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My drivetrain required total capacity is 14.

(folding bicycle Dahon Eco C6, 14-28 rear, 1x front)

The rear derailleur (Shimano TZ50, which I purchased), has total capacity = 34 and a long cage. Are there are any disadvantages, in such overkill, when total capacity of the derailleur (34) is much higher than required (14).

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There's a huge disadvantage for a Dahon and any other small-wheeled derailer bike, which is that the cage is gong to be unfortunately close to the ground no matter what you do, and having a longer cage than necessary makes the problem worse.

Shorter cages also manage chain slap better and are lighter.

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  • Related - longer derailleurs on small-wheeled bikes seem to get even dirtier faster than any other wheel size. – Criggie Nov 17 '19 at 10:59
  • Why the original dahon eco c6, (eu.dahon.com/bikes/eco-c6), use the long cage cave (Total Capacity: 34 instead of 14)? – Michael D Nov 17 '19 at 11:46
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    @MichaelD Likely because very basic rear derailleurs like that typically only come in long cage. – Nathan Knutson Nov 17 '19 at 18:05
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As you no doubt know, the cage length is a significant factor in determining the link capacity of the derailer. But having a longer than necessary cage has several disadvantages.

One disadvantage is that a longer cage tends to result in less precise/less "crisp" shifting. Another disadvantage is that the longer cage is easier to damage.

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Chain tension and control over bumps is reduced because the mere is greater leverage in the cage vi the lower pulley.

On a folding bike with small wheels you may find you have ground clearance problems.

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