Just How Important Is Chain Wrap Capacity?
 

I understand what chain wrap is and why it is important. I also understand that a manufacturer's capacity ratings are typically very conservative and an experienced rider/mechanic can exceed the rating without worry, but by how much?

I'm working on my Jack Taylor tandem project and changing parts of the drivetrain. It was originally equipped with:

• 49/44/27 TA triple
• Suntour XCD 600 RD
• Suntour Compe V FD
• Suntour 14-32 6s freewheel

It will have:

• 49/44/27 TA triple
• Shimano STX RC RD
• Campagnolo Victory touring triple FD
• 11-32 8s freewheel

I don't know what the chain wrap capacity of the XCD 6000 was, but I doubt it was any more than the STX RC's rated capacity of 39t. The total chain wrap of the new setup is 49t. That's a substantial difference, but if I only use the largest 3 or maybe 4 cogs with the granny ring then I should be fine, right?

Edit: Oops. Total chain wrap for the second setup would be 43t.

I come up with 43t total chain wrap (49-27=22) + (32-11=21) = 43, but that could be my math. :p

IMHO I think everything should shift pretty well; you shouldn't get any upper pulley rub on the 32t with the STX-RC
derailleur, and your gearing really isn't too extreme so the total chain wrap is still within the capacity of the derailleurs
to shift it.

I have a very similar setup (28/45/50 with an 11-32 8s cassette), and with an older XT mid cage derailleur similar to
yours it handles the range nicely. Your STX-RC has the long cage so you should be even better off!

Measure the chain length based on Large/Large, stay with your idea of using the largest four cogs with the granny and
it should be smooth sailing.

I'd like to know how it works for you; are you setting your drive train up to index? Mine's friction and I like it a lot.

:thumb:

Oh, wow. How embarrassing. I'm an accountant! Yes, your math is correct and that's only a 4t difference from rating which makes a lot more sense.

Yes, I'm setting it up to index with 2011 Veloce 10s Ergos.

With a triple being built for your own use, avoiding the extremes is probably a non issue. A bike shop assumes that a client might make a mistake.
If assembling a bike for someone else, then pilot error has to be considered.
Then there are duplications or near duplications.
Consider those and ultimate chain wrap capacity is probably more moderate than it is on paper.

I also don't like a chain that is so slack that any small bump in the road causes the chain to slap the chain stay.

How frequently do you cross-chain? Or more precisely, how confident are you that you will never ever cross-chain even by accident? Total wrap capacity should be the difference between little-little and big-big, neither of which you want to use intentionally.

I select chain lengths for two criteria: 1. So that the derailleur is stretched but not to a breaking point in the big-big combination; 2. So that the cage isn't wrapped all the way backwards in the little-little combination. The first of these is more likely to break something and more likely for me to hit accidentally, so I err on the side of a longer chain. If the RD can't handle those extremes to my satisfaction then I look for a different RD or different gear combinations. But since I like long-cage RDs anyway this is rarely a problem.

Yeah, I know, that didn't answer your questions.

Colonel, we used to do what you were originaly considering. We explained and showed it to our customers who needed extreme gearing. We'd shift into the small/small combo and show them the slack and then go gear by gear untill the slack was taken up and tell them the remaining gears were all they could use in the small ring.

In almost all cases if you need that low of a gear you probably aren't using the big chainring to it's potential and thus should go to a smaller big ring to help with chain wrap issues.

Jim, a 27/32 gear is super low. I can't imagine using it on a tandem. I fear we'd fall over going that slow. It's what the bike is equipped with at the moment, though, so I won't change it until I have a better feel for the gearing we'll need.

That is low...I can remember runnig 24/32 and maybe even lower on some touring rig's....crazy low.

I'm by no means a tandem expert, but have a fair amount of experience riding tandems. Don't be surprised if you actually use that 27/32 combination on some climbs. This will be dependent on how well matched you are with your stoker, but if you have it available, you'll likely use it and will be happy you did.

