Timing Belt: Update #3

See something that doesn’t look quite right with our Calfee in the photo at right?  Need a hint: look at the cranks.  That’s right, they’re not quite in-phase… which is not how it looked when we left the house today.

Apparently, we started having some belt slip yesterday while climbing out of the saddle on a 12% grade.  I didn’t realize the belt had slipped until today when it happened again.  I was also having a lot of clicking from the front eccentric bottom bracket, which was a bit disconcerting and the focus of my attention after yesterday’s ride.  So the ‘schlunk’ noise we heard and felt while climbing yesterday was simply one of those things that was too unique to isolate, lacking any further evidence of a problem.

After arriving home after our ride on Saturday and in an effort to eliminate the clicking, I pulled the eccentric and front bottom bracket for cleaning & lubrication, to include pulling the dust seals off the front BB and repacking the bearings with grease.  I Loctited the Phil Wood adjusting rings when they were re-installed in the eccentric and greased the inside of the eccentric shell before reinstalling the eccentric as both of those steps usually eliminate any spurious noises that come front the front bottom bracket when everything else is otherwise in order.  I also re-greased my Speedplay Frogs with my Speedplay Speedy Luber as they too can be the source of ‘clicking’ noises when they’re in need of lubrication.  Everything was reassembled with normal belt tension & the Bushnell eccentric fixing bolt was set to the 90 in lb torque spec.

On today’s ride, the clicking noise was gone…  but on an 8% grade we typically take out of the saddle, the belt jumped twice before we realized what it was. We sat down, geared down and spun our way to the top of the hill where  we stopped and dismounted to investigate. Seeing that our cranks were now a bit out of phase with my cranks leading and the belt now had a lot of slack that didn’t exist at the start of our ride, I had to assume the eccentric rotated in the shell under the torque loads on the climb: after all, carbon belts don’t suddenly stretch, they break.  Unfortunately, I didn’t have the tools needed to adjust the belt on the road (i.e., 6mm & 4mm long-handle Allen wrenches & pin spanner), so we had to nurse the bike back  home by spinning up hills in low gears.  We did fine, except at one stop where I hadn’t geared down enough and, upon starting under a heavy pedal load the belt slipped with a resounded ‘schlunk’ again.

Once we arrived back at home, I re-adjusted the crank alignment, re-set the belt tension and re-torqued the eccentric at bit beyond Bushnell’s spec of 90 in lbs to see if that will “hold” when we next ride at the Southern Tandem Rally’s Friday afternoon ‘warm-up’ in Florence, Alabama.

Ultimately, I’m inclined to suspect the higher than historically ‘normal’ torque created by the smaller pulleys used on our experimental sync belt is placing a much bigger pulling force on the eccentric vs. belts (or chains) installed on larger diameter sprockets like those marketed for tandems by Gates and most OEM chain drives, including our 34t daVinci cranks.

Therefore, as our friend Bob Thompson noted, it is essential our eccentric be able to hold fast to maintain the proper  tension such that the belt will not deflect more than 0.47 inches  when a downward force of 17-19 pounds is applied.  Again, from Bob we know that Gates has designed the 8 mm x 12 mm carbon GT belt we are using to work on a two axis wrap with 30t pulleys and easily accomodate 1.5 hp…  far more than I could generate, noting 1 hp = 600 watts and being mindful the sync drive only transmits the captains power to the rear crank axle.

Bottom Line: It appears as though our eccentric may not be holding fast under out-of the-saddle efforts, so more investigation and closer attention to eccentric tightness / belt tension will be required.  I’ll definitely need to keep a closer eye on our eccentric, as eccentric slip definitely seems to be the most likely root cause of our belt skip.  I suspect going from a dry interface between the eccentric and eccentric shell to one with a layer of Phil Wood waterproof grease may have been the biggest contributor to today’s issues, whereas Saturday’s may have been the cumulative effect of some eccentric movement during several hundred miles of belt use since the last time the belt / eccentric was adjusted.  I’ve left the grease in place with the increased torque on the Bushnell eccentric, but I may take it apart again and remove the grease to go back to a dry (aluminum on aluminum) interface for STR or replace the grease with some FSA Dynamic Paste for a bit more bite… belt and suspenders, if you will.

Looking ahead to our ride on Friday, I’ve marked the eccentric shell so we can check for eccentric movement if we have any issues during/or after the ride, and I may also stuff those extra tools needed to adjust the belt on the fly in my jersey pocket “just in case”.  I’ll also bringing along our conventional left-side daVinci cranks with 34t toothed timing rings, our sync chain and my 108mm front bottom bracket in the unlikely event we need  to switch back to the chain drive as another “just in case”.  After all, it seems that when you’re prepared for problems, problems never seem to crop-up so that’s my real strategy.

More to follow….  Other than that, the belt drive has been performing just fine and remained trouble-free.  I’m guessing some additional attention to the eccentric will eliminate this one blip in the evaluation process.


About TG

I've been around a bit and done a few things, have a couple kids and a few grandkids. I tend to be curmudgeonly, matter-of-fact and not predisposed to self-serving chit-chat. Thankfully, my wife's as nice as can be otherwise we'd have no friends. My interests are somewhat eclectic, but whose aren't?
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