As mentioned in Update #5 last week, after using the sync belt and 33t sprockets for about 500 miles we switched back to the sync chain and timing rings for about a 35 mile ride with some friends on Saturday just to get re-calibrated. After doing so, both Debbie and I came to the conclusion the chain had a noticeable reduction in drag compared to the belt. It seemed as though our cranks were spinning with almost no resistance, a noticeable difference from previous rides with the belt. The latter was somewhat disconcerting as when we started the evaluation and switched over from the chain to the belt, neither Debbie nor I detected any difference in pedaling effort / drag from the belt.
As for why we didn’t detect any difference in drag at the beginning of the evaluation, I’m left to conclude either the belt’s very quiet and vibration-free performance created a placebo effect that caused us to not recognize the change in drag, OR the increase in drag developed during the testing. In regard to the latter, as I swapped-out the bottom brackets while converting back to chain I noted the Phil Wood stainless steel rear bottom bracket’s timing side bearing had a gritty feel that it didn’t have when I first installed it to support the belt . The front BB — a Phil Wood Ti-Mag model — also had a slightly gritty feel on the timing side that I didn’t detect when we started the testing as well.
Therefore, at present I suspect the 33t sprockets were just too small to be practical for use on a sync drive, which would also be true of a sync chain supported by 24t timing rings. In other words, it wasn’t a belt thing, it was a small sprocket/chain ring thing remembering that, the smaller the sprocket or chain ring, the higher the force on the belt or chain… and when sprockets or chain rings get really small, the force they generate gets really big.
So, in summary, I think our experience with this particular experimental 33t sprocket w/12mm wide Gates 8MGT-1792-12 belt system was less successful than I’d hoped for a couple of reasons:
- Small sprockets and chain rings just aren’t a good choice; they simply generate a lot of added force on the sprockets and belt for no real benefit and often times to the detriment to other equipment and sync drive efficiency.
- Our Calfee — even with its couplings — may not be as robust as it needs to be to support a high-force generating, smaller timing ring/belt sprocket sync drive.
Even when the belt tension was set at the higher preload values that precluded ratcheting, e.g., 15lb or there abouts, if I locked the rear brake and applied a moderate load to my captain’s crank the bottom run of the belt would go slack… for whatever reason, there was just that much elasticity in our Calfee’s frame and bottom bracket bearings and axles. As noted in early updates, the additional force generated by the small sprockets clearly caused the eccentric to rotate, which was more evidence that the small sprockets were stressing the sync drive and associated components under peak load. As for the wear and tear on the bearings, I suspect a chain with a pair of 24t chain rings would have been equally hard on the Phil Wood BB’, etc. Belt pre-load alone just didn’t strike me as a major contributing factor, even on experimental system with the small sprockets and higher pre-load. After all, the drive-side bearing on the same bottom bracket has to deal with some truly massive loads and I’d guess that our Calfee’s Phil Wood Ti-Mag rear bottom bracket has somewhere around 6k miles of pretty stout wear and tear at this point.
As for trying a larger set of sprockets and longer belt, our friend Bob came up with an alternative configuration based on a 59t sprocket and longer belt that would have worked. However, it may have required even wider bottom bracket axles vice the ones we were using with the 33t. After mulling it over, and given that we were both at our limit for Q-factor on the Calfee with the 33t sprockets, we decided the 59t sprocket configuration was pretty much a non-starter for us. Bear in mind, I’m most comfortable using bottom brackets that are no wider than 108mm in combination with our daVinci cranks where most tandems come with 118mm sprockets and a proportionately wide tread or Q-Factor, if you prefer. Debbie is most at home with 108-113mm and was very happy when I switched the 119mm bottom bracket out for her 111mm one at the same time we put the chain back on. I think I mentioned in the Paketa V2r thread that Calfee has apparently adopted a different stay design that’s deeply dimpled to work with the Gates 69t sprockets without using a super-wide set of bottom brackets, so they too recognized that sync belts do best with some special frame accommodations.
So, we are now at an end to our sync belt experiment and the belt and sprockets are on their way back to Bob. While I am somewhat disappointed that our experience was truncated, I think it was worthwhile to explore the alternative system and smaller sprockets, if only to see if they might be viable. We could have taken a second run at it with more conventionally sized sprockets but, even though there were many positives for the belt, unless we found ourselves riding in adverse weather and doing a lot of sync chain maintenance, the tried-and-true sync chain drive simply works just fine for us.