So I've read a few good threads on this but I want to be sure and not spend money and realize it won't work. I have a Jamis xenith with a compact double 105 group. It's 50-34 and 11-25. I'm newer with knowing exactly what will fit what but I'm learning from here and I'm somewhat mechanically inclined. I'm trying to make a hill climber with this bike because that's all that's around me. The LBS told me that I'm limited to a 28 rear cog. From what I've read here I could do a sram 12-30 cassette with a ultegra long cage RD and longer chain. I'd like to go to a 32 or even 34 but again the LBS told me I couldn't use a mountain derailleur with this set up. If the 12-30 would be just a new/used cassette and chain I'm game. Even if possibly a new/used derailleur. If trying to get an 11-34 is too much money and b.s. then that's ok, but if it isn't that much more cost or hassle then I'm all for it.

What number of speeds is this?

If it's 9/10-speed, you can use a 9-speed MTB derailer and run as big of a cassette as you please.

If it's 11-speed a GS derailer will handle a 32t. A Tiagra RD-4700-GS is even rated to handle a 34t. With either one you may be able to get a couple more teeth out of it depending on your bike.

OK, let's go with the basics.

First of all, though specs may appear to limit your options, that's not necessarily true. It's very possible to run derailleurs beyond the rated capacity, and many do so without ANY issues at all. But you have to know what's what.

Shift to low gear, and pull the lower chain loop froward to swing the RD cage counter clockwise as far as it will go (the lower loop will be a straight run to the bottom of the inner ring). Note the radial clearance from the upper pulley to the 25t sprocket. That's the limiting factor and you can safely use a larger sprocket figuring that each tooth will add 1/12" to the radius of the sprocket.

So if you have 3/4" to play with you can go up 9 teeth. Keep in mind that you still need some clearance, so don't go the limit. But I expect that 32t is very feasible.

Now, you have to sweat chain length and TD take up capacity. The ABSOLUTE minimum chain length is determined by looping the two largest sprockets plus 1". Odds are that once you lengthen (replace) the chain, the RD will not be able to take up all the slack using the 34t ring with the smaller rear sprockets. This isn't a crisis, and you simply won't use the crossed combinations.

The number of unusable gears depends on the extent of over capacity. If the total combination (big+big minus small+small)is 6t larger than the rated capacity, and your smallest rear is 12t, your RD will start taking up the slack on the 18t (or a bit smaller).

So, you now know the considerations, and can make an informed decision about the limits of what you can achieve.

May Have to swap the Road RD for a Mountain Long cage to fit cassettes with a 34t low gear cogs ,
then low is 1:1 x the size of the wheel ..

A Virtual unicycle gear..

It's a 10sp. I saw one guy even changed to an ultegra 6800hub, pg 1070 11-36 cassette, an XT 9sp RD, and shimano 6701 10sp chain. I'm wondering if I could just do the sram 12-32 and call it a day and just watch my big-big and small-small pedaling. And if it doesn't quite work try a med cage RD. Would I almost certainly need a longer chain?

For some reason the LBS said I can't use a mountain derailleur

You can't use a 10-speed MTB derailer. You can use a 9-speed MTB derailer. Don't try to pinch pennies running too short of a chain. You should change a chain when you replace a cassette anyways, and you will regret it the day you accidentally shift into big-big and kill your drivetrain.

Before you go out and spend a heap of time & money....
How does the bike climb hills as it is?
What are you lacking?

34x25 or 34x28 gearing should get many riders up a lot of moderate hills.

What is your weight? Are you carrying cargo?

An option might be to go out and practice climbing hills with your current setup, and you might find that you don't need some fancy custom configuration after putting in a little work.

FBinNY: how do I know the rated capacity? Sorry if it's a stupid question. The number of your equation if I were using a 12-32 should be 36 (50+32)-(34+12)=36

Yes I need more time in hills but I have some serious hills. Some I'd imagine are certainly 20%. But I'm 5'4" 130lbs. And I'm in decent shape. I've read many posts on how so many don't like the compact double and want a little higher gear for climbing and it was exactly what I had been thinking. I'm sure a 28t would help but if I could squeeze a 30t or even better a 32t with moderate costs I'd like to do it.

Sheldon Brown's Bicycle Gear Calculator
Plug it into Sheldon's gear calculator.
Get your smallest gear at or below 30 inches
Try to keep your steps in single digits.
Keep your tightest steps from 70 to 85 inches, because that's where we live

You don't need an 11t cog, that's 120 inch gear.
12-30t will give you 30 inches to 111 inches - that's pretty perfect

+1

If you are just riding for fun and exercise or to get somewhere and not racing, shaving split seconds off your times or worried about keeping up with the latest and greatest, the easiest and cheapest thing to do is probably to switch to friction with any long cage derailleur and then you can you pretty much use anything you want with anything there is. Really, you just need to match the chain to the cassette and everything else should work just fine. It is so liberating!

If a pro can use 34x32 on the steep stuff, it makes sense for a cyclist with half the FTP to use an even lower gear.

Have you ever tried friction shifting on something that ramps up to 20% grade?

You could fit one of these and put a yuuuge cassette in back: RoadLink ? wolftoothcomponents.com "is optimized for use with ten or eleven-speed 11-36t and 11-40t cassettes ..."

The rated capacity of RDs is usually found on the maker's website. However, the rating isn't relevant to the conversion, which you're doing by direct observation. The only use for the rating is to predict the outermost rear sprocket you'll probably be able to use with the inner.

BTW- you earlier asked about the odds of needing a longer (new) chain. Going up 2 teeth or so, you can usually get by, but don't bet on it. More than two teeth odds are you'll need the new chain.

You can predict this by shifting to the big/big combination, then pulling up the lower loop, each 1/2" you can pull forward is worth 2 teeth on the largest sprocket.

lol. I don't need a 42t in the cassette. I just want a little more help for the steep stuff.

20% hills are intense, but relatively rare, or frequently only steep for short sections.

I know that a lot of smaller women like triple gearing on the crankset, so it may well be that both the smaller riders, as well as the super-heavy riders benefit most from the low gears.

I have heard that derailleur maximum cassette sizes and wrap may be conservative. So if you wish to try the x-30T rear cassette, then you may just choose to first buy the cassette. See how it works on your bike. If it fits without the derailleur rubbing, then you may be just fine.

Oh, also buy a new chain as a short chain is a recipe for accidental damage.

I believe that one can pedal somewhat higher gearing on hills when using foot retention (toe clips, clipless, etc).

You'll be fine with a 32 tooth cassette if you put on a 10 speed mountain derailleur and a new chain.
I'm running an 11-32 on my touring bike with tiagra shifters and a deore LX derailleur. It shifts fine.

Thanks everyone for the help. Just one more silly question. What chain length. Or is there a basic equation for that also?

Equation, maybe, but nobody does it that way.

Scroll back up to post no.3 where I referenced how to determine the minimum chain length. In your case, since you're over capacity the minimum is what you have to go with.

It's the engine not the bike that makes a hill climber.

I wouldn't spend a cent.

That's why I climb 20% grades on a fixie geared for track.

Gotcha. Thanks.