Shimano Compacts w/12-27...a "dr" question

[trophyoftexas]

I searched around the older posts about rear dr's, compacts, and 12-27 cassettes but most of them degenerated into arguments about all kinds of crap...so I'm starting a new thread and asking this simple(?) question. I'm running a 12-27 cassette and am thinking about installing a Shimano compact 50/34 in the front. There seems to be a difference of opinion among the lbs guys that I've talked with about dr's with this combination....some say standard in back and front, some say longer cage in the back is necessary....what is the consensus of opinion amoung our forumite Shimano guru's? I've got an FSA crankset on this Ottrott and have never been very excited about it, figured now would be a good time to try a compact which is something I've never used. I don't care at all about loosing on the high end, if I'm going fast enough to spin out then I'm going too darn fast for someone my age anyway! Lots of nice, steep but fairly short climbs coming up for Hellweek and thought this would be interesting to try....taking a couple of other bikes in case the experiment is a flop!

[stevep]

i used it w/ a short cage in europe the last time.
worked perfectly.
also used a regular front der.

[sellsworth]

i used it w/ a short cage in europe the last time.
worked perfectly.
also used a regular front der.

Ditto - except the europe part

[Sandy]

1. I started a thread previously "Riding with my compact crank". There are relevant posts on the thread. My first post compared using the 50/34 Shimano crank and a 12/27 cassette, to using the normal 53/39 with the 12/27 cassette. I compared gearing and what you gain and lose with each combination.

2. To specifically answer your question- I made no changes in the Dura-ace 10 speed relative to derailleurs, front or rear. The setup has worked very well.

I hope this helps.


Sandy

[FierteTi52]

I use the Shimano compact, D/A front deraileur 12-27 cassette with the standard D/A rear derailleur. The method I used to determine the chain length was threading the chain through the big ring and largest cassette cog, skip the derailleur and overlap 2 links. This is the shortest chain length you can use and not tear off the RD is you cross shift to the extremes. This all works fine, but if you use the smallest 2 cogs on the cassette while on the small front ring, the chain will become slack. At this point you should be on the big ring anyhow. Not worth using a long cage RD IMHO.
Good luck,
Jeff

[Birddog]

I know this wasn't your question but I can't help myself. I would suggest either a 50/36 or a 48/34. The 16 tooth jump is frustrating.

Birddog

[trophyoftexas]

1. I started a thread previously "Riding with my compact crank". There are relevant posts on the thread. My first post compared using the 50/34 Shimano crank and a 12/27 cassette, to using the normal 53/39 with the 12/27 cassette. I compared gearing and what you gain and lose with each combination.

2. To specifically answer your question- I made no changes in the Dura-ace 10 speed relative to derailleurs, front or rear. The setup has worked very well.

I hope this helps.


Sandy
Hey Sandy, THANKS! I didn't turn your thread up in my search and it was full of interesting comments. Not much was made of the dr question but I'm beginning to think the guys with the larger caged rear dr's for sale probably just want to move them! Next question would be, I guess, would the longer cage be "better" even if it was not totally necessary....meaning would it result in smoother shifting than a standard even if the standard would do the job?

[Sandy]

Glad to help. I really am not qualified to answer your new question.


Sometimes Slowly Shifting Sometimes Speedily Shifting Shimano's Shifters,

:banana: Shifty Shiftless Serotta Sandy :banana:

[RPS]

Hey Sandy, THANKS! I didn't turn your thread up in my search and it was full of interesting comments. Not much was made of the dr question but I'm beginning to think the guys with the larger caged rear dr's for sale probably just want to move them! Next question would be, I guess, would the longer cage be "better" even if it was not totally necessary....meaning would it result in smoother shifting than a standard even if the standard would do the job?Shimano rates the short cage rear derailleur (SS) at a total capacity of 29 teeth. If you are going to stay with a 12-27 rear cassette (that's 15 teeth difference) and run 50-34 front chainrings (that's 16 teeth difference), then the total capacity you need is 15 + 16 = 31 (hence you’ll be exceeding the rated capacity by 2 teeth). Technically, if you go by the Shimano ratings, you should use the triple’s longer cage derailleur (GS).

Having stated that, I'd expect that Shimano may be a little conservative with their ratings because on some bikes the actual chainstay length may lead to less-than-perfect chain length to optimize capacity. IMO the worse case scenario is that you may not be able to cross the chain without problems. You can always try it on a bike stand ahead of time to check which gears won’t work.

In my case, I ran a 53-39 up front with an 11-27 at the rear while being able to hit all combinations, even though it exceeded the published capacity by 1 tooth. Maybe you can get 2 teeth out of it.

[Sandy]

Excellent clear explanation. I enjoy reading communications that are so clearly done. I never use crossover gears, but as I recall that is not really a problem on the 50/34, 12/27 setup. I am normally the problem.


Sandy

[stevep]

i guess the ultimate question on the short cage if you are...no offence, an idiot...and try to use the thing in the 50-27...then you deserve what you get.
the set up works well if you have any idea what you are doing.
( not meant to be offensive... just accurate )
fahgettaboutit

[tiger]

I agree with all above. My experience is that it works with the short cage rear deailler. I used the "large chainring, largest cog" method for chain length as well. I do not find the chain too slack in the small-small, but my chainline has the chain hitting the inside of the large front chainring when I'm in the small-small so I am quickly reminded to shift by the noise.

I found the shimano compact front crankset to shift much better than a FSA for me. It takes a while to get used to the compact, but now that I have used one for a while I find the shifting much more precise than my triples, and will probably migrate all my bikes to compacts over time. Only thing I miss is a little top end on downhills.



