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Old 05-28-03, 07:28 AM   #1
ChipRGW
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A Rear Derailleur/Chain question

I got this photo of a bike I am interested in.

Is this derailleur position normal when on the big ring? Or is there a problem? Chain too short maybe?

Any ideas?
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Old 05-28-03, 07:53 AM   #2
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When the chain is in the small cog/large chainring position, the der pulleys should align perpendicular to the ground. Having said that, you should be able to run the chain in the large ring/large cog combo. I don't think you have enough chain in this pic.
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Old 05-28-03, 08:42 AM   #3
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Could it be anything else?
Or just short chain?
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Old 05-28-03, 03:50 PM   #4
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The chain is obviously too short here........looks almost impossible to get into large cog/large ring combo (not that you would ever want to use that combo). Aside from that, the derailleur MAY be mounted without any setback. There should be a small set screw near the derailleur mounting bold.
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Old 05-28-03, 04:35 PM   #5
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Chain is to short. Either the person installing it was a bonehead, or they put a new cassette on or something and didn't account for the change needed in chain length. Either way you look at it the chain is to short and somebody is a bonehead...Not that I've never made that mistake myself.
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Old 05-28-03, 05:00 PM   #6
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I disagree with all of the other posters.

It looks to me like the chain length is fine. You have the chain on the big ring in front and a middle cog in back and the derailleur arm is approximately vertical. It looks to me like you will have enough chain length to safely cover the big/big gear combination.

My bet is a "B" screw adjustment is in order. That will pull the derailleur body down so that it is parallel with the chain stay.
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Old 05-28-03, 07:44 PM   #7
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Quote:
Originally posted by Spoke Wrench

My bet is a "B" screw adjustment is in order. That will pull the derailleur body down so that it is parallel with the chain stay.
Could be so,

Put the bike on a stand or something to see if you can make it into big ring/big cog combo..Pay attention to how much noise is being made, and how tight the chain is, you don't want to mess anything up. If it goes in to the combo with no problem (assuming the derailleur is adjusted) then the chain is okey dokey, check the "b" screw as Spoke Wrench suggested.
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Old 05-29-03, 01:16 AM   #8
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It almost looks to me like either the B-pivot spring is broken/disengaged, or the screw has missed the intended resting place on the RD hanger entirely. Did you rotate the derailleur back while bolting it down, so its screw gets behind the stop as intended? Here's a rear view showing how the B-tension screw should rest on the dropout:



If the derailleur's mounted properly, then the spring in the upper pivot should rotate the whole derailleur backwards if you introduce a lot of chain slack by taking the chain off the front chainrings and resting it on the bottom-bracket shell. If it isn't doing that, something's amiss, possibly a broken spring (unusual but possible).
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Old 05-29-03, 01:57 AM   #9
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I don't know that the "B" screw adjustment and its associated spring can do much when the chain is physically a little on the short side. As long as you haven't completely straightened out the deraileur wheels in line with the chain, leaving absolutely no slack on the biggest cog you will ever use, then I wouldn't bother too much about it. You should only be going further than the 3rd biggest cog with big chainring in a racing emergency, after all.

I think shorter is better than longer for crisp shifts and better and more reliable tension over rough roads...especially when going fast in smaller cogs. I think most people (myself included) tend to err by making the chain too long. Check out some photos of some pro bikes...they are 'similar' to your setup.
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Old 05-29-03, 03:24 AM   #10
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Quote:
Originally posted by ChipRGW
Is this derailleur position normal when on the big ring?
Another vote for the B-screw adjustment. Is the guide pulley contacting the cassette when on the larger cogs? It's easy to adjust if nothing's broken, but the guide pulley may be damaged or worn and need to be replaced.

Read more about it in Barnett's, chap. 32.
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Old 05-29-03, 05:04 AM   #11
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Chain length should = Wrap around big cog and big ring + 1 link without going through derailure. different gearing will put the derailure at different angles when looking at it the way you all are.
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Old 05-29-03, 09:59 AM   #12
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Thanks everyone for the input thus far.
This is NOT actually my bike, but one I was considering buying.
It looks like a pretty good deal, for a well-equipped bike. I saw the der. and thought it looked odd. I just wanted to make sure there wasn't a potential for a serious problem, If I did actually buy it.
I take it, then that the worst case scenario, is replacing the derailleur?
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