Just bought a new road bike and would like to have a better really low gear for those long hills. Is there any reason why one cannot simply replace the stock 12-26 rear cassette with a 11-32 mountain bike cassette to go with the 52-42-30 front gears?

You can do that, but if you do, that may not be all you need to do. From what I remember, most road rear derailleurs have a limit on the difference between smallest and largest cogs that they can handle, and so an 11-27 is about at the limit. Putting a mtn bike rear derailleur on there ought to work just fine.

Another option would be to change your chainrings (after consulting your front derailleur's capacity and making a gearing chart to check it for being practical). But, a cassette change and perhaps a longer cage r.d. to take up the extra chain slack sounds better. You might make up a gearing chart anyway to check for redundancy and shifting patterns with your new cassette. Go to https://www.sheldonbrown.com/gears/ for an automated calculator.

*** Changing chains along with your new cassette is a good idea...you may well need some extra links anyway to accomodate "bigger" cogs.

Your local shop will be able to advise if the rear derailler will need changing, and that is a possibility. The other thing is a longer chain aswell, so yet another expense. Taking it that you want to retain the 52 on the front, then this is your best option.

I have campy components with a long cage der. I went from stock 12-25 to 13-29 and changed to a Wipperman chain. Works beautifully. I am now normally using the middle cogs a whole lot more.

I like this calculator better.
https://www.panix.com/~jbarrm/cycal/cycal.30f.html

It gives more info and allows you to change the front chainrings as well. Use sheldon's for the listing of the number of teeth on each ring of standard cassettes.

I have an older 7-speed 105 After a couple emails back and forth with Sheldon Brown I replaced the 12-25 (original) with a 13-30 and of course a new chain. Oh what a difference! Harris cycle list a 9 speed 13-30 which was my first choice but then I would have needed new shifters too and didn't want the extra expense. The LBS wasn't sure of the change but I was confident with Sheldon's advice.

I don't know why you'd want an 11 cog.
You can go with a 12-32 or 12-34 if you use a mountain rear derailleur. I don't like mountain cassettes on a road bike because the large ratio jumps play havoc with your cadence. However a mountain cassette may be the best choice if you ride in mountains.
The 13-30 idea would be my recommendation.

Al

Instead of changing to 52-42-30, why not simply a road double with 48-38? Your current derailleur may be able to handle something like a 13-27 cogset with this combination, without exceeding its wrap capacity. This would give you a very nice gear range of 38 to 100 gear-inches, which was pretty typical of nonracing road bikes of the bike boom era (e.g. Schwinn Varsity, 52-39 / 14-28).

--- I successfully replaced my original 12-30 five-speed cassette with a 14-34 six-speed cassette on my 1970's road bike. The new cassette fit between the stays and the original Suntour derailleur handles it fine. The new cassette was hard to find so I ordered it from Sheldon Brown.
By the way, I also replaced the chain.

He's aleady got the 52/42/30

As Al has said,there is hardly any point to an 11 cog, especially with that kind of granny, and your RD might not handle a 32,and if that is the case you would also need a mtb RD.Also consider dumping extra bacon,eating more spinach and training harder.

The issue that you might run into is the upper derailleur pulley hitting the biggest rear cog. You might be able to avoid that by dialing in the "B" screw on the back of the rear derailleur and you can eliminate the problem for sure by using a mountain rear derailleur.

You'd also be well advised to resize the chain. Even though most riders never use the big/big combination, if you ever try to shift into that combination by accident and your chain is too short, it can get real expensive. Make sure that your chain is long enough to safely cover the big/big.

I have a campy mirage triple (30-42-52) with an XT 11-34 rear cassette on a Shimano rear wheel and hub. The campy is a long cage derailleur. It works fine, except I do need to adjust the shifters a little more often than usual.

A cheap way to get lower gears is to replace the 30T granny ring with a 26. The change is easy and far less expensive than changing a cassette and rear derailleur.

Yes, you will exceed the published "capacity" of the rear derailleur but all that means is you can't use the granny X the smallest two or three cogs. That's no sacrifice at all and the consequence if you do is that the rear derailleur goes slack which is only annoying, not damaging.

I've made this change on numerous 7, 8 and 9-speed drive trains and it works very well. BTW, all granny rings, including the factory 30T ones, are "flat" with no pins or ramps to enhance shifting, so a flat 26T replacement ring makes no difference.

Don't see any reason why you couldn't do that.

FWIW, I had an 11-30 cassette, I found a scrap 34T cog and simply replaced it (Sheldon Brown has a great page explain what to do to break apart a cassette)

Folks can feel free to jump in and correct me but here's what I've learned about rear derailleurs:

They have a maximum cog size. I don't know all the factors this is affected by but assume arm length is one of them. A short cage Sora rear derailleur accomodates up to about a 29T rear sprocket. A long cage Deore goes to 34T.

Rear derailleurs will have a "chain capacity" also called "range" which does NOT denote the "range" of rear cogs.

IOW, a rear derailleur with a "max 14T difference" does NOT mean you can only have, say an 11 to 25T rear cassette. It means you can only have a 14T difference from highest to lowest FRONT CHAINRINGS.

The idea being, a longer cage rear derailleur "contains" more chain over it's rollers and through the cage and therefore has the ability to take up or let out enough chain to accomodate different sizes of front chainrings.

You're never worse off for having a long cage rear derailleur (unless ground clearance becomes a factor - likely not for road riding). Upgrade away!

One thing my recent drivetrain upgrade schooled me on was the interconnected nature of all components. Making a front chainring change meant I had to look at my rear derailleur to see if it could handle the jump between front rings. The new sprocket meant ensuring I had a long enough chain and a rear derailleur that could handle the 34T sprocket.

To go bigger than 30 in back you would need a new rear derailer. You would also need a new, longer chain.

A better, easier, cheaper way to get lower gearing is to replace the granny gear. (The term "granny gear" refers specifically to the smallest chainring of a triple crank.)

Get rid of the silly 30 tooth ring, and replace it with a 26 or even a 24. This is WAY cheaper, and also lets you keep the close-ratio cassette gearing for the normal cruising range.

For more detailed instructions, see: https://harriscyclery.com/74

Then, if you find you still need a lower gear than that, it would make sense to consider a different cassette. My "Century Special" 13-30 is very popular for this sort of application.

See: https://harriscyclery.com/9

Sheldon "Jump Stop" Brown

Rather than switching out the rear derailleur, try to put on a smaller chainring. I dropped mine down from a 30 to a 26 and have been enjoying the easier rides on my long hills.