From my years of off-roading, there's kind of two trains of thought.

Don't use 4x4/lockers until you need them. That way if you get stuck in 2x4, you can use 4x4/lockers to back out and go back the way you came. If you get stuck in 4x4, you're probably screwed.

The other possibility is that going back the way you came isn't an option or you simply don't want to. In that case, the goal is not to get stuck at all. Use every tool at your disposal to get through.

When I used to go wheeling, unless it's extremely thick, deep mud, my goal is to always go forward. I'll engage 4x4/lockers and power through. With that said, I had front and rear lockers and a 12,000 lb winch, so I didn't really care if I got stuck, I would just pull myself out. I wouldn't risk that in my F150, not unless I had someone there to pull me out

 

(P.S. Seems like there's always someone shame-posting text from the manual on the assumption that the OP didn't read it, that the manual's version of things is self-sufficient and so complete there's nothing left to be said.)

 

I know this has all been discussed before, but I still find it very interesting. I previously drove an STI and of course it delivered power to all 4 wheels continuously, and was a great help on dry pavement in various situations. When I ordered my f150 FX4 I just assumed I would be able to leave 4A engaged all the time.


Clearly these 4x4 systems are different, I will have to look more into that. Wondering if there are any advantages for the f150 having this particular 4x4 system that cannot be engaged all the time like an STI could?

 

It's the same with a "manual locker" on very low coefficient of friction surfaces.

I'll use ice as that's the easiest to understand.

In an open differential on ice, one tire will spin and the vehicle will move forward very slowly or maybe not at all. The vehicle still has lateral stability (IE:front and rear of vehicle stays in line with each other) as the wheel that is not spinning still has some friction between the tire and the surface to keep the vehicle pointed where you want to go.

Now take the same vehicle and lock up the rear differential so both rear tires have drive. On smooth ice, both will spin as both loose traction easily. The difference to the open differential is that now you don't have that one "non spinning wheel" giving you lateral stability at the rear. The overall effect is that the rear of the vehicle comes out of alignment with the front, or "fish-tailing" if you prefer to think of it that way.

The reason it fishtails (other than just the loss of traction) is that with both wheels spinning, the rear of the vehicle will follow any type of grade it is sitting on. If you have a 1-2% grade, the rear may slide slowly. If you have 3-5%, it may swing rather rapidly. The front tires stay fixed, the rears follow whatever grade is there.

It's this swing that can make it difficult to control/dangerous. You can slide into something beside you, a ditch, a hole, a curb, off a bank, etc if you're not ready for it. Sometimes you can be ready for it and still hit something. Once the tires are sliding, you're at the mercy of the forces in play. Even getting your foot off the throttle can still mean a few seconds before the vehicle comes back under control.

A few seconds with 4-6,000 lbs not in total control is the difference between just messed shorts and a lesson learned or a complete tragedy.

You do want to use a locker when in low traction situations, but you need to be selective and be aware that on very low friction surfaces it can bite you. Slow, steady and smooth wins the race of low friction surfaces.

As you get into surfaces that offer a bit more "bite" (deeper snow, gravel, mud etc) you can lock 'er up and let 'er eat! Just be aware that each surface is going to behave a little differently than the others and drive accordingly.

 

A full time 4x4 system, as in most Subaru, Audi Quattro, and some luxury minded SUVs, involve a center differential to allow the front and rear axles to spin at suffering rates. Such systems are great for the road and decent for soft roads. When the going gets rough though, the weakness of these systems is evident as if any one wheel loses traction entirely by sitting in the air, buried in mud/sand/snow, etc., then all wheels lose traction. Limited slip differentials can help to mitigate this, but those differentials tend to operate on torque ratios rather than an absolute minimum value of torque transfer. That is, a mechanical limited slip differential might allow a torque bias of 8:1 in worst case conditions, but if the bad wheel has near zero torque capacity then you've still got only near-zero torque going to the solid wheels.

