Short cage RD on triple - why not?
Discussion
I've got a short cage 105 derailleur about to be fitted to a triple and am wondering if it is going to work correctly. The largest sprocket on the cassette is well within the stated limit, and the maximum total capacity is 33, exactly what I have.
Having done a bit of research it seems everybody says you must get a long/medium cage rear derailleur for a triple, but without saying why. The maximum front chainwheel tooth difference will be exceeded between the largest and smallest only, however I am not entirely sure how it is relevant given the total capacity is within spec.
Having done a bit of research it seems everybody says you must get a long/medium cage rear derailleur for a triple, but without saying why. The maximum front chainwheel tooth difference will be exceeded between the largest and smallest only, however I am not entirely sure how it is relevant given the total capacity is within spec.
Edited by TheInternet on Saturday 5th July 18:54
The issue isn't so much sprocket size per se- although that's part of it.
Typically a medium/long cage mech is required with a triple as the amount of chain that needs to be taken up by the arm of the mech is significantly longer than when a double chain set is fitted.
Your short cage mech will have a maximum amount of chain that it can safely handle- and this may well be exceeded when you change down to the inner ring that it wasn't expecting, as it were.
Typically a medium/long cage mech is required with a triple as the amount of chain that needs to be taken up by the arm of the mech is significantly longer than when a double chain set is fitted.
Your short cage mech will have a maximum amount of chain that it can safely handle- and this may well be exceeded when you change down to the inner ring that it wasn't expecting, as it were.
13+20=33, you're bang on the limit but according to Shimano it should work - there's only one way to find out, ultimately.
My concern would be that if the chain is long enough not to straight-line the cage in 50/12 it might be folded right back in 30/25.
However- that's less of a difference (low/low) than a 34/28 on a standard MAMIL-wagon.
So, I reckon you'll be fine, just set it up carefully.
My concern would be that if the chain is long enough not to straight-line the cage in 50/12 it might be folded right back in 30/25.
However- that's less of a difference (low/low) than a 34/28 on a standard MAMIL-wagon.
So, I reckon you'll be fine, just set it up carefully.
Dammit said:
My concern would be that if the chain is long enough not to straight-line the cage in 50/12 it might be folded right back in 30/25.
Those gears will be fine, it's 50/25 and 30/12 that would be problematic if you were to accidentally select them.I do think a short cage will work perfectly well, but for peace of mind I think I'll probably change it for a longer one. The main problem is being bothered to return it.
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