Anatomy Of A Truck – Off Roading 4×4 Vehicle Breakdown

In the world of  4×4 trucks and off-road vehicles, there is a fault line running between Jeeps, and everything else. And some people never get over it!

For the rest of us, due to the obvious subjectivity of the question, many pros further split things into what 4×4 truck is best, is a 4×4 SUV capable, etc. so they don’t have to fully answer the question!

Avoidance is the best way to sell magazines and not tick of your readers!

However, like Bill Clinton once taught us our answer has got to depend on what “is-is.” Is the budget sky-high? Are we a beginner looking to get into the sport? Are we mudding or Overlanding or rock-crawling?

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Or, are we asking which 4×4 is best as a means to drive safely in the snow, in this case, and in cases including “go fast” dirt sports AWD type vehicles come into play.

This is going to be quite a lengthy discussion, but I want to touch all the bases and leave no stone unturned! So here is a brain dump. It may be edited in the future. But it is an open discussion on the anatomy of the best 4×4 vehicle from my point of view. Here we go.

4×4 Truck Driveline

Let’s delve into how the power gets to the wheels! Here I’m not going to discuss those parts like drivelines that are maintenance items and extremely cheap to switch out, nor brakes that are very easy to upgrade and are so federally regulated that most are very similar.

The Devil is in the Differentials

I want to ask first, exactly what is the four-wheel drive?

4×4 is a familiar symbol and some people that off-road is never more concerned with it than knowing which button to push to turn it on.

Yet, that “4×4” denotes there are 4 wheels and selectively 4 get power (versus AWD where they always do), against typically rear-wheel drive 2×4 which means only two wheels ever get power out of four and there is no button; as we’re about to find out it’s not even that simple!

4×4 is a kind of a lie.

A road-going car or truck goes around curves, and if you think about it the wheel inside the turn goes a shorter distance than the one on the outside because they draw different sized circles. To compensate, the differential is designed to let the inner wheel slip in a variety of ways like illustrated below:

Open differentials are the simplest way to compensate for this.

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To spare a bunch of mechanical terms the layman and I confess even professional normally doesn’t need, what you need to know is that these work to make all things equal. And it stays equal no matter what!

This is where they get into trouble. Say you are on ice or dirt and one tire is hopelessly slipping and the other has a good grip? The differential will poor the same amount of torque into both wheels; it has to equalize things. So what ends up happening is the slipping wheel that needs .01 lbs power to spin will spin and set max power, yet the wheel with grip will just sit there not moving under .01lbs power.

Because of this, many on forums and the inter-webs say a 4×4 with open diffs is only running on “2 wheels” one front and back. This is a bit of an oversimplification; however, it is true that if you have one front wheel and one rear wheel off the ground the vehicle will just spin the tires in the air helplessly!

Limited Slip differentials work to correct this by the use of an internal clutch, oil pressure, or other means. These work to change the balance and pour power into the wheel that isn’t slipping.

They are called positraction, or “posi’s”, and a few other brand names it’s easy to see why these are an upgrade over the open differential. While they still don’t put all available power to the ground when tires are slipping, they allow the non-slipping tire to at least move!

Most AWDs have this type of system balancing all wheels.

Full Time 4×4 Lockers

Locking Differentials are the “full-time” 4×4 option.

They are open differentials with mechanisms to “lock” the pinions together so that both of the axles spin off the drive-line at the same “speed” and power. They undo the features that allow the car to turn in the circle with wheels at different speeds.

The trade-off, though, is that if one tire spins the other tire is oblivious. It really can power out of slippery situations; lockers can’t corner without dragging the tires so they are often set up with on-off switches of various means.

This is a big consideration when asking “what will the vehicle have to do?” Open differentials (diffs) are virtually bulletproof and very cheap to make and maintain. For light-duty work they may suffice; however, they are not a good off-road option and in some respects not even very good for the “I bought a 4×4 to avoid using chains” rational I often hear.

Limited-slip differentials, on the other hand, are very well rounded, and with some training, the brakes can be used to make them act like a full locker. And Lockers are off-road kits for rock crawling!

This also means 4×4 is a tricky term to apply. Some vehicles come with options that mean one has a limited-slip and another doesn’t. So 4×4 on any given vehicle may not match it on a different example of the same model.

NOTE: with newer cars “traction control” is becoming much more popular; however, depending on the system they all have plus or minuses. Some selectively brake to adjust power levels, some just cut engine power. These systems cannot overcome these basic differential issues if you’re thinking of that option.

But the axles get the rubber to the road

In this realm of mud, snow, and rocks durable tends to equal “heavy.” But heavy is a trade-off that contributes some undesirable characteristics like less range and MPG, more wear and breakage, and more inertial effects like roll-over, worse stopping, and beefier suspensions that are more primitive, etc.

So some of the game is to ask, what does the vehicle need to take?

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AWD Is The Best Option For Many

For those who are going to only stick to the pavement or near pavement like graded gravel roads or dessert trails, AWD is a lightweight option that allows the retention of a highly engineered suspension. Just think of the Subaru’s racing around! There is some mileage hit; this system already pops in the best of the heavier “posi” systems without primitive and limited straight axles.

And the automakers know this. Even the dedicated 4×4’s has slowly moved from large solid axles to CV joints and differentials culled from AWD programs because most customers will never take a Jeep off-road, let alone an F-150, and they realize that people are using it to avoid putting chains on or status symbols.

Independent Front Suspension

Independent front suspension has two main bloodlines.

