Compact crankset chainline issues
 

Hi all:
Last winter I bought a Shimano compact crankset (50/36t) on sale and decided it would be perfect for my road bike, which is about 1990 vintage. My live-in bike mechanic (answers to "Honey") put it on, and took a Dura-Ace derailleur (about the same vintage, but mint condition) out of his stash for the 13-27t freewheel.
My knees are vintage and damaged, so I appreciate the lower gear for hills, but I'm a little disappointed that I can't get the 36/15 gear combination. I wouldn't expect to use 36/13, but shouldn't I be able to get the next cog up without the chain rubbing on the larger chainring? Is this a rear derailleur or chain wrap problem? Or is this common with compact cranksets? The chainstays are 16-3/4" from bottom bracket to hub center.

If the problem that you are having with the 36/15 combo is the chain rubbing on the larger chainring, then this has nothing to do with the rear derailleur or chain wrap, and is all about chainline. Since you said that it is a Shimano compact crankset, then I assume that it is the Hollowtech II style with an integrated axle and external bottom bracket cups. If so, then you can't really change the chainline of that crankset. I therefore can't see a way to solve your problem except to go to a triple crankset, which will yield many advantages in addition to being able to access the whole cassette from the middle ring. I hate compact cranksets because the gearing combinations needed for most people's general riding speeds encourage a lot of cross-chaining and double-shifting.

You happen to have very short chainstays, which have a problem with chainring clearance with a large difference between big and small ring. The shorter the chainstay, the greater the likelihood of chains rubbing.. It's not the cranks fault, as the chainline is where it is "supposed" to be for a double. It's just the combo of the rings and short stays.

While I have never had to do this, you could try using a freewheel spacer of 1, 1.5 or 2mm. This goes on before you thread the freewheel. It may help alleviate the angle enough to prevent the rub.

Since you need the lower gears , you have these options off the top of my head:
1. Get a different 110bcd or 110/74bcd crank. Get a square taper though, as this allows chainline adjustment. New or used would do. You can buy a triple for the price of the double, but you don't have to install the third ring, just install it as a double with a double or triple BB length. You may need to try different BB's to get the right chainline. This said though, because of the severe angles of short chainstays, you may be better of with a triple, so you can use close range rings. . . say 26/36/46 or 48. I suppose you could just use a 36/46 double too.

2. Use you previous crank and a wider range freewheel, though you'd need a different rear D.

But, with a triple you need a new RD also, and a new FD. . . so heck, u pick 'em.

:cheers:

The compact double on my Klein did that. I spaced the bottom bracket over with a 2 mm shim. (I think it was intended for a freewheel).

Check the chainline from the side of the seat tube to the tip of a tooth on the big ring, then add 1/2 the ST diameter. It should be 47-48mm.

Your chainstays are NOT short at all. All of my bikes have 406mm or 16 inch chainstays and I have no problem with chain rub, even using a 50/34. I can use the 34/12 with an 11-25 cassette.

Thanks guys, lots to consider. I'll look for a shim of some sort. Or a triple.

Short to me:lol:

Shimano's integrated double cranks give you the standard chainline of 43.5. The owner can take it or leave it, but not change it. 47-48mm is a mtb triple chainline.

That's the downside of double integrated cranks, no chainline adjustment.

Here's the spacers(first three listed), but they are available from any shop.

http://www.biketoolsetc.com/index.cg...-and-Freewheel

http://wheelsmfg.com/content/view/517/42/

While we're throwing out options, have you considered replacing your rear wheel with a freehub version and getting an 8 or 9 speed cassette? A 12-27 9 speed would move the 15 tooth cog into the fourth from the chainstay position making it easily accessible. Of course, there are numerous other benefits to a 9 speed cassette as well, along with a few minor drawbacks.

Wrong again. 43.5mm is the distance to a point out in space, between the two chainrings - a stupid way to define chainline. The 47-48mm is to the tip of a tooth of the big ring - something you can measure with a scale. You take the 43.5 and add half of the typical 7.5mm chainring c-c spacing.

Dave,what I stated are the standard industry chainlines. If you have your own method, thats fine . . but this isn't third grade . . .so lets spare the reprimands:lol:

The point IS the chainline on a Shimano Compact crank cannot be altered. If you have a way to solve the OP's problem, please share.

My girlfirend's bike has the same clearance issue with a 50/34 FSA Gossamer crank and a shimano 9 speed 12-25 cassette. With her chain on the 34, it rubs against the 50 on her third smallest cog, a gear which I think she should be able to get. I know you are not supposed to cross chain too much.

My question to the mechanics out there: what if I put some sort of a spacer (wouldn't need to be more than 2mm I'm guessing) around the spindle between the crank and the drive side external bearing? Are there any problems with that and do crank spindle spacers exist? Thanks.

You didn't comprehend what I wrote and told me that I was wrong, but the reverse situation is rude?

I clearly stated the point to be measured to - the tip of a tooth on the big ring. Triple crank chainlines are smartly defined to the middle ring, which can be easily measured.

My point is that something must be wrong or this would not happen. The chainstay length is not a problem. There's no mention of a possible chain mismatch, like a 9 speed chain with a 10 speed crank. A frame that old might be out of alignment. There are lots of possibilities.

You know DaveSSS, this is what sucks about communicating in forums by the written word. I misunderstand what you're saying, you misunderstand what I'm saying . . . then it goes where no one intended.

My intention was not to conflict with you , but it turned out that way . . . for that I do apologize.


That said, yes it hard to diagnose someones bike issues online. Ideally you'd like someone to post their complete drivetrain, and weather the alignment is proper or not. That's not usually the case though, as people are usually desperate and not really thinking the devil is in the details. But, you gotta start somewhere to learn all this, and these days it's easier for many to pick up a mouse and type in a forum than it is to research what the problems may be. Even with the knowledge of what you find, you may not know what to do with that knowledge. So you're back to the forum:)

:cheers: