For example: Pulling a 4,000-pound vehicle up a 50-degree slope that was in deep mud would take over 6,300 pounds of pulling force. On the other hand, if the vehicle was on a 10-degree slope on pavement, it would only take a pulling force of a bit over 800 pounds.
Actually, it's kind of fun to play with the different factors. It sure gives you a better idea of just how hard your winch has to work. One thing that the spreadsheet can't do for you is figure out how much pull is necessary when the tire encounters an obstacle such as a large rock. The size and shape of the obstacle, tire diameter, and even air pressure all have a cause-and-effect that's best figured by common sense and experience. Be sure to keep in mind that the figures on the spreadsheet may be higher, depending on the "stuck" condition of the vehicle, and are only to be taken as guidelines.
Now, let's go back a bit and assume you figure you can get by just fine with a 9,000-pound capacity winch. Remember what I said about the pulling capacity of the winch being diminished by the number of layers of cable on the winch drum? You really need to check and compare these actual capacities per layer with the figures that are provided by the particular winch manufacturer you're interested in.
Here is a simple example of how this works. Let's say you figure you want a winch with a 9000-pound capacity rated at the number-one bottom layer cable on the drum:
| Cable layer |
1 |
2 |
3 |
4 |
| Capacity (1,000 lb) |
9 |
8 |
7 |
6 |
| Line speed (fpm) |
6 |
7 |
8 |
9 |
From this simple chart, you can see that your 9,000-pound winch may only have a 6,000-pound capacity on the top layer, so on a short pull, your winch may not have the necessary capacity, especially on a pull in deep mud. On a long pull, you may run out of pulling capacity as the winch cable is spooled in, and you may have to relocate the pulling vehicle or your anchor point.
Something else to keep in mind is amp draw, as it has a major effect on winch performance. Amp draw is the amount of current that the winch motor requires to perform a load-pulling function. The harder the pull, the more amps are required, up to the point of the motor actually stalling as it reaches its maximum pulling power or capacity. Permanent-magnet motors draw less current than series-wound motors and can be built smaller. However, their drawback is that they- overheat quite quickly, so for serious long pulls without more frequent cool-down periods, the series-wound motors are generally the way to go.
Winches draw a lot of amps, some times in the 400-amp range under full load. So it is not hard to figure that a battery with a 55-amp rating doesn't have much reserve capacity. That's where our alternators come in. Keeping the battery charged to full capacity when doing a hard winch pull is nearly impossible, but because very seldom does one make a long continuous pull at full load, an alternator in the 100- to 150-amp capacity seems to work just fine.
Actually, you could overtax the alternator to the point of meltdown if you were doing a lot of winching without a cool-down period. But then again, that would mean a lot of winching. Yes, we could get into the fact that as the battery voltage gets drawn down, amp draw increases to meet the wattage demand, but we are not going to go that far. In reality, pick the best battery you can find within your financial range that will fit in your battery tray, and then keep in mind when the winch motor gets hot to the touch that it's time to give the motor and alternator some cool-down time.
Obviously, the more storage capacity of the battery or batteries and the larger output of the alternator, the more winching time one will have. When picking a winch, you might want to consider comparing amp draw and different loads. You just may find that a 10,000-pound winch pull draws fewer amps on, say, an 8,000-pound pull than a 8,000-pound winch does at full capacity.
Next month, hopefully I will get into proper rigging and ways to lessen the strain on your winch.