Tiger

[Sandy]

i guess the ultimate question on the short cage if you are...no offence, an idiot...and try to use the thing in the 50-27...then you deserve what you get.
the set up works well if you have any idea what you are doing.
( not meant to be offensive... just accurate )
fahgettaboutit

We are not all on the same page of the learning curve simultaneously.



Sandy

[stevep]

We are not all on the same page of the learning curve simultaneSandy


probably true.
hence my gentle explanation.
the term " idiot" is used frequently by me and is not meant to offend you sandy.
for you i would use
sweetly successful smooth shifting sandy

overall i would reckon, however, that anyone who gets to the point of owning or riding a nice bike will rapidly figure that the outside cross over gears are neither necessary nor desirable. hence my use of the offensive term.

[tiger]

anyone who gets to the point of owning or riding a nice bike will rapidly figure that the outside cross over gears are neither necessary nor desirable.

Not necessary or desirable, but occasionally shifted into at mile 97 of a hilly century when I'm spent and near hallucinating...hence I set up my bikes to tolerate stupid shifting without breaking.

[Dave]

RPS covered the topic pretty thoroughly. What is often overlooked is the fact that chains can only be shortened or lengthened in 1 inch increments. A 1 inch difference in chan length is the same as a 1/2 inch difference in chainstay length. Four sprocket teeth are equivalent to 1 inch of chain. Frames with in-between chainstay lengths could have up to 2 teeth less wrap capacity than another frame with the perfect chainstay length. That's why a combination that works on one bike may not on another.

To get the maximum capacity from a RD, set the chain length in the little ring and the smallest useable cog, with just enough length to keep the chain from hanging loose or rubbing on the upper chain guide tab as the lower section of chain passes under it. If in doubt whether to remove another inch of chain, shift up 4 teeth and you'll have the same tension as removing an inch of chain. I would also check the the big/big combo to be sure the chain is not overextending the RD.

[hypnos]

I know this wasn't your question but I can't help myself. I would suggest either a 50/36 or a 48/34. The 16 tooth jump is frustrating.

Birddog

+1

[RPS]

RPS covered the topic pretty thoroughly. What is often overlooked is the fact that chains can only be shortened or lengthened in 1 inch increments. A 1 inch difference in chan length is the same as a 1/2 inch difference in chainstay length. Four sprocket teeth are equivalent to 1 inch of chain. Frames with in-between chainstay lengths could have up to 2 teeth less wrap capacity than another frame with the perfect chainstay length. That's why a combination that works one bike may not on another.

To get the maximum capacity from a RD, set the chain length in the little ring and the smallest useable cog, with just enough length to keep the chain from hanging loose or rubbing on the upper chain guide tab as the lower section of chain passes under it. If in doubt whether to remove another inch of chain, shift up 4 teeth and you'll have the same tension as removing an inch of chain. I would also check the the big/big combo to be sure the chain is not overextending the RD.Exactly, just because it works great for one person, it many not work so well for someone else of equal talent and attentiveness. That’s one of the limitations of depending on someone else’s experience under (unknown) different circumstances.

I think implying that a rider is an idiot (not referring to your comments) because he gets distracted looking at wildlife or talking shop with a friend and forgets which gear he is in seems harsh. I think that’s why manufacturers rate their equipment to be “fool proof”. We’ve all probably had one of those memorable moments at some time.

P.S. -- Not the question asked itself, but if 12-27 doesn't work well with the 50-34 for Trophyoftexas, an 11-26 cassette and/or a 48-34 may work fine because of the difference you state above. It's partly luck.

[dbrk]

12-27 with short cage DA and "regular" front der on a 50-34: no problem. This has already been said and, not for anything, this is one of my preferred set ups. I see no downside unless you are riding flats where the 34 is not really great---I prefer 48-36 overall unless you need the gears.

dbrk

[RPS]

12-27 with short cage DA and "regular" front der on a 50-34: no problem.dbrk, are you saying that because it was not a problem for you (and perhaps others you know) that it will not be a problem for anyone?

If you are correct and it's never a problem, then Shimano has not rated their equipment correctly -- IMO. It can't be both ways.

[regularguy412]

My experience has been that Shimano errs on the conservative side when listing the max tooth capability of their rear derailleurs. You can usually get a one or two tooth greater capacity out of the rear derailleur than rated. If you are worried about the RD binding on big-big in an accidental shift situation, you might loosen the B-tension screw all the way off. That will let the cage come all the way forward with less pressure on the chain, the cogs and the whole system.

Mike in AR

[RPS]

My experience has been that Shimano errs on the conservative side when listing the max tooth capability of their rear derailleurs. You can usually get a one or two tooth greater capacity out of the rear derailleur than rated. If you are worried about the RD binding on big-big in an accidental shift situation, you might loosen the B-tension screw all the way off. That will let the cage come all the way forward with less pressure on the chain, the cogs and the whole system.

Mike in ARAgreed. All that is being said by some is that because it usually works, it doesn't mean we can count on additional capacity beyond the listed 29T. Any additional capacity depends on chainstay length and actual gear sizes.

[RPS]

Lots of nice, steep but fairly short climbs coming up for Hellweek and thought this would be interesting to try....taking a couple of other bikes in case the experiment is a flop!Trophyoftexas, if you have a chance, please look me up during Hell Week -- I'd like to meet you and perhaps ride together for a while. I will be the only one there riding a blue Schneider with rear passive suspension -- should be easy to spot (except that unlike last year when I started all the "A" rides, this year I plan to mix it up, so I don't know which rides I'll be doing).

As to your desire to go with a 34/27 for climbing, I can relate. Last year I started with a close-ratio triple (30/21 low gear) and after four centuries in a row, I switched to a 12-27. And I was thankful I did when doing the Leakey Death Ride and Fred's Ride later in the week.