In my quest to learn more about the Gates belt drives I’ve consumed a lot of information from various sources on synchronous drive belts. One of the things I started to notice was the somewhat ‘generic’ belt pre-load tension measurements used on single bike drive belts, tandem sync belts and (wait for it) Harley Davidson secondary drives. Yup, ~10 pounds of force applied mid-span to the non-tooth side of the belt using one finger with ~.5″ of deflection is about right for all three of these very different synchronous drive belts.
The recommended method for checking pre-load tension is using the Gates (Dayco) Krikit (pronounced Crickett) Tension Tester. Gates has detailed instructions on how to use the Krikit that anyone can download from their website and that you can find HERE. I’m somewhat amused that the Carbon Drive Krikit appears to be a Gates Krikit I with some stickers that cover the calibrated scale on the Krikit I. Those stickers seem to add about $20 to the MSRP of a Krikit.
Update: In the comments you will find that a reader was kind enough to check with Gates on the differences between the Dayco Krikit sold for automotive V-belts and those sold for bicycles. According to Gates, ” …the bike version with the stickers over the engravings uses the same aluminum casting as the auto version, but with a different spring–so the scale and spring tension are different between the two devices. And that the bike version is more expensive because of the lower volume, and that the bike spring is installed by hand (given the low unit volume).” If I had enough interest and owned a Gates Carbon Drive belt system designed for bikes, I’d almost be tempted to buy one of each so I could compare the difference.
Regardless, the Krikit made for Gates bike belts is probably a good tool to have on hand for anyone who owns a belt and who doesn’t have a good way of judging what 10 pounds of downward pressure feels like. The following video from Dayco shows how the Krikit should be used (in this case, the larger Krikit II)… remembering that the Carbon Drive model adds the “red zone / green zone” sticker instead of specifying the tension level hidden under the sticker.
As I mentioned, Harley Davidson motorcycles also use a synchronous drive belt that uses a similar 10 pound force to measure deflection using a slightly different tool (below) that you can see someone using in this video.
When I thought about the different length Gates Carbon Drive belts offered for use as a drive belt for bicycles and the much longer sync belt, I began to wonder why both belts had the same pre-load tension: coincidence or is it just a relative amount of tension? More specifically, the following is what I asked:
TG: I’m curious as to why the tandem sync drives developed with Co-Motion seem to have the same tension pre-load specification as bicycle primary drives?
Is the pre-load simply relative, i.e., 5-10lbs of downward force / .5″ deflection on a bicycle’s drive belt or a tandem’s sync belt is “about right” without regard for the length of the belt, size of the sprockets, or span between the sprockets? Or, does it just work out that way by coincidence?
Case in point, I’m doing some testing with a tandem sync drive using a Gates 8MGT-1792-12 GT belt & 33t sprockets sitting on a pair of axles that are ~30″ apart. The tensioning specification is 17-19lbs of downward force / .47″ of deflection which, given the smaller sprockets, seems to make sense. While I suspect the higher pre-load may reduce bearing life on the bottom brackets, that’s obviously part of what we’re investigating with the small sprockets.
Just trying to understand more about the belts and related maintenance, care and feeding.
I was pleasantly surprised to get a fairly prompt response from Gates, and even more surprised at how simple their feedback was:
Gates: It has been our experience that the smaller sprockets require a higher tension to function properly. As for bearing life, it has depended a lot on the bearing quality. If the bearings have a high tolerance then there seemed to be no reduction in life span. The bearing with looser tolerances and more play definitely wear out faster.
Pretty straight forward. In essence, the sync drive belt needs to have enough tension to keep it from ratcheting (aka, slipping) under load. If the belt ratchets, it’s not tight enough. If you make it too tight, then you are putting unneeded loads on the belt, sprockets, and bottom brackets. If you have good bearings with tight tolerances in your bottom brackets, they’ll do just fine. If you bottom brackets or cranks aren’t up to snuff in the first place, you’ll have the same problems you have with chains in terms of wear and tear. BTW, from all accounts thus far, the belts do not ever ‘break-in’ and stretch one iota.
FWIW, I did an informal poll of a few Gates sync belt users a short while back at BikeForums.org and, to a person, all of them used the old calibrated thumb / finger method to check the pre-load tension. Another person who has installed several belts noted they were using an Ultimate Digital Scale, another belt user relied on belt pitch by plucking the belt, and a few others in the original poll who used finger pressure were considering the use of a more accurate method of measurement in the future. Other than a couple reports of needing to fine tune the belt pre-load tension to eliminate some perceived drag, no real issues were reported and most are genuinely happy with their belts. So, similar to torque wrenches, while there are precision tools available to help set the pre-load tension on belt drives, it remains to be seen just how sensitive belts are to ‘approximate’ tension over the long haul. However, it’s also pretty clear that it doesn’t take a lot of brute force to apply the necessary pre-load to the belt: pin spanners and the like are more than adequate.
Finally, anyone who owns a belt-equipped bike or who is considering one should note that, while Gates’ Carbon Drive belts sometimes touted as being maintenance free, the tandem belts and sprockets do seem to want for some cleaning every once in a while, i.e., soap and water to remove road grime. Additionally, some folks have also reported a little belt squeaking: to resolve they revisited their sprocket alignment to address the noise while others have simply applied a little silicone spray or in one case run a bar of soap along the sides of the belt every now and again to eliminate the noise.