It seemed to me that our timing chain was sagging a bit too much, so I decided to tighten it up....
Well, this turned out not to be as straight-forward as I thought it would be. I had first gone back to R+E to talk to the mechanic and had him show me a Bushnell eccentric so that I could see how it works before I started to unscrew things. Anyway, the wedges and wings on the eccentric seem to grab and release pretty quickly but I'm not sure how tight the adjusting screw should be. It didn't take much force to unscrew it, so I didn't try to reef it very hard when I tightened it back up. I rotated the eccentric until I had the amount of slack that I wanted but the chain tightened up when I tightened the eccentric. I think this is because the wings are at back of the bottom bracket shell and they force the eccentric forward when tightened. It doesn't take much movement in the eccentric to take the slack out of the chain! After dinking around with it, I finally got it close to where I wanted and we went out for a 25-mile ride. The captain's crankset creaked the entire time and I didn't have an allen wrench, so I couldn't do anything about it.
This whole process seems pretty simple, so what did I do wrong? Will a too-tight timing chain cause this? Or did I not tighten the eccentric enough and it was moving in the bottom bracket shell?
jgg3
02-18-08, 02:40 PM
The creaking is a sign of not tightening enough, but not knowing the eccentric you have, I don't know what it could be.
swc7916
02-18-08, 02:58 PM
The creaking is a sign of not tightening enough, but not knowing the eccentric you have, I don't know what it could be.
I guess that I didn't specifically say, but I thought I indicated that I have a Bushnell eccentric.
JanMM
02-18-08, 02:59 PM
Too tight on part of the rotation, maybe? On our KHS, it's easy to get the chain tension just right, but then turn the cranks and it sags. Requires fine-tuning of the rings. Or, the eccentric is not tight enough? No creaking before your adjustments? If the chain is too tight all the way around, that might creak.
TandemGeek
02-18-08, 03:21 PM
Here's a link to the Bushnell eccentric installation page:
http://www.bushnelltandems.com/eccinstructions.pdf
Having owned Erickson and Ventana tandems with Gen 2, Gen 2.5, Gen 3 Bushnell eccentrics -- and with one of the first Gen 4's that I use as a paper weight in my home office that should have Todd Shusterman's name (daVinci) attached to the refinements -- I can tell you that they need to have a generous amount of grease (Phil Wood Waterproof is a good one) applied to just about all of the mating surfaces and then torque must be applied per spec... if not a scootch over, otherwise they can and will creak.
The installation instructions are actually somewhat new and really do address all you need to know. I had a Bushnell Eccentric info page up on my web site for several years when there was a dirth of information on it as it is quite different from the others and the more you understand about how it works, the easier it is to work with it.
FWIW: Here's a link to a collection of previous postings on eccentric and sync chain adjustment
http://www.bikeforums.net/showpost.php?p=1790025&postcount=31
BTW, reference a thread a while back, I've since learned that Bushnell Cycles was acquired by Dan / R&E cycles, to include intellectual property and the like. I really do need to follow-up with Dennis, as I'm still not sure of all the ramifications.
swc7916
02-18-08, 03:42 PM
I don't know anything about R+E acquiring Bushnell Cycles, but both Dennis and his wife work there and R+E produces the Bushnell eccentric. I've seen and handled one of these eccentrics and I fully understand how they work; a bolt that has right-hand threads on one end and left-hand threads on the other drives two wedges that press two wings outward. Mine is newly installed so I have no reason to remove it and apply grease. I did read the installation instructions (they are on the R+E website) but I missed the torque specification. I don't have a torque wrench anyway and the assembly mechanic doesn't use one. In fact, he told me that there is no torque specification; I will print out the installation instructions and show him.
When you start to loosen the adjusting bolt things go easy as it backs the first wedge out then stiffens when you reach the second wedge. As you continue to loosen it, the eccentric suddenly comes loose; there is nothing gradual about it. I don't have a feel for how tight it needs to be; I'm afraid to overtighten it and it untightens easily, so I didn't crank on it very hard. The mechanic said to tighten it until it was firm and give it another 1/4 turn, but I don't know what "firm" is.
regomatic
02-18-08, 05:40 PM
I've been having some issues of late with our Bushnell eccentric as well.
No problems at all for the first couple of thousand miles with only expected adjustments. Then, what I thought was "firm" started loosening and creaking after 25-30 miles. The "new firm" is lasting 40-60 miles.
I already had the instructions downloaded and have been thinking about what steps I might take to eliminate or reduce the need for such frequent adjustment. TG's comments suggest I need to break it down, re-grease and re-assemble. Not all bad, because every time I do one of these new projects I learn a bit more and become one step closer to mechanical self-sufficiency.
I didn't know squat about bike mechanics until I started riding tandem and exploring the knowldge offered on this and other tandem related sites. Heck, I couldn't even clean and lube a chain properly until I found http://www.sheldonbrown.com/chainclean.html and bought one of their chain cleaning kits (Ride In Peace, Sheldon).
I'll probably invest in a torque wrench as well, for this and other adjustments. Thanks to the OP and replies.