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Old 09-02-06, 12:39 AM   #1
jpdub
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is this for real?

I just brought my Klein into the local bike shop for a tune up and I was told the rear cassette was in pretty bad shape, so they suggested replacement of it and the chain. After riding it, I found with the big gear in front and back the chain would jam. I took it back and suggested maybe the chain was too short, they said they would check and let me know. This is the response from the bike shop, "you have a triple chain ring setup and a double rear derailer and they are not compatible... ....you will need to go with a longer cage"

Does that make any sense? The bike worked fine before I brought it in. I don't have a problem with the front derailer shifting between the 3 rings, just the rear, when I go big in front and back. What would the rear derailer have to do with it. All it does is guide it to the 8-9 rings in the rear. Even if I had a double chain ring setup, how would that help the jamming when I go big-big? I assume a double setup would just eliminate the small ring in front......

I plan on calling them tomorrow to ask for an explanation. Thanks for any input you can provide.
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Old 09-02-06, 04:11 AM   #2
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I think what they're talking about is this: Assume you are not shifting the rear gears, only the front. With a triple instead of a double, you have a lot more change in chain slack that the rear derailleur has to take up as you shift from the big chainring to the small chainring. In order to do this, the rear derailleur has to swing back to take up the slack. To take up enough slack, the cage (swing arm) has to be longer than what might be required if you only had a double chainring up front. Make sense?
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Old 09-02-06, 04:30 AM   #3
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Yes, but I don't feel that's the problem here. Because if it worked properly before the LBS "fixed" it and it's not working properly now, then the only explination is that the shop "fixed" it wrong. Sounds like the new chain is too short.
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Old 09-02-06, 04:43 AM   #4
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Quote:
Originally Posted by jpdub
I. This is the response from the bike shop, "you have a triple chain ring setup and a double rear derailer and they are not compatible... ....you will need to go with a longer cage"

Does that make any sense? The bike worked fine before I brought it in. I don't have a problem with the front derailer shifting between the 3 rings, just the rear, when I go big in front and back. What would the rear derailer have to do with it. All it does is guide it to the 8-9 rings in the rear. Even if I had a double chain ring setup, how would that help the jamming when I go big-big? I assume a double setup would just eliminate the small ring in front......

I plan on calling them tomorrow to ask for an explanation. Thanks for any input you can provide.
As the bike was working fine before the changes- I have my doubts. The explanation they have given does not sound right. I will agree that if you are working with a large range of rear sprockets, and you have a short cage derailler- then problems do exist. BUT it was working fine before-unless the shop put on a larger rear cassette that takes it out of the range of the derailler.

Easy way to see if the chain is too short- Large ring on the front and Large ring on the back. +1 or 2 links to allow it to reach that combination. If the chain does not link up over that combination- Then it is too short.
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Old 09-02-06, 05:34 AM   #5
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Look at the info on sheldon Brown's site, I believe a short cage DR only determines the max size rear cog supported. Usually 27T. If your largest rear cog is larger than 27T you would need a long cage DR. However, I think they are giving you a bunch of BS. I believe that somewhere Sheldon states there is no such thing as a double or triple RD.
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Old 09-02-06, 05:46 AM   #6
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also could be a case of cross chain-- you really don't want to ride the large chain ring with the large cog on a triple (or on a double, for that matter)

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Old 09-02-06, 05:59 AM   #7
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There may not be such a thing as a "double or triple rear derailer", but the cage still has to take up the slack when you shift from the large ring to the smallest. Derailer specs normally list the max cog size, followed by the max number of teeth. You subtract the smallest cog from the largest, and do the same for the chainwheels, then add those two numbers together to determine how many teeth you need to take up the slack chain if you ran the small/small combo. Some people will accept some slack in order to have a granny gear that is only used on rare occasions. So on my bike with a 48-38-28 and a 14-32 freewheel, I need (48-28)=20 plus (32-14)=18 or 38 teeth. My road bike with 52-42 and 14-28 freewheel only needs 24 teeth.

All that being said, I agree with you that they are giving you a line of bull. If the bike shifted okay before and not now, they messed something up. It sounds to me like the chain is too short, maybe just a couple links.
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Old 09-02-06, 06:24 AM   #8
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Cross chaining is the real culprit..... Big to big is a no-no. A brand new chain may not do this as well as an old worn out chain would on an old cogset. The Rear Der. story is BS unless the shop put a larger cog set on the back as stated above.... 27 is max.
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Old 09-02-06, 07:49 AM   #9
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Did I hear the term "Cross Chaining"....slowly I turned, half-step by half-step.........

Blast, Pompiere beat me to the lecture on derailleur capacity. If you'll supply us with your big ring/little ring sizes and the range of your cog set,and the model/model number of your rd, it will be no problemo to check out the validity of the shop saying you need a longer cage derailleur. At first glance, it sounds like el crappo to me. Shorter cages shift a tad quicker and are a little less "out there" for damage, plus you already own it so why buy new if you have felt no previous need.

Most of us avoid cross chaining most of the time, but if you prefer to slam through the gears without setting limits for yourself, and some riders do (we don't discriminate against them LOL)-- a longer cage won't hurt.

