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Chain length and rear derailleur tension arm angle

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Chain length and rear derailleur tension arm angle

Old 06-09-16, 08:42 AM
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Chain length and rear derailleur tension arm angle

Here is something I have never quite understood. Pardon if there is still sense making going on while I formulate the question.

In my experience, a bike can operate well enough with a range of change lengths. A wider gear range needs more chain and vice versa, so rear derailleurs have a return chain takeup arm that tensions the chain. Too much chain, and the derailleur will not have enough capacity to take up the chain. Too little chain, and the bike won't shift. I usually start out by wrapping the chain around the big ring and big cog and adding a link, but this can be inexact.

When adjusting the chain length from this point, my bias has always been to use as little chain as possible within reason. I don't count on myself (or others) to avoid using the big-big gear combination, so I usually set up the bike with just enough chain that the bike can shift into and out of this combination. This can leave the tension arm at an extreme angle--even maxed out or almost maxed out--in this combination.

Is this unhealthful for the derailleur or other drivetrain components? Should I be biased toward having as much slack as possible? What is your approach to this or rule of thumb?
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Old 06-09-16, 08:51 AM
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If you're conscientious about what you're doing, you won't be cross-chaining big-big or small-small often. As long as the chain's long enough to cover big-big without straining, I wouldn't worry.

Personally, when I used closer ranged cassettes (11-23, 11-25) I cut my chains a little looser. Now that gives me some slack to go climbing with on an 11-28.
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Old 06-09-16, 11:43 AM
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The bike must go into big-big but it's not used as a routine so maxing out the rear derailleur tension in that gear isn't a problem. You can use a longer chain to avoid it but you run the risk of having the chain go completely slack in small-small which, isn't a hazard.
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Old 06-09-16, 11:51 AM
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Chain must be long enough to go into big-big without ripping the derailleur off, and short enough to go into small-small with the derailleur still providing tension and the chain not rubbing on itself. If you meet those criteria then you have nothing to worry about. Edit: I said exactly what HillRider said... but I'm prettier.
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Old 06-09-16, 12:04 PM
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Originally Posted by Roll-Monroe-Co
Here is something I have never quite understood. Pardon if there is still sense making going on while I formulate the question.

In my experience, a bike can operate well enough with a range of change lengths. A wider gear range needs more chain and vice versa, so rear derailleurs have a return chain takeup arm that tensions the chain. Too much chain, and the derailleur will not have enough capacity to take up the chain. Too little chain, and the bike won't shift. I usually start out by wrapping the chain around the big ring and big cog and adding a link, but this can be inexact.

When adjusting the chain length from this point, my bias has always been to use as little chain as possible within reason. I don't count on myself (or others) to avoid using the big-big gear combination, so I usually set up the bike with just enough chain that the bike can shift into and out of this combination. This can leave the tension arm at an extreme angle--even maxed out or almost maxed out--in this combination.

Is this unhealthful for the derailleur or other drivetrain components? Should I be biased toward having as much slack as possible? What is your approach to this or rule of thumb?
I recommend you ride more.
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Old 06-09-16, 12:24 PM
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Originally Posted by Wilfred Laurier
Chain must be long enough to go into big-big without ripping the derailleur off, and short enough to go into small-small with the derailleur still providing tension and the chain not rubbing on itself. If you meet those criteria then you have nothing to worry about. Edit: I said exactly what HillRider said... but I'm prettier.
To go along with this, If you size the chain so it meets both of these criteria, and then add as many links as you can so that you can use small+small without rubbing, then the RD angle on big+big is less extreme. If you are within the limits of the take up capacity of the RD this should be possible.
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Old 06-09-16, 12:31 PM
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Originally Posted by redlude97
To go along with this, If you size the chain so it meets both of these criteria, and then add as many links as you can so that you can use small+small without rubbing, then the RD angle on big+big is less extreme. If you are within the limits of the take up capacity of the RD this should be possible.
This is true... however the angle on the bottom run of chain is not really that important as there is no tension on the bottom (not compared to the top, anyways) and so that is not where wear happens. Another advantage of a longer chain is that if the chain breaks and you have to remove a damaged link, your chain will probably still be long enough that you can still safely shift in into the big-big (like when your brain is fried at the end of a long ride, for instance).
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Old 06-09-16, 12:39 PM
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Originally Posted by Wilfred Laurier
This is true... however the angle on the bottom run of chain is not really that important as there is no tension on the bottom (not compared to the top, anyways) and so that is not where wear happens. Another advantage of a longer chain is that if the chain breaks and you have to remove a damaged link, your chain will probably still be long enough that you can still safely shift in into the big-big (like when your brain is fried at the end of a long ride, for instance).
I hadn't thought of this.
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Old 06-09-16, 12:40 PM
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Thanks, everybody!
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Old 06-09-16, 12:45 PM
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Originally Posted by Wilfred Laurier
This is true... however the angle on the bottom run of chain is not really that important as there is no tension on the bottom (not compared to the top, anyways) and so that is not where wear happens. Another advantage of a longer chain is that if the chain breaks and you have to remove a damaged link, your chain will probably still be long enough that you can still safely shift in into the big-big (like when your brain is fried at the end of a long ride, for instance).
It makes a big difference in the noise generated and the roughness IME in the big+big when you are at the very limit of RD capacity as the upper pulley is pulled close to the cassette compared to with some chain slack
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Old 06-09-16, 01:24 PM
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It very well could be putting undue stress on the rd springs and derailer hanger to max it out often, but I've never seen a spring so worn out it just wouldn't pull enough tension on the chain, and it's pretty tough to tell exactly what puts an alluminum rd hanger out of whack. Even cheap jockey wheels last a long time, and I'd guess that the force that goes into the chain from the cranks does more to wear a chain than any derailer could do. So I'm not really concerned about this issue as far as drivetrain lifespan goes.
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Old 06-09-16, 01:42 PM
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Originally Posted by HillRider
The bike must go into big-big but it's not used as a routine so maxing out the rear derailleur tension in that gear isn't a problem. You can use a longer chain to avoid it but you run the risk of having the chain go completely slack in small-small which, isn't a hazard.
On 10- and 11-speed compact setups, the chain often rubs on the inside of the big ring in small-small, so that's the one I never use. I've also found that flipping around with the front derailleur with the chain on that end of the cassette is more likely to result in a dropped chain.
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