Winches And Such
When I first started fourwheeling back in the dark ages (it was referred to as "jeeping" then), very few people had winches mounted on their vehicles. There were a few PTO winches available from long-forgotten names and some electric winch companies, but the ones I had any contact with were usually homemade modified military-surplus "bomb winches" with 12-volt starter motors attached in place of a crank.
Just in case you don't have a clue to what a "PTO" winch is, perhaps an explanation is necessary. PTO (power take-off) winches probably originated with military vehicles. A separate gearbox is attached to either the transfer case or the transmission, and from it a driveshaft was snaked past the front (or in some instances the rear) axle to a gearbox on the winch. To engage the winch, a lever inside the cab was shifted. Winch speed was dependent on engine speed. On today's 4x4s, snaking a driveshaft forward past the suspension, exhaust system and front axle is pretty difficult, and about the only place you see PTOs is on military trucks, construction equipment and tow trucks.
Arthur Warn most likely was responsible for the development and expansive use of the modern electric winch that is so prolifically mounted on the front of vehicles today. In 1959, he teamed up with the Belleview Manufacturing Company to produce the now long line of electric winches under the Warn name.
Enough of the history lesson. What I really want to talk about is winch capacity. A commonly question asked is, "How much winching capacity do I need?" Generally speaking, the minimum capacity you want can be figured as:
1.5 x (vehicle weight or pulled load)
For instance, if your Jeep weighs in at 4,000 pounds, then a 6,000-pound winch is the minimum capacity of pulling power you need. If your truck is tipping the scales at 6,000 pounds, then a 9,000-pound winch is necessary. Again, generally speaking, bigger is better (up to a point, anyway), so perhaps in some instances you would need to go to two times the vehicle weight.
Most winch pulls are well under the maximum spooled-on cable length of 75 to 125 feet. It takes at least three full layer wraps of the cable around the drum to hold this much cable. However, as the cable overwraps itself, the rotational ratio on the drum becomes higher, thus reducing the overall gear ratio and, in effect, the pulling power of the winch. Winches are rated as to their maximum capacity at full amperage draw and on the bottom layer of cable. By the third or even fourth layer, the capacity is significantly less.
Actually, there is a lot more involved when picking a winch's capacity besides vehicle or pulling load weight. We can start out with angle or slope, or in simpler terms, how steep of a hill are you pulling the load up? Gravity is not our friend here, so vehicle weight is the first factor, then the slope. You have rolling resistance, starting with that which is inherent to the vehicle itself. Other factors are smooth surfaces, dirt or sand (wet or dry), as well as the different types of mud and their depths. The distance and angle of the pull also need to be taken into account. All these may present an instance where you need to use a factor of two times the load equals winch capacity.
There are several different formulas for factoring in these parameters that go beyond my desire to do that much math, but the 4x4 Icon (http://4x4icon.com/offroad/winch/index.htm#loads) has this way-cool spreadsheet that does it for you with your input of numbers.