Modern 4x4 winches boast some big load ratings from small packages; winches with a capacity (the maximum amount of force the winch can safely exert) of 12,000lbs (5,400kg) or more can be plucked off the shelf at just about any 4x4 store for less than $1,000.
Given that your average four-wheel-drive, even when fully loaded, weighs less than two-thirds of this, a 12,000lb winch should be more than enough for any 4x4, right? Not quite.
The key is in how that 12,000lb is measured, and like so many things, what looks good at a glance on paper is not how things work in reality.
Winch basics
No doubt you're familiar with a winch's basic construction and workings: a motor and gearbox on one side, a drum in the middle, and 100 feet or so of steel cable or Dyneema rope that spools onto the drum.
Spooling a rope of this length into the minimal space available on your average winch obviously requires that the rope be spooled on over multiple layers, and this layering is where the problem comes in.
Winch capacity by layer
The layers of rope on your winch effectively work as a gearbox.
Picture using a vehicle's manual gearbox. Starting in 1st gear (the shortest) the engine pull is slow and strong, with plenty of torque. When you shift up to 2nd gear (a taller gear), speed increases, but torque decreases. This process repeats as you change up to 3rd gear, 4th, and so on.
When pulling in the line of your winch, the same thing is happening. The first layer of rope on the drum is effectively "1st gear", and with every layer of rope that is added, it goes "up a gear", reducing the amount of torque (the rotational force applied by the winch motor).
How do we work this out?
We can demonstrate how quickly the capacity of a winch drops with every layer of rope with a few simple calculations.
Torque (turning force), is calculated as "Force x Distance = Torque". In this formula "Distance" refers to the radius from the winch drum, to the top rope later.
And to calculate Force, we use "Torque / Distance = Force". This is where your winch capacity starts to get reduced.
An analogy you will know from everyday experience is that it takes more force to loosen a stuck bolt with a short wrench than a long one. A longer lever exerts more force on the bolt, and so, a higher rope layer exerts more force on your winch, thereby reducing its capacity.
The exact amount of loss will vary depending on the size of the winch's drum, and the diameter of the rope, but a rough rule of thumb is that for every additional layer of rope, the capacity of a typical 4x4 winch is reduced by 15%. Using our example 12,000lb winch, this works out as follows:
- First rope layer: 12,000lbs
- Second rope layer: 10,200lbs (15% reduction)
- Third rope layer: 8,670lbs (28% reduction)
- Fourth rope layer: 7,369lbs (39% reduction)
- Fifth rope layer: 6,264lbs (48% reduction)
You can see how dramatically each layer of rope affects your pulling ability; for a winch with 5 layers of rope, when pulling on this top layer, as you might be towards the end of a recovery, nearly half of your winch's capacity is gone.
While this will still be ample for plenty of recovery situations, steep inclines and deep mud holes may require more, especially if towing a camper or caravan.
Image credit: Super Cheap Auto
Solving this with a snatch block (or not)
Some would argue that the above is easily countered with a snatch block and performing a double-line pull. This is fair enough if there is an anchor point close enough to use.
While using a snatch block or winch ring doubles your pulling capacity, it also halves the length of available rope. This is further reduced to a third of the original length if using a second snatch block for a triple-line pull.
At the end of the day, it doesn't matter how much mechanical advantage you gain by adding pulleys if you can't reach an anchor point to winch off of. Depending on the areas you're off-roading in, this is something to give proper consideration.
You can, of course, get winch extension straps, but this is one more thing to store and carry, taking up precious cargo space, and is an additional expense that could have been spent on a larger winch in the first place, potentially eliminating the need for the extension strap.
Go for the bigger winch
If you only engage in mild off-roading where using your winch is going to be a rare event, and aren't towing a caravan or camper, in all likelihood a modest capacity winch will serve you fine.
That said, the cost of a more powerful winch compared to the same brand's entry-level model is often only 5-10% extra, but may increase the rated capacity by 20% or more.
If you're planning on tackling terrain where needing your winch is likely, driving mountain tracks or through deep mud, going with a larger capacity winch is a good investment, in this wheeler's opinion. For the relatively small additional cost, often less than the cost of a winch extension strap, a winch with a higher capacity will provide a bigger safety buffer and give you more flexibility in whatever recovery situation you find yourself in.
# winch capacity by layer, winch rope layers power, winch layer power loss.
