Front shifting problems

[Russell]

Right...

I've got M900 shifters and mechs. A Cook Bros standard drive chainset, a 127mm BB axle and a Sram PC58 chain on my Zaskar.

Its shifts fine from inner to middle and vice versa but it is a real struggle to get it to shift to the outer ring, and then when I downshift it skips the middle ring altogether and dumps the chain on the inner ring. The chainset is 110BCD and spacing between the rings (centre line of teeth) is 5mm.

I've tried two different sets of m900 shifters and I get the same problem with both so I'm guessing that the fault is mech/bb/chainset compatibility or my shoddy mechanic skills.

Whats going on!!!! Heeeeeeeeelp!

(Or should I just abandon STI and get some nice reliable thumbies??)

 

[orange71]

is 127mm too long, I wonder?

 

[Russell]

is 127mm too long, I wonder?

I thought that. I discounted it though because all three chainrings are within the mechs range of motion. I hope it's not that because if I fit a shorter BB axle, the inner chainring will be damn close to the frame, if not fouling it

 

[orange71]

Sometimes I find if I change the angle very slightly on the front mech so it's not quite parallel to the bike helps - I find the leading edge needs to be slightly 'further out' than the trailing edge i.e. trailing edge points slightly more towards rear wheel.

Sorry not sure if I've made myself clear there, hope you get my drift!!

 

[Russell]

This probably helps...

Mech and shifter in granny ring position, everything lined up fine. Mech clear of frame.. ready to go!!

Shifter to middle ring pos, mech obviously overshifting as mech is a bit too far towards the outer ring.

Shifter to outer ring pos. Mech blatently far too far over.

I have tried using a different M900 shifter pod that I had and the same thing happens which leads me to think that the problem is with the chainset... I thought that it was standard drive but the rings seem to be too close together, hence the mech appears to over shift. BCD is 110/74 and spacing between the rings is 5mm.

Anyone do anything with this info? :? thanks you!!!

 

[cchris2lou]

as somebody mentioned change the angle slighty for the chain to go on the outer ring .

as far as the mech going too far , have you tried to restrict its movement with the screws ? , you should be able to set it so that it doesnt go this far .

 

[Russell]

If I turn the 'high' adjuster screw in anymore, it doesn't allow the shifter to complete the shift. If I angle the mech, it fouls the rear tyre.

I'm sure that there is a simple solution to this but I'm having one of those moments where I just can't see it!

 

[cchris2lou]

what about the low screw . can you make it start lower as you only use the inner ring with a few sprockets at the back . if your mech start lower it will finish lower too .

 

[Russell]

what about the low screw . can you make it start lower as you only use the inner ring with a few sprockets at the back . if your mech start lower it will finish lower too .

yeah, thought of that too... The mech is adjusted so that it clears the frame by less than 1mm when its on the granny ring. Not sure if you can see this in the pic?

I've now noticed that the outer cage of the mech fouls the crank arm when its shifting to the outer ring aswell.

I'm sure that this is a compatibility issue but I don't know with which bits!

 

[cchris2lou]

your mech and shifter should be compatible , it has got to be the chainset and BB . 68 or 73 mm ? have you got the right one ?

on a different note your cage seems very wide , but it might just be the picture .