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What cage length do you need?
The ‘cage’ refers to the part of the mech that hangs down, with the jockey wheels held within it. Its length varies according to the set up that it’s designed to be used with. A long cage mech is required when using a triple chainset, as the mech needs to take up more chain slack in the smaller gears. Long cage mechs are used almost all the time on mountain bikes. A double road set up uses a short cage mech. You will occasionally come across ‘medium cage’ as an option in some products and this is recommended for use with larger cassette ratios, e.g. 12- 29 teeth, or to keep tension if you are using compact gears on the chainset, where the inner ring at the front is smaller.
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For road bikes, your choice is fairly straightforward. If you plan to use a double crankset, stick to a short cage unit. For triple cranksets, you'll want to use a longer cage rear derailleur to handle the increased chain wrap necessitated by the wider range drivetrain.
On a mountain bike, your choice is slightly more difficult. Less experienced riders, or riders who ride frequently in steep terrain should probably stick to a long cage rear derailleur. This, combined with proper chain length, will allow the bike to be safely shifted into every gear on the bike. Racers might appreciate the lighter weight and slight shifting improvement a short cage rear derailleur offers. However, these benefits come with a tradeoff - because of the decreased capacity a short or medium cage rear derailleur offers, it may not be possible to safely use the small chainring in tandem with the smallest cogs. (The derailleur simply can't take up enough slack in the chain)
For mountain bikes, when in doubt, choose a long cage rear derailleur, unless you are certain you understand the limitations of a short/medium cage derailleur.
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