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Dumb question RDR related

Old 12-13-11, 12:57 PM
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Dumb question RDR related

Like the title says, this is probably a dumb question but I'd really like an education on this.

What's the big difference functionally between a short cage and long cage rear derailleur?

Is it to do with the size of the rear cogs used?

I've built a lot of bikes but 99% of the time don't change anything because I don't need to, I just service what's there and it goes back together.
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Old 12-13-11, 12:59 PM
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Long cage lets me use a 11-34 on my road bike.

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Old 12-13-11, 01:09 PM
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Originally Posted by 10 Wheels
Long cage lets me use a 11-34 on my road bike.

Interesting. I thought cage length had nothing to do with capacity. I was always taught that cage length is for chainwrap (short for double cranks, long for triple) and that capacity is really determined by MTB vs road derailleurs.
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Old 12-13-11, 01:59 PM
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Originally Posted by Puget Pounder
Interesting. I thought cage length had nothing to do with capacity. I was always taught that cage length is for chainwrap (short for double cranks, long for triple) and that capacity is really determined by MTB vs road derailleurs.
That's why I'm asking, both my bikes have long cage derailleurs, both have double chain rings and nothing real extreme for cogs in back.

I'm doing a little shopping on E-bay for my next build for myself and the RDR I like is really tough to find in a long cage.
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Old 12-13-11, 02:22 PM
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A long cage derailleur can handle a bigger difference in chain length. Consider a 52-40 crank with an 11-32 cassette. The small chain ring and smallest gear on the cassette gives a total of 40 + 11 = 51 teeth. The large chain ring and the largest rear cog give a total of 52 + 32 = 84 teeth. The difference between the two is 84 - 51 = 33 teeth which is the Total Gear Capacity. That number is one of the specifications on a rear derailleur: What is the largest difference in chain length a chain can handle, as expressed as the difference between the max amount and min amount of the teeth on the cogs.

I am getting a close ratio freewheel for my vintage Raleigh Super Course because by doing so I can get the Total Gear Capacity down to a value that can be handled by a short cage Suntour Cyclone rear derailleur in my parts bin (to replace a longer cage Suntour aRX). I think there is more flex and more weight in a long cage derailleur, so shifting can be a little sloppier and the derailleur is more prone to damage.

If that value is not specified in the ebay sales you are looking at, you can try looking at www.disraeligears.co.uk; they list Total Gear Capacity for the derailleurs they show.
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Old 12-13-11, 02:49 PM
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Originally Posted by Puget Pounder
Interesting. I thought cage length had nothing to do with capacity. I was always taught that cage length is for chainwrap (short for double cranks, long for triple) and that capacity is really determined by MTB vs road derailleurs.
Right. Cage length does not determine max cog size. Cage length determines max chainwrap. The distance from the axle centerline to the guide pulley determines max cog size. It's just that a larger max cog size typically also necessitates more chain wrap, so the two attributes become correlated. But it's entirely possible to have large cogs with small chainwrap (like a fictional 24-26-28-30-32-34 cluster with a typical double crank), as well as small cogs with large chainwrap (like using a corn cob in back with a wide-range triple up front).
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Old 12-13-11, 02:49 PM
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Thanks, I'll check it out.
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Old 12-13-11, 08:36 PM
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I've been running a triple with a short cage but you can't really cross chain it or it will start to skip. If you try to run the small front and the smaller back gears it doesn't take up the slack and the chain gets loose cause there's not enough arm to pull the excess.
I'll get around to putting a longer cage on there eventually so the shifting is a little more idiot proof. Till then I'm working on not being an idiot.
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