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chain meshing (or not) on the 39T chainring


By Slowpup - Posted on 27 June 2010

NB: Originally posted elsewhere on the Global Riders Network and appears via syndication.

I've just put a new drivetrain on my scummer, well everything except the crankset and chainrings, and had an interesting problem this moring.

The bike was 8 speed when I got it, and I replaced the crankset and chainrings with a carbon Truvativ Rouler, with new 10 sp FSA rings that I had hanging around. After about 500 easy k's I've upgraded to 10 speed. Dura-ace chain, Ultegra 10 sp FD, 105 cassette. Everything appeared perfect on the bike stand, and on a short test ride around the block yesterday.

Today, with the rear in about the 5th cog, when changing down at the front, after about 16 k's the chain shifted so the inner row of side plates was running on the top of the chainring teeth. Hence no drive. This wasn't chain skip or anything, there was literally no drive, jsut the sound of a chain sliding over metal. I couldn't change down at the back as the cassette wasn't turning, and back pedalling didn't help, so had to tap the chain with my heel. Looking down, the chain was actually being held sideways (towards the little ring), but had a bend in it which allowed it to run hard against the big ring or so it appears?

Later, with the rear in the 2nd largest cog a similar occurred, except that now the outer row of side plates was over the top of the teeth? A tap of the shifter fixed this..... Almost dumping the chain off the chainring, but magically not.

Does anyone have any thoughts? Could I have ended up with a directional Dura-ace chain, even though it was supposed to be the older series, and it is on upside down? I can't see any odd markings of shape. Should I change my little ring, even though there is no visible wear, or measurable either? Do I have a poltergeist in the drivetrain?

I don't know how it was possible to run against the big ring either, the pick up ramps/pin should have stopped this....... The big ring is definitely not on backwards.

Is it possible that the spacing on an 8 speed crank arm is different to a 10 sp, so even though the rings are 10 sp, the crank arms could be 8 or 9 sp width?
'Tis tres embarrasment to be cranking up hill and then fall into neutral.....

Or could it be caused by a lack of chain tension? I just checked again, and the chain is where I'd have it in terms of length on the MTB, but could lose a link and still shift big-big OK?

Check that you have your little ring the right way around. Some little rings have a slight offset, so if it's the wrong way around there could well be an ever so slightly larger gap between the rings than what there should be, causing the poor shifting.

Spacing on crank may well differ.

8-speed chains are wider, so this would make sense. We need fankles to chime in on this to confirm.

Thanks for both points. I can machine the crank spider arms to compensate if required, will check the direction on the little ring tonight.

Cheers.

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