So, what are those rings and cog sizes?
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Old 09-02-06, 08:11 AM   #10
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Quote:
Originally Posted by stapfam
Easy way to see if the chain is too short- Large ring on the front and Large ring on the back. +1 or 2 links to allow it to reach that combination. If the chain does not link up over that combination- Then it is too short.
+1 Correct for determining the correct length.

However, a worn chain might have had enough slack to allow for shifting.


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Old 09-02-06, 08:39 AM   #11
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As has been stated previously, if you're riding in the big chainring and the big cog on you're cassette, you're putting alot of stress on the components. This will cause the chain, chainring and cassette to wear out prematurely. You can get almost the same gear inch by using your middle chainring and a different cog in the back. As for the rear derailleur, yes there is a long cage deraillleur and it is made for a triple setup. It is what I use on all of my bikes. Sorry, everyone. but out here in Seattle with all of ours hills a triple, just makes climbing them easier. I agree with Grampy, a worn chain and rear cassette may have allow for the combination of big and big. And with you new chain it isn't. If, you go on ebay, you can get a new triple rear derailleur, fairly inexpensively.
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Old 09-02-06, 10:02 AM   #12
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Thanks to all for your replies. THe bike is still in the shop so I don't know the cog sizes... WHat puzzles me about all the talk of a larger cage is that suppose the chain is lengthened, now I wouldn't have an issue, only when I went small-small, then there would be too much slack with a small cage. right? I never use the small ring in front anyway. It's about 9am now so I'll call the shop and see what they have to say.
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Old 09-02-06, 11:12 AM   #13
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ok, just called and they lengthened the chain so big-big should not jam anymore. The concern is IF I go small-small there will be too much slack since the rear derailer has a smaller cage. And, since I never really use the smallest gear in front it's probably fine. We don't have anything really steep in Sac..........
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Old 09-02-06, 12:16 PM   #14
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Chain length is fairly critical and I ran into a problem last year when I was one link short on the tandem. We never crosschain so it would not be a problem of big/big. EXCEPT- the new chain showed I had a problem on the middle ring and it slipped like crazy on any gear in that middle ring. Problem was that we did not find out until the first hill about 5 miles into the ride. Had to ride in Big ring or granny. Soon learnt that you don't even attempt the big ring if you have this problem. As I have said- we never crosschain- but we were on that ride. Nothing for it but to keep finding hills to stay in the granny for the shakedown ride after a rebuild.
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Old 09-02-06, 12:44 PM   #15
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If you are running a triple front then you need a long cage rear derailleur to pickup the extra chain and still have the correct chain tension throughout the gear ranges from large ring to small. Is your rear derailleur a short cage or long. I do not doubt that with adjusting the chain length and proper derailleur setup that it could work but the long cage was designed for this situation.
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Old 09-02-06, 01:57 PM   #16
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I'm no bike mechanic, but I tend to agree with kk4df. The same problem was intermittently happening to my LWB bent, which has lots of slop in the long chain, (uses 3 regular chains to replace the LWB cahin) and my LBS advised me to replace my rear deraileur with a longer one to better handle the slack- I can't recall the model (a Shimano high end rear derailleur) to take up the slack better.
1900 miles later it had never failed to shift absolutely perfectly!
Whether the same thing can apply to a DF bike rather than a LWB bent I haven't a clue. But it sure worked great for mine, anyway, for what it's worth.........
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Old 09-02-06, 04:36 PM   #17
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My suspicion, based on the fact that the old setup worked and the new didn't is that the shop replaced the existing cassette with a wider-range cassette, or at least a cassette with a larger "big" cog. If that is the case, then the shop (incorrectly) put in an identical-length chain back on the bike, and the big-big ratio caused jams.

Upon return, the shop installed a longer chain to accommodate the big-big cog combination. Once the longer chain was installed, then the rear derailleur had insufficient take-up capability to keep the chain from sagging on the small-small cog combination. The shop then pontificated that the rear derailleur was meant for doubles - not triples, and asked the OP to consider a new derailleur.

If this is indeed the situation, AND the OP is willing to stay off the small-small combination, then the problem is academic.

However, to restore "original condition" status, the shop should be asked to replace their "new" cassette with one identical in range and maximum cog size to the one removed from the bike, and restore the chain to the original length. That would allow the rear derailleur to adequately wrap all chain in all combinations.

All this is conjecture on my part, and I could be wrong. The scenario does, however, account for the situation to date.
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Old 09-03-06, 06:19 PM   #18
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Quote:
Originally Posted by jpdub
ok, just called and they lengthened the chain so big-big should not jam anymore. The concern is IF I go small-small there will be too much slack since the rear derailer has a smaller cage. And, since I never really use the smallest gear in front it's probably fine. We don't have anything really steep in Sac..........
Most derailleurs will have a specified number of teeth of "chain wrap capacity" or "take up" in slack. For example, if you have a chain of correct length for a 50/28 combination, and want to run small-small of perhaps 28/12, you will need sufficient take up for (50+28) - (28+12) = 38. Shimano Deore and XT/XTR derailleurs have capacities of up to 45 teeth. On the other hand a Dura-Ace only has about 29 or so. Cage and pulley lengths determine not only the maximum tooth for the rear cassette, but also the amount of slack that can be taken up.
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