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Can I Switch Back and Forth Between a 24 and a 27 or 28

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Can I Switch Back and Forth Between a 24 and a 27 or 28

Old 01-05-09, 11:06 AM
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Can I Switch Back and Forth Between a 24 and a 27 or 28

I have a 24 big gear on the back, and want to be able to go with a 27 or even an SRAM 28 on the back on occasions when I have a real long ride with major hills. I run a compact 34/50 on front. Shimano 10 speed group.

Can I switch cassettes back and forth between a 12/24 and a 12/27 or 12/28? Will the derailleur (Shimano Ultegra standard) pick up the slack between what's needed for a 27/28 vs the 24?
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Old 01-05-09, 11:08 AM
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Yes
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Old 01-05-09, 11:11 AM
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I think there might be slight issues with a 28 if your chain was set up on a 23. If it was set up with a 25 or 27, then you should be fine.

The 'problem' if there is one is probably going to be on the big ring, and only when you're completely cross chained. You should try the big big combo on a stand with 0 load on the RD to see what happens. You might have to remember never to shift to that combo or your RD could explode.
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Old 01-05-09, 11:32 AM
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Originally Posted by ridethecliche
I think there might be slight issues with a 28 if your chain was set up on a 23. If it was set up with a 25 or 27, then you should be fine.
Along these same lines, don't forget about the "B-tension adjust screw" or whatever the Hell it's called; the one that controls the vertical distance between the RD and the cassette cogs. You need to make sure there's clearance between the RD and the largest gear you're going to use. If you've set the RD to clear a 23-tooth gear and then install a 27 or 28, you may run into trouble. Conversely, if you're setup for a 28-tooth gear as the largest you may notice that shifting isn't as crisp when using a 23.
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Old 01-05-09, 11:55 AM
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As long as you set it up for the largest gear you are going to use and you are within the chain wrap capacity of the derailer you can change between cassette sizes without problems.
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Old 01-05-09, 12:02 PM
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You should never use the 34/12, so adjust the chain length to the maximum that creates a small amount of movement (and chain tension) from the RD cage when the ends of the chain are brought together in the 34/13. This will give you the absolute maximum wrap capacity from the RD.

The minimum length change is 1 inch, that is equal to 4 cog teeth. The Shimano short cage should handle the 12-27 or 12-28. If you never shift into the big/big combo, then it most certainly will.
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Old 01-05-09, 12:12 PM
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Originally Posted by DaveSSS
You should never use the 34/12, so adjust the chain length to the maximum that creates a small amount of movement (and chain tension) from the RD cage when the ends of the chain are brought together in the 34/13. This will give you the absolute maximum wrap capacity from the RD.

The minimum length change is 1 inch, that is equal to 4 cog teeth. The Shimano short cage should handle the 12-27 or 12-28. If you never shift into the big/big combo, then it most certainly will.
Should, and don't are two very different things.

If I'm racing and in the little ring, I'd rather shift to the 12 than have to shift between chainrings up front if I need to stay in the little ring for the majority of the time.

Just like the 53/27 shouldn't be used, but doesn't mean that I won't use it in a crit if there's one hill that I can power through on it.
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Old 01-05-09, 12:18 PM
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Originally Posted by markwebb
I have a 24 big gear on the back, and want to be able to go with a 27 or even an SRAM 28 on the back on occasions when I have a real long ride with major hills. I run a compact 34/50 on front. Shimano 10 speed group.

Can I switch cassettes back and forth between a 12/24 and a 12/27 or 12/28? Will the derailleur (Shimano Ultegra standard) pick up the slack between what's needed for a 27/28 vs the 24?
yes to the 27; not suggested on the 28 unless you are running a SRAM RD; SRAM indexing is different than shimano IIRC.
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Old 01-05-09, 01:06 PM
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Originally Posted by ridethecliche
Should, and don't are two very different things.

If I'm racing and in the little ring, I'd rather shift to the 12 than have to shift between chainrings up front if I need to stay in the little ring for the majority of the time.

Just like the 53/27 shouldn't be used, but doesn't mean that I won't use it in a crit if there's one hill that I can power through on it.
A one tooth change can't be adjusted for, except with a one inch (4T) shorter chain. It's rare that the chain might have tension in the 13 and not the 12, but I suppose it could happen. Even if it did, a slight sag in the chain would not keep it from driving the bike or shifting up to the 13.

If you use the little/little then be smart and check that combo. Seems like a no brainer. With a compact crank the chain will usually rub the big ring in the that combo and I've even had the big ring attempt to pick up the chain.

Last edited by DaveSSS; 01-05-09 at 01:15 PM.
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Old 01-05-09, 01:13 PM
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Originally Posted by skipmcne
yes to the 27; not suggested on the 28 unless you are running a SRAM RD; SRAM indexing is different than shimano IIRC.
SRAM and Shimano cog spacing is the same 3.95mm, according to all reports I've read. Of course you can't put a SRAM RD with Shimano shifters, since the shifters pull different amounts of cable.

When pushing the very limit of a RDs wrap capacity, some bikes will have up to 2T more wrap than others, depending on the exact chainstay length. That's why some people report that a setup works and others claim it doesn't.

The maximum cog size listed by the manufacturer is rarely the actual maximum. In the case of Shimano, it's listed as the largest cog they sell, but you can generally get at least another 2T.

Last edited by DaveSSS; 01-05-09 at 02:18 PM.
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Old 01-05-09, 02:08 PM
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I run 50/34 with 12/27 on the same Ultegra. I've been known to crosschain 50-27 to no ill effect.
With my chainline on short stays it is less an issue than using the 34 with even the 3rd smallest cog.
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Old 01-05-09, 02:44 PM
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Originally Posted by skipmcne
yes to the 27; not suggested on the 28 unless you are running a SRAM RD; SRAM indexing is different than shimano IIRC.
I use a SRAM 11-28 with my Dura-Ace.
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Old 01-05-09, 08:07 PM
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I now use the 34/12 a lot - like someone else said it's a lot easier jumping down 2 to the 12 than moving to the big on front.

It's set up with chain length that works with a 12-24 in back right now, so my real question is will I have to use a new chain with a little longer length now to accomodate a 28 on the back? Remember I wanna switch back and forth between the 12-24 & 12-28 without modifying my chain each time.
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Old 01-06-09, 06:02 PM
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Originally Posted by markwebb
I now use the 34/12 a lot - like someone else said it's a lot easier jumping down 2 to the 12 than moving to the big on front.

It's set up with chain length that works with a 12-24 in back right now, so my real question is will I have to use a new chain with a little longer length now to accomodate a 28 on the back? Remember I wanna switch back and forth between the 12-24 & 12-28 without modifying my chain each time.
I guess you haven't been reading closely. 4 teeth equals 1 inch of chain. If your RD has enough wrap capacity, then you don't need to change chain lengths. Just set it up to the maximum length that won't hang loose in the 34/12. I'm assuming that you also use the 50/28? If not, then any short cage will work. If you might shift into the 50/28, all you can do is try it - easily on the workstand first.

I use the 34/12 too, but I have an 11-25 11-speed cassette, so the 34/12 is quite useable. I would never use the 50/25, but of course it won't cause any damage to shift into it. You will hear the noise though.

Last edited by DaveSSS; 01-07-09 at 07:48 AM.
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Old 01-06-09, 10:04 PM
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Htfu
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Old 01-07-09, 04:51 AM
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I change from an 11-23 to an 11-27 combination without problems. I even change just the last 3 largest cogs.

Cheers!
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