Rear mech question - medium or short?

Hi all - after some advice please.

The bike I'm building up has 7 speed cassette 12-28 & triple front 24 - 46. It had a medium cage Shimano rear derailleur on which I'm looking to upgrade.

I've a chance on a short cage derailleur. The capacity is shown on cage as Max 28T & Total 28T. If my understanding is right from my googling - that's 28T max on cassette & 28T max difference on chainrings. Meaning a short cage would work. Have I understood that correctly?

Catalogue also mentions Difference avant of 20 - not sure what that is?




If I have understood it, is there a drawback to choosing a short over a medium derailleur for the gear range on my bike?

It's a no go!

rwm1962":391ver82 said:

The capacity is shown on cage as Max 28T & Total 28T. If my understanding is right from my googling - that's 28T max on cassette & 28T max difference on chainrings. Meaning a short cage would work. Have I understood that correctly?

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no. It says max 28 sprocket and max difference 28 (both front and rear added together)

rwm1962":391ver82 said:

Catalogue also mentions Difference avant of 20 - not sure what that is?

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total difference rear as far as I can tell.

rwm1962":391ver82 said:

If I have understood it, is there a drawback to choosing a short over a medium derailleur for the gear range on my bike?

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it will "work" but there will be a good chunk of the gears that won't be available. And some that'll either snap your chain or mangle the mech. Depending on how driveline aware you are!

I've run a similar ss set up for years with no issues, but I've seen others destroy them within weeks

agree. it will work. But if you like to ride big chain ring big cassette sprocket then it will be a issue. You should always keep the chain as straight as possible.

Marin used SS cages with those rings.

It will work fine, as it is within the limits. Typically you can get away with a couple of teeth more than Shimano tech docs which are on the conservative side.

Re:

i'd say yes it can work, but back in the day they length of chain was set a little long which meant with your specified sizes the chain will hang down in the small/small gear (which you shouldn't use anyway) but this will mean if you go in to big/big then the mech shouldn't be over stretched which would cause it to fail, although the chain line will be awful.

basically if you use your gears correctly as in small front ring, 1st 4 sprockets, middle ring any sprocket and top ring last 4 sprockets then you should be ok with no issues.

i prefer the look of the shorter mech but i prefer the use of the longer mech.

hamster":1u9cr6p5 said:

as it is within the limits.

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It's really not. An SS mech only has a 28 tooth range.

I've decided to go with the long to be on the safe side.

mattr":napllnar said:

hamster":napllnar said:

as it is within the limits.

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It's really not. An SS mech only has a 28 tooth range.

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Whoops, I lost 10T somewhere while working out the difference. I blame the beer.
:xmas-wink: :facepalm: