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I just got a pair of Deore LX ST-M580 dual control levers on the cheap and I'm trying to figure out if I want to put them with a top-normal rear derailleur or a bottom-normal.

It seems to me like I would want to be able to shift two gears lower at a time for things like climbing rough hills. So my intial instinct is to go with top normal. However, the default setup in the Shimano manual is with a bottom normal (allowing shifting to higher gear by twos), so there must be a reason.

So I ask all of you: why would I want to run a top-normal with this set up and why would I want to run a bottom-normal with this setup? Experiences? Theories? Thanks
 

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I think that low-normal (or bottom-normal) is the way to go with dual control levers. Shifting into a lower gear demands only a quick flick of the wrist (or N quick flicks if you want to shift N gears at a time.) Shifting into a higher gear requires pressing downwards on the lever. I find the wrist flicks for shifting to a lower/easier gear to be quicker and easier than pressing down on the lever.

It's easier on your equipment too. If you run a top-normal derailleur with a dual-control lever and want to shift two or three gears at once, you'll press the lever downwards two or three clicks. This will cause the tension to increase on the housing and the cable. Eventually, this tension will be released, but only after the shift has completed. (I.e. you're using your cable and housing as a spring.) Using a low-normal derailleur in this situation allows the derailleur's spring to do the job.
 

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Well, the XTR 960 shifters were setup like these, 2 in one direction 1 click in the other, and when the 970's came out they made it two in both directions. People were like you and wanted two click in both directions.

Basically, the low-normal setup, while only giving you one downshift click at a time, is better for drivetrain life if you have a tendency to shift under load. If you use the top-normal derailleur you can force the bike to shift two gears under load and possibly break a chain/warp a cassette.

Another thought is that if you're used to gripshift then using a top-normal is more intuitive, the shifting motions are similar.

And yet another thought is that when you want to upshift to a faster gear you'd rather push down, wrapping your hand around more as you increase speed as opposed to releasing your grip in your up-flick while increasing speed.

Also, if you setup with a top-normal your down push on your left shifter will be an upshift, but your right shifter wil be a downshift.

I'm sure it won't take more than a few rides to get used to whatever you decide to do.

FWIW I ride dual control with low-normal (rapid rise).
 
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