Colonel, we have a 28/32 combination on our tandem and we use it a lot. It's good for maybe 5.5 to 6.5 mph, not so slow that falling over is a problem. Sometimes I wish we had one click smaller but there are diminishing returns for lower gears.

Ah, good to know. I was puzzled to see a 49/14 high gear on this bike as other road bike tandems I've seen have some pretty impressive high gears. I believe this model is referred to as a Tourist versus the Marathon and Super Touring models they also offered.

I don't cross chain on purpose and don't have issues doing it accidentally. I also set my chain length using the big/big as a guide, but after having some indexing issues recently, I'll be selecting the chain length for bikes equipped with Shimano RDs according to the tech docs. See the chain length section at the bottom of this document.

FWIW, our Peugeot's original high gear was 52/14, and it still is. I know some couples really fly along on their tandems. We have seen the low 30s mph occasionally downhill but never on a road which appears flat enough to pedal instead of freewheel down. Though I have shifted into the 52/14 combination once or twice for the heck of it I have never felt a real need for it. On the right kind of road we will pedal in the low to mid 20's mph, but even that is usually with the 52/17 (or whatever the next cog is). I run "compact doubles" on my solo bikes too for the same reasons, a high of 48/14 which I almost never use and never actually need. (The Bianchi and the Masi may make me have to alter that statement a little.) I ain't racin' anyone, and around here if I/we ever get going so fast I/we spin out the 52/17 then I will likely to have to downshift again in less than 12 parsecs anyway.

That's what I tell myself, and it works well until I make one unthinking RD downshift shift when I'm grinding uphill on a road with heavy traffic. :eek: A few moments later I discover what gear I'm in. :o That's when I'm really glad I set up the chain to accommodate the big-big combination. Otherwise it might have broken something. I don't worry about cross-chaining the other way though because I almost never use the little cog anyway.

One final thought. About balance, I have been surprised at how easy it is to balance, maybe even more stable than a solo bike. Riding slow is not a problem.

Ergo shifters are great, but they give no tactile feedback to what gear you are in. Sometimes it's hard for the captain to visually check the gearing, and stokers cannot always be relied upon. Even on a single, I have accidentally cross-chained while riding at night at the end of a long day, which is also when you get tired and the mental capacity diminishes. Because of these, I much prefer bar-end shifters on tandems and long distance bikes. I would be conservative and make sure all gears are safe on a tandem.

All good points. I'll make sure large/large is accessible.

Good to know, Jim. I'm a complete tandem newb.

Whatever you do, make sure the chain is long enough to accomodate the big-big combination. You'll never use it on purpose, and you'd think that there's no risk of using it by accident, either. I used to think that until I learned otherwise in fairly dramatic fashion.

Very good point. The captain can't see the rear at all and can see the chainrings only poorly. The captain also can hear the chainrings poorly and can't hear the rear at all. If you can't judge from the levers then you have no confirmation of what gear you are in.

MetinUz is correct. I have managed to drop the chain a couple times on my Burley when I wasn't paying attention to what I was doing and because I had cross-chained to a degree that it wouldn't cleanly drop onto the smallest ring going up a hill. I've not used bar-ends on a Tandem, but have used DT shifters on my Davidson. My newest tandem acquisition has bar-ends, so I'll see how I like them on that bike.

+1 Dropping the chain on a single is one thing, but on a tandem the potential of getting hurt goes up significantly.

All of my road bikes have top gears in the 93.6 to 98 range: 45/13, 49/14, 50/14, and 47/13.

I wrenched for 12 years but it only took me a few to realize 'chainwrap' was nice for conversation but in the real world it had limitations. I make all my chains as long as possible as it decreases drivetrain wear and helps shifting...

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The point about having some feedback as to what gear you're in is a good one and something I'll consider. I'm not the first one to install Ergos on a tandem, though. I think I'll go with them initially and if I don't find them to my liking I'll install indexing bar end shifters.