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Long cage RD


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Hope I have got this right!

 

What is the difference/purpose of a long or short cage rear derailer?

 

How do you relate the steerer tube length of the front shock to the head tube length? How much do you need to allow for the headset?
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The length of the derailer cage you need is determined by the size of your largest gears.  Road bikes with a low gear of 25 teeth use a short cage while MTB's with larger gears tend to use long cages.

 

Steerer tube length should relate to the riders position rather than the head tube length.  You can either add or remove spacers to get the right length before cutting the tube.  There must be some point at which the tube length can be "too long for the frame" if the frame is far too small for the rider.  On carbon road steerers it is considered unwise to have more that 40mm of spacers between the top of the headtube and the bottom of the stem, but I haven't a clue as to how long you can go with aliminum.

 

If the steerer is too short you can play around with stems with a steeper angle to compensate.

 

 

 

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Derailer cage length is not so much determined by the size of the largest sprocket, although it happens to be true for most cases. It is determined by the difference between the biggest and largest gear combinations on the bike.

 

For instance, if you have a 34-11 at the back, the difference in cogs* (teeth) is 23. On the front you may have a 44-28. The difference here is 16. Add the two differences and you have a total of 39. The derailer's spec  should be able to handle a total difference of 39 cogs.

 

The longer derailer cage sweeps through a large arc with a long arm and can thus take up the necessary chain slack that comes with such a large difference. Should the cage be too short for the job, you can't access all your gear combinations. Otherwise, it will still work fine.

 

* Cogs. Most people confuse a cog with a gear or sprocket. However, a cog is a single tooth on a gear/sprocket.

 

 

 

 
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We don't know what your setups look like.

 

Calculate the maximum cog difference as I showed above. Now go to the Shimano website and find the "range" of the derailer you have or want to use, and see if it falls within the range.
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Generally you can use a short cage on a hard tail (if you don't cross chain too much) and a long cage on a full suss to allow for chain "growth" when the suspension moves...the other explainations are also true, on a road bike the only need for a long cage RD is if you are running a triple crankset to allow for the jumps between the rings.

 

A steerer tube's length is determined by your setup and the stack height of the head set and the depth of the stem and spacers. Basically if you buy a new fork set it up to your previous handle bar height and then cut the steerer 3-5 mm below the top of the stem, If you are buying a "pre-owned" fork the make sure it'll match your current set up.
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i see, i have hardtail with LX shifters, XT front derailer. still figuring out what RD to get, XT long or short cage.  the crankset is a deore m443, cassette is a mid range shimano hyperglyde.  i'll try and workout the cog difference, but off hand any ideas of which i need to go for? 

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You should be able to use a short cage, just make sure you stay away from the big ring and 3 largest sprockets at the same time and you'll never have hassles.

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