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Tandem Timing Chain Problem...(22 posts)

Tandem Timing Chain Problem...yfoiler
Jan 1, 2003 11:03 AM
I'm building up a new tandem.
When I install the timing chain, it's either too short or too long. The eccentric can't take the required slop out of it if I set up the chain as short as possible. And with the eccentric in the shortest adjustment position the chain is a quarter inch too short to put the link in.
So am I missing something here? Seems like I'm screwed no matter what I do. I was able to set up chain short and "roll" it on. I hated to do that but I needed to see if that would work. It was BAR tight and unusable so I had to take off a chainring to remove it.

Any suggestions will be greatly appreciated as I can't very well ride around with a two inch slop in the timing chain---and that's two inches on the bike stand, not even under load.

TIA

Marty
re: Tandem Timing Chain Problem...Spoke Wrench
Jan 1, 2003 11:34 AM
You have something going on that I don't understand. Chains can be shortened or lengthened in 1 inch increments. Your eccentric should be able to take up that much slack. Is this a Cannondale frame by chance? I've never worked with one but I've heard some comments regarding their peculiar eccentric.
re: Tandem Timing Chain Problem...yfoiler
Jan 1, 2003 1:21 PM
The frame is a KHS "Roma" tandem. It's 7005 AL with a CF fork.

As for the eccentric---the guy I bought the frame from forgot to ship me the eccentric. So it came later. But I can't see where the eccentric could make a difference. There's only so much room to make up in the BB shell. When I rotate the eccentric it takes up all the play you could inside the BB shell so I think it is correct.
And the BB and cranks are in and fit fine. I'm stumped...

As Doug suggested, If I go to 39 tooth rings, I'll use less chain length but won't the ratio of loose/tight stay the same? I've never tried this.

As for my timing chain I used two SRAM chains with the gold power links. I took out one gold link and pinned the two chains together as one and just use one gold link to be able to take the chain on and off easier. Did I cause a problem with this type of chain? It would still measure correctly at 12 inches exactly with two power links or one. BTW, they are new chains and never been riden.

This is a weird one for sure...

Marty
Eccentric is the key.Spoke Wrench
Jan 1, 2003 2:52 PM
The bottom bracket isn't mounted in the center of the eccentric, it's off center. You should be able rotate the eccentric in the bottom bracket shell so that the bottom bracket is either in front of or behind the center line of the bottom bracket shell. Start with the eccentric rotated so the bottom bracket is as far back as you can get it so that you can install the timing chain with a bit of slack in it. Then you can rotate the eccentric to snug up the timing chain.

Don't be afraid to snug it up fairly tightly, that whole boom tube will curve sideways and cause the chain to loosen a little when you put power to the front set of pedals. Check out some photographs of tandems. When they are standing still, both the top and the bottom of the timing chain is nice and tight. When under power, the bottom run of the timing chain has noticeable slack.

The next thing is timing the two cranksets. All of the tandem teams that I know today are in phase - both right cranks are at top dead center at the same time. There is a theory, and I think it has merit, regarding phasing the cranks 90 degrees apart - usually the captains crank is 90 degrees ahead of the stoker's crank. That way, when you are grinding up a hill, one of the riders is always in the power portion of the crank stroke. Sure looks funny though.
Eccentric is the key.yfoiler
Jan 1, 2003 3:54 PM
Thanks Spoke,

OK, I put the chain on but there was no way to do it other than link the chain first and then (and this hurts), roll it onto the front chainring. (exactly in phase---we have no mountains in Florida so I'm not even considering the out of phase thing---not to mention my wife and I learning our "phase" relationship while cornering! Ha!)

So now the timing chain is BAR tight with the eccentric rotated all the way "aft" so as to give the least tension possible.
To give you an idea of how much tension there is on the chain, with my one hand I can grab the top and bottom of the chain mid distance between the chainrings and squeeze no more than 1/4 inch in on both top and bottom of the chain---and I'm squeezing hard to get this much.
When I do this I think I can see the back of the bike flex a bit to the side. Maybe that's the keel tube bowing that you were talking about? So I don't think this can be right, as there is a ton of load on my BB bearings with the bike just sitting there.

