Derailleur cage length.

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Dave W
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Derailleur cage length.

Postby Dave W » 20 May 2015, 9:29pm

What determines the length of the rear derailleur cage - anyone know? Would it be the size of the front ring?

Brucey
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Re: Derailleur cage length.

Postby Brucey » 20 May 2015, 9:46pm

it is just one of the factors that determines the total capacity of the rear mech.

If it appears to have any correlation to the max allowable sprocket size, that is IMHO more of a coincidence.

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Dave W
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Re: Derailleur cage length.

Postby Dave W » 20 May 2015, 9:51pm

:cry: what's that in English?

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531colin
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Re: Derailleur cage length.

Postby 531colin » 20 May 2015, 10:11pm

If you have a big range in the sizes of your chainrings and sprockets, you need a long cage mech. to reel in all the slack chain you can generate.

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gaz
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Re: Derailleur cage length.

Postby gaz » 20 May 2015, 10:16pm

Last edited by gaz on 20 May 2015, 10:17pm, edited 1 time in total.
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Dave W
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Re: Derailleur cage length.

Postby Dave W » 20 May 2015, 10:17pm

I see. Is there a method? Is there a recognised angle of mech when on large front and large back? I suppose when on the smallest rings the cage runs out of tension when nearly horizontal.

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531colin
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Re: Derailleur cage length.

Postby 531colin » 20 May 2015, 10:30pm

The specs. for the mech. will probably give "total capacity"....ie difference between biggest and smallest rings and sprockets added up....which you can often exceed, if you don't use big/big, small/small

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Sooper8
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Re: Derailleur cage length.

Postby Sooper8 » 21 May 2015, 6:34am

531colin wrote:The specs. for the mech. will probably give "total capacity"....ie difference between biggest and smallest rings and sprockets added up....which you can often exceed, if you don't use big/big, small/small


When I was trying to work this out, people pointed me to Velobase and Disraeli Gears web sites ,where you can find the spec of almost any derailleur (front and back). As the above post says, it will show total capacity for your combination.
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Re: Derailleur cage length.

Postby simonineaston » 21 May 2015, 10:01am


That's such a great site - a tribute to one man and an obsession! :wink: Super photos. :D
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drossall
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Re: Derailleur cage length.

Postby drossall » 21 May 2015, 11:09pm

Dave W wrote:I see. Is there a method? Is there a recognised angle of mech when on large front and large back? I suppose when on the smallest rings the cage runs out of tension when nearly horizontal.

Roughly, yes.

As hinted by others, the derailleur does not only move from side to side, to change gear. The cage also swings forwards and backwards, to take up slack caused because the chain length to run round a large ring and large sprocket is greater than that for small ring and small sprocket.

If there's a large range in the size of sprockets and rings (generally on an MTB, touring bike or similar - racing bikes have a smaller range of gears), the derailleur needs more capacity to take up slack, which is provided by a longer cage, which has more "swing" forward and backward in the lower jockey wheel.

You don't want the derailleur (mech) to reach its limits anywhere. As the slack to be taken up increases, the mech doubles the chain back on itself. As the rings/sprockets get large and there is less slack to take up, the cage swings forward to the limit of its travel. When fitting a chain, you use the number of links that will prevent either of those limits from being reached. Chains are supplied longer than any normal bike set-up will need - although someone will be along in a minute to tell you about his bike, which needs one-and-a-half chains.

As stated, manufacturers state capacity, which indicates the difference in the sizes of ring and sprocket that can be accommodated. For example, 11-tooth sprocket plus 40-tooth ring is 51T (teeth). 26T + 52T = 78T. So you'd need a capacity of 78T - 51T = 27T.

Also worth noting is that the mech has to hang low enough to get below the largest sprocket. Racing mechs with limited capacities will probably hang too high to get underneath a 32T sprocket - because they are never going to be used on a sprocket of that size.