Bicycle Mechanics - Can a DA triple rear der handle a 32 cassette?

The cheapest way to turn a road bike with a double chainring into a climbing specific bike is to add a mountain bike rear derailleur and a 11-32 mtb cassette. Has anyone tried the same trick with a road specific rear derailleur? The stated capacity is 27 teeth, but Shimano is always conservative. I was thinking about taking the derailleur off my tandem and putting it on my road bike for an upcomong trip.

It can be done, but the deraillieur will not last long.

It can be done, but the deraillieur will not last long.
It might shift a 30,but 32 is doubtful and a inexpensive. mtb derailer will work as well.

I use 48-34 chainrings on my road bike with an 11-32 cassette and Shimano Deore derailleur. Works great.

Maybe I'm not understanding why it wouldn't last, but if they can put a Mtn. derailluer on a tandem then they should last on a road bike. I thought that the only real difference between the two types were the cage length...

I do not know where I got the technical data but in my notes I have it that the road Shimano Dura-Ace rear derailer can handle only up to 26 teeth with 9 speed shifting. The road Shimano Ultegra can handle up to 37 teeth cassett. I have a Trek 5200 which has the road Ultegra nine speed shifting. I am handicapped and have put on the 11X32 cassett just to handle hills and I have no trouble at all with it. Of course it requires a change of chain length and screwing in of the B screw.

I think many of you are missing the original poster's question. Phil from VA wanted to know if you could use a road-specific rear derailleur with a 32T cog and not necessarily if you could somehow run a 32T rear cog in general. I remember reading a few people running upwards of 29T or 30T if they use cyclocross chainrings but just as another poster mentioned, a 32T is probably out of the question. With 39/53 chainrings I'd be reluctant to push anything beyond a 28T even with a medium caged rear road derailleur.

The number you really want to look at is the total capacity. For the Shimano Dura-Ace Triple (RD-7700GS), this is 38T.
(http://www.bikepro.com/products/rear_derailleurs/rear_derailleurover.html)The total capacity is calculated by subtracting the size of the smallest chainring number from the largest chainring and adding it to the difference between the largest and smallest cog in the rear. For example, if the front chainrings were 24-36-46 and the rear cluster were 13-28t then, the front difference of 22 (46 minus 24 = 22) would be added to the rear difference of 15 (28 minus 13 = 15) to achieve a Total Capacity of 37 teeth. Total Capacity is a rating to express the most extreme comfortable working conditions for a given rear derailleur.

I do not know where I got the technical data but in my notes I have it that the road Shimano Dura-Ace rear derailer can handle only up to 26 teeth with 9 speed shifting. The road Shimano Ultegra can handle up to 37 teeth cassett. I have a Trek 5200 which has the road Ultegra nine speed shifting. I am handicapped and have put on the 11X32 cassett just to handle hills and I have no trouble at all with it. Of course it requires a change of chain length and screwing in of the B screw.

Actually the RD-7700SS (short caged) will handle a 27T. I know... I have a 12-27 with an RD-7700SS on my bike. The same goes for the GS (medium length cage) model. Supposedly I can push it to 29T if I was really careful about never selecting the big-big. The Ultegra rear derailleurs are also rated to the same as Dura-Ace (27T) in both SS and GS versions. The difference between the SS and GS is the cage length which effects total capacity and not max sprocket. You can find more information on Shimano's Road Group (http://bike.shimano.com/Road/index.asp) website.

Maybe I'm not understanding why it wouldn't last, but if they can put a Mtn. derailluer on a tandem then they should last on a road bike. I thought that the only real difference between the two types were the cage length...

Road rear deraillieur will not last on a mtb because of the rough ride.

I think Robert Gardner answered my question. He is running a 9 speed rear road long cage derailleur with a 32 rear casette. Since this is a one week fix, longevity is not an issue.
I've set up friend's road bikes with an XT rear derailleur with a 32 cassette and it worked pretty well but looked pretty bad.

I do not know where I got the technical data but in my notes I have it that the road Shimano Dura-Ace rear derailer can handle only up to 26 teeth with 9 speed shifting. The road Shimano Ultegra can handle up to 37 teeth cassett. I have a Trek 5200 which has the road Ultegra nine speed shifting. I am handicapped and have put on the 11X32 cassett just to handle hills and I have no trouble at all with it. Of course it requires a change of chain length and screwing in of the B screw.
ALL current Shimano 9 speed road derailers are speced for a 27 max cog.Spec is CONSERVATIVE, and they will usually handle a few more. Deralier hanger length is also a factor,and the longer ones let you get away with a few more teeth. Running a 32 large cog with a 9 speed ultegra RD may be possible in some cases,but not always.Cage length is also irrevalent,as even the long cages have the same lage cog spec.

Road rear deraillieur will not last on a mtb because of the rough ride.

he's not putting it on a mtb. where did you get that idea?

Maybe I'm not understanding why it wouldn't last, but if they can put a Mtn. derailluer on a tandem then they should last on a road bike. I thought that the only real difference between the two types were the cage length...
No it's not cage length. MTB deraliers have a different parallelogram design and track a greater lateral angle across the cassette in order to shift the larger cogs.

I think Robert Gardner answered my question. He is running a 9 speed rear road long cage derailleur with a 32 rear casette. Since this is a one week fix, longevity is not an issue.
I've set up friend's road bikes with an XT rear derailleur with a 32 cassette and it worked pretty well but looked pretty bad. You MIGHT get away with it. Test it before the trip and make sure the chain is long enough.

Maybe I'm not understanding why it wouldn't last, but if they can put a Mtn. derailluer on a tandem then they should last on a road bike. I thought that the only real difference between the two types were the cage length...

what they said was - a road bike derailleur might not handle a mountain bike cassette. If you want to use a mountain bike 11-32 cassette on a road bike you will probably have to use a mountian bike derailleur as well.