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Old 06-27-06, 02:05 PM   #1
lupson
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rear derailur question

can i use a long cage rear derailur with a double chain ring. thanks
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Old 06-27-06, 02:24 PM   #2
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Sure. It'll just shift an eensie-weensie bit slower then a short-cage derailleur would.
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Old 06-27-06, 07:12 PM   #3
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Quote:
Originally Posted by KevinF
Sure. It'll just shift an eensie-weensie bit slower then a short-cage derailleur would.
Just curious... does anyone know why this is the case?

I guess it must be, othewrwise they wouldn't have short AND long cage options right?
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Old 06-27-06, 09:55 PM   #4
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As far as I know, long cage is just for running larger cogs on the rear wheel. They extend further back to accomodate the larger gear. I assume that's why it shifts a tad more slowly, because it's moving a longer distance (we're talking milimeters here)
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Old 06-28-06, 06:18 AM   #5
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Long cage is all about chain wrap when you're in the smallest front ring.
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Old 06-28-06, 08:06 AM   #6
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Yes, cage length is all about chain wrap. For cogs larger than 30t you need a mountain bike derailleur, which is also long cage.

Al
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Old 06-29-06, 03:44 AM   #7
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Yep, cages come in different lengths, but a long cage will always work (so you don't need a short cage).

But why are short cages faster at shifting? For a given cassette they are moving the same distance.

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Old 06-29-06, 04:18 AM   #8
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Quote:
Originally Posted by urbanknight
As far as I know, long cage is just for running larger cogs on the rear wheel. They extend further back to accomodate the larger gear. I assume that's why it shifts a tad more slowly, because it's moving a longer distance (we're talking milimeters here)
That's not necessarily so.

Rear derailleurs have two different capacities: Largest rear cog cpacity and chain slack take up capacity. While somewhat related, they are different things.

Largest rear cog is determined by the parallelogram and the path it takes as it moves inward. All Shimano road derailleurs are listed as having a 27 tooth big cog capacity. Since they are rated the same, I assume that the parallelograms are the same and I don't see why they would shift any differently.

Chain wrap up is determined by the length of the arm. The simplistic view is that long arm derailleurs are for triple cranksets and short arms are for doubles. Typically, a long arm derailleur will weigh around 10 grams more than the equivlent short arm.
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Old 06-29-06, 10:20 PM   #9
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Just to clarify Retro Grouch's great info:

Road derailleurs vs Mountain derailleurs

When you shift across with a road derailleurs, the pulleys tend to stay "level", i.e. almost at the same distance from the hub. They work best with close-range cassettes. If one were to use a road derailleur on a 11-32 cassette, the top pulley would be far from the 11 and 13 cogs, making imprecise shifts, and would litterally walk on the 32, making it very noisy. Indeed, depending on the exact geometry of the rear end, the road derailleur probably won't work well for a cog larger than 30 and may crash into the large cog, break and...

On the other hand, when you shift across, the top pulley moves up and down, so it's closer to the axle at the top end and further away from the axle at the bottom end. So it's perfect for a 11-32 cassette because it means that the top pulley travels always at the same distance from the cogs. On the other hand, if one were to use a mountain derailleur with a 12-23 cassette, the pulley would be very far from the cassette at the "23" end, meaning sloppy shifts.

As to why having the pulleys too far away from the cogs result in sloppy shifts, think of handling a dog with a leash. The shorter the leash, the better you can control the dog.


Short cage vs long cage

A short cage derailleur has a "better grip" on the chain, so the chain starts to move more rapidly than with a long cage derailleur. With modern systems, it may make a difference if you like to shift aggressively during hill climbing or sprinting while standing on the pedals. Otherwise, it's a non-issue, so buy a long cage derailleur because it allows more flexible gearing. But...

A long cage derailleur is a bit low and hangs low on small-wheel bikes. I have a long-cage LX derailleur on a Piccolo (20" wheel) because I had it at home, and it's very easy to have the chain drag about 1" off the ground. While I don't see it as an issue on a bicycle with a 26" or 700c rear wheel, it definitely is an issue with small-wheel folding bikes or recumbent bikes.
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Old 06-29-06, 11:23 PM   #10
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Michel,

That's great info thanks. I will now have to go and more closely examine my bike which has a Deore rear derailleur and a 105 ten speed cassette.

Phil.
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