Back on Feb 27th I posted an item based on a BikeForums discussion where a team that was an early adopter of the Gates Carbon Sync Drive was looking to hear from anyone who knew what to look for to determine when a belt needed to be replaced. I posted a follow-up to that entry on March 5th where at which point Mr. Pink was still waiting to hear back from Gates after they took a look at some photos of their sprockets, now with some 10,500 miles of wear and tear.
Since then, Gates has looked at the photos and, as the Gates Rohloff Manual mentioned in the Feb 27th entry suggested, the sprockets were wearing out faster than the belt and the wear had reached a point where Gates was recommending the sprockets and belt be replaced. When I last checked, a set of sprockets and belt was about $456.00 at Webcyclery.com; Yikes!
While interested readers at BikeForums.net have not yet seen the photos of Ms. Pink’s worn sprockets, another one of the early adopters of the Gates sync drive — merlinextraligh — was prompted to take a look at his tandem’s belt and sprockets which had accumulated about 8,000 miles of hard training, road racing, time trials and some big climbs to see how much wear they were showing. While his front sprocket didn’t look all that worse for wear, the rear sprocket was a different story: one of the photos that Merlin shared with us in an entry to the thread is at left. Interestingly enough, the wear pattern is very similar to what you find with a chain driven sync drive. This seems to support part of what we observed with our belt drive experiment, in that the belt-driven tandem sync drive seems to behave less like a true synchronous belt drive system and more like a chain drive in terms of how the typical elasticity of a tandem frame’s boom tube causes the belt tension pre-load to drop-off during use. If that wasn’t happening, the rear sprocket wouldn’t be showing the significant asymmetrical wear pattern.
Again, we’re eagerly waiting to see Ms. Pink’s sprocket photos as she described their sprockets as showing a lot more wear and tear, such that the sprockets were now ”worn to teeth” with no remaining evidence that there was at one time a flat ramp between each of the sprocket sockets when they were new (at left).
Getting back to Ms. Pink and Merlin, both of them wondered if they couldn’t extend the life of their sprockets in much the same way that tandem teams double the lift of conventional, chain-driven tandem timing rings by swapping the rings front-to-back to reverse the wear pattern. The folks at Gates were asked this question and their initial response was:
“I’m not sure that rotating the pulley will help or cause problems. Like a chain system, the parts start to wear together. If you rotate the pulleys you run the risk of the belt not tracking correctly because of different wear patterns on the front and back.”
However, at the same time, Gates has not yet had a chance to inspect Ms. Pink’s worn belt and sprockets, something they’ve asked to do when Ms. Pink’s tandem is fitted with the new, replacement belt and sprockets. As of her last update back on March 15th, they had received the new belt and sprockets but had not yet installed it and were still riding their back-up tandem.
Like a lot of folks following the Gates Carbon Drive sync drives, I’ll be very interested to see what Gates analysis of the worn belt and sprockets yield. I believe that a lot of folks who were excited about the belt drives saw them as a way to eliminate a dirty chain, shave some weight off of their tandems, and save a few bucks on chain replacement. While the belts clearly achieve the first two objectives, the 3rd one — life cycle cost reduction — may now be a Con instead of a Pro for the belt drives. I’m pretty sure it would take about 12,000 – 15,000 miles to run a pair of timing rings and 3 chains to death ($150.00?) if someone was so inclined vs. changing out the sync chain every 2,500-3,000 miles to preserve the timing rings. Both of those scenarios seem to provide a lower-cost alternative to a $456.00 belt system replacement every 10,000 miles or so. At least for Ms. Pink’s team, their all-weather riding needs coupled with the performance of their belts has kept them sold on the belt.
Also, FWIW, it appears as though Gates will be rolling out a tandem sync drive based on their new Center Track system at Interbike 2011. So, if you’re still Jonesing for a belt drive, the extra-cool Center Track may be worth waiting for… along with the complete Gates analysis of Ms. Pink’s well-worn, 1st generation belt & sprocket autopsy.