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I'm building a kind of new bike and would like to put a triple or compact crankset on it.

I've still got this NOS huret derailleur set (front, rear and shifters) laying around which would match the bikes flair pretty well; but I'm worried it won't fit.

How can I calculate the maximum size difference in front and rear sprockets this derailed can handle (by looking at it's dimensions).

The derailleur model is the Huret Svelto. I'm looking into buying a crank with 34t and 48t sprockets and have a 7 speed rear cassette of 13t - 24t (but these are loose sprockets and can be changed).

What is your advice?

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Mount it on a bike and feed a (broken) chain through it. Wrap the one end around the cluster to anchor it, then pull the other end until you judge you're at "minimum tension" (granted this is a judgement call). Next stretch the chain out nearly taut and count how many links pass a given point forward of the derailer as you do this. Won't be exact, but pretty close. –  Daniel R Hicks May 29 '12 at 15:03
    
I'd add to Daniel's answer that it's not a good idea to calculate it. Too often what looks viable on paper turns out not to work in practice. Either the max travel is less than it looks as though it should be, or there's too much or not enough spring tension for it to work. Recumbent bikes get this a lot, as they want a wider range than other bikes usually do, but many derailleurs don't work well at the limits of their capacity. So you get poor shifting and a tendency to break. That said, Daniel's suggestion gets my vote. –  Kohi Jun 4 '12 at 5:43
    
With a double and a 13-24 you'll be fine. With a triple you'll be pushing it. –  joelmdev Jul 28 '12 at 17:51
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1 Answer 1

As a general rule a Short cage derailers should only be used with a single front ring, medium cage with double front rings and a long cage for tripple.

Naterally this can be effected by the amount of travel your bike has at the back. If it is a full suss then as the rear wheel moves through it's travel the chain line will get longer, placeing more tension on the rear derailer.

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+1 I agree with your first paragraph, but the last one isn't quite true: 'chain growth' isn't affected by travel, it's affected by the suspension linkage. On a single pivot (link actuated shock or not) bike, it's affected by the size of the gap between the BB centre and the swingarm pivot centre. Multi pivot bikes tend to adjust the rearwheel path to compensate (e.g. Lapierre's PendBox system on the DH series). –  cmannett85 Sep 8 '12 at 20:25
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