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D/A Rear Der & 12-27 Cassette(22 posts)

D/A Rear Der & 12-27 Cassettejagiger
Apr 1, 2002 10:53 AM
I just got a D/A rear Der which I thought was compatible with the Shimano 12-27 cassette, according to the Shimano website. Reading the RD-7700 (ss) directions, the spec say it works with 26T sprocket. Is this doc correct & will this work? Also, any tips regarding set up & chain length would be appreciated. Thanks in advance.
re: D/A Rear Der & 12-27 CassetteChen2
Apr 1, 2002 11:22 AM
I've been running 12-27's on my Dura-Ace for the last 4 years with no problems, and I'm sure the rear derailleur will go more than 27. It may require adjusting the "B" adjustment screw and possibly a few links of chain. Any of the maintenance books or web sites can explain chain length better than I can. Try www.sheldonbrown.com or the Park Tool website.
~Al
re: D/A Rear Der & 12-27 CassetteKEN2
Apr 1, 2002 11:22 AM
There's always a little caution in these guidelines... I have a DA rear derailleur, Uletgra 12/27 cassette with Ultegra 53/39 chainrings and have had no probs.

Standard way to size the chain is to wrap it around the large chainring and the largest cog (not through the rear derailleur), overlap it by 2 rivets (may be 3--remember that there are inner and outer plates that have to mate), remove excess chain then thread normally through derailleurs and install.
Source?grzy
Apr 1, 2002 11:23 AM
At www.shimano.com both the SS and GS versions of the DA rear der. list the max. tooth as 27.

In any event it will work fine - 1 tooth is not going to suddenly cause a problem. If you're running a triple or some strange gearing then you would need the larger capacity. A 39/53 with a 12-27 will present no problems.

Set the bike up cross geared with big/big combination - although you don't want to run this way if you don't allow for the possibilty you can jamb the drivetrain and bend the rear der. (been there). Wrap the chain around this combo, skipping the der., and allow one extra set of links. As a cross check now set it up on small/small through the der cage and see if it will take up most of the slack - the chain will probably show some sag. Again this isn't a gearing combo that you should be running.
Extra Chain Linkjagiger
Apr 1, 2002 11:42 AM
Just to understand all aspects of this, what would the effect be on added a second link as already suggested. Would this mean less precise shifting? Does a shorter chain reduce the chances of chain drop?

BTW, I have a new Shimano Ultegra chain to go with the new Ultegra cassette.
Extra Chain LinkKEN2
Apr 1, 2002 11:53 AM
You need that extra link to prevent bending the rear derailleur cage if you shift into big/big. Think about this: if the chain "just fits,", i.e. it wraps exactly over the teeth with no slack, what will happen when the derailleur tries to carry it *over* the cog teeth to get there? Yep, bent derailleur cage or hanger. The springs in the cage handle the extra chain length in smaller combinations--just this much longer chain will not increase the incidence of chain drop.
What He Saidgrzy
Apr 1, 2002 2:57 PM
KEN2 is right on.

The added link is your insurance policy against bending expensive parts. Having it has absolutely no effect on shifting performance - one of the purposes of the der. cage is to take up the slack. Since a triple has a large range of gearing more slack needs to be in the system and thus taken up. You will meet some from the fringe element that maintain that your chain should be shorter since one should never shift into the big/big combo. Never say never if it can happen.

You need to slap that thing together and spend some time understanding how it works with it up on a stand. A qucik way to understand is to just install the larger cog set and then run the bike through all possible gearing combos BY HAND ONLY. You will then see what we're talking about. Be very careful when you do this - you can mess things up when you go to big/big. Now sperate the old chain and determine what is the minimum number of links that need to be added. Always use the correct repair links when putting a chain together - re-using original links leads to a premature chain failure down the road somewhere.
What I Meantjagiger
Apr 1, 2002 6:44 PM
I knew about the negative aspect of going with too short a chain, but I didn't know if 2 or 3 additional links effected things that much versus just 1 addtl link, which would be the min. reccomendation. If you add 3 links is shifting effected? Would adding 2 links provided any insurance vs 1 link only?

Thanks again for your input.
What I Meantgrzy
Apr 2, 2002 9:22 AM
2 or 3 extra links will be too many and the chain will hang quite slack when you get close to the small/small combo. It will literally be flopping in the breeze - then you can get problems with it jumping, skipping and jaming - not all the time, just when you least expect it. With your gearing combo there is really only one correct chain length.
My source is the instructions that came it! (nm)LC
Apr 3, 2002 10:02 AM
It will work fine with 12-27,and 106 links, done it (nm)Paul
Apr 2, 2002 9:06 AM
Careful...Mike Prince
Apr 2, 2002 6:47 PM
If your chainstays are the exact same length as Paul's this prescription may work. Each frame is a little different and the right chain length for your gearing combo and frame geometry is as described in all of the previous posts. Better to spend a few extra minutes to make sure the big/big combo is functional before applying the "what works for one bike works for mine" approach. For example I am running the exact gearing described here and if my chain has 106 links it's pure coincidence.

