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Mechanics and Fixing Any Questions Answered

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    Thanks so much for all the helpful ideas! In response to the comments so far:

    • Will check for stiff links when I get the time to pull the bike apart, but I would have thought stiffness should be far more evident on regular use rather than occurring once every so often?
    • Chain line is fine (or at least I haven't had issues with it and when chain and sprocket are new bike is entirely silent) - running a 120mm hub with spacers in the dropouts. Might add that bike has been running fine for a year and a half prior to the problem surfacing!
    • Rear wheel spins fine, freely and noiselessly; what do seized bearings feel like?
    • Could be stones carrying all the way around but both the tires I was running (Schwalbe Duranos and now Zaffiro Pros) don't have much in the way of tread to catch something all the way around..
    • Would run without mudguards but this is my commuter so slightly defeats the purpose if I can't use them!
    • I use one of these lockrings (https://www.halowheels.com/shop/spares/hub-spares/fixed-sprocket-lockrings/) which has a recessed section (unless I was somehow meant to install it the other way..). I did check both the lockring and cog and both are on fairly tight. Was thinking about the cog jumping but in each case the spinning should be in the direction of my pedaling - so surely it should feel almost like the float you get when you're between engagement pawls on a freewheel, rather than a resistance which jams the wheel?

    Chainring is the only thing I haven't really thought to change.. more detailed pics attached! Come to think of it, this problem seems to only have surfaced after I changed out the chain (worn cog) then the cog a month later.. But both are 1/8; chain was new and cog still loads of life left, if that helps.

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    the spinning should be in the direction of my pedaling - so surely it should feel almost like the float you get when you're between engagement pawls on a freewheel, rather than a resistance which jams the wheel?

    Not necessarily, pedalling can tighten the cog against the hub leaving it some space to undo until it hits the lockring and jams again and it can tighten itself thoroughly enough against both the hub and lockring that the movement isn't immediate. Like, it'll jam so tight against the hub that you can get a bit of resisting the pedals before it breaks free and then conversely it jams so tight against the lockright that you get a bit of pedalling before it breaks free.

    I don't know that this fits with what you are describing though, when it's happened to me it's a rather soft feeling, what you are describing sounds harsher.

    Looking at the photos you've posted 2 things strike me,

    1. There's a bit of paint loss on your chainstay around where the end of the spider would pass, is it possible that the crank arm (or a bulkier chainring nut) is contacting the chainstay here when you're riding and get a bit of flex?

    I don't think this is very likely btw as the paint loss doesn't seem bad enough for this.

    1. I think this one is more likely, could the top/front of the mudguard be jamming between the brake caliper/brake bridge and the tyre?

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    Yeah I would have expected a soft feeling is what I mean! Never equated the chain stay with the spider but you definitely have a point. Probably not the flex (and not at that point of the stay surely?) because this didn’t happen when climbing, but when going straight..

    Also have approx 2-4mm clearance on the mudguards so should have been quite alright on that front I would have thought! Either that or the rubber straps holding them to the chainstay should have popped off..

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