This was the question that a contributor to the tandem sub-forum at BikeForums.org posed last week. In addition to being an early adopter, this particular tandem team logs lots of miles each year and with nearly 10,000 miles on the belt experienced some minor ratcheting on a recent ride. You may recall from my earlier posts on our belt-drive experiment that ratcheting is the toothed-belt version of a chain slip.
I for one was very interesting in hearing what, if any, others who had logged high-mileage on their Gates Carbon Drives had to report in terms of how long they were able to use the belt before it required replacement. Sadly, there were no replies from anyone who had hit a replacement milestone.
Lacking that type of essential feedback, and out of curiosity, I decided to see what Gates had in their FAQs and various manuals on the subject of “when to replace the belt”. The only really detailed information I’ve found on evaluating wear and tear on the Gates Carbon Drive came from the manual Gates produced for the Rohloff hub users. I’m not quite sure why there wasn’t a similar manual for Gates Carbon Drives in general. Regardless, here’s a link to the .pdf file.
If you search on “replace” you’ll find a number of references to when a belt should be replaced, to include making mention of a “ratchet event” being cause for replacement. However, since the Rohloff and single bike systems use a very small driven sprocket with a larger drive sprocket vs. tandems using two larger drive sprockets, I’m not sure if the same assumptions regarding presumed damage apply. In theory, a ratchet event could be a very bad thing IF the belt was fully tensioned and there was no other elasticity in the system. However, as we discovered during our belt experiment, eccentrics can slip over time, leading to a loss of tension and under very heavy loads, tandem boom tubes do have a tendency to deflect which would also lower the belt’s tension pre-load to a point where there could be enough slack to allow a ratchet event without actually “stretching” the belt and damaging the carbon fiber cords that give it its strength.
However, what I did find interesting was in Section 3 / Page 30 of the manual where Gates goes into some detail on how to check the sprockets and belt for wear. They show a gauge that can be used to check both the belt and sprockets for wear which would probably be the most reliable and technically correct method for evaluating wear.
But, what I found most interesting in reading the manual was a comment regarding sprocket wear. In short, it suggests that, despite what intuition would suggest… the sprockets typically wear out before the belts and that the sprockets and belts should be replaced at the same time.
Again, I don’t know if that is unique to the drive belts or if would also be true of the sync belt application. Bear in mind, unlike a drive-side belt where a large driving sprocket is mated with a smaller driven sprocket, a sync drive uses a pair of evenly matched sprockets with a 1:1 drive ratio which, in theory, should not cause nearly as much wear and tear as a drive-side belt & sprockets.
I’m still waiting to hear back from the O.P. in the hopes that Gates was able to provide her with some definitive information for tandem sync drive applications. Again, I suspect that the wear and tear on a sync drive should be different than what a drive side, single speed or internally geared hub belt system would experience.
More to follow…