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Discussion Starter · #1 ·
I have an 07 Rock Hopper and the chain began jumping off the 32T chainring when climbing rough terrain. So I upgraded to a Deore crankset, replaced the cassette, and replaced the chain all within about 20 miles of riding. Chain still jumped so checked and double checked Acera front and Alivio rear adjustments, cables, and hanger. Chain still jumping. Blah blah blah.

Only thing I can think of is the cage on the Alivio is not keeping the chain in enough tension since the chain won't jump off the larger 42T chainring. So, upgrade the rear derailleur or am I missing something? Thanks.
 

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The only things....

that put tension on the chain are the rear derailleur and the act of pedalling. The only time the chain should be affected by the front derailleur is during shifting in the front. There are three common things that could be causing your problem. First is too long of a chain, second it improper front derailleur adjustment, and third is a worn out tired rear derailleur.

For the first, the rear d can only take up so much slack, if the chain is too long it can cause issues such as this. To check it shift into the small ring front and small cog rear. Then look at it from the side. The chain shouldn't have a noticeable bow in it when at rest. Also push up on the rear jockey wheel cage. There should be a bit of movement left before the derailleur stops. That indicates that the chain is the correct lenght and even in the smallest diameter gears it is still applying tension to the chain. It won't be much movement but there should be some.

Next is the front derailleur adjustment. Shift the front d to the middle ring. Then shift the rear to the largest cog. Eyeball the clearance between the chain and the inside plate of the front derailleur cage. The chain should just barely clear that inside plate. The maximum amount of clearance between the inside plate and the chain should be no more than half the total width of the chain. But the ideal is to have just enough clearance so that the chain doesn't rub the inside plate when you are in 1st gear (largest cog) in the rear. If you have more than that then turn the shifter barrel adjuster counter clock wise until you achieve that clearance. Another adjustment that could be off and cause the problem is derailleur height. If the derailleur is clamped to high on the seat tube then the plates wouldn't line up correctly (be a bit high) and allow the chain to slide past. To check this shift the front into the middle ring and then eyeball the clearance between the front plate of the cage and the teeth of the big ring. There should only be a gap of between 1 and 3mm between the lower edge of the front plate and the large chainring teeth at the point of closest approach. It it's larger than that loosen the clamp and move the clamp height down until you are within that spec.

For the third possibility, there's nothing for it but to replace or upgrade the rear derailluer. The main spring in the rear d is what provides the tension to bring it back down the cassette when you click the trigger and allows the derailleur to take up the slack as you move to the smaller cogs both front and rear. It also provides a bit of tension on the chain. These springs get tired over time, and they get tired faster on the lower end components. Usually though the derailleur pivots slop out before the spring causes issues. But it is possible. If you are getting nice quick crisp shifts in the rear it's unlikely that this is your problem, but check it anyway.

Once you have all of these double checked and are sure that every thing is set right you can go from there. There are other issues that could be causing the problem, things like chainline. But they are less likely.

Good Dirt
 

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Discussion Starter · #3 ·
Thanks for the reply Squash.

I've already taken out as many links as safely possible on the chain and triple checked the front derailleur. Thanks for clearing things up regarding the rear d. With that I'll upgrade it and see what happens.
 
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