So the first thing I'm going to do is get rid of my 42's.
Tomorrow I'm going to order a pair of 39's because I can't find 40's in the 130mm bolt pattern. I've read that you should have an even number of teeth on the timing side because the wear pattern is even as opposed to shifting around with the odd teeth. (argue this one with Sheldon Brown, his explanation makes sense I just can't explain it properly). Guess I'll just live with a set-up that wears the chainrings faster---no biggie. (that is if the 39's solve my problem)

Thanks for taking the time and the help.
If the 39's don't do the trick I may be back begging for more help.

Thanks again,

Marty
Somethings wrong here!Kerry
Jan 1, 2003 4:27 PM
Changing chainrings will not help one bit! When you change both rings, you reduce or increase in 2 tooth increments - exactly one full chain link. If your chain is super tight with the eccentric all the way "back" then you should be able to add one full link (1 inch) to the chain and rotate the eccentric toward the front, finding a point where the chain runs smoothly. You may find that the tension changes just slightly as you rotate the cranks due to slight eccentricities in the cranks/rings/etc. Also, the way to install the chain is to remove one ring, put the chain on the rings, and then move the loose ring into position on the crank arm spider. There is some trial and error required here to get the ring engaged with the chain so the ring is in the right relationship to the crank.

BTW what is "BAR tight"?
sure about that?DougSloan
Jan 1, 2003 5:42 PM
When the size of the rings increases, the diameter is larger, making the chain go further by the number of teeth added; however, the increase in diameter also puts the distance between opposite ends of the rings further apart, but that increase in distance is not the same as the increase in teeth; adding a tooth to each ring will not put the opposite ends of the rings 1 inch further apart; it's more like about a quarter inch (what's the actually increase in diameter of the ring?). In other words, I (respectfully, but not confidently) don't think your analysis is correct.

BTW, I think Sheldon confirms this:

Since the eccentric has only a limited range of adjustability, in some cases, it may not be possible to use even-size synch chains: if you replace a pair of 39 tooth rings with a pair of 40s, you need to adjust the eccentric 1/4" closer to the fixed bottom bracket, or you can add a link to the synch chain and move the eccentric 3/4" farther. Some eccentrics may not permit this amount of adjustment.
http://www.sheldonbrown.com/synchain.html#chainring


Anyone care to do the math?

Doug
I blew it!Kerry
Jan 2, 2003 5:28 PM
Here's how to think this through: each timing chain has exactly 1/2 of its teeth engaged with the chain. 40t timing rings engage a total of 40 "half links" of chain, or 20 full inches. If the distance between the centers of the timing rings remains the same and you add one tooth from each ring, you now have 20.5 + 20.5 teeth engaged, for a total of 41 "half links". You've added 1/2" to the chain requirement, so if you add a full link, you've got a spare 1/2" to make up for what the eccentric won't handle. Going up or down one tooth on the timing rings is the way to go. Sorry about last night's brain fart.

However, looking at the data you provided (chain 1/4" short with eccentric all the way "back"), it sure seems like the eccentric should be able to handle it. Adding a full link would leave you with 3/4" of "slack" in the chain. Your eccentric would have to move 3/8" forward to take up that much slack. This is because when the eccentric moves forward, more chain is needed on the top and bottom of the timing loop. IIRC our tandems had that much movement in the eccentrics, but I don't have one around to measure to confirm it. By inference you are telling us that your eccentric has less than 3/8" forward/backward movement.
Somethings wrong here!yfoiler
Jan 1, 2003 6:48 PM
Yea, you're right, something is wrong here and I'm sure trying to get it sorted out. BTW, by Bar tight I meant the chain is as rigid as a steel bar--- in other words damn tight!
(it's an old expression, not to be confused with BAR as in pressure)

Kerry, when I have the eccentric rotated so the Captain BB is all the way towards the rear (the loosest adjustment) the chain will not reach. It comes within one quarter inch of me being able link it up. So, no biggie you say---like I said to myself. I put in a "link pair" (one inch of chain) and put the chain on the chainrings. I rotated the eccentric forward thinking it would take up all the slack---and it does take up some, just not enough, the chain is so damn sloppy that the first bump I hit it's going to come off. The eccentric can not take up the slack in the one inch of chain that I added! I don't know why?

