Centering timing chainrings
 

I wanted to share with you my way of centering the chainrings on a tandem.
I know that the late Sheldon Brown has some wonderful web literature on this subject as well. I've read his methods and mine is slightly different and ended up working better for me.

http://bikeforums.net/attachment.php...hmentid=352201

With the chainrings loosened to hand tight (5 bolts on each chainring), find the "high spot" by spinning the crank; it's where the chain has the most tension. In that position, put a zip tie or a rope around both top and bottom of chain, bringing it together a bit to provide extra tension and force on the loosened chainrings (see photo). This will force the chainrings to come together as much as possible, reducing the high spot. DO NOT PUT EXCESSIVE FORCE ON THE CHAIN WITH ROPE/TIE BECAUSE THE MECHANICAL ADVANTAGE PUTS EXTREME FORCE ON CHAIN AND CHAINRINGS. Now tighten the chainrings, at least 3 of the 5 bolts on each. Remove the rope or zip tie and spin cranks.

If the high spot is still in the same location as when you had the chain tied, then you've done the best you can. If the high spot is somewhere else now (usually on the opposite side), you can continue fiddling by loosening only one chainring and using the technique above or a different technique.

In my case I was able to get the chainrings so perfect that the chain moves up and down about a millimeter and the chain droop is no more than 5 millimeters or so.

Sheldon mentions rotating the chainring to a different position (meaning, lining up the 5 chainring holes with the 5 spider holes differently), but in my case that was not needed. The rotation of chainring will work especially if the chainring itself is not round.

http://bikeforums.net/attachment.php...hmentid=352209
Tandeming since 1975; over 240,000 miles so far. Have never been concerned with 'centering'.

Indeed, perfection is not everyone's goal.

In my case, however, the chainrings were pretty far off from round when I inherited the bicycle. I decided to do something about it.

Only centered the syncro rings once during the ten years we rode a KHS. Not really sure it needed to be done but I was able to even out the chain tension.
With our recumbent tandem, chain tension is controlled by adjusting the position of an idler. Much simpler/easier.

Thanks for the post. I never would have even considered such a thing. I guess I fall well into that group for whom perfection is not the goal. I'm happy if things are pretty good, but I do like to know what I'm giving up. Maybe I'll even take a look and see how far off my tandems are.

Well, here's the reason I paid attention to my timing chain tension.
So like I said, they were quite a bit off from being perfectly round. As a result, the ideal chain tension would be such that it's fairly tight on the high spot (as tight as possible without causing a noticeable increase in drag) and fairly slack on the low spot. The danger, at least in my opinion, is having the slack in the chain when, for example, you are out of the saddle sprinting, putting out a combined 2500 Watts let's say, and going 30 miles per hour. With the frame flexing etc. there is an increased change that the timing chain will derail, which has the potential to be fairly catastrophic.

Now on the other hand, if you never put out 2000+ Watts out of the saddle, it may not be such an issue.

I want to get the chain so that it is taught all the way around for safety reasons.

Must be why it hasn't been an issue for me. Our propulsion is supplied strictly by human beings.

on a more serious note, I've just never seen measurable variation on our setup even when I've looked for it fairly carefully.

Seems like if the chain is very very slack at one point, that the chain could potentially bounce off on a section of really rough road surface, regardless of massive or puny power output.
Curious to know how common timing chain derailing is.

I think you're right regarding derailment of chain even at lower power.
I used the high power and standing up and going fast as an example because in that situation you can imagine how badly things could happen...

Another reason to do this is if you have a timing belt instead of a chain. If the rings are reasonably well centered, the variation in tension as you turn the cranks may be much greater than the manufacturer's recommended tension range - so the belt will run at too high and too low tensions in different spots, and rarely at the recommended tension. For this reason, I spent some time getting the timing rings centered on our belt drive.

Have derailed/dropped timing chain twice in 240,000+ miles, so we are not really concerned.
Apparently the most round chain rings are made by/for daVinci.
Perfection in anything is difficult to achieve; at ages 81/78 we are as close to perfect as we've ever been!
Just ask our great-grandkids . . .

We want to grow up to be you!

Not really the issue, 'centering' is a completely different beast from 'chain slack removal'. An idler system or an eccentric BB only address the chain slack. Like others we have noticed a degree of cyclic increase/decrease in timing chain tension which could likely be reduced/eliminated by either Sheldon's or the o.p.'s centering procedure but... I haven't bothered. Hasn't been an issue. FWIW.

H

True - the idler adjustment does only address general chain tension. Easier than adjusting an eccentric BB. Probably easier to fine tune than an eccentric BB. I'll have to reconsider how this relates to the not-quite-centered synch ring issue.