ericm979

I switch from a 12-27 to 12-30 cassette for the Everest Challenge stage race. The extra gear helps quite a bit when your legs are tired and you are faced with steep ramps at the end of 15,000' of climbing. I could do any one or two of the biggest climbs with a 12-27 but doing 29,035 feet in two days is tougher.

Also, the 12-30 cassette's last three cogs are 24,27,30. The 12-28's last two are 24,28. There's a big step from the 24t to the 28t cog. When you're doing a lot of big climbs it is important to have the right gear. Climbing at too low an rpm will tire your legs out more quickly.

There's no drawback to bringing the 12-30. You don't have to use the low gears if you don't need them.

volosong

I went from 28 to 30 and will never go back, (unless I'm doing a TT). We only have a couple of HC climbs around here, but plenty of 1 and 2 climbs, (did an HC this last Saturday - GMR/GRR to Mt. Baldy Village). Being on the bigger size, (6'3", 195 lbs.), and getting on in years, (61 next month), I need all the help I can get, short of a petroleum fueled motor. After I switched, I definitly felt the difference with those two teeth. It's not much use on grades less than 7-8%, but I'm not afraid of 15% grades anymore, (well, that is if they are less than a quarter mile or so). It ultimately depends on if you have the discretionary income. If not, you can probably get by with 28. If so, 30 will help you on those lengthy +10% grades.

znomit

30T no question...Or HTFU and use a 23.

contango 

I'd reckon it will help, you're talking about dropping your gearing by about 6-7%.

If I were riding the Alps I'd be looking for ways to reduce my gearing any which way I could. I've made it up 20-25% grades in my 30/32 granny combo but even then it was hard work. Don't know what you'll face in the Alps, but even if it's not as steep as the hills I've done I'll bet they are incomparably longer.

Dee Tee

Yup-I have a 2011 Trek with 105 (50/34 and 11-28) and just made this exact same change from a 105, 11-28 to Tiagra 12-30. No adjustments needed. Smooth transition and well worth the effort to make the swap. I noticed a measurable difference on the low end.

znomit

5700-A rear derailleur is new and officially supports 30T, older stuff is 28 max.

https://techdocs.shimano.com/media/te...9830746860.pdf
vs
https://techdocs.shimano.com/media/te...9830702204.PDF

storckm

If your derailleur doesn't have a long enough cage, it will fold up all the way with the smallest cogs. Not a big deal in practice, at least in my experience.

ColinL

There's two ways to do it... make sure your chain is long enough for big-big and then you will lose at least one gear*, small-small, or you can size the chain smaller and give up big-big... BUT if you shift into big-big by mistake you can tear the derailleur off the hangar and then you've got a bit of a problem.

It's a bad idea to exceed the derailleur's chain wrap capacity. *In reality you often lose two or more gears in your small chainring and that really sucks.

storckm

That really depends on how you use your smallest chainring. I seldom use it, and only when I need a really low gear: I doubt I've ever used the small cog with anything other than the largest three cogs. I got the idea that mountain biking might involve more use of the small chainring with smaller cogs, but haven't ever seriously mountain biked.

davida

I run 12-30, compact cranks with standard 6700 Ultegra RD and a slightly longer chain and it still shifts perfectly well. The mechs will say it doesn't work based on tech data, but when you actually do it, it works fine. Great for going up steep hills, or going up lots of hills with a higher cadence. A bit of a jump between the low gears is the only disadvantage. I slightly miss the 11 cog, but only for a few seconds a ride, whereas I use the 30 cog for an hour on a long hilly ride.

fstshrk

+1. I have done this as well.

wphamilton

I did switch from 11-28 to 11-30, technically exceeding the specs of the RD but it works without issue. The one caveat is that I have a lower end Shimano road derailleur which oddly allows more range than the more expensive ones.

I could be off-base, but it seems to me that provided you can actually shift ok to the big cog, the worst case is you lose the big-big cross-chain gearing.