With the eccentric adjusted as tight as I can get it, there is still several inches of up/down play in the chain midway between the two timing chainrings. It's really loose! It's got me baffled for sure... I don't see how one inch of chain can make so much difference.

BTW, I don't see how the eccentric could have any more adjustment to it. The BB is all the way on one side of the eccentric---I don't think there is room to machine it any further off-center than it already is. Everything fits nicely but something is dreadfully wrong. It's almost like they welded up the bike frame with the two bottom bracket shells to close together on the keel tube.

The thinking in going to the 39's is that, like you say, the chain gap will remain the same because we're only going down two teeth, (one inch) but maybe because of the diameter of the chainrings being smaller, it may come into the adjustment range of the eccentric---I makes sense that as the chainring diameter gets smaller and smaller the ratio of smaller chainring diameter to the fixed diameter of the eccentric becomes bigger. In theory if I went to two inch chainrings the eccentric would probably have a two to one adjustment ratio.
I don't know, but it's all Iv'e got left to try.

Anyway, I need help big time, because it's un-rideable the way it is...

Thanks guys,

Marty
still think the rings is the way to goDougSloan
Jan 1, 2003 8:03 PM
Try 1 tooth larger rings and see what happens; can you borrow some just to check?

Doug
still think the rings is the way to goyfoiler
Jan 1, 2003 8:19 PM
I think the rings are the way to go too, however I just found out my buddy that has the same frame I have (only a better paint job damn it) has the 39's for his timing chainrings and all is well. I originally thougt he had 42's. So this gives me incouragement that it is the chainring size that is my problem. In my previous explanation I think it makes sense that the smaller you go in chainring size the larger the effect the eccentric has. Taken to extreme you can imagine if I had a three inch chainring my 3 inch diameter eccentric would have an enormous effect on tensioning.

So... I have 42's now, if you are suggesing a set of 43's that would be a pretty hard beast to find. I think I'll order the TA 39's I found at Mud Sweat N Gears for only 26 bucks a piece. I'm going to make sure I can return them if it doesn't work out.

Thanks, and I'll know in a few days I guess...
ya, try the 39's; can you borrow your buddy's to try? nmDougSloan
Jan 1, 2003 8:29 PM
Almost forgot...yfoiler
Jan 1, 2003 8:23 PM
Doug,
Almost forgot...

That C-40 you are selling is to beautiful to ride.
Man, I don't think I've EVER seen a more beautiful paintjob/frameset.
If I only had the $,$$$'s oooh....
(and even "I" know Shimano makes an Italian threaded BB) heh heh...

Good luck with it.

Marty
thanksDougSloan
Jan 1, 2003 8:31 PM
People usually love it or hate it. One of those things.

Doug
I'm learning something new today.Spoke Wrench
Jan 2, 2003 7:04 AM
It had never occured to me that timing ring size might affect your ability to adjust out the slack with the eccentric. Doug's quote from Sheldon Brown, however, seems to indicate that's true. If it was my bike, I'd certainly try a pair of smaller rings.

For whatever it's worth, my Santana has 39 or 40's.

The general consensus among tandem riders is that bigger timing rings are better because the leverage differences result in less tension on the chain which should result in longer chain and chainring life. I'm thinking this chainring/eccentric relationship might be a reason why manufacturers, like Santana have settled on 39 teeth rather than use 53's or something like that.
2 ideasDougSloan
Jan 1, 2003 12:25 PM
1. Try a connector link that basically allows a 1/2 inch change instead of a full inch.

2. Try different size rings.

Doug
re: Tandem Timing Chain Problem...Calvin
Jan 2, 2003 11:23 AM
The chain ring sizing makes sense. It will be interesting to see how much a difference it makes.

