Bicycle Mechanics - Can I put a 11-32 on a Trek 1.5?

Tried a search but not much came up and did read Sheldon Browns articles but am still a bit unclear.

Did a 8km hill yesterday, and I was struggling even on 34/26. My bike [Trek 1.5 2009] has a compact 50/34 and a 11-26 SRAM cassete. I suspect that its a mountain cassettes [came with the bike], because SRAM has only a MTB 11-26:

http://harriscyclery.net/page.cfm?PageID=49&action=details&sku=FW6177

Now I was thinking of getting a lower geared cassette like maybe a SRAM 11-32

http://harriscyclery.net/page.cfm?PageID=49&action=details&sku=FW6154

The Trek website shows the original cassette as a SRAM PG 950 11-26

http://www.trekbikes.com/us/en/bikes/road/1_series/15/

So couple of questions:

1. Will I be able to replace the 11-26 with a 11-32 or even 11-34?
2. If possible, is it easy enough to change the cassette to use it only when I go to the hills? IOWs does it involve different length chains etc?

The real question is: Can the Tiagra RD you have handle a 32T cog? Other than that there is no problem. How do you do this? Here:

http://bicycletutor.com/replace-cassette-cluster/

You can do it but you'll need a mountain type rear derailleur (Deore, Deore XT, or XTR). You may or may not need a longer chain. The chain needs to be long enough to handle the big to big combination just in case you accidentally hit that combination.

Al

Probably, I have ran an 11-32 cassette on a Cross Check with the stock Tiagra derailer. I just put that cassette on my Monocog with a short cage Tiagra derailer and it works fine.

Yes - I was pretty sure the Tiagra would handle it. You've confirmed it. I think I've seen stock set-ups running 32T with Tiagra RD's.

You may want to try it with your current rd before buying a "mtn" one. I have done this conversion on several bikes and some worked with Shimano "road" triple and some didn't. The deciding factor was whether the pulley on the rd bumped the 32t cog. Some did, regardless of chain length. and those needed the "mtn" rd.

Thanks guys.

Does the RD vary depending on whether you have a double or triple crank? IOWs are there different Tiagra RDs for a triple and a compact?

Trek says your bike came with the Tiagra GS model - GS inferring what is known as medium-cage derailleur. This would be typical for use with a triple chainrings, with the SS model being a short-cage best suited for double chainrings/compact.

Hi Deepak
What's an RD?
Arvind

RD = Rear Derailleur

If you run a triple you need a long cage RD. I am running an 11/34 SRAM cassette on a touring setup with a Campy long cage (90mm) RD and a triple campy crankset with T/A Specialties rings 50/39/28. I am using Campy 10spd ergos for the "shimano" 9spd type cassette. Works very smoothly as the cog pitch c-c is very close. You need to experiment with any setup that differs from stock, but it is sometimes surprising the combinations that work well.
The only truly critical indicator of appropriateness is the chain length and RD takeup. Never compromise with a short chain because you will bust an RD and possibly the hanger, chain, and spokes.

One other consideration for rear derailleurs is the angle the parallelogram is designed to travel.

Modern derailleurs follow the cogs closely to maximize chain wrap (the number of teeth the chain engages on each cog) and will only handle a certain range of gears while maintaining this optimum. Granted, this is only crucial with newer designs such as SRAM or Shimano Shadow series that have a semi-fixed upper knuckle, but even with long cage derailleurs and sufficient chain length, the parallelogram angle remains the same and design limitations can be exceeded, causing chain jump, rubbing on the large cog, etc. Here's a diagram that shows the difference between "mountain" and "road" designs:

http://i95.photobucket.com/albums/l156/Wordbiker/Derailleurrear-1.jpg

Whether certain combinations can be or should be run is up to how particular the individual is regarding wear, function, reliability and frequency of maintenance. Running components out of manufacturer's spec may work, but it's definitely a Class 'B' fit and will require a compromise somewhere.

Personally, were I finding the same limitations as the OP, I'd be looking at either a triple up front or an LX rear derailleur which can handle up to a 34t cog.

Another factor as to whether or not a road derailler will accept a MTB cassette (28+ tooth rear cog) is the placement of the derailler hanger. This is not consistent across all bikes so your success rate may vary from someone else's.

I have run many Shimano derailleurs, both F & R, at well over their Max T. And they work like they were born there. Latest is an XTR FD that has a rating of 44 - 48T. It's smooth as silk on my 52T Ultegra (triple). Seen many examples of the Ultegra RD (Max 27T) running on 32T cassettes - smooth as silk.

Bottom Line: Try it & don't believe the hype.