HornedFrog

5k miles on 9speed Shimano transmission, it's been rough in the higher gears for a few months now...especially noticeable on the trainer.

replaced the chain and cassette last night, still feels rough. before anyone says it... yes i did route the chain properly through the rear derr jockeys so it's not rubbing on that tab.

bottom bracket was replaced ~4months ago... so all i can think of is the only other thing it could be is the crank. the roughness appears in all chainrings, even the inner one which hardly ever gets used, so i'm bit puzzled there. the roughness is most noticeable in gears 6-9, in all chainrings.

i have cheaper stuff, so i can replace the whole crank and chainrings for $60, so i'm thinking why not just do it...

any advice would be greatly appreciated!

edit: when i spin the transmission in reverse, i don't feel/hear anything...so it's only when transmitting torque do i feel it... so could it be the bottom bracket?

joejack951

Sometimes a new chain and cassette need a little meshing period before they smooth out. Small burrs and sharp edges on both chain and cassettes cogs being the culprit. If you can pull the crankset yourself, you might as well check the bottom bracket, too. I mention pulling the crank to check it because I've never had luck felling for slight roughness with the arms attached (too much leverage). Using my fingers to rotate the axle tells the real story even when it seems fine spinning with the cranks on. You might have just gotten a bum BB which failed early. It's happened to me.

HornedFrog

I actually wonder if its the deflection of the cassette/chainstay due to clamping on the trainer that causes what i'm seeing.

i noticed at high rpm in a higher gear that the chain looks like it's vibrating in a standing wave. i don't think that would happen from just wear/profile differences of components. haven't ridden on the road with the new chain yet, and won't do so this weekend since it'll be raining the whole time.

FBinNY 

You get the standing wave pattern because of how chains work - see chordal action - and because of chain/sprocket misalignment in most gears. When the chain is not aligned with the sprocket, the teeth alternately press on the close fitting and loose fitting side plates. That means that one tooth is holding the chain off line until it clears, while the next allows it to pull straight. That sets up a side to side vibration, and if you dial in the forces and speed you can establish a standing wave.

The vibration diminishes with age because side wear on the sprocket teeth, allow the chain to move back to line more smoothly. It's like the difference between riding your bike off a 2" driveway step vs. one that was blended into the road better.

The effect is most pronounced with the greatest alignment error, and non-existent when the chain is running inline like on a SS or IGH drive.