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Uneven chain-ring wear with Speedhub?

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ndt44:
Hi all,

I'm currently on a London - Tokyo tour with a Thorn Raven Nomad, and I've just noticed some odd behavior from my drive-train. I've done about 5000km so far, and am about to do the first oil change. When I back-pedal the cranks, I notice the chain goes slack-tight-slack with each revolution. I've pulled the chain off to give it a good clean, and I can't see any 'wobble' of the front chainring, side-to-side or off-centre. There does seem to be some significant wear on the chainring though. The crank axle isn't bent, and looking at the edge of the teeth as I spin the cranks, I can't see any variation in the profile of the chainring. The only solutions I can think if for this behavior are:

* Uneven chain-ring wear, making the gear somewhat elliptical - some teeth have been worn deeper than others, such that the 'rolling diameter' varies, despite the circular appearance

* Uneven chain stretch (highly unlikely, if not impossible in my opinion)

The chain is really quite slack at one point of a revolution, and then very tight at the other extreme (180 deg later). I have at least another 10,000km to go, so I'm a bit worried about this. I may try rotating the ring 180 degrees to see if I can wear the other side equally, or flip the chain-ring over and start 'fresh' - but I wonder how this could have happened in the first place...?

Edit: I should mention that I have tensioned the chain twice so far, being quite careful not to 'over-do it'. However I did notice some faint pedaling resistance after the initial tensioning - kind of like a stiff link in the chain - but every time I checked the chain it was no tighter than I was shown to do it at the Thorn shop. I have a feeling that I've had this lop-sided behavior for a while, and that my chain-checks were all at a point on the crank rotation where the chain was relatively loose.

Any ideas / advice welcome.

Cheers,
- Nigel

Al Downie:
Hi,

Is yours an XT chainring, from about a year or two ago? I can't remember the exact model code, but there was one set of XT chainrings which was made from *very* soft alloy which wore out incredibly quickly. After a huge number of complaints, they could hardly give these rings away. I'm wondering if yours has worn corresponding to the two 'maximum force' segments of the revolution? (the same segments that 'Biopace' was meant to affect)

ndt44:
Hi Al,

No, my ring is the Thorn 38T super-thick item, and the tension only occurs at one point of a complete revolution, rather than at two (a-la Biopace).

I've combed the bike shops here in Istanbul for a new 38T ring, but came up dry (unsurprisingly)

I used to have a Biopace set on an old MTB from the late 80's - people used to hate them, but I never found too much to complain about...

Thanks for your input!

- Nigel

stutho:
My first guess would be that the chainring isn't quite centred on the spider.  My second guess would be that there is some damage to the spider or chairing.  However from what you have said you have checked out these possibility.

I'd rotate the chainring by 2 chainring bolts.  If the tight spot moves with the chainring then you know the fault is with the chainring, If the tight spot stays aligned with the crank position then there is a problem with the spider or the bottom bracket alignment

Best of luck

Stuart

ndt44:
Hi Stuart,

Thanks for your ideas - yes, I've had a pretty thorough look at the spider, axle, ring, chain & sprocket. Everything looks pretty good and centred, the only noticeable issue is the wear on the chain ring. I'll pull it off and rotate 2 holes as you say, and check where the tautness occurs, then report back here.

Cheers,
- Nigel

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