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Perplexing fringe gearing mod

Old 10-09-14, 06:20 PM
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Perplexing fringe gearing mod

I have recently come back to cycling after a 4+ year bout with back problems. During that time I had little if any exercise. So, I am basically starting over riding.

My Litespeed Classic I bought in ’97 had the following set-up when my back issues pulled me off the bike:

Dura-Ace 7700 RD (short cage) and 7700 FD
Shimano Ultegra 6500 Crank 39-53
Shimano Ultegra 6500 Cassette on Ultegra 6500 hub wheels

The original 12-23 cassette has proven to be a little optimistic in the lower ranges to get back in shape. So, after researching and finding the original RD installation instructions (in my spares closet) I decided to try a 12-27 cassette. Saw where others have used this same set-up, even though it is out of the recommended chain wrap range. So, I ordered and installed a new stock CS-6500 12-27 cassette on the original wheels.

Adjusted the “B” screw (now maxed out) to allow for the larger cog and checked the chain length, per the Shimano installation sheet for the RD. All looked pretty close and on the work stand, shifted smoothly and sounded quiet with no load on the old chain. Did a quick test ride around the hood and all was good again, quiet and smooth.

Felt it was a good idea since I had one in stock to install a new chain, CN-HG93. Used the same length as was on the bike. Since installing the new chain when on the 27 cog I hear a slight rumbling sound when on the road. It is very feint and as soon as I shift to the 24 and smaller all is smooth, quiet and buttery like has always been the case on this bike. It also sounds quiet and smooth on the work stand under no load.

Any ideas? I'm not seeing any excessive wear and the sound is not that intrusive, but would like to dial it in further if I've missed something. I’ve read I may need a longer “B” screw or should I try adding another chain link even though I never cross chain and that same chain length was working fine with a 12-23?

TIA,
Mark
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Old 10-09-14, 07:01 PM
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Since you kept the same chain length as when you had a 23T cog, before you ride the bike any farther you should determine that your chain is long enough to shift to your big ring/big cog combination without straining anything; even if you never intend to use that combination you may forget and shift into it anyway.

Since your B-screw is maxed out you may need a longer screw or to turn the existing one around so as to push the derailleur farther; I suspect that this may eliminate the noise.
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Old 10-09-14, 07:24 PM
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I suspect that dsbrantjr is correct in thinking that you're suffering the classic and common pulley knock. When the upper/guide pulley has a slight contact with the underside of the large rear cog. Shimano is particularly sensitive to this. One can conjecture why but my opinion is that Shimano doesn't like using the extreme of their range with many of their components.

Careful chain length experiments, reversing the "b" screw to add length, going to ma 25T/26T large cog, going to a long cage der (oh! the horrors) are all other options. Andy.
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Old 10-09-14, 08:20 PM
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Originally Posted by Mark9

Any ideas? I'm not seeing any excessive wear and the sound is not that intrusive, but would like to dial it in further if I've missed something. I’ve read I may need a longer “B” screw or should I try adding another chain link even though I never cross chain and that same chain length was working fine with a 12-23?

TIA,
Mark
If your chain length was correct with the 12-23, you should have added 4 links (2 inner, 2 outer) to make the length right for the 12-27.

I'm familiar with the Shimano instructions that indicate the pulleys should be vertical when on the large front chain ring and the small cog, but obviously that will not account for a large cog that is outside the normal range and would in fact result in the same length chain for both cassettes. Obviously that won't work well.

your capacity with your current setup is:
27 - 12 = 15
53 - 39 = 14
= 29

The total capacity for the rear is 26 and the max cog size is 26.
You're cheating a bit and can probably get it to work, but not with a chain that is too short.

Here's your guidelines: https://www.shimano.com/media/techdoc...9830610603.pdf

If you don't get enough wrap, then only add 2 links. You will need to experiment.

Last edited by andr0id; 10-09-14 at 08:28 PM.
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Old 10-10-14, 06:54 AM
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Doesn't matter now but since you were replacing the chain you should have adjusted the length to match the new setup so all of you other adjustments could be in the center of the range.
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Old 10-10-14, 09:56 AM
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I assume you are running 9 speed as that is a 9 speed chain you refer to?
I also assume you are talking about using the small chainring when you are on the larger cogs?
If your chain is short that combination shouldn't affect the issue. You should carefully shift to the large - large combination and make sure that you can lift the derailleur and create slack in the chain. If that's OK then your chain length at least won't cause any damage and is useable.
It should be easy to verify if you have pulley to cog clearance. If not, you can get a longer B-screw or reverse the B-screw.
You should also check your limit screw adjustment - that could at least affect the 27 cog.

Does adjusting the rear derailleur cable tension change the noise at all?
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Old 10-10-14, 09:59 AM
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Originally Posted by spdracr39
Doesn't matter now but since you were replacing the chain you should have adjusted the length to match the new setup so all of you other adjustments could be in the center of the range.
Since the cassette's large cog is outside of the published derailleur limits, it stands to reason that some of the adjustments (B-screw for example) could be expected to "hit the stop". Running with a possibly too-short chain will also cause things to be out of adjustment range, not to mention the possibility of derailleur or frame damage.
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Old 10-10-14, 10:34 AM
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Thanks for the input guys. Here's where I am diagnosing the "pulley knock", didn't know the term to describe it last night.

After installing the cassette and before and after the new chain I checked/adjusted the cable tension, B screw and both limit screws. Also, checked to see if I could cross chain big to big (which as I said I never do)...it did no problem. However, the chain might be slightly short for that combination (per your comment Slash5).

Found a metric allen head screw in my RC sailplanes spares stock that was longer than the original B screw. Installed that this morning, did a couple of spins up the hill here in the neighborhood and it does seem to be a bit quieter. I might try adding a link or two to the chain, but I need to pick up a few connecting pins.

dsbrantjr your comment of derailleur or frame damage has me concerned. What should I be looking out for or should I just pack it and ride my mountain bike until I can use the 12-23 again?

What has me baffled is that I'm sure I didn't hear the knock before putting on the new chain. Can't tell you how many miles were on the old one, but it didn't check to being over stretched. I have always been pretty good about replacing my chain on a regular basis.
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Old 10-10-14, 01:37 PM
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Do you have a picture of your setup? Disraeli Gears lists a 27T maximum cog for the short-cage 7700 RD, so there's no reason you should need to max out your B-screw, much less swap it for another part.

https://www.disraeligears.co.uk/Site/...700_SS%29.html
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Old 10-10-14, 03:38 PM
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Don't have any pictures, just what would you want to see ThermionicScott? Could probably get some later.

Added a link to the chain and it's even better, quieter. When I had the chain broke I inserted one link and then two using the connector pins to hold it. Two links would have been too much...the cage would have rubbed on the chain when cross chained small to small.
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Old 10-10-14, 10:16 PM
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Originally Posted by Mark9
Don't have any pictures, just what would you want to see ThermionicScott? Could probably get some later.
That's a fair question -- I was mostly picturing the RD straining to handle the big-big combination while cocked back as far as possible with the B-screw.

Added a link to the chain and it's even better, quieter. When I had the chain broke I inserted one link and then two using the connector pins to hold it. Two links would have been too much...the cage would have rubbed on the chain when cross chained small to small.
Sounds like you've got it most of the way solved now! FWIW, I don't worry too much about slack when in the small-small combo since I never use it and I like to install as much of the chain as I can, seeing that I paid for all of the links.
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Old 10-12-14, 03:27 PM
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ThermionicScott,

I have a few pictures, but get an error each time I try to upload them. They may be too large...looked around the forum and can't find a max file size or what else I may be doing wrong.

If the lighting good tomorrow may see if I can shoot them again at a lower resolution.

BTW, compared the length of the B screw that I trimmed to and it's about one full thread. That would make the adjustment about .7 mm more than the stock length, if my estimates are correct.

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Old 10-15-14, 03:33 PM
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Originally Posted by ThermionicScott
That's a fair question -- I was mostly picturing the RD straining to handle the big-big combination while cocked back as far as possible with the B-screw.
Finally got a chance to reshoot some pics. Had my camera set for high res, so the file size was too big as I suspected. Sorry about the shadows. Did a full test ride today...everything seems OK, but since I have OCD it's not like running the 12-23 cog set. Guess that's a price for getting old.
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Old 10-15-14, 06:12 PM
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Looks pretty good to me, but I can see how close that jockey pulley is to the big cog in the 2nd pic. Any remaining knock might still annoy you if you listen for it, but shouldn't be a problem. Great-looking bike.
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Old 10-15-14, 06:51 PM
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Yep, just not quite as smooth as the 23. As you said I can hear a slight knock, when on a quiet country road. Can't feel it at all and any car traffic masks it entirely. But, I'm liking the two extra lower gears.

Thanks for the comment on the bike...never have had buyers remorse. Even though many people at work back in '97 thought I was nuts to spend what I did. It's such a sweet ride.

Those are the original wheels with who knows how many miles. I may put the same set up on my Mavic Ksyrium SL's now.

BTW, strange why Shimano had a 26 cog limit on the DA 9 RD. In my searches never saw reference to a 26...25's and 27's in DA cogs, but no 26?
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Old 10-15-14, 07:34 PM
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Originally Posted by Mark9
Yep, just not quite as smooth as the 23. As you said I can hear a slight knock, when on a quiet country road. Can't feel it at all and any car traffic masks it entirely. But, I'm liking the two extra lower gears.

Thanks for the comment on the bike...never have had buyers remorse. Even though many people at work back in '97 thought I was nuts to spend what I did. It's such a sweet ride.

Those are the original wheels with who knows how many miles. I may put the same set up on my Mavic Ksyrium SL's now.

BTW, strange why Shimano had a 26 cog limit on the DA 9 RD. In my searches never saw reference to a 26...25's and 27's in DA cogs, but no 26?
I just now caught the link that andr0id had in post #4 , and Shimano's official 26T spec trumps the Disraeli Gears one. Since Dura-Ace is Shimano's "race" group, I imagine they wanted to optimize for the tighter cassettes and higher gearing racers would be using (probably rare for them to use >23T during that period unless they were climbing something especially steep). Now the knocking with a 27T isn't sounding so far-fetched. Good thing it works, though!
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Old 10-16-14, 08:17 AM
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I actually have the original hard copy of those instructions when I installed the RD on my bike. OOH, did I mention I have OCD?

Before I ordered the 12-27 cassette I did quite a bit of research to see if it might work. Now we all know there are many self appointed experts that frequent forums, but below is a link to a thread that lead me to give it a try.

12-27 cassette and Dura Ace rear derailleur?
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Old 10-16-14, 10:31 AM
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I dont see fringe, you just put on a bigger low gear cassette., but for the self-described Obsessive Compulsive type, it may to You.


My 3 x 3 speed drive train build in 1960 would be more out there ..
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Old 10-16-14, 06:15 PM
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Originally Posted by fietsbob
I dont see fringe, you just put on a bigger low gear cassette., but for the self-described Obsessive Compulsive type, it may to You.


My 3 x 3 speed drive train build in 1960 would be more out there ..
By "fringe" I was referring to being outside the recommended set up parameters.

3 X 3? Do you care to expound?
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