Mechanics and Fixing Any Questions Answered

My chain has been slipping forwards off the cassette. It looks like the chain doesn’t seat into the teeth of the cassette.
I’ve just had a new chain and cassette fitted and it’s still doing it!!! Usually in granny ring.
The chainrings were new not long ago. The derailleur hanger doesn’t seem bent.
I have an eccentric bb so wondering if there could be an issue there.
Any suggestions?

came off the other week and thought bike (fairlight strael) was largely ok except for a fucked fender. i was just waxing my chain and realised the drive side crank gets VERY close (basically rubs) the chain when it is in the smallest cog at the back..... clearly something afoot here...

given the bike fell onto the right side i assume (and am hoping) the crankset is just bent inwards slightly. before buying a new rh crank is there something i can do to check it isnt the frame or something? no dings or dents in the frame... doesn't look out of plumb? has anyone observed this with a shimano crankset before?

The usual string from the dropout to the head tube, measure the gap from string to seat tube and repeat on the other side. You could also take the crank off and put the frame on a table with the rear end hanging over the edge, the BB flat and measure the height of the dropout from the floor and flip, repeat. That would be more of a pain without removing parts though

You could just measure the gap from the end of the crank arm to the seat tube on both sides. Or the downtube or both. The frame would be unlikely to be bent in both vertical and horizontal planes but the other measurements would rule it out.

Another one, you could measure the gap crank to chainstay, remove the crank, install it backwards so the drive side is on the left and see if the smaller gap to the crank arm stays on the right or follows the drive side crank

thanks @PhilDAS - appreciate the advice

measurements all seem to be somewhat ok. when putting the crankset on backwards i do think the gap stays with the drive side vs non-drive BUT it is actually quite hard to say because the chainstays are ever so slightly different formed (i.e. dimpled for drive side and slightly different curve for the non-drive to make room for the disc)

i think i'll just have to try another crankset annoyingly. hope someone in my cycling club has a spare

Yep Shimano cranksets are known for this. Another brand or model will probably be fine.

my ultegra crank touches the chain

borrowed 105 crank clears by 2-3mm :-)

assume frame is ok so will hunt down a new crankset/rh arm

• edit - misunderstood the problem
  

is it possible to put r8000 (11sp) chainrings on a r8100 (12sp) crankset? just looking at options for new cranks and potentially putting a PM on my summer bike

additionally - it is possible to run 11sp chain on the 12sp rings yes?

Worth it because fun.

If saving the cost of three EW-SD50s and a B junction, and it takes less than say, two to four hours (depending how you price your time), the fun is pure bonus.

Like I say, if you're the type who hangs onto old USB cables for the wire, why would you pay for such spendy stuff as Di2 cables?

Go to your LBS and get it inspected under warranty, said it “bent” when riding (if it’s 6800/R8000)

i will :--) thanks

The new crank is basically a relabelled 12 speed one with 11 speed chainring.

saw a video on youtube; seems the chainrings arent cross-compatible :( i.e. the 12sp will not fit on the 11sp crank and vice versa

Should be no problem running a 12s crank on an 11s bike; worst case scenario you have to shim to get the right gap, but that's unlikely.

was more concerned about the 11sp chain on a 12sp ring but according to various people (incl. youtube) it seems fine

All chains from 9 speed to 12 speed have the same internal width of 2.18mm. For once YouTube is right.

thank you sir :-)

I'm currently failing to sell a brand new in box R8100 chainset, 50/34 170mm. £140 posted if it's helpful?

appreciate that but too short for me :-)

Are you some kind of giant?

6ft 3 . i typically go for 172.5 after a bike fit though !

after a bike fit though

Probably the right choice then.

Yeah supposedly, Shimano 12 speed chain are terrible for longevity also, best to go for an aftermarket brand.