I think I have this figured out, but will beg for some of your superior knowledge. I'm riding a Giant Defy, compact double 50/34 front, 11/28 10 speed cassette. I'm not the strongest rider, but getting better every month. Yesterday was hill day (go ahead and laugh at the FL guy talking about hills), and it finished my decision making process about needing a slightly lower gear. My plan is to get a long cage derailleur, 11-34 cassette, and new chain. I only want to run the 34 in the hills, and that's where the questions start. Can I combine a road cassette and mountain cassette to keep some of the 1 tooth spacing? (Depending on the first answer) Is it preferred to keep 1 chain and cassette together (in other words, swap chain and cassette when going from flats to hills). I have read of people having success using the short cage derailleur and using the B screw to clear a 32 tooth cassette, but don't really want to experiment that much. What am I forgetting?

The cross compatibility of MTB and Road RD did not carry over to 10 speed , maybe you can find a 9 speed MTB RD * and make it work

Reading about Touring bike selling companies they get that combo..

Or lose the whole crank , a 30-46 gets sold thru VeloOrange a 46:11 gear should be high enough for most but the downhill racer ..

derailleurs dont have speeds they just move X amount as the lever detent spacing commands .

my only indexed shift is a S-A 3 speed.. , derailleurs, all friction, R'off sorts it all out in the hub itself.

If you aren't sure how much easier gearing you need, you can get the Tiagra 12-30 cassette for $20. That may be all you need. Otherwise, there is a good chance a 32 will work with your current rd.

OK, I'll laugh. The only hill in Florida I know of even close to worthy of the name is Sugar Loaf Mountain Road west of Mt. Dora and it's pretty puny compared to most places. The only other climbing I've seen there are overpasses on I-75 and I-95.

Joking aside, what you are proposing is probably overkill for the area and a bit more riding time and conditioning will make your 34x28 seem plenty deep. As a short term stop-gap the 12x30 Tiagra cassette shelbyfv recommended is the cheapest solution.

I do get annoyed at the posters who say "HTFU" when anyone mentions problems with hills but, in this case, the hills are quite modest and you will soon find them manageable unless you have a significant physical impairment.

I agree that the 34 seems like overkill, but if you really want it the answer to your question is yes, you can combine road and mtn cassette components to make a low range, closer ratio cassette. I ride a 9-speed 13-26 Shimano cassette put together with parts from XTR, Ultegra, and D-A cassettes. It shifts perfectly. I could put together a 14-34 if needed.

A long cage road RD will probably not work with a 34 cog unless it is a 9-speed (or pre-10-speed) mountain RD. The cage length is not the cog size limitation.
A 10-speed mountain RD is not compatible with road shifters due to a change in RD actuation ratios.

Totally agree.

Tiagra long cage derailers have some serious clearance, but you truly don't need it. 34/28 could get most riders up most anything without trouble.

Podagrower, There are many ways to accomplish what you want, some fairly expensive. Switching to the 12-30T cassette is a good suggestion because you'll gain ~7% fewer gear inches in bottom and lose the 11T top cog, which is possibly little used. This difference maybe just all that you need/want.

Brad

"Hill" repeats is the logical solution.

Shimano 10 speed road and 10 speed mountain are incompatible.

But, you can totally use a 9 speed mountain derailer with your 10s Shimano road shifters. It will index perfectly.

My favorite rear derailer for this situation is the Deore XT RD-M772. It can fit a 36t cog, and the Shadow design is nice and low-profile.

Remember that if you put on a larger cog, you'll have to replace your chain with a longer one.

Take a guess at where I live, and what century ride I'm training for...the 34x28 seemed incredibly high 5 months ago, coming from a bike with a triple. I am way stronger now, but I'd prefer not to walk 1/2 a century.