This is a critical step in both the conduct of a winch recovery (because you need to be absolutely sure that EVERY single piece of equipment used in the task is strong enough and will not fail and thereby endanger life and limb), and in the selection/purchase and decision to use a piece of recovery equipment. It is a calculation where, quite frankly, the manufacturer's recommendations are woefully inadequate. Why? Simply because if they let you in on how large the forces really are, it would leave you realizing that they are unable to economically produce a winch of sufficient capacity in anything resembling a small, light, or economical enough package. They get away with it, because, as I said, there are virtually no regulations or standards governing the industry. I'm not saying all 4x4 winches are inadequate, dangerous, or useless. But I am saying that the forces involved are often much greater than the manufacturer's would have you believe, and you will be far more capable and SAFER if you approach your 4x4 recovery KNOWING THIS, and knowing the real numbers. Realize, that for reasons of practicality and economics, your 4x4 recovery equipment is almost certainly undersized.....you can still do the job, using the correct techniques, but you will be much SAFER if you keep this in mind. Enough of the pre-amble.
Most, if not all, winch manufacturers will tell you to select a winch based on 1.5 times the gross vehicle weight. This often leads to less than satisfactory results for 2 reasons:
1) Most people are terrible at actually estimating the gross weight of their rig as it sits on the trail, full of gas, tools, equipment, food, camping gear, people, the dog...everything. Heck, in some cases the real figure can actually exceed the GVWR of the vehicle. Simple advice here - either err WAY on the heavy side, or get your rig weighed in trail trim.
2) More importantly, the "effective weight" of a "stuck" 4x4 is very often FAR more than 1.5 times the GVW. The following data on how to more accurately estimate the "effective weight", is taken from the world of professional heavy recovery - the guys that recover Tractor-trailers that have flipped on their side for instance, as well as U.S., Canadian, and UK Military recovery manuals.
Once you have accurately estimated or measured the trucks loaded weight (LW) you can calculate the resistance to be overcome in any recovery situation (this is commonly known as the ROLLING resistance). There are 4 types of resistance that must be accounted for to accurately assess the resistance that must be overcome. These are surface resistance, damage resistance, mire (stuck) resistance and grade (slope) resistance. Calculate them all as follows: