crank choice/chainline and oval chainring shifting?

[mr4fox]

Im looking for a new power meter and also need new cranks for a new build. I'll be running Dura ace 9070 di2 except for the rear mech which will be ultegra R8050
On my BMC TMR01 i have a power2max NG with absolute black oval 50/34 rings and have really reliable shifting. not dura ace smooth but its still quick and doesnt drop the chain (have a chain keep fitted). I tried fitting 52/36 but just couldnt get reliable shifting. id like 52/36 on at least one of the 2 builds so am wondering ....

Could it be the wider 45mm chainline that Rotor uses that causes the poor shifting with the larger rings? Do oval rings work any better on a more "normal" 43.5mm chain line?
Would say a quark Dfour or shimano chainset with 43.5mm chain line help? I seem to recal it struggled to lift the chain up onto the 52.

I would go straigth for a power2max NGeco with rotor cranks again if i though it would work but at this stage im skepticle

[alcatraz]

Did you take your time adjusting the FD after swapping chainrings?

Apart from the rings being different or maybe chain being a bit short, I can't really see why 52/36 rings would behave a lot different than 50/34.

It is true that 34t rings are nearly round even when they are called oval. Possibly the 36t is moving the chain up and down more on the FD cage causing something when trying to shift up.

Maybe someone else has an idea what to check.

- chain tension
- fd position and adjustment of cable tension
- compare shifting to big ring in different places on the cassette
- make sure the rings are mounted in the right position, 36t being more oval it will be less forgiving than the 34t if mounted wrong.

/a

[mr4fox]

Loads of time!! Normally i have Dura ace rings shifting perfectly in about 10min but i spent hours trying to get reliable shifiting withthe 52/36 it would work but would drop multiple times every ride when when shifting up....or it just wouldn't up shift and would make a racket trying and slipping.

Chain is correct length acording to shimanos instructions.
Its a Di2 9070 front derailleur so no cable tension to check
form memory thrying to shift up at the front while in the largest couple cogs at the back (shiftin into a cross chain position) made it slipp and not shift up more i think.
Rings were mounted in the right position...quadrupel checked it....would have felt a bit lumpy otherwise too.

guess ill jsut order the quarq Dfour and try again.

[alcatraz]

Sounds like that 52t ring has a problem with it's ramps/pins. How about trying a different set/brand of oval semicompact rings?

The chain tension is usually adjusted on the rear derailleur. If you encounter the problem again and suspect chain length you can try to adjust your B screw fully in/out first and see if your drivetrain shifting is improving or same/deteriorating. Avoid small+small gear combo if you have a slack chain.

[Hexsense]

Get Rotor QXL's FD shim.
http://www.starbike.com/en/rotor-qxl-ri ... ces-black/
Use only the thin piece to move your FD back and tilt a bit up on the FD cage's tail.

Then tell us how it works.

[mr4fox]

I did try one of rotors shims without any luck. But i think I only received the large one. I’ll have to dig it out at the weekend and have a look.

I had rotor q rings before switching to Absolute black and have much better shift quality and reliability with Absolute black. They don’t use pins, and the 52/36 set was new when I got them.

I think the problem is that when the derailleur is adjust to not run on the 34:28 gear (11-28 cassette) the derailleur can’t throw the chain enough laterally to get a reliable shift onto the 52. I think tyebovality increases with chainring diameter so assumed that why the 52 has problems. Could totally be wrong though.
I have only tried the 52:36 combo on my rotor cranks (with P2M) so haven’t tried yet with a 43.5mm chain line crankset.

Have just ordered a Quarq Dfour (43.5mm chain line) and will try in a couple weeks when the new frame arrives and update here.

Thanks for the help weight weenies!!