[tosa]

I have a CX frame that uses disc brakes. I'm using a Shimano 9-speed double road drivetrain in the front (brifters/FD/crankset) with 9-speed MTB drivetrain in the rear (long cage RD/34t cassette) on it. The wheels are 29er disc wheels (135mm rear hub width).

Using the small chainring/large cogs or large chainring/small cogs is fine, but using the middle cogs causes with either chainring causes the drivetrain to bind up. I think it's due to using a 68mm BB shell (versus a 73mm shell on MTBs) with a 135mm rear hub width. The crankset is Shimano FC-4550, so the width is not adjustable (I think). How can I make this work?

 

[fissiontofallout]

How/where is it binding? It seems odd that a chain line offset would negatively affect the middle cogs, I would expect either the large or the small cogs to be most negatively affected. The first thing I would check would be chain length, followed by derailleur or derailleur hanger misalignment.

If you want to adjust the chainline, the cheap thing to try would be to add 1 or 2mm of spacers behind the drive-side bottom bracket cup (assuming that still leaves enough spline sticking out of the non drive side to solidly attach the left crankarm).

 

[kapusta]

Define “bind up”

 

[tosa]

I think I wrote the symptoms wrong, but I don't have the bike with me to test until the weekend. I'll try to write to the what I recall until I can test the bike again.

The biggest 2-3 cogs seem to work well with the small chainring Let's call the inner most (34t) cog #1 and the outer most (11t) cog #9. I can shift and go up steep hills. Once I shift to cog #4 while still in the small chainring, the chain starts getting caught in something and the rear dearailleur arm starts getting pulled up and I need to shift back to cog #3. I think I could also shift to the large chain ring to make it work (assuming the hill is not steep). I can't recall if I can use the outermost cogs at all (even with the large chainring), so I may have mis-written that in the original post. I will try to take a picture of the drivetrain when I get hold of the bike to show the chainline.

 

[kapusta]

I think I wrote the symptoms wrong, but I don't have the bike with me to test until the weekend. I'll try to write to the what I recall until I can test the bike again.

The biggest 2-3 cogs seem to work well with the small chainring Let's call the inner most (34t) cog #1 and the outer most (11t) cog #9. I can shift and go up steep hills. Once I shift to cog #4 while still in the small chainring, the chain starts getting caught in something and the rear dearailleur arm starts getting pulled up and I need to shift back to cog #3. I think I could also shift to the large chain ring to make it work (assuming the hill is not steep). I can't recall if I can use the outermost cogs at all (even with the large chainring), so I may have mis-written that in the original post. I will try to take a picture of the drivetrain when I get hold of the bike to show the chainline.

I don't think this has anything to do with the chainline being off 2.5mm.

Do you mean the body of the RD is getting pulled back and up into the cassette,

Or do you mean the cage (tension arm with the pulleys) is being extended forward and up?

Or do you mean that the cage swings so far back that the chain is rubbing against itself?

You say this happens in the big ring as well?

Show a picture of the RD when you are in 4th gear.

BTW, the biggest cog is called "1st", the smallest is "9th".