Rough Drivetrain in Highest Two Gears
 

I'm getting close to finishing up my current build---a secondhand Gunnar Roadie built with an Ultegra SL compact double groupset. I've adjusted my derailleurs to where the shifting seems spot-on, but I've noticed that the drivetrain runs pretty rough in the highest two gears, running more smoothly once I move up to the third smallest cog.

It seems like a chainline issue, with the chain traveling over the big ring and small cog at enough of an angle to create resistance and sound "chunky." I've measured the chainline, though, and it seems reasonable. The front measures at the normal 43.5mm. The middle of the cassette is within a millimeter or so of this.

Is this normal for a double chainring setup? Or is there something else I should be looking at?

Thank you.

Try giving the barrel adjuster at the RD a 1/2 turn clockwise. Sounds like the tension just needs some tweaking. Also make sure your limits are set up correctly.

I'll give it another go when I get home tonight, but as I mentioned, I'm pretty sure cable tension and limit screws are adjusted properly.

From Park Tool website:

B-Screw Adjustment
After setting the L-screw, check the "B-screw" for an adequate setting. The B-screw controls the derailleur body angle, hence the name, B-screw. Shift to the innermost rear cog, which is the largest cog. View the upper pulley relative to the largest cog. If the pulley is rubbing against this cog, tighten the B-screw to increase upper pivot spring tension, which pulls the pulley back and away from the cog. If there is a large gap between the upper pulley and cog, loosen the screw. To find a good setting, loosen the B-screw until the upper pulley begins to rub, then tighten the screw to get clearance.

Oops, I forgot to mention that I had set up the B-screw properly as well. The pulley is not contacting the cog.

Speaking of the pulleys, though---does it matter which pulley goes on the bottom of the cage and which goes on top? I've noticed that the two pulley wheels are different (different bearing types).

FWIW I have always found the smallest couple of cogs to be a little noisy. I have always thought it had to do with the tighter bend that the chain is making around the smaller cogs. Limit screws control where it ultimately stops. The high limit could have an effect on the smallest cog but not the next one up. Cable tension could have an affect, but if the chain is well centered and quiet on the rest of the cogs this is not the issue either. What you are describing sounds normal to me and although it is apparent in the workstand, you will not notice it on the road.

Are the cassette and chain new? I've had this sort of problem with a new chain and an old cassette.

This makes sense. On my singlespeed mtn bike, where chainline is more important, I had similar issues until I switched to a BB with a longer spindle length. And you're right, I never noticed it while riding---only on the workstand.

All drivetrain parts are brand new.

I'm having the same problem with a bike I just built up. All the parts are new and I think I have it all adjusted right.

Once in a while you can get a chain that for some reason doesn't work as well as it should.

I'm having a similar issue with my SRAM stuff. I'm going to try a KMC chain, which I've had great luck with on my Ultegra ride.

The pulleys can be different. http://www.parktool.com/repair/readhowto.asp?id=76

Well, I re-checked everything: tension, B-screw, limit screws, pulleys. Everything seems to be as it should be, but it just seems like there's too much noise coming from the drivetrain compared to my other Ultegra 10-speed bike (triple). It's just not nearly as smooth.

I'm stumped.

Mine seemed to be a little noisey too when I 1st got it and checked it out on my stand. I too have a full Ulgegra group, and after putting about 100 miles on it this week (with quite a bit of climbing) it seems to have quieted down some. As long as it's shifting smooth, see about putting some good miles on it and then check it out. You can spin it all day long on your workstand, but there's no substitute for miles on the road under load.

FWIW, Here's the exact text from an email from my LBS:

"if everything is new and the drivetrain seems noisy and not silky smooth, the chain is almost always at fault (even brand new). I see this with about 1 out of 5 Shimano chains, 1 out of 20 SRAM chains, and never with a KMC chain."

Have you tried just riding it and seeing if the chain needs some break-in time? I know that's worked for me with a new drivetrain I installed a few years ago.