By the way, this is not criticizing Paul's theory as it sounds like the 106 link solution works great for him. Just a different perspective on the issue.
Good Pointjagiger
Apr 3, 2002 7:06 AM
There are quite a few things to keep track of... I'm still learning & it's good to get an all points view of the situation. Thanks
re: D/A Rear Der & 12-27 CassetteLC
Apr 2, 2002 12:24 PM
Mine said the same thing, 26T max. i am not sure if there are two versions of the RD-7700, but I could not get the 27 to work without just a touch of noise from the upper pulley being too close to the 27 cog. Got the B-tension screw pretty far in too.
How bad is it?jagiger
Apr 3, 2002 7:11 AM
I'm probably going to put mine on & test it. Do you notice added wear? Is it only something that you notice? Have gotten use to it?

By the way, Shimano make a SS type & a GS type RD-7700.
How bad is it?LC
Apr 3, 2002 9:57 AM
I rode it only couple times and the vibration was enough to make me avoid the 27 and just suffer in the next gear, the 24 instead. The vibration felt like it actually may be doing damage to the pulley. Since then I put on a 12-23 and just learned to suffer a bit more up the hills. Seattle area has lots of steep hills, and there are a few I avoid with the 23. The legs have gotten stronger, but I will probally get a 12-25 and hope that is enough for rides like Mt. Rainier with 10,000 ft of climbing and Mt. St. Helens were I attempt to climb 6,240 ft with a double, instead of last year with a triple crankset.
Questiongrzy
Apr 4, 2002 2:46 PM
I guess I don't understand how everything is fine with the 26 but doesn't work with a 27 - usually things deteriorate on a contimuum. Normally it's a fairly straight forward adjustment of the B-screw to bring the top pulley away from the gear - you may need to go ALL the way in - you are at the limit for the mechanism. I know you can do this easily with an Ultegra der., but I haven't had a DA setup that way. A possible related factor is your spring tension in the der. - some setups have more that one position for the spring. In any event there are two versions of the 7700 rear der. - SS & GS. The jump from a 27 to a 26 is less than 4%, but the jump to a 25 is approaching 8%. Kind of significant if you're dealing with steep hills.
12-26Chen2
Apr 3, 2002 11:28 AM
You can make a 12-26 by using the [20-23-26] out of the middle of a 12-34 mountain cassette. Actually I'm running a 13-26 also using a 16 out of the XTR cassette.
~Al
Is XTR == to Ultegra? Can I just get 26? [nm]jagiger
Apr 4, 2002 6:40 AM
Is XTR == to Ultegra? Can I just get 26? [nm]Chen2
Apr 4, 2002 12:14 PM
XTR is more like Dura-Ace, it's Shimano's most expensive cassette. We did have it on my wife's road bike last year. You can probably buy a single 26 from www.sheldonbrown.com but it will be an aftermarket cog. It would make some sense to add a 26 single to a 12-23, but you would have to lose something else. You could order a 13 with built-in spacer to replace the 12 and 13, then stick the 26 on the back. Another possibility would be to start with a 12-25, replace the 12 and 13 with a 13 with built in spacer and add a 16 in the middle, resulting in a 13-25.
~Al
Adding Spacers, etcjagiger
Apr 5, 2002 11:12 AM
What do you mean by adding spacers? I have a 12-27 which came as an assortment of cogs & not as a unit that would slide in place. I thought that I would disassemble my current 12-24 and build the 12-27. Is this right? If so, could I just replace the 27 with a 26 & be done? Thanks
Adding Spacers, etcChen2
Apr 5, 2002 12:17 PM
All of the cassette's that I have been working with are Shimano 9-speed. The 9-speed 12-27's are built with individual cogs, 12,13,14,15, but the larger cogs are combined on common carriers which reduce weight and add stability. The 17 is riveted to the 19, and the 21, 24, and 27 are rivited together. It's not practical to try to take them apart and use them individually. If you will go to the Shimano European bike web site there are exploded view diagrams and charts that show how their cassettes are assembled. There are a lot of interesting possibilities but buying individual parts is difficult. It may even be cheaper to buy two or three complete cassettes to get the parts you really need. Each cog needs to be separated from the next with a spacer, but some of the spacers are "built-in". The smallest cog of a cassette must have a special built-in spacer for the lock ring to work. Sheldon Brown has most cogs and spacers but they are not necessarily Shimano, but they are compatible. I would rather my cassettes have a 16 in the middle than a 12 on the front. This gives me close-ratio shifting so that I can keep my cadence in that 85 - 100 sweet spot. I've got my wife using a 14 - 28 built from a 12 - 25 Dura-Ace with Sheldon Brown's 14 and single 28 and a single 16 from an XTR cassette. Fits her needs.
~Al