Chain tension and swapping out chain rings?

Swapping chain rings means moving your front derailler, that's even assuming they have the same bolt-circle-diameter. Way too much hassle. Changing cassettes is the better option.

capwater said:

Swapping chain rings means moving your front derailler, that's even assuming they have the same bolt-circle-diameter. Way too much hassle. Changing cassettes is the better option.

Right and I understand that. That help me with the low end, but my concern is with the high gears. As configured, I've got a 50x11. The 52x11 would be nice, but as you say might be too much hassle. I could always got 52x38 permanently (I'm sure the LBS could make that swap for me), and then switch the cassettes on my own as needed for climbing or flat land riding....

L

I've got the 50/34 on my bike for wintertime training, and then once race season starts i change to a 52/36. the 36 is still pretty low for climbing when i'm training, but i get that 52x12 for the sprints. So yeah swapping out chainrings every day or so would be a pain, but FSA makes the 110BCD rings in sizes up to 52 i think.

12-25 for training and hilly road races, 12-21 for flat crits and 12-23 for everything else. Cassette swaps take maybe 2 minutes even if you stop to pick your nose. It's all about gear ratios, not necessarily how big or small your rings are; 50, 52, 53 are minimal changes.

Thanks for the help. I did some messing around on sheldon brown's gearing page and I think I'm good (Unless I meed to go 40mph.... 😉 )

Quoted post said:

but i get that 52x12 for the sprints.

The 50x11 is a bigger gear than the 52x12...

L

Leave the 50/34 crankset and choose a cassette 11-21 or 23 for the flat race work.

On my 650c wheel bike my biggest gear is 53-12,
on my 700c bike (Felt) 52-13,
so 50-11 should be heaps big enough on flat roads.

If you want to lower the rolling resistance, consider Conti GP 4000 tyres. 😎

Chain tension is determined by the ability of the rear derailleur to take up the chain slack.

1. Subtract your smallest front chainring from your biggest. (52-38=14)
2. Subtract your smallest rear cog from your largest. (27-12=15)
3. Add the two answers. (14=15=29)

Your rear derailleur needs to be able to take up 29 teeth of chain slack. Most Shimano short arm rear derailleurs are rated for 29 teeth.

Retro Grouch said:

Your rear derailleur needs to be able to take up 29 teeth of chain slack. Most Shimano short arm rear derailleurs are rated for 29 teeth.

So what you are saying is that I need to get the 50/36 (instead of the 50/34)compact to run a 12-27 rear cassette.

50-34 = 16
27-12 = 15
15+16 = 31 > 29 = too much....

L

Lonnie Utah said:

So what you are saying is that I need to get the 50/36 (instead of the 50/34)compact to run a 12-27 rear cassette.

50-34 = 16
27-12 = 15
15+16 = 31 > 29 = too much....

L

Derailleur capacity recommendations assume that the chain is the proper length. You can get by with larger cogs or smaller chainrings if you use a shorter chain.

I suspect that your combo will probably work outright with a 50/34 and 12-27 cassette if your chain is not too long, but you can definitely make it work if you run your chain a bit on the short side and avoid using the 50/27 gear combination, which you shouldn't use anyway. Basically make your chain short enough that the derailleur can take up the slack in the 34/12 (or 34/11 if you go with an 11T small cog), and avoid the 50/27 (or 25 if that makes the chain too tight).

Sweet. Thanks for all the advice guys. This site is awesome.

L

John M said:

Derailleur capacity recommendations assume that the chain is the proper length. You can get by with larger cogs or smaller chainrings if you use a shorter chain.

I suspect that your combo will probably work outright with a 50/34 and 12-27 cassette if your chain is not too long, but you can definitely make it work if you run your chain a bit on the short side and avoid using the 50/27 gear combination, which you shouldn't use anyway. Basically make your chain short enough that the derailleur can take up the slack in the 34/12 (or 34/11 if you go with an 11T small cog), and avoid the 50/27 (or 25 if that makes the chain too tight).

I'm going to disagree with John to a point.

It's important that the chain never be too short. If you ever accidentally shift into the big/big combination, a too short chain can cause a couple hundred dollars of damage (sometimes more) in less time than it took me to type this sentence.

If you're going to cheat on derailleur wrap up, do it with a slightly too long chain. You can live with a little accidental slack if you happen to shift into the little/little combination. In fact, my chain rubs on my big ring when I'm in the second littlest rear cog. I use that as an audible warning that it's time to shift into the big ring.

Where we agree is that often the derailleur specs can be exceeded with no penalty at all. I agree there's a good chance it'll work for you. Even if it didn't, I'd definitely run the cogs you want and I wouldn't worry if my chain turned out to be a couple of links too long to wrap up completely.

Retro Grouch said:

I'm going to disagree with John to a point.

It's important that the chain never be too short. If you ever accidentally shift into the big/big combination, a too short chain can cause a couple hundred dollars of damage (sometimes more) in less time than it took me to type this sentence.

If you're going to cheat on derailleur wrap up, do it with a slightly too long chain. You can live with a little accidental slack if you happen to shift into the little/little combination. In fact, my chain rubs on my big ring when I'm in the second littlest rear cog. I use that as an audible warning that it's time to shift into the big ring.

Where we agree is that often the derailleur specs can be exceeded with no penalty at all. I agree there's a good chance it'll work for you. Even if it didn't, I'd definitely run the cogs you want and I wouldn't worry if my chain turned out to be a couple of links too long to wrap up completely.

I'll accept Retro's comments. He makes a good point that damage could be done with a dangerously short chain, but a chain that is a link or two short could definitely be a bit tight in the big/big combo but unless it is way short, it shouldn't cause damage. There is a difference between a little shorter than ideal and too short. A chain that is too long may be loose enough on the small/small combo that it would dropped on a bump. This is all really an academic discussion though, the true test is what works safely and acceptably on Lonnie's bike.

So I just popped the 12x27 on and everything "seemed" to work. I did however get a bit of chain skip which was remedied when I went back to the 11x23. Ideas? Seems to me, I've got 2 options. Switch to a 12-25 rear casette or a 36 tooth small ring. I kinda like the second option since Shimano reccomends no more than a 14 tooth jump between the big and little ring.

L

Lonnie Utah said:

So I just popped the 12x27 on and everything "seemed" to work. I did however get a bit of chain skip which was remedied when I went back to the 11x23. Ideas? Seems to me, I've got 2 options. Switch to a 12-25 rear casette or a 36 tooth small ring. I kinda like the second option since Shimano reccomends no more than a 14 tooth jump between the big and little ring.

L

If the chain is skipping in the middle of the cassette (13, 14, 15, 17, etc..) when on the small chainring, then it is not a problem with the chainring size or derailleur capacity, because chain tension would not be an issue in the middle combinations. The chain being too tight would only be a problem in the big ring and bigger cogs. The issue of derailleur capacity, as discussed by Retro, is having a long enough derailleur cage to handle the longer chain that is needed to wrap around bigger cogs and chainrings.

Even though cassettes of the same brand/model should be equal, sometimes you need to make small adjustments with the derailleur cable adjsuting barrel to fine-tune the shifting. You can do this at the derailleur itself or at the downtube adjusting barrel. Another possibility is that you may need to adjust your "B-screw" a little bit to move the derailleur body a little lower (away from the 12T cog).

See this link for help.

http://www.parktool.com/repair/readhowto.asp?id=64

After further inspection, it might be a spacer problem.

L