I have an FSA Energy Triple 53/39/30 and a 12-25 rear. I took off the 30, but still have the crank attached to a 118 BB. My rear mech is a 105 long cage. I am thinking of replacing it with a short cage 33 tooth chain wrap rear mech.

Can this be done with the 118 BB?

With the 30t granny gear removed, a short cage rear derailleur can be used. However, a short cage rear derailleur don't work any better than GS models, it will just a couple of grams lighter.

I read somewhere in the forums that it is do-able as long as you stay out of the big-big combo. Going there might overstretch the rdr and jam it. More function to a long cage besides picking up slack in the granny.

Cross chaining is bad Mmmkay.

A 105 long cage is about 229g a short cage replacement is 199 grams+/-.

No, if the chain is installed to the correct length, this will not be a problem. However, the chainline will be an issue when on the big ring of the triple crank. Removing the granny chainring does not resolve this issue.

Yeah, it will work. But is it worth the 30g? I'd say leave the long cage on there. It isn't hurting anything.

Since he's not using the granny his total chain wrap is 14 (53-39) plus 13 (25-12) = 27 so the short-cage derailler would work fine. In fact, since it isn't designed to accommodate a 21-tooth difference in cassette cogs, as it would with an 11-32 cassette, it sweeps across the cassette at a shallower angle than a long-cage derailer, thus keeping the guide pulley closer to the cogs, leading to more precise shifting. The only negative would be a chain line disparity when using the 1 thru 4 cogs with the 53-tooth chain ring. But then, that's what the 39-tooth ring is for! Good to go!

Regards,
Bob P.

Chain would need to be too short to reach Big/Big combo , to not go slack..

just buy a long cage RD, and be done with the problem in question.

the Mountain bike Deore ..

Nope - a 105 long cage isn't designed to handle a 32t rear cog.

It's not designed for a 32 tooth cog, but it will fit and operate perfectly on most bikes. I say "most" bikes, a few bikes with a short hanger will have problems.

So that I understand.

The rear d will work as long as I am not in 53/25/23/21/20 or 39/12/13/14/15.

I can see it not working in 53/25/23 or 39/12/13 but the others shouldn't be a problem 'cause that's how I shift now. Basically once I hit the middle of the cassette it's time to go to the big ring.

maybe if one chainring,.. and as the chain climbs to the big cog on the cassette
the short chain pulls the tensioner around and away from the cog.

ran a single chainring with a short cage 80's Chorus and a big rear cluster,
and it worked OK..
tensioner 's swing axis and top pulley were concentric on that RD.

You should be able to use any combination without immediate failure if the chain is sized correctly. The chain will possibly wear more quickly on the small-small combination if it rubs against the big chainring.