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12-27 cassette install question(10 posts)

12-27 cassette install questionRyan
Jan 7, 2004 8:07 AM
I received a new 12-27 Shimano cassette for Christmas. My Bike currently has a 12-25. My current chain only has about 1,000 KM, so it still has a lot of life left in it. When I install the new cassette will I have to also install a new chain or can a just add a couple of links to my existing chain? If it is okay to just add a couple of links how many should be added?

Thanks in advance.
Depends on current chain length...biknben
Jan 7, 2004 8:34 AM
Unless you chain is already too short, you shouldn't need to add links. Install the cassette, put the chain on the 27, and see how your Rr. Der. cage looks.

Many riders swap wheels and cassettes while using the same length chain. I swap 12-23 to 12-27 without problems.
re: 12-27 cassette install questionSaddle_Sore
Jan 7, 2004 8:37 AM
I'm no expert, but are you sure that you will need a longer chain? I would have thought that your existing one should cope - but I may be corrected.

If you do, then I personally would plump for a new chain. Adding links in would involve breaking the chain, dropping the links in and then stitching it back up, potentially creating a weak spot on the chain where you've removed and reinstalled pins.
re: 12-27 cassette install questionjrm
Jan 7, 2004 8:53 AM
I never changed the lenght of my chain when i change back and forth between my 12-25 and 12-27. However you may improve the shifting using a better quality chain with te new cassette.
re: 12-27 cassette install questionNEIL
Jan 7, 2004 9:29 AM
You do not need to change your chain length going from 25 to 27.

To gauge proper chain length shift in your 53/12. Big ring on front, smallest cog on back. Look at your rear derailleur from the side. With proper chain length you should be able to draw an invisible line that is perpendicular to the ground and intersects both pulleys. If it's off, you need to add or subtract links. If you have a long cage derailleur, this method doesn't really work, but with a 12-25, you shouldn't have a long cage version anyway.

If you are in your lowest 2 or 3 gears (largest cogs) in back, you should also be in your small chainring up front which allows more slack in the chain. If not, you'll be crossing gears, (big front-big back) and yes, your chain will appear too short and probably not shift well due to too much tension on the chain.

As mentioned, it is a good idea to change cogs and chain at the same time to keep a consisant wearing pattern on both parts and maintain better shifting.

better way to set chain length...C-40
Jan 7, 2004 9:35 AM
Two simple tests will determine if the chain is the correct length. First, it must not hang loose in the little ring, little cog combination. Wrap the chain around the small chainring and through the derailleur in the normal manner. When the ends of the chain are brought together, some movement of the lower derailleur pulley should occur, indicating tension is being applied. Two more links (another inch) may need to be removed, beyond the point of absolute minimum tension, to keep the chain from rubbing on itself as it passes under the upper derailleur pulley. Once this is done, the chain is set to the maximum useable length. Removing additional links will do nothing but reduce the derailleur's capacity.

Second, the chain must be long enough to avoid overextending the rear derailleur when shifted to the big ring and biggest cog combination. If the chain is set to the maximum length as described, it should always pass this test, unless your setup exceeds the derailleur's stated capacity.

If you want to see how much lower pulley movement will occur, without removing the extra two links, shift up four teeth (11 to 15 or 12 to 16). This has the same effect as removing two links.
FWIW: That's the way I do it...biknben
Jan 7, 2004 10:30 AM
I do it exactly as you describe in your first paragraph. My chain it as it's maximum "useable" length.

Argueably, it may be a couple links longer than needed. I've seen what happens to people who make the chain too short and then shift into big x big ring combos. It doesn't look healthy.

It also provides a little room for error if you have "issues" out on the road. If the chain were to break I can give up a link, put it back together, and still be fine.
Ditto.KG 361
Jan 7, 2004 10:34 AM
I always KNEW you were a 'friggin genious. =)
re: 12-27 cassette install questionMichaelM
Jan 7, 2004 9:28 AM
I'm going to jump in and say you should put in another link.

Just put a new chain on myself, and for the first time ever, I read the instructions that come with it:

Run the chain around your largest front chainring and around the largest rear sprocket, pull the ends together and it should overlap by 2 links (or 1 if your using a powerlink or similar).
Then run the chain through the rear derailer and connnect.

If you set it up this way on the 25, and then replace the cassette with the 27, you'll find you need to put in some extra.

re: 12-27 cassette install questionWoof the dog
Jan 8, 2004 12:04 AM
I don't think anybody mentioned that you should also check for the upper pulley hitting the largest cog.

I really never ended up solving the problem, so there is a little bit of contact when riding a 25, and even more with a 27 (not that I really use 27 often).

so if anyone knows how to tune it, let me know. And no, the b screw (i think thats what it is called) doesn't do SH!T.