I have busted a couple of Deore long cage derailleurs, on tree stumps and roots, since they only have about an inch clearance from the ground. I have a SRAM Dual Drive on this trike, and use the lowest gear on that rather than on my cassette, to keep the derailleur away from the ground. Combined with a 30, 40, 52 crank set, and 12-36 cassette, it gives me a lot of gears and a broad range. Running a Dual Drive, with an 11-38 cassette and a Schlumpf Mountain Drive (1-2.5x) and a 50 tooth front chain ring gives me a range ov 7.2 to 115.6 gear inches, or 1600% range! AS it is, i can climb a wall with my trike. I had to put a 20kg cast iron weight on the rear rack to maintain grip on the steepest sections of my old driveway, which was about 25% gradient. With these types of numbers I reckon I could drag a piano up a wall, and could certainty drag the heaviest touring load up the steepest road I am ever likely to encounter.
Sheldon Brown's calculator gives some rough estimates.
