In the past 1000 miles all over SoCal, I've found that generally I don't need the 11-cog....In fact I use it exclusively when I'm going downhill. However, when I go uphill I find I'm pushing myself. As I'm trying to be more consistent on the cadence, I find that 25 simply isn't enough for me when I'm tackling hills.

I currenyl have this bike with a 11/25 cassetee running on an ultegra derailleur.

Looking around, it seems that 28-cog should be a safe move in that it will work out of the box. 12/30 is possible, but it depends on the bike (hence why I put mine in the link). Is there anything larger than that that would be reccomended or am I looking at more than simply replacing the cassette for ~100 bucks?

I looked at BikeCalc.com - Cadence at all Speeds for any Gear and Wheel and it seems that there is about 5-rpm difference between -28 and -30 cog sizes for the speeds I'd be typically going up hill (5-10mph).

Thanks :-)

There are two capacity issues when using larger or wide range cassettes.

The first is chain take up capacity, which is expressed as the sum of the differences largest to smallest of the cassette and chainrings. ie. a 12-25 used with 39-53 rings would need take capacity of 13+14 or 27 teeth total.

The other is the largest sprocket the derailleur itself can handle.

Both these have a bit of fudge room, and the RDs take up capacity can be increased slightly by substituting a larger pulley in the lower position. You can also, run over capacity as long as you err to the right side. Specifically measure the chain based on the large/large combination + 1" (even if you plane NEVER to use them together). This will mean the chain is too long for the inner ring with the smallest few rear sprockets, but that's not a real loss anyway.

Also, don't forget that a larger cassette may require a longer chain. Failing to check that could cost you the RD, and a bent axle, so it's not to be overlooked.

I don't know exactly which rear derailleur you have but based on the rest of the build you probably have a 6700SS. The specs for the 6700SS list a total capacity of 33 teeth and a maximum cog of 28 teeth. Your proposed setup would have a capacity of 34 teeth.

You are exceeding Shimano's recommendations but it might work.

However if you replace your rear derailleur with a 4600SS, it is designed to work with the 12-30 cassette.

You can get the Tiagra 12-30 cassette from Ribble for $21.00. It's worth a try. As mentioned, you will probably need a new chain.

Shimano is usually conservative on the max cog. I think you will be ok with the 30. You will need a longer chain to avoid problems if and when you shift into the large-large combination.

I just finished up building a 2012 Trek Madone 5.9 with Ultegra parts, including a 50/34 compact chainset. I can't tell for certain if I got the 6700A or 6700 (it does NOT have an "A" stamped with the part number on it), but I had little trouble adjusting it to run on an Ultegra 12-30. The clearance is fine and the b-screw has plenty of room I could still turn it in. The chain length will need to be exact...there isn't any leeway as I learned when I had the chain an inch too short at first and tested the big-big combination (good thing I put it together with a Missing Link, so I still had the Shimano pin it came with to add a link back). I haven't ridden yet, but in the small-small combination there is just enough tension in the chain that I don't think there will be a problem. Rest assured...you really should only be using about 16 or 18 of your 20 gears anyway (no small-small, big-big = 18), but just in case you don't want to risk ripping your derailleur off the bike, breaking the frame or anything else when it happens.

my guess your rd using ultegra ss which shimano recommendation max 28t, if you trying go 12-30 t need longer rd with GS code like my tiagra 4600 gs 12-30t work fine. my suggestion go for 12-28t without changing your rd.

You should be able to run the 12-28 cassette with the stock derailleur, chain length will set to handle the larger rear cog and with a 34/50 compact you should have a decent climbing gear.

I ride with a 40/28 on my road bike and it gets my old busted up ass up most things.

My campy chorus rear der (short cage 9 speed) maxes out at 27 with a 53 front chainring.

There's no reason to use the big-big/small-small combinations anyway.

Back in the day, a 53/42 and 13-23 front and rear were standard. For me, a 42/23 low was sufficient for just about everything, including long 7 and 8 degree grades, and occasionally steeper.

Now I'm getting back into the sport, and am hoping a 34/28 low will be adequate. I would opt for a triple, but few (if any) road bikes I'm interested in are spec'ed with a triple this model year.

Just a short update - I did get a 12-28 and its nice - its a huge difference on hills that were previously un doable. The extra 3 cogs translated to extra 5-6 rpm, and it makes a big difference even though it doesn't seem like it!

That said, I want to move along as close to my pet cadence as possible I forsee another upgrade to a custom cassette in the future (and another thread asking questions as I explore this!) - I definitely want to see if I can run a 32T or 34T. I could easily give up 12,13,14T and take in 30T 32T 34T. As it is I never use my 50 Chainring up front...always the 34 ChainRing. If I needed speed I could shift up on the chain ring :-)

There are several people in my local club running 12-34T mountain bike cassettes already. This isn't a stretch...heck, even Shimano now has a 11-32T in the 11-speed lineup. Everybody that is running the 12-34 though is using a Shimano mountain bike derailleur...so that's how you do it.

Custom cassettes are pretty much a thing of the past so you are generally stuck with an 11 or 12T smallest cog even with a 32 or 34T largest cog. The problem with these wide range cassettes is the huge gaps in the interior gears.

Since you never use your 50T big chainring, consider attacking this from the other end by getting an MTB crank with a 42 or 44T largest chainring and a smaller than 34T inner or granny chainring.

You could always build your own.

You can't get the individual cogs and, if you assemble one by using cogs from two or more complete cassettes, not only will it be very expensive but the shifting gates won't phase well and shifting will be poorer.