Also, for setting the tension on tandems see http://www.parktool.com/repair_help/tandem.shtml
Even vs. odd timing ring teeth.Spoke Wrench
Jan 3, 2003 5:27 AM
I layed awake half the night trying to picture this problem in my mind. I thought I was pretty brillent when the answer finally came to me. Now I find that two different guys already had it figured out but had phrased the answer in terms that my feeble mind didn't understand.
Sheldon Brown on "Even vs. Odd" timing ring teeth and more.yfoiler
Jan 3, 2003 9:29 AM
Odd or even timing chainrings...does it matter?
(The following is from Sheldon Brown's website under "Tandems")

One can make a case that it is better to use even numbers of teeth for synch chains, and keep the chain always set the same way on them. As a chain wears, it elongates, but the elongation only happens between the rollers separated by outer links.

As the chain wears, it deforms the chainweel teeth to match the increased pitch of the worn chain. On an odd numbered chainweel, each tooth is alternately in contact with a "stretched" half link and a normal-pitch half link, every other revolution. The "stretched" half links deform all of the teeth, which then no longer mesh properly with the un-stretched half links.

With an even numbered chainring, only the teeth that correspond to the "stretched" half links get deformed, and by doing so, they work fine with the elongated half links. The alternate teeth don't wear as much, since they are dealing with normal pitch half links.

As long as you don't derail the chain and put it on out of its normal phase relation to the sprocket teeth, a considerably worn chain/chainrings can run smoothly and efficiently.

>>here's the part that relates to my problem and our discussion...MT<<

Since the eccentric has only a limited range of adjustability, in some cases, it may not be possible to use even-size synch chains: if you replace a pair of 39 tooth rings with a pair of 40s, you need to adjust the eccentric 1/4" closer to the fixed bottom bracket, or you can add a link to the synch chain and move the eccentric 3/4" farther. Some eccentrics may not permit this amount of adjustment.
[end]...

So, there you have Sheldon Brown (in theory) solving my eccentric problem.

Also of note: The front timing chainring teeth wear on their leading edge and the rear timing chainring teeth wear on their trailing edge. When the chainrings get worn, you just swap front to back and have new "surfaces"... cool huh?

My FSA 39 tooth chainrings arrive today. I can't wait to put the theory aside and get a wrench on this issue.
I'll post ASAP on how it turns out!

Thanks everybody!!!

Marty
Sheldon gets really esoteric sometimesDougSloan
Jan 3, 2003 9:40 AM
This issue of how the teeth wear is a pretty small issue, isn't it? Well, it may not be a small issue if you expect not to have to replace any parts for 30,000 miles or so.

The counter to Sheldon's recommendation is that if you use odd tooth rings, all the teeth bear some of the wear from elongating chains, thereby making the rings last twice as long.

With the tension on the sync chain, how long would it take to get slippage?

Doug
I think so too.Spoke Wrench
Jan 3, 2003 1:18 PM
Since the timing chain is always in line - it doesn't get deflected to shift gears like a rear chain, they tend to last a lot longer regardless of the number of teeth on the chainrings.
My 39's Arrived---AND THE FINAL ANSWER IS....yfoiler
Jan 3, 2003 3:33 PM
Hello to everybody that helped kick in all their thoughts and ideas on my timing chain problem. My two FSA 39 tooth timing chainrings arrived this afternoon and I could hardly get them on fast enough....

Well...THAT DID IT!!!!
I had to take out one more inch of chain, dropped in my SRAM power link and Voila!! The chain can now be adjusted from very loose to VERY tight !! So apparently the size of the timing chainrings dictates how much adjustment a given eccentric will have. As it looks now, I think I could have gotten away with 40 teeth, but FSA doesn't make a 40 and I should consider myself lucky that the 39's match the FSA cranksets.

I can't tell you all what a load off my mind this is .

And I also want to thank everyoe that stuck it out through this one. I think we all learned a few things about eccentrics and tandem timing chains, and there was lots of positive energy put into this one---much appreciated guys.

All the best to you all for another safe riding season.

Marty