Robo has suggested having a one speed edirtbike. With rear suspension, you will most likely need a chain tensioner for a derailleur elimination method. An unacceptable way of achieving a chain tensioner would be to use a rear derailleur as this would hang out in harm's way and be no better than having a working derailleur. There is a variety of chain tensions already made in the bike component market but I could not find one that would work with a 48T rear cog. The Surly will take up to a 24T cog. These items seem to be made for taking up chain slack when running an internal gear hub.
I recently order and received the SRAM EX-1 derailleur, shifter and cassette at prices beating Amazon and get this: actually "in stock" from WWC. The cassette is far more durable than the SunRace steel that works okay for hill work as it is milled from one piece of steel. But at $123 for a derailleur I decided to go with one-speed gearing when riding the rough -- no trail -- but sage and mountain mahogany brush on the side slopes of hill & gully riding.
Here are some items to consider when making a chain tension for a large cog 40T - 50T.
1. The chain tensioner will have much better protection if it resides inside the stays or width thereof.
2. A set up that angles backward would likely not get bent out of alignment as easily as one that hinged and projected forward.
3. The closeness of roller to rear cog is better as this will allow for little misalignment due to chain rattling.
4. A return spring strong enough to keep the slack out of the chain for the rate you hit the bumps and create slack or tension.
5. Optional: create a set up that can swing out of the way when you decide to change the chain length and add a derailleur.
6. Possible unforeseen Measurement? How much chain take-up or loosing occurs during the full range of suspension movement? To determine the needed chain length you can remove a rear shock pin and swing the rear cage from uncompressed position into full compression for a trial chain length. You will not want the chain to get overly tight during this range of motion.
Low tech Solution: I was able to figure out a chain tension assembly that met the above criteria. I did employ a DMR chain tensioner (with the red pulley) fastened to a coaster brake arm that is gear clamped to lower stay and a hardware store spring for keeping the chain tight.
So far the chain tension has worked without failing on some very rough riding through some brush. Top speed with this 48T gear is 21mph. The 48T, 44T, 36T sub-assembly steel cassette is homemade from steel Cyclone chainrings and the cassette offers 9 speeds.