’s for now): they smooth out the bumps, and corner much better. The steering is much lighter. I’m kind of surprised how different it is. Before it required a heavy touch, almost muscling it, to negotiate corners. Now it’s fairly light and much more responsive. Similar to the difference between a front-wheel drive car with heavy understeering to a more neutral feel. I’ve not dinged the pedals (yet), so no problems there.

    Reply
  • Todd

    that’s great, mike. there are a lot of people lauding 700c-650B conversions these days, but nobody talks about 700c-559 conversions — yet. if you send photos i’ll publicize ‘em. i have long felt that xtracycles call for much lighter/quicker steering than the conversion leaves them with, such as you get by going to smaller wheels. meanwhile the load-bearing ride quality should go up quite a bit with the extra tire volume.

    Reply
  • patrick

    not to hornswoggle this discussion, but…what Kogswell porteur?

    Reply
  • patrick

    never mind. I googled. Wow.

    Reply
  • Mike

    OK, we can lay this topic to rest: I’ve put 150 miles on the properly configured X/SM and everything is working great. The chain tension has stabilized at a point I still consider a bit tight, but not too much. Cable tension is still nice and taut. And the bike is a ton of fun to ride. The weather has been very mild, so the bike hasn’t been subjected to nasty salt/slush/snow yet.

    I prefer the big battery for the 20 mile one-way commute. It delivers a generous amount of power in the beginning, and continues to deliver plenty of power for the duration of the ride, with a decent amount leftover for side trips, etc. I can cruise at 20+mph for the duration of the ride, and keep my total workday (time leaving house to time returning to house) down to 12 hours or so, and still get a nice ride/workout in.

    Well done Todd!

    Stoked in WI

    Reply
  • big dog

    Clever! I would also recommend some galvanization.

    Reply
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