First Impressions: Co-Motion 4-Screw Eccentric


Co-Motion Internal 4-Screw Eccentric

One of the Co-Motion FAQs that does bear some attention is the one entitled, How do I adjust my timing chain?

This one may be in need of an update given how many Co-Motions are now fitted with FSA’s MegaExo SL-K carbon cranks in combination with Co-Motion’s internal 4-screw (two right, two left) eccentric.  As a reminder, the eccentric is the cylindrical component on a tandem and, more recently, single-speed bikes, that sits in an enlarged bottom bracket shell (see film strip, below) into which the bottom bracket is installed.  Given the off-centered position of the bottom bracket in the eccentric, the eccentric can be rotated clockwise or counter-clockwise in the shell such that the crank axle is moved fore and aft to adjust chain tension.  There are a wide variety of different eccentric designs in use, some like the Co-Motion and Bushnell models shown here with their integrated retention systems that allow them to be installed in a closed frame shell, whereas others use grub screws in the frame shell or even a split frame shell with pinch bolts.  Co-Motion, for example, uses three different types on its tandems: internal 4-screw, 4-point grub screw and pinch bolts.  Santana uses grub screw models, Cannondale and Trek have typically used wedge-types. Calfee developed its own model for its tandems but more recently adopted a larger eccentric shell that allows for the use of a Bushnell eccentric. All of them work, but some are easier to work with and/or make for a more robust front shell than the others.

Calfee tandem frame's eccentric shell with Bushnell eccentric, Phil Wood square taper bottom bracket and daVinci cranks. Note: Older Calfees do not have the aluminum sleeve in the ecentric shell; a design change was made in early 2009.

Getting back to the Co-Motion eccentric and FSA carbon cranks, during our July 4th Tennessee weekend I had an opportunity to put my hands on a beautiful Ferrari Red, Co-Motion Robusta tandem that was fitted with the Co-Motion’s 4-screw eccentric, FSA’s carbon SL-K MegaExo cranks and the 1st generation Gates Carbon Drive sync belt and pulley system.  This photo of the bike was actually taken by Jon back in June 2009 at the Tennessee Tandem Rally when it was brand-spanking new and included in Jon’s TTR Blog Entry.

Photo by Jon (http://twotoneatl.com)

The subtle nuance that exists with the combination of the 4-screw eccentric and the FSA SL-K cranks is that the SL-Ks molded-in crank spider precludes the ability to get any type of standard 4mm hex key (aka, allen key, hex or allen wrench) into the two left-hand screws used to secure the eccentric.

4mm Hex Wrench Cut Down For Eccentric Adjustment

While it’s possible to use a ball-headed T-handle hex wrench to get to the screw sockets, you run the risk of marring the cranks & rounding a socket.  Moreover, there’s a pretty good chance you won’t be able to generate sufficient torque on both left-side screws to keep the eccentric from creaking during hard pedaling efforts.   Therefore, what you need if you own a Co-Motion with this particular combination of eccentric and cranks — with or without the Gates Carbon Drive — is a 4mm hex/allen wrench that has had the shorter-end of the wrench cut down from it’ typical 1 inch length to about a half-inch. The one pictured at right is the one I cut down with a hand grinder in Tennessee to work on the Robusta. The shortened end allows you to slip it in behind the crank while still having enough ‘meat’ on the shaft to apply the needed torque to secure the eccentric bolts without damaging the key or socket flats.

Finally, a note on the Gates Carbon Drive. The belt drive on this tandem was what I refer to as 1st generation, in that it was fitted with the 71 tooth timing pulleys (aka, timing rings). The 71 tooth spec was Gates’ recommended sizing for Co-Motion’s 28.5 inch crank-to-crank length boom tube spec and in practice, it makes for a really tight fit… really tight. Even with the eccentric in the fully reward position, it takes a lot of patience and finesse to work the toothed-belt on to the pulleys without violating any of Gates’ cardinal rules of belt handling. The 2nd generation Gates drive now uses 69 tooth pulleys that eliminate the snug installation issues.

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About TG

I've been around a bit and done a few things, have a couple kids and a few grandkids. I tend to be curmudgeonly, matter-of-fact and not predisposed to self-serving chit-chat. Thankfully, my wife's as nice as can be otherwise we'd have no friends. My interests are somewhat eclectic, but whose aren't?
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One Response to First Impressions: Co-Motion 4-Screw Eccentric

  1. Jon says:

    Great post and thank you for the link!

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