Chain Alignment

woolwich

Folks,

I have fitted a new Sram Apex groupset to a frame and have hit a snag. All components are Sram except the cassette which is new 105 as thats all the LBS had.

Installation was easy, clear instructions, nice components ect. I had the indexing/ hi-low and b screw adjusted in moments to achieve a nice crisp change both ways whilst pedalling normally.
However if I back pedal in the highest gear, big ring and small sprocket, the chain rolls off the ring and hangs on the crank. It becomes lifted off by the teeth of the ring as if the chain/ring and sprocket alignment was off.

I have checked the chainset is running perfectly parrallel to the frame and am confident the BB faces are aligned.
The BB is 68mm, therefore I have added the two spacers that Sram provide to bring the bearing faces out enough.
I have had straight edges and string all over it and and the rear wheel seems perfectly aligned with the frame, as does the cassette on the freehub.
There appears no manufacturing defect with the rear mech cage and it is adjusted in the high position to only just bring the chain onto the smallest cog and no further.
The rear mech hanger and alignment seem to be good.
So I am completely lost for an explanation. The bike seems to ride well with no untoward noises at all. Although I cannot help but think that excessive wear is being caused somewhere.
Are Sram chains notorious for lack of side to side play for a couple of hundred miles until bedded in?

I would be grateful for any thoughts.

Twostage

woolwich wrote:

Folks,
However if I back pedal in the highest gear, big ring and small sprocket, the chain rolls off the ring and hangs on the crank.

Don't do it then.

nicklouse

Err road cranks and GXP BB do not need any spacers.

woolwich

Twostage, Yep just not doing it crossed my mind but you'd be amazed how often it's helpful to back pedal half a turn. Clipping in, getting set for quick away from the lights, getting the cranks level for hopping speed humps, lubing chains ect. Besides, I am a nerd and knowing its just not right would bug me.

Nicklouse, can you confirm this? I am all over the tech drawings and for all Sram kit, except the latest press fit, the story is the same, for a 73mm wide BB no spacers, for a 68mm wide BB space it out. Doesnt seem to be any mention of whether or not its a road or MTB crank. Also without the spacer wouldnt the problem be worse?

Thanks for your thoughts though.

nicklouse

woolwich wrote:

Nicklouse, can you confirm this? I am all over the tech drawings and for all Sram kit, except the latest press fit, the story is the same, for a 73mm wide BB no spacers, for a 68mm wide BB space it out. Doesnt seem to be any mention of whether or not its a road or MTB crank. Also without the spacer wouldnt the problem be worse?

Thanks for your thoughts though.

stop looking at the MTB specs and look at the road specs.

there are no road frames with 73mm BBs it is a MTB thing only. Apex is road. and yes it will move the chain line out.(but you say)

the chain rolls off the ring and hangs on the crank

so the crank is not out enough.

also you will not be overloading the bearings. these do not use preload and there should be a gap on the drive side.

woolwich

Nicklouse,

Thanks for your perseverance with me here.
Having just driven myself nearly insane, the penny dropped. Rather than label their manuals with words or highlight the text, they use an illustration in the top left corner. A dropped bar bike for road, a full susser for MTB. Actually pretty bleeding obvious when you know the convention or a hole waiting to fall into if you don't. I suspect I'm not the first but feel a bit of a fool anyway.
20 minutes later and the problem is practically solved. Actually it has still ridden off the ring if i spin it backwards quickly but I'm confident its going to be fine in normal use.

Cheers

John.T

My Dura-ace 9sp used to do when first fitted. It stopped after a few weeks use. Things may just need to bed in a little.

nicklouse

Yep as parts wear out they flex more reducing the side loads.

John.T

nicklouse wrote:

Yep as parts wear out they flex more reducing the side loads.

In my case it sorted itself out in about 200 miles. Long before things started to wear out. Just initial bedding in.