The lack of a center differential in part-time 4x4 systemseans that an entire axle can be worthless for traction but the other axle is required to spin the same rate, thus pulling (or pushing) you out. A front or rear locking differential lock both wheels of that axle together.

In short, you can think of basic full-time 4x4 systems as still limited by the wheel with the least traction. Basic part-time 4x4 systems are limited by traction of two wheels - one at the front axle, one at the rear. A full-time system with an optional center locker is basically the same as a part-time system. Add in a rear locker to a part-time system and you've got to defeat three wheels before you get stuck. Add in a front locker and now all four wheels are spinning at the same rate no matter what the ground is like.

 

The STI is an AWD system. Our trucks are 4x4. It is a completely different system. If you want to know all the details, do some research on AWD vs 4x4.

To save you a bit of time, here's the basics.

In 4x4, power is sent to one front wheel and one rear wheel. Now if you engage the rear locker, equal amounts of power will be sent to both rear tires. If you drive in a perfectly straight line, this is fine. As soon as you turn, the inside tire spins slower than the outside tire because the outside tire needs to cover more ground. That's fine if you're on slippery conditions, as the inside tire will just slip and everything will be fine. If you're on dry pavement, the inside tire will struggle to slip. You'll feel the truck bind and you may even break something. The tires cannot spin at the same speed while turning unless one slips, which is hard to do on dry pavement.

In an AWD system, power is sent to all 4 wheels, but it's not equal amounts of power. Sensors and computers determine how much power to send to each wheel based on traction. So if one tire needs to spin faster than the other, the computer will allow that to happen.

So to sum it up, with your locker engaged on your truck, both rear tires are powered equally. It doesn't matter if they need to spin at different speeds while turning, the truck will always send equal amounts of power. In AWD like in your STI, different amounts of power are sent to different wheels.

Each system has their advantages and disadvantages. 4x4 generally allows for maximum traction in off road situations. AWD is generally preferred for traction while driving under normal conditions.

It is fine to use 4x4 auto when it snows though. It will only kick in if the tire slips, in which case nothing will break. Generally, 4x4 is okay even on dry pavement unless you're turning sharply or going fast, then it will bind and maybe break. Still, there's no reason to use 4x4 on dry pavement, so I would not recommend it.

 

Thanks both of you for the thoughtful explanation. After reading your comments, I went and did some additional reading and I think I understand now. 6 years of casual track days and who knows how many years of wrenching on cars and I never even considered this lol.


In summary, this is what I've learned:


1.Our trucks come with open differentials which are not capable of any sort of torque vectoring your would come to expect in an AWD type car like a Subaru or Audi Quattro.


2. The benefit of this setup is realized when you are in very slippery conditions where it is possible for 1 or 2 wheels to lose traction completely, and you are equipped with an e-locker. In this case a limited slip diff will not help very much, but a locked diff will get power to the wheels you need.


3. All 4WD does is get power to a third wheel in the event you need it. I would also say it helps to distribute the power more evenly when you do have traction so as to minimize rutting yourself to begin with.


Interesting stuff, and it makes perfect sense now why there is little point in engaging 4WD unless you really need it, same for the e-locker.


While this will be my first truck, I am not exactly new to off roading, having driven trucks for work in some challenging off-road conditions. If it were me, I think I would have 4WD engaged all times I was off trail or in snow, and only engage the locker if I were a. stuck, or b. the route looked slow and sketchy. If I were going through deep loose sand (like a wash) I don't see the point in locking the diff if your strategy is to keep momentum and never lose traction in a wheel to begin with.


My 2 cents from a newcomer

 

Plus if you're in 2H and bury up, you have to switch and get started from a dead stop. In low traction situations, momentum is king... if you lose your momentum, even 8 wheel drive might not be sufficient to get you out.

I go 4A if the situation is sketchy or the traction is shifting dramatically (crossing roads, etc.) If I'm in a low traction environment continuously, 4H.

For our brush trucks (F550s with aluminum bodies and 300/400gal. tanks... 10-12K lbs.) we go to 4H and lock the hubs as soon as we get off the road.