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12x25 or 12x27

Apr 19, 2009
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In a month i have a heavy day of steep climbing in a ride i used to do yearly but haven't in about 6 years. Columbus fall Challenge.

Its not the alps or the rockies but its pretty hard for a guy with climbs only a half mile. The question is what cassette to get, i went to 10speed this year and i can't decide if i should get a 25 or 27 tooth.... i used to be able to do this ride with a 23 but that was when i raced.....

thanks in advance.
 
Mar 18, 2009
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Black Dog said:
If you are not sure then go for the 27. You don't have to use it if you do not need but if you have a 25 and need more then you are in for some pain.

+1. The extra gear always comes in handy and will get you through the day less fatigued. If you don't use it, then no problems because you haven't lost anything. But if you needed it and didn't have it, then you would have problems.
 
Mar 17, 2009
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elapid said:
+1. The extra gear always comes in handy and will get you through the day less fatigued. If you don't use it, then no problems because you haven't lost anything. But if you needed it and didn't have it, then you would have problems.

++1 You'll be cursing if you need the 27 and don't have it.

Have a great ride!
 
Jun 9, 2009
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If you recently upgraded to 10 speed, are you certain that your chain-rings have the same number of teeth as your previous ones?

If so, then everyone agrees in the 27.

If not, then we need to do some more math.
 
Apr 19, 2009
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Good question.

The crank is a 39x53 and is what i have always used. One question am i going to need a longer chain for the 27.....i ride with a 12x23 today.

BTW i am convinced on the 27
 
Unlikely to need a longer chain - most bikes are set up with chains that a few links longer than a race mechanic would use anyway.

I have know proprietry knowledge here its just been my experience over the years. Something about longer chain wearing better but race mechanics being more intersted in weight and performance I guess. Others here will explain this better than I.
 
May 13, 2009
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euphrades said:
Good question.

The crank is a 39x53 and is what i have always used. One question am i going to need a longer chain for the 27.....i ride with a 12x23 today.

BTW i am convinced on the 27


Probably be ok, I switch out between an 11x23 and a 12x25 all the time, and have been fine. Is your der. a short cage or long cage? Might make a difference if it is long cage.
 
Aug 16, 2009
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27, unless all the slack is pulled in on your current 25, chain should be fine. Just avoic x-chaining, and short-cage should have no trouble.
 
Mar 10, 2009
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Take a picture of your chain setup in the 53x27 combo, and some people here can comment on the position of your rear der and see if its too tight.

I'd say if you are using a 23 now as biggest rear cog and want to go to a 27, you might have to use a new, longer chain, but it depends on how much slack you have in your current setup.
 
Apr 19, 2009
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Well today the chain length is determined based on the chain in the big chain ring and the small cog in the back, and the deraileur pulleys are in a straight line perpendicular to the ground. I can take a picture later.......kind of thinking getting a new chain anyway as this one will have about 1500 miles before doing this ride.
 
Jun 9, 2009
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If you are going to get a new chain at a local shop, the mechanic should install it at no extra charge, or for a nominal fee. They will be able to determine that your shifting will be okay with the new set-up. Be sure to take the bike into the parking lot and run it through the gears before you leave, just to be sure.
 
Mar 10, 2009
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euphrades said:
Well today the chain length is determined based on the chain in the big chain ring and the small cog in the back, and the deraileur pulleys are in a straight line perpendicular to the ground. I can take a picture later.......kind of thinking getting a new chain anyway as this one will have about 1500 miles before doing this ride.

I was assuming you were going to use your old chain, and to see if it fits without disconnecting the chain, the 53x27 combo would pull your chain as tight as possible. The position of the derailleur could then be so, that the jockey wheels are 'too horizontal'. But as a mechanic, you already knew that ;)

If you have a new chain, you can obviously just measure it the way you indicated. I always used the big x big combo without running it trough the derailleur and have a link overlap.

If you ocassionally go from 27 to 23 you could measure your new chain by using a 25 maybe? Then it's never too tight for the 27 or too loose for a 23 and saves you an extra chain?
 
Jun 18, 2009
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euphrades said:
In a month i have a heavy day of steep climbing in a ride i used to do yearly but haven't in about 6 years. Columbus fall Challenge.

Its not the alps or the rockies but its pretty hard for a guy with climbs only a half mile. The question is what cassette to get, i went to 10speed this year and i can't decide if i should get a 25 or 27 tooth.... i used to be able to do this ride with a 23 but that was when i raced.....

thanks in advance.

I hear ya. I used to ride a 13-21 or 13-23 back when I raced (many years ago). Today I ride a 12-27 and love it. Many of my rides have some 20% grades on them and the 27 is just right. Like others said, if you don't use it, you really haven't lost anything.

I wouldn't think you'd have any chain issues (though you should check).
 
Apr 19, 2009
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Bala Verde said:
I was assuming you were going to use your old chain, and to see if it fits without disconnecting the chain, the 53x27 combo would pull your chain as tight as possible. The position of the derailleur could then be so, that the jockey wheels are 'too horizontal'. But as a mechanic, you already knew that ;)

If you have a new chain, you can obviously just measure it the way you indicated. I always used the big x big combo without running it trough the derailleur and have a link overlap.

If you ocassionally go from 27 to 23 you could measure your new chain by using a 25 maybe? Then it's never too tight for the 27 or too loose for a 23 and saves you an extra chain?

Yea the chain today is a bit tight for a 53x23 combo....think the 27 would be too much.....I think I will get a new chain anyway. This way if the one i have is too short then I can put the new one one. Besides its not like the chain won't get used soe day
 
The Dura Ace rear derailleur can handle the 39/27 ratio. However that is the limit for the short cage. Depending on the length of your current chain you may need a new chain. If your old chain is old then this is a good idea anyway with the installation of a new cassette.
 
Black Dog said:
The Dura Ace rear derailleur can handle the 39/27 ratio. However that is the limit for the short cage. Depending on the length of your current chain you may need a new chain. If your old chain is old then this is a good idea anyway with the installation of a new cassette.

Just for info, the manufacturers recommendations are very conservative. a 11-28(now offered by shimano in 10s) or even a 13-30 is really no problem for a short cage DA rear derailleur.
 
Jun 26, 2009
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Bustedknuckle said:
Just for info, the manufacturers recommendations are very conservative. a 11-28(now offered by shimano in 10s) or even a 13-30 is really no problem for a short cage DA rear derailleur.

Agree. Have DA 7800 with short cage and normally run 12-25. Ran a SRAM 11-28 for a REALLY hilly course in August and everthing was fine. Switched back to 12-25 for training purposes though as I like the spacing better and the 28 cog is too tempting to cheat with...!