Your chain probably needs to be lengthened when you add that many teeth. You may blow out the springs in your derailleur bending it that far. However, go too long and you may get rubbing in the 52-11 combo.
Two things:
1. Think about what combos you need. Did you ever ride in the 52-12 and need a higher gear ratio? If not, no reason to have an 11.
2. You should try to avoid "cross chaining". The bike should be able to shift into any combination safely, but you shouldn't be riding large-large or small-small in general because it puts a lot of extra stress on the drive train and just puts you in a middle gear anyway.
If what you need is just a lower bottom gear for a really steep hill, you might consider changing the inner front ring for a 28 as well. Also note that it's good practice to change your chain and cassette at the same time, unless the chain is pretty new.
At least the Shimano specs for the RD-5700 GS (current 105) say that the max sprocket teeth number is 28. Also, the maximum capacity is 39 T. I don't know what dérailleur you have, but the specs will be in the same ballpark (I think the older 105 had 27T as maximum but same capacity) and you already had a cassette that was at maximum (together with the crank set). As DaveM suggested, you could go with 12-32T (as this seems to just work) or maybe get a smaller granny gear in the front (28 T). Note that the chain will then be either too short for the 52-32 combo or too long for the 28-12 (you shouldn't use both anyway, make sure you remember during the race!). Another possibility is getting a MTB rear dérailleur as those have a higher capacity.
I am not really sure that the 32 T would work in the rear, The movement of a rear dérailleur is complicated, so I find it hard to anticipate where it would end up (if it is a close thing as in your case).
IMO, you should change the 30T chain ring to a 24 for climbing. That's a very tall granny 30T gear. Its way to much crossover across from the 52F to the 32R. You should use your smaller crank chain-rings more.
AS it is now the chain is too short, for the extreme crossover. BUT as I said before you should not go there. Try half way in back than shift in front.
Never Give Up!!!
There are many of us who run combinations that would be destructive to the bike if they were used. Do NOT try to use the 52 / largest cog at the back no matter what.
On our tandem; the front triple is 28/44/54 the back is 28 - 12 7 speed currently (we're planning on going 34-11 9 speed in the future to get two more lower gears.
* The 28 front can only be used with the four largest cogs at the back. This ring is only used for climbing.
* The 54 front can be only used with the four smallest cogs at the back. The 54/28 combo results in a locked up system on the bike stand, and would result in a destroyed rear derailleur probably taking out the rear wheel if tried on the road. The 54 is only used for speed, on the flat, down hill, tailwind, etc.
* The 44 front can be used with any of the cogs at the back. 90+% of our riding is in the 44. 44/28 is fine for starting on the level.
To your question, it will probably by okay with 42 - 32; but reserve the 52T chainring for the top half (smallest cogs) of the cassette. Use the 42T chainring across all the cogs, reserve the 30T for the bottom half (largest cogs) of the cassette.
Nigel
This was rather an older post. As said above nigel opints out that using those gear combinations are not recommended! Very hard on the drive train. I have a 26/36/52 triple and a 11-28 on the rear on my GT Outpost and I can say it doesn't lockup, BUT you wont seeing me use anything but the smallest 2 cogs (6th and 7th gear as some knows it){smallest 3 in quick need}.
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
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