Just Belt Me… Really.


After seeing the black CenterTrack sprockets on a Calfee and a few other tandems at the Tandems East Expo two weeks ago, being pinged by BikeForums.net brother USPSPRO on the results of our custom Gates belt test from a few year back, and then spending way too much time analyzing the drive train noise coming off our sync chain & rings during a Saturday ride this past weekend my interest in fiddling with the belts was piqued once again.

Worse yet, I made the mistake of building a little spreadsheet-based table to figure out what size sprockets would be needed on our tandem to work with the 250t / 2000mm Gates CarbonDrive tandem sync belt. Son-of-a-Gun, if my math was correct it would appear 60t CDX sprockets would support a nearly perfect fit on our 30″ boom tube.

I’ve submitted questions into Gates as well as da Vinci to see if all of the various bits and pieces I’d need would be available and if there’s any past experience in mounting CDX sprockets to the da Vinci cranks to know if there would be any interference issues, i.e., can the sprockets be mounted either in-board or out-board on the da Vinci spiders.

Unfortunately, there was just one HUGE problem: I forgot the tandem CarbonDrive / sync drive system and the regular Gates CarbonDrive / drive-side systems use belts and sprockets that have a different pitch: 8mm for the tandem application and 11mm for the drive-side. Doh!!!!

[edit] However, while inquiring about the 60t sprockets via Email Steve P. at Gates and being reminded of the pitch incompatibility he shared that, “You can achieve a 30″ boom length with a new tandem sprocket we have coming out this summer, a 66 tooth tandem sprocket.”  When I initially posted this entry I was cautiously optimistic that we might have a near-term belt option, even though my numerically challenged mind was telling me that my original calcuation of needing a 60t sprocket was still correct, the more I’ve thought about it the less optimistic I’ve become.  Just doing the math, a 66t sprocket would appear to support only a 28.97″ (29″) boom tube assuming it relies on the existing 2000mm long, 250mm tooth tandem sync belt, i.e., 66t *8m  pitch = 528mm, less 2000mm = 1472mm / 2 = 736mm (29.97″).  This is just how you back into the forumla that you use to calculate belt length.  So, I’ve sent off another note to Steve P. at Gates to double check and see if there’s something I’m missing in all of this.

If there is a factory solution, I’m also told “no worries” on the daVinci cranks + 130 bcd spider + Gates sprockets; they work fine together. There has been some talk but no movement as of yet on a spiderless CDX sprocket for da Vinci. Wouldn’t that be nice.

I also have no qualms about using square taper BB’s. The day I / we can generate enough wattage or torque to bring BB axle deflection into play as a concern is the day I quit my day job and begin training for the Nationals & sign-on for the TransRocky challenge. I’ve only had one rear bottom bracket — a very, very light Erickson Ti BB — ever show clear signs of axle deflection. Never a hint of it with Shimano UN72s, UN73s, UN52s, Race Face TaperLock or our Phil Wood SS or Ti BB’s. Being a lightweight, recreational team apparently has its advantages. Don’t get me wrong, I like the simplicity of the MegaEXO, GigaPipe, Hollowtech and other crank interface systems, but I really get tired of hearing creaky tandems with under-torqued BB bolts or stories of chewed up BB bearings due to alignment or low-torque issues.

Just my narrow view… as I look forward to June and some additional FARKLE for our tandem.  If there’s not a factory solution, then our good friend Bob Thompson had shared that he would be able to provide us with some 60t sprockets that bolt-up to our daVinci cranks without a spider that would use the Gates 250t/2000mm tandem sync belt, so we have options!


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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7 Responses to Just Belt Me… Really.

  1. cbratina says:

    You can’t beat a good sealed bearing, square taper bottom bracket such as Edco, SKF, and Phil Wood for reliability, adjustability, and low drag. Octalink is junk. External bottom bracket bearings require a lot of maintenance and are prone to dirt caused failure, unless you go Phil Wood. And they demand a wide Q Factor so you can fit any imaginable bike with one axle length.

  2. Richard James says:

    My tandem has a 36 in boom tube (center of BBs), so it might be awhile before we have any belt options to consider. I do like the idea of the Gates belt, but it seems a lot less flexible than the lowly bicycle chain…

  3. FredC says:

    I just bit the bullet and ordered a CDX system from Ric at HoT. This will be a replacement of the CDC system that came on our CoMo tandem in 2010. The belt and pulleys were showing wear. i decided to go with the CDX based on the good reviews it is getting online. My hope is that it will track better at lower belt tensions and be a little bit less of a maintenance headache. For some reason, the CDC system on our CoMo was always very sensitive to tension. If under tensioned, it tended to walk off the pulleys. If over tensioned, it became noisy.

    I do run the FSA Mega Exo bottom bracket. I haven’t had any bad experiences with it (yet). I do try to torque components to the manufacturers specs. Maybe that has kept us running silently so far (knock on wood). A noisy bike means a miserable ride in my case.

  4. AdamS says:

    I’m currently spec’ing a new Calfee and also pinged Steve at Gates about supporting a longer boom tube, (which I need.) Here’s what he told me, “Currently we offer systems for two boom lengths, 28.5″ and 27.75″. We are offering a 3rd which will be for 30.0″ that will be available by Summer.” I forwarded this to Calfee to factor into the proposed geometry of my new frame. Hope this helps for any retrofits as well!

    • TG says:

      Steve has since corrected his information / understanding of the newest belt spec. The 66t sprocket will support tandems with a 29″ boom tube, not a 30″ like ours. They would have to offer a 60t sprocket in 8mm pitch to support the 30″ boom tube.

      Quoting from my request for clarification

      Hey Steve

      After reflecting on this I’m wondering if the 66t sprocket will only support a 29″ boom with the 250t 8mm belt.

      I think my earlier calculations were correct in that a 60t sprocket would be needed but missed the finer point that the standard drive side belts were 11mm pitch, hence my error in assuming the 60t CDX sprocket that y’all do make might work.

      Anyway just wanted to confirm the design spec.

      ….and Steve’s response:

      The The 66 tooth tandem system will tension up right at 29″. That part will be available in June. The 3 boom lengths we will have available are 27.75″, 28.5″ and 29″. All tandem parts use a 8mm pitch and all single drive parts use an 11mm pitch.

  5. John says:

    TG, has anything changed in the last year. Is there an option to run a belt with a 30″ boom ?

    • TG says:

      To the best of my knowledge, not from Gates…

      The only alternative would be one of Bob Thompson’s custom systems using a different Gates belt and custom-made sprocket/pulley. The one you may have seen on our Calfee was not the standard Bob Thompson configuration; that was a special project to see if a very small / low tooth count sprocket / pulley would work.

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