8 Speed Cassette, 9 Speed Rear Derailleur ?!?

You want the chain to match the cassette for best shifting. The 9 speed Alivo will work fine, the shifter determines where the derailleur is indexed. I would go with a longer cage with that set up and cut the chain to the correct length.

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8 Speed Cassette

Shimano 9 Speed Cassette

Yes you can use a 9-speed derailleur on an 8-speed cassette. All Shimano derailleurs use the same pull ratio, which is 2:1. So pulling an inch of chain moves the derailleur a half inch. Since 9-speed and 8-speed cassettes have different distances between gears, the shifters for each will pull different amounts of cable between detents. So the shifters, specifically the rear shifter, is the part that must match the cassette type.

Derailleurs don't have "speeds" the shifters do. As long as your shifters are the same brand as the derailleur, there will be no compatibility problem. The chain should match the cassette for good shifting, so you need a chain designed for 8 speed cassettes. As long as your rear derailleur is reasonably good, your shifters have far more to do with shift precision than the derailleurs. Many people believe that upgrading a rear derailleur will work wonders, often you will see a bike with a 105 group but an Ultegra rear derailleur and think that the bike is much better. In reality, it only costs the manufacturer a few dollars to make this "upgrade" Better shifters,crank, or brake calipers would make much more of a difference, but would cost much more, which is why these parts are often downgraded

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RE:

8 Speed Cassette, 9 Speed Rear Derailleur ?!?

I already have an 8 speed cassette(Shimano HG30), i want to buy an 8 speed gear changer, and i also want to buy the new shimano alivio m430 9 speed because in reviews it is better than m410 ! I do not know any derailleur with 8 speed and medium cage better than this so i could buy ! So my question...

Alivio will work fine, but get the long cage, med cage works for road bikes.

IG 50