I'm going to be using a Shimano compact crankset (50-34) on frame set I'm getting built up. I've got an Ultegra front and rear derailleur that I used with the crank along with 12-27 cassette on another bike. I'd like to use a 12-32 or 12-34 cassette on the new build. Do I need I med-long range rear derailleur to use with the larger cassette? It's 9-speed if that matters. There are no numbers stamped on my Ultegra derailleur. How to I determine if it's short or medium range?

You need a longer cage derailleur. But I don't believe you can use the current 11 speed Ultegra rear derailleurs; they won't work with your 9-speed system. You can use any 10 speed or lower derailleur, including so-called mountain models.

So the indexing is part of the shifter not the derailleur??

Both.

The derailleur moves a fixed increment for each click that is the width between cogs. The ratio changed between 10 and 11 speed so parts are not interchangeable.

I know that some folks have done well with a long cage Ultegra RD when shifting a 32 or 34 rear cog, but on my Cannondale ST, a Deore worked better. That might be because the RD hanger on that frame is not as long as some steel frames. Deores are relatively inexpensive at $35-40 and are intended to work with cogs that size.

I actually used an Ultegra long-cage derailleur with an 11-34 cassette on one of my other bikes with no apparent problems. I didn't realize when I installed the cassette that Shimano rates the RD as 27 teeth max for the cassette. However, I don't have another long-cage Ultegra RD, so I would prefer to buy a derailleur that is properly sized for the larger cassette.

tarwheel, A quick measurement on three RDs:

6400 short cage: just under 5 cm between pulley axles.
6500 long cage: just under 7 cm between pulley axles.
XT mtn. cage: you guessed it, just under 9 cm between the pulley axles.

Brad

PS I don't have a RD meant for a compact crank set to measure and I don't know iif they're different. I'd lean towards a mountain group's RD.

You need a better ruler ... or you really mean centimeters.

OOPS! Yes cm.

Brad

For an 11-34 cassette, get a long-cage (SGS) MTB rear derailleur.

For 9-speeds, the Deore XT RD-M77x models are my favorites.
M771 - Standard geometry
M772 - Shadow geometry. (tucks under the cassette more for better clearance, but doesn't include a tension screw. add an inline adjuster for use with road shifters)

The Deore-level 9-speed RDs are also good.
M591 - Standard geometry
M592 - Shadow geometry

These RDs can still be found at most bike shops/websites.

You could also use a 9 or 10-speed mid-cage (GS) road RD, but you're pushing their 30-32t capacity with a 34t cog. Many folks do this successfully, but it can be affected by all kinds of stuff (cable run, derailleur hanger length/dimensions, etc.)

That's kind of confusing.

The shifter determines the indexing; it's speed count must match the number of cogs in the rear.

The derailleur just needs to be compatible with the shifter...

The M772 and M592 are rated for up to a 36T big rear cog, making a 12-36T cassette a safe possibility.

The M592 is my go to RD.

Thanks, Tim. That helps a lot.