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Old 01-14-08, 03:57 PM   #1
Psydotek
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Which rear derailuer? I'm confused... (light touring bike build)

Again, i appeal to the mechanics forum gurus for help...

I'm planning on building up a Salsa Casseroll for light touring duty. I'm wanting to put on a MTB 9-speed cassette with a 34t big cog. However in my research i'm looking at the Nashbar rear derailuer page and i can't figure out which one to get...

http://www.nashbar.com/results.cfm?s...%20Derailleurs

I'm looking at the Shimano Deore XT derailuers, namely the m770 and m772 models but i can't figure out the difference between the Low and Shadow Top models... Which one do i need?

I'm so far planning on using downtube Dura Ace shifters (though if i can score some STI levers for cheap i might get those if it makes a difference with the RD choice).

Also, i'm going to be using either a compact 50/34 crank or a triple crank...

And as an ancillary question, which cage length (the 770/772 seem to come in medium and long cage) would be best suited for my drivetrain?
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Old 01-14-08, 04:15 PM   #2
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I personally would get the traditional top normal RD, long cage. I run an XT (M750) with 9 speed DA bar end shifters on my Long Haul Trucker.
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Old 01-14-08, 04:25 PM   #3
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Is the m772 "Shadow Top" the same as the traditional top? Or is there something i'm missing...

I see the XTR 971 Top-Normal (Traditional) derailuer but i'm not sure if i'm willing to shell out nearly $200 for it just yet...
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Old 01-14-08, 04:27 PM   #4
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Have you been to the Shimano websites to compare these derailleurs? There is a lot of useful information there. As you know, you'll need a mountain style RD to handle the 34 tooth cassette cog, or any cog bigger than a 30. The cage length needed is a function of chainwrap capacity or "total capacity" which is the difference between the largest chainring and smallest chainring added to the difference between the largest cassette cog and smallest cassette cog. Shimano's total capacity and largest cog limits are conservative meaning that you can usually exceed them a bit. Your choices of crankset and cassette will affect the total capacity, just do the math.
A "low normal" mtn. style RD is designed to line up with the lowest (largest) cog when all of the cable is out of the shifter or when the cable is detached from the derailleur. When shifting a low normal to a higher gear you will be using the shift lever in the opposite sense from the normal way that a road derailleur would work (STI brake lever for higher gear instead of the normal small lever).
With the Shimano mountain type cassette you can use 9-speed STI shifters, 9-speed downtube indexed shifters, or friction shifters. IRD makes some 10-speed mountain type cassettes compatible with Shimano. This would be a more expensive choice and would require 10-speed shifters and chain.

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Old 01-14-08, 04:29 PM   #5
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Is the m772 "Shadow Top" the same as the traditional top? Or is there something i'm missing...
Yes, top normal. If you read further into their description it states so.
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Old 01-14-08, 04:33 PM   #6
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Thanks! The low/top normal explanation is what i was looking for. I'm used to road bike parts but in my quest to attain low gearing i want a MTB derailuer (which i'm not too familiar with).

Yeah, i visited the website and know about cog/chainwrap capacity. I just have (well, had) no idea which derailuer (low/top) would be most compatible with regular road shifters...
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Old 01-14-08, 04:45 PM   #7
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Thanks! The low/top normal explanation is what i was looking for. I'm used to road bike parts but in my quest to attain low gearing i want a MTB derailuer (which i'm not too familiar with).

Yeah, i visited the website and know about cog/chainwrap capacity. I just have (well, had) no idea which derailuer (low/top) would be most compatible with regular road shifters...
From my experience with rental touring bikes it just doesn't make much difference if the rear derailleur is low normal or high normal. My wife and I adjusted our brains to low normal quickly and I see no big advantage either way. The low normal may be easier to adjust to for a Campy rider. I ride both Shimano and Campy so I stay confused anyway.
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Old 01-14-08, 04:48 PM   #8
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Okay, when looking at a rear derailleur, there are two major factors: 1) Max cog limit (determined by upper parallelogram) and 2) Total capacity (determined by cage length).

For the first two factors, just use this chart:

Road SS ("short" cage)............Double chainring....27T or smaller cassette
Road GS ("medium"/"long").....Triple chainring.......27T or smaller cassette
MTB GS ("short"/"medium").....Double chainring.....28T or larger cassette
MTB SGS ("long" cage)............Triple chainring........28T or larger cassette
Models are "upwardly" compatible (that is, a given model will also work with the setups listed above it).

With a 34T cassette, you definitely need a MTB model. With a double, the GS model would work, however a 50/34T crank and 11-34T cassette setup is far exceeding the MTB GS total capacity of 33T. Your compact setup has a capacity of 39T [(50-34)+(34-11=39)], so the SGS would work best. If you go with a triple, you definitely need a MTB SGS, like the Deore XT SGS.

You also mentioned two other, less important, factors: 3) Top Normal (traditional) vs. Low Normal ("Rapid Rise") and 4) XT/XTR Shadow vs. traditional.

3) Most bikes use the traditional Top Normal design in that the rear derailleur naturally/normally sits at the smallest cog/highest gear/"top" of the cassette. You pull the cable to move it to the lower gears. Rapid Rise Low Normal is just the opposite: it sits at the low gears and you pull it to the high gears. It really comes down to preference, though I prefer the traditional Top Normal design because with STI levers I can upshift one cog at a time as I accelerate, but quickly downshift up to three cogs at a time when coming to a stop. "Rapid Rise" Low Normal, however, is supposed to make downshifting easier (especially under load) because the derailleur can shift better than you can. I'm guessing it won't matter too much for DT shifters (however, Rapid Rise would mean both shifters would move in the same direction for up- or downshifting).

4) The XT and XTR "Shadow" RDs use a new design in which the RD sits closer to the frame and does not protrude past the quick-release nut. The main advantage to this is that if the bike falls to the right, there is less chance of dinging the RD or bending its hanger, which is appealing to MTBers. Be aware that these RDs do not have barrel adjusters so they may be difficult to set up with down tube shifters.

Edit: Took me so damn long to write all that, most of it's been covered already!
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Old 01-14-08, 07:48 PM   #9
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Awesome, thanks for the writeup. Now just to make a decision on which way to go with the shifters then...

I guess i'll be going with a top-normal derailuer to keep things consistent with my other two geared bikes.
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Old 01-14-08, 08:11 PM   #10
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Since you'll be going 9-speed, I'd look for some new Tiagra shifters. They have the improved feel of the newer 10-speed 105, Ultegra and Dura Ace models. You can still find older 9-speed of the better models, but I don't think they feel as good, and I've heard good things about the Tiagras.
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