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Chain length Question. Chain rubbing on the rear derailleur cage.

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Chain length Question. Chain rubbing on the rear derailleur cage.

Old 01-28-12, 12:35 PM
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Chain length Question. Chain rubbing on the rear derailleur cage.

I was doing a quick tune up on my friends bike and I noticed that on the 39x11 the chain is slack that it was rubbing on the cage of the Red rear derailleur. The derailleur in the picture isn't the one with the problem, but I don't have the bike in front of me to photograph it right now


photo by YCantelar, on Flickr

Basically, the cage is parallel to the ground and the and the chain is rubbing on the piece circled, in the direction shown by the arrow.

I'm pretty sure the chain is a few links too long, but he took it to the shop that installed it and they said "You're not supposed to use that gear, and we do that because when you store the bike at night, you need to have it in the 39x11 to not wear out the springs, and by leaving the chain a bit longer, it has less pressure on the springs in the cage."
I do try to store bikes in the 39x11 in order to keep unneeded tension off the cables but leaving the chain longer just seems crazy to me.

While it's always good to avoid cross chaining, it does occur at times and I'm pretty sure the chain shouldn't be rubbing through the carbon cage on the derailleur when it happens.

I told my friend that we'll ask the collective wisdom of bikeforums to see whether the mechanic is right, or if I should shorten his chain for him. Thanks.
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Old 01-28-12, 12:52 PM
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OK, possibly the chain is too long, but equally possibly is that the RD has insufficient capacity for the gear range. You need to check the chain length on the big/big combination before cutting.

Shift to the big/big combination, and pull the lower loop forward and see how far you can go before the RD cage reaches it's trabel limit, ot the lower loop is straight. A straight lower loop is 1" too short, and likewise you don't want to go to the absolute limit of the RD travel, so add back an inch, and whatever is left may be safely be removed.

If the chain is already at the shortest safe length, then it's an RD capacity issue, but shouldn't be a crisis, since the small/small combination is rarely. if ever, used. In many cases the small/small combination causes the chain to brush the outer chainring on it's way to the inner and so is unusable.

Chain length is a judgement issue with no one absolutely correct answer, except that the chain must be able to loop the big/big combination with room to spare. It isn't that one would intentionally ride big/big, though it's OK, though less efficient. The real reason is that one might accidentally shift into big/big and if the chain is too short that will destroy the rear axle, cassette, RD or chainrings, or more than one of these.
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Old 01-28-12, 01:47 PM
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The "advice" to always leave the bike in 39x11 to avoid wearing out the springs is nonsense. Springs don't "lose tension" from being stationary in one position unless they are used past their design limits and nothing you can do with a derailleur will do that. People are still using Simplex and Huret derailleurs from the '50's and 60's and their springs still work and weren't made nearly as well as current ones. Leave the bike in the gear you want to start out in on the next ride.

As FBinNY noted, be sure the bike will shift into big-big and shorten the chain only if it will do that with noticable excess.
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Old 01-28-12, 02:30 PM
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Originally Posted by HillRider
The "advice" to always leave the bike in 39x11 to avoid wearing out the springs is nonsense.
.
I missed that nonsense about spring wear, which as Hillrider pointed out, is 100% USDA Prime, certified pure BS. Springs stay in various stages of compression all the time, and nothing changes. If it did, you'd have to recalibrate the bathroom scale daily.

However, I'm a long chain fan, and if the RD capacity allows, cut chains to the longest length that the RD can take up in the small/small combination (which on my bike is rideable), without the chain rubbing anything it shouldn't. On that bike it's probably only 1" too long form my taste.

I've been in the bike industry since 1967 and one of my biggest frustrations is mechanic who do things wrong either intentionally or through inattention to detail, then try to justify it with some BS explanation of why wrong is better than right. While a bit of rubbing might be acceptable as a compromise with an alloy cage, it's totally unacceptable with a carbon cage, and to me is an indicator of poor work ethic on the part of the mechanic who measured it (unless the RD is at or above it's capacity).
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Old 01-29-12, 10:12 PM
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You may want to try screwing the derailleur tension screw (or what ever the top one is called) in a bit
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