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12 speed chain replacement


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On 7/29/2021 at 12:22 AM, Mountain Bru said:

Some good advice in the previous comments. 

I'd recommend when you next wander into a bike shop, or order stuff online, buy yourself a chain checker. It'll be like R100, and will save you thousands. (Something like this: https://www.cwcycles.co.za/product/marvel-chain-checker).
It'll show you when your chain is .5% worn, and when it's 0.75% worn. (Have a look at this for more info: https://www.youtube.com/watch?v=gXd-3UnqoaM&ab_channel=ParkTool)

Basically, chains stretch over time due to loads of different reasons. It's not only km dependent, but also how clean the chain is, how much you ride with it "skew" (as in flexed laterally), if it's lubed correctly, how muddy the conditions are etc etc. So some guys might get 400km from a chain, and some 1500km or more. But wear is measured in stretch, not km. It might seem like a tiny difference in length per link, but it really matters. And once it's stretched, you now you sit with a chain doesn't mesh lekker with the gears anymore because it's stretched and the pin to pin distance is too big. And when you ride it, that "not lekker mesh" eats away at the gears and causes them to have a different profile. The old chain will still work though because as it's stretched, it's gradually eaten the gears to match it's new pin to pin distance. 


But when you then replace the chain with a new shiny one that should mesh nicely, it doesn't because the old chain deformed the teeth, and then your new chain will jump, and your shifting will suck. Sometimes it'll only be in certain gears on your cassette.... surprise surprise the ones you ride in most often and therefore screwed up the most with the worn chain. Hence the reason for buying a chain checker, and replacing the "cheap" chain before it screws up your expensive chain rings and cassette. Based on the fact that you broke your previous chain, it's possible that it was worn out, and might have chowed your cassette and chain ring. 

So based on what you said though, my advice would be:
1. Buy a chain checker and routinely check your chain.
2. Clean your drive train really nicely, and put some new lube on the chain
3. Make sure your gears are correctly indexed. Also make sure the derailleur tension is correct. I know nothing about Eagle, but on Shimano 12sp, there's a line on the derailleur cage that helps you get it right. Might be a similar thing for eagle.
4. If it still jumps, post pictures here of your cassette and chainring. I'm amazed by how well some guys can identify worn components just from pics.
5. If the cassette is new, and everything else is in order, you'll probably need to replace your chain ring

@Mountain Bru thank you for all the advice, so it was the chainring. Fitted a new one last week and all fixed now, the previous one was second hand so i guess it was time.

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