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!!!!
 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!