Rear derailleur cage length, what's the formula?

caw35slr

Can somebody point me to a formula or set of rules for working out how to choose the correct rear derailleur cage length for a Campag 30/40/50 triple?

I've seen this on the JE James site:

Short cage derailleur for combining a double/compact crankset with a 13-26 sprocket set or smaller.
Medium cage derailleur for combining a double/compact crankset with a 13-29 sprocket set or smaller or triple crankset with a 13-26 sprocket set or smaller.
Long cage derailleur for combining a triple crankset with a 13-29 sprocket set or smaller.

But this doesn't seem to take into account chainring sizes (i.e. the difference between big and small rings of a 30/40/50 versus a 30/42/53).

nicklouse

you need to know the range you need to cover. total of the two big cogs (one front and one rear) and the total of the two small cogs (one front and one rear) that will give you the range you need to cover.

the mech makers site should give the range that it covers.

caw35slr

Well, that's where my question arose. Campagnolo words it differently:

Triple crankset + medium cage derailleur: all Campagnolo sprocket sets except the 13-29 set.
Triple crankset + long cage derailleur: all Campagnolo sprocket sets.

That doesn't sound like it's telling the whole story, but perhaps it is!

itisaboutthebike

there isn't one really.....................but

with a 14 tooth front chain difference.....................

a short cage rear mech (Campag or Shimano) will cope ok up to 27 rear sprocket (Shimano probably 28).

Anaything greater you'll probably (and I say 'probably' 'cos they'll be some on this forum whose experice is different) need a medium cage rear mech(big 'S' only do a long cage rear mech tho)

Hope this helps

piedwagtail91

I'm using 30, 40 , 50 with 13-26 with a medium cage. works fine.
it's unlikely or not recommended to use the big ring and sprocket so i don't think the 3 tooth difference between a 50 an 53 will make much difference.

Rich-Ti

I'd say don't run a short cage but either of the other two would be fine

meagain

What Rich-Ti said IMO.

caw35slr

Thanks all! :-)

Sfelt

I am just getting away with a compact 50/34 - 12/27 on a short cage but the springs are getting a workout, I'm sure if funds permitted a medium would be a lot more comfortable.