[niteschaos]

I have a 105 rear derailer with a 12-23 cluster on the back. I'm running the longest chain I can and have the b-screw maxed out. I was wondering what the largest cassette was that I could run. I have a race up some mountains in Boone, NC, and I know that the 39x23 combo isn't going to suit me well. I'm trying to decide between going with a 12-27 Ultegra cassette or if a 11-32 shimano 9 speed cassette would fit. I'm thinking about just canabalizing the two into one cassette so that the gear changes are smoother. I'm no slouch in the ITT, but being 6'2" and 185 pounds I'm having trouble keeping up with some of the other guys on the team when things get steeper than 10%. Some of the grades will hit 18% for short distances.

 

[Nater]

12-27 tooth

With your current rear derailleur set up, I think that a 12-27 cassette is the biggest that you can use (as Shimano recommends it anyway). I'd stick to that with a normal short cage road rear derailleur. If you switched to long cage road or a mountain rear derailleur, you could get away with the larger cassette.

 

[Rusty Coggs]

With your current rear derailleur set up, I think that a 12-27 cassette is the biggest that you can use (as Shimano recommends it anyway). I'd stick to that with a normal short cage road rear derailleur. If you switched to long cage road or a mountain rear derailleur, you could get away with the larger cassette.

That is just spec. The 9 speed 105 usually handles a 30 and even a 32 in some applications. Cage length is about chain wrap, as the long cage has the same large cog capacity.

 

[Al1943]

Cassettes

Assuming your 105 is 9-speed, there are several choices you could make. A 12-27 may be all you need but you could easily combine two or more cassettes into one, all of mine are like that. If you will be doing any flat land drafting there will be an advantage to keeping the ratios close, as few gaps in tooth count as possible and still get the job done. As for climbing 10%+ grades you could try adding a 28 or 29 or 30 to the mix, the largest I've tried on a road derailleur is a 28, but others have run 30's, don't believe I'd try a 32 without a mountain derailleur. As Rusty pointed out it's not actually a cage length issue, it's a cog size limit built into the upper part of the derailleur. If you can get by without the 12 then start with a 13 (must have built-in spacer). Starting with a 12-25 you could convert to a 13-28 or 13-29. My wife runs a 14-28 9-speed made from a 12-25 D-A with some cogs from http://www.sheldonbrown.com Harris Cyclery. My favorite 9-speed cassette is a 13-26 with cogs from Ultegra and XTR cassettes, and the 13 from Sheldon. I don't like running a mountain bike cassette on a road bike because of the big gaps and ratio jumps from cog to cog, unless you are going to use it for a special mountain event.
~Al

 

[niteschaos]

Yes I have the long cage derailer and this is a special event.

 

[Al1943]

32

Yes I have the long cage derailer and this is a special event.

In that case, for this event, I would probably throw together a 13-32 mixing in some other cogs or just go with a 12-32 and in either case use the mountain derailleur.
~Al

 

[CycloPathic]

that's what they have triples for.

Going from 23 to 27 yields 17% lower gear and from 23 to 32 39%. In 39x32 10% will feel like 7.1% and 18% as 12.9%. Triples will give lower range you need and close ratio, consider. And if triples are out of question, FSA compacts perhaps?

 

[niteschaos]

Thanks for taking your time to run the numbers. Idealy I would have just put on a 13-25 rear and a FSA compact and be done with it, but I don't have the funds for a new cluster AND crank. It is close to time for me to replace my rear cluster anyway, so I was going to try to find a stop-gap to fix my climbing delema. Right now it looks like if I can fit a mountain cassette with me derailer, then that's what I'm gonna have to do to save me knees up those 18% switchbacks.

 

[torquecal]

I took the compact crankset option

When you get a chance you might want to check out one of the compact cranks. I use the FSA carbon 50/34 and it does wonders for my gear range. There are other companies that make less expensive versions of the same thing.

Thanks for taking your time to run the numbers. Idealy I would have just put on a 13-25 rear and a FSA compact and be done with it, but I don't have the funds for a new cluster AND crank. It is close to time for me to replace my rear cluster anyway, so I was going to try to find a stop-gap to fix my climbing delema. Right now it looks like if I can fit a mountain cassette with me derailer, then that's what I'm gonna have to do to save me knees up those 18% switchbacks.

 

[ruly62]

Go FSA COMPACT cranks!

 

[TTU-XC]

Go FSA COMPACT cranks!

Or any other standard MTB crank that has the same 110 bcd. Then take the granny off, buy a 50 and you have the 34 already most likely

 

[CycloPathic]

Thanks for taking your time to run the numbers. Idealy I would have just put on a 13-25 rear and a FSA compact and be done with it, but I don't have the funds for a new cluster AND crank. It is close to time for me to replace my rear cluster anyway, so I was going to try to find a stop-gap to fix my climbing delema. Right now it looks like if I can fit a mountain cassette with me derailer, then that's what I'm gonna have to do to save me knees up those 18% switchbacks.

If you have an MTB to cannibalize borrow MTB cass and RD. On 11-32 replace lower 2 cogs with cogs from your 12-23. It will be the easiest fix, just make sure you have enough chain.

If no MTB buy 12-27 Ultegra. In race situation with adrenaline pumping, you'll suffer but should get by, on a long run it is better choice as 12-25. Both have the same top 7 cogs, and 27-24 are better escape gear then 25-23, you don't need them often. As a last resort you can get 38t smaller ring 130BCD good luck.