Bicycle Mechanics - Long Cage vs. Short Cage

After reading some of the previous posts of the long cage vs. short cage debates I'm still a bit confused. I have a double front crank (48-38T) and a 12-27 rear cassette. It looks like I have a 25-tooth chain-wrap capacity (48-38=10, 27-12=15, 10+15=25). When I view the shimano spec sheets should I be looking at the "Total Capacity" for if I need a short or long cage derailleur? It looks like a short cage should be fine from previous posts.

I've added a link to one my previous posts if it helps:

http://www.bikeforums.net/showthread.php?t=410910

you are totally fine with a short cage. While the 12-27 cassette is on the wide range for road bikes, the 48-38 is only a 10 tooth difference, typical road double is 53/39 (14 tooth diff).

There are 2 specs to be concerned about-
1. Chain wrap capacity
2. Max cog size

You're good for both with a short cage.
IF you were to get much larger with the max cog size (>28-30 or so), you'd need to go to a long cage.

Umm. No.

Cage length has nothing to do with max cog size. All Shimano road RDs have a max cog size of 27T (practically good out to 30T). To shift a larger cog he'd have to go to a RD in the mountain bike line.

Edit: Typo = 27T

Umm. No.

Cage length has nothing to do with max cog size. All Shimano road RDs have a max cog size of 25T (practically good out to 30T). To shift a larger cog he'd have to go to a RD in the mountain bike line.
According to Shimano's site, they have a max cog size of 27. And yes, the later models have some wiggle room in there. I'm on a 28 with no problems. However I did destroy an 8 speed Ultegra RD by cross shifting a 34/28 combo.

Thanks for the help guys. I really do appreciate the time you've spent.

I'm looking to minimize the cost since this will be a rainy season commute bike. Is there any advantage to buying a mountain bike derailleur vs. a road bike derailleur since this is a touring bike and the original rear derailleur was a Deore XT? Also is there a disadvantage to buying a long cage? (I see a lot of long cages in my price range are on sale right now)

Long cages work every bit as well as short cages and they can wrap more chain. Some people claim that short cages shift just a little bit 'crisper', but what use is that if you can't get the gearing you want?

Thanks for the help guys. I really do appreciate the time you've spent.

I'm looking to minimize the cost since this will be a rainy season commute bike. Is there any advantage to buying a mountain bike derailleur vs. a road bike derailleur since this is a touring bike and the original rear derailleur was a Deore XT? Also is there a disadvantage to buying a long cage? (I see a lot of long cages in my price range are on sale right now)

There is almost never a reason not to run a long cage derailleur. It simply has more capacity. The only *slight* advantage you get with short cage is it may shift slightly quicker and it weighs less.

MTB derailleurs will work just fine, XT, 105+ above is decent stuff.

Thanks for the help guys. I really do appreciate the time you've spent.

I'm looking to minimize the cost since this will be a rainy season commute bike. Is there any advantage to buying a mountain bike derailleur vs. a road bike derailleur since this is a touring bike and the original rear derailleur was a Deore XT? Also is there a disadvantage to buying a long cage? (I see a lot of long cages in my price range are on sale right now)

Just a quick note. The Shimano specs say they recommend Rapidfire shifters for the Deore XT rear derailleur I want to replace. However when I bought the bike it came with Shimano 105 shifters that I don't want to replace. Can I match the 105s with a mountain bike derailleur for this Cannondale T1000 touring bike or is this a serious no no?

The one's I am looking at are:
ULT RD-6500 GS REAR DERAILLEUR
XT M760 REAR DERAILLEUR
XT RD-M761 REAR DERAILLEUR

There is almost never a reason not to run a long cage derailleur. It simply has more capacity. The only *slight* advantage you get with short cage is it may shift slightly quicker and it weighs less.

MTB derailleurs will work just fine, XT, 105+ above is decent stuff.

Sweet! I've been riding for a couple years, but I'm not in dire need for anything too lightweight or quick shifting just yet.

I bought up a couple Ultegra GS at $39.95/Shp. Use them on my circa-1982 road machine and my Trek FX. Excellent derailleur - I usually hate Shimano. Want one?

http://www.cambriabike.com/shopexd.asp?id=68265&page=SHIMANO+ULT+RD-6500+GS+REAR+DERAILLEUR

No, I don't own stock in the company - either of 'em.

Just a quick note. The Shimano specs say they recommend Rapidfire shifters for the Deore XT rear derailleur I want to replace. However when I bought the bike it came with Shimano 105 shifters that I don't want to replace. Can I match the 105s with a mountain bike derailleur for this Cannondale T1000 touring bike or is this a serious no no?

Not to worry. Shimano road brifters will shift a Shimano mountain bike rear derailleur just fine. As a matter of fact, that's a common spec for many tandems.

According to Shimano's site, they have a max cog size of 27.

Yes. It was a typo.