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When pedalling in higher gears, the crank or cassette seem to completely lose grip for a second, then grip then lose it again. I could probably pedal all day in 1st and any front ring but when start to go in 6/7/8th it will do it.

This is not a gear change problem, all gears change perfectly and when the bike is on the stand everything works perfectly.

In order to try to cure the problem, the front and rear derailleurs have been renewed including cables, the chain renewed, the derailleur hanger also, but the problem remains. I tried swapping the back wheel with known good one off another bike to eliminate the cassette, but the problem stayed the same.

The front crank has a tiny bit of play but the teeth look ok - none obviously more worn or broken than others - the biggest ring on the crank has been bent and straightened, not perfect but it has been working fine previously like that, plus the problem exists on the two smaller rings too.

Any help appreciated

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    So, did you try with new chain and two used cassettes? Sounds like it's time for new cassette. – ojs Mar 25 at 9:18
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    Sounds to me like a worn chain hanging up on the chainring. – Daniel R Hicks Mar 25 at 12:22
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    Have you checked the frame for cracks? – renesis Mar 25 at 13:16
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The comments above are probably correct; if your chain is new, then the issue is likely a worn cassette. Because they wear out as a pair, chains and cassettes are often replaced simultaneously. A new chain on an old cassette can lead to skipping, just like an old chain on a new cassette can lead to skipping.

Another possible cause of "losing grip" could be a damaged or missing pawl inside the freehub body's ratcheting mechanism. If, for example, one out of your three pawls is chipped or has broken off, the freehub will engage fine under light load, but heavier force (like pedaling hard in higher gears) can cause the hub to "freewheel" briefly in the wrong (pedaling) direction.

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  • Ok, thanks for all comments and assistance. Yes it is a new chain, but still the old cassette, The thing is, it had the issue before any parts were replaced.. Which lead me to suspect the cassette internals, but I thought I had illuminated that by swapping rear wheel/cassette but identical problem remained.. As the general consensus seem to be cassette then I guess that should be my next purchase and I will also check the frame thoroughly for cracks.. Any thoughts on chain length? As when chain was replaced I just took it out box and fitted it, regardless it had the issue with old chain. – Bodaeshus Mar 28 at 6:44
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    While the cassette itself does not have any moving parts, the freehub body on which the cassette sits does (e.g., pawls). Regarding the chain, if you took the chain out of the box and put it on the bike without shortening it (removing links) to match the length of your old chain, then it is almost certainly too long. A chain that's too long can cause a handful of problems, including skipping/slipping off gears. If you already threw away the old chain, you can find loads of resources for sizing a chain to your bike on this site, Sheldon Brown's website, or ParkTool's videos, to name a few. – Poquontchn Mar 29 at 9:48

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