There is the Twin-Traction Beam that Ford used until the mid-1990s, including my Ford Ranger, that many hate. They are actually rather tough (for Dana 35’s/ 44 hybrids) because the steering knuckles are spared extreme angles, the internals are of a dana 60, and they came up to even an odd-ball Dana 50 on the 350 4×4.

Most of the hate is actually because to upgrade to a Dana 60 or something requires a total front-end swap, and they are very hard to align if modified or worn.

CV Axle Setups

These benefit from the RND spending they share with AWD and all front-wheel-drive cars. They put all the weight on the suspension arms and dedicate the axle solely to turning the wheel. So they are as strong as the control arms, etc., and are pretty much capable of about 3/4ths (in the affordable road-legal build) to do what a straight axle can.

Best yet, because they are the same as an automotive front-wheel-drive setup they can have struts and “modern” shocks as OEM features making most of the big canister shocks mostly just redundant for non-enthusiast.

They also keep the weight low and the road manners high. The new F150’s match cars for stopping distances and Road and Driver even had one beat a Civic!

Solid Axle

Solid axles benefit from being old-hat with off-road enthusiasts. They like them and are familiar with them which means they are plentiful and have wide aftermarket support. Yet, these range in strength and some are weaker than IFS setups. This is in part because many axles throw all the weight onto the axle shafts in a design called “Semi-Floating.”

Torque and resistance mean breaking stuff. A full-floating solid-axle adapts for this and throws the weight onto the tube so the axles only have to spin the wheels like the IFS and AWD, and the good news is most can be converted if you have enough cash.

The initial trade-offs tend to be that they are hung under very basic suspensions with less adjustment and road manners, think “rides like a truck.” They also are not optimized with MPG in mind like modern components and have many places where mechanical drag is unavoidable.

Even more so, they are the epitome of durability at the cost of weighting a ton which exacerbates the MPG issue. Solid axles probably chop off 25% from the top in efficiency rating.

The real hesitancy issue with the Solid Axle setup is that in stock form will not handle extreme abuse any better than the others. And if lifts, big wheels, etc. are in the equation the price difference between a built solid differential and a tricked-out IFS depends on the Craigslist fire sales, and the “opportunity cost” is determined by your vacation itinerary and nearby environs. But remember, you have to gas it up year-round unless it’s dedicated.

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Across the nation…. manuals are currently dying.

Yet I’m sure 98% of the public couldn’t or wouldn’t drive one and is oblivious to this tragedy; therefore, this isn’t much of an issue if you are shopping for something new.

For us budget-crunchers though, we have a few more options shifting through the parts bin!

Manual Transmission

I drive a manual sports car; I drive an automatic truck. Manual transmissions are fun because you can really control and select how much power gets down to the wheels and when. This is especially helpful and noticeable with a smog-bog engine in traffic in the mid-range 20-30 mph zones. Yet, there’s a reason that “automatic-manuals” now that the MPG is equal are fast replacing them. All the fun and less failure.

Manual transmissions first off simply have less ability to handle power than their automatic counterparts, especially if you’re going to go the high torque route. That’s why it is very hard to find a classic muscle car paired up to a four-speed.

The weak point that can’t handle the power, and causes the most general headaches have always been the clutch. It’s a disk of material similar to brake pad linings that has to “skid” and catch constantly. A large amount of torque can break it free, and in first gear, the issue of stalling often leads to either dropping or riding the clutch causing it to wear prematurely.

Just imagine both a bunch of stones and a steep incline. You’ll roll backward with changing power demands. I think you can appreciate what kind of wear troubles they’ll eventually give you.

Second, the clutch needs to be served by master cylinders, slave cylinders, throw-out bearings, pressure plates, and pilot bearings. While maybe a tad bit simpler on the inside (but I would argue much harder to fix with needle bearings, etc.), manual transmissions have a lot of external complexity that will be out in the elements and mud.

That said– there is still a case for:

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Manual Off-Road Transmission

Mainly, they don’t need a battery to start which comes into play more harshly with the nearest store 50 miles away! Best I could cook up!

Additionally, some very capable rigs are powered by rather gutless 4-cylinder power plants. These need to be held in low gears to really make it over obstacles, and there are other times that the gearing in a manual works out technical trails for you.

And for just running light trails and circle tracks they offer way more of a fun factor on the power slides with more control about when the back end will lock back up (important with top-heavy vehicles especially).

However, the main reason I often hear given for preferring a manual to an automatic is that the manual is far simpler and generates less heat and therefore “will much less likely break down on you.”

Automatic Transmissions

I call B.S. on that last point!

When properly maintained an automatic transmission will always be better than an unmaintained manual with an iffy clutch. Moreover, with a trans cooler in the radiator, and an external transmission cooler, they are very shielded from their main point of failure; which, funny enough… is burnt internal clutches! DANG CLUTCHES MAN!

Just take a look at the pros in off-road racing, rock-crawling, and many other 4×4 motorsports. Pretty much no one uses a manual.

In fact, from my experience as a shop mechanic the only automatics I would not trust are made by only a few companies who in truth I don’t trust anything from!

What I did see a lot of, however, was owners who never maintained their transmission or change the fluid treating them as “set and forget” have abused automatics blow up on them…

Here’s how to change the ATF

NOTE: I think the owner’s behavior explains about 80% of the reliability issues between automotive brands. Some brands just attract lazy customers who blame them for their laziness. Others get the anal-retentive type and take all the credit for it.

Running 4×4 throws the transmission into its “heavy” maintenance cycle, and events like submerging the transmission let water into the breather tubes, meaning the case needs to be drained far more often than some would think.

Luckily, most trucks run the cheaper fluids like plain Mercon/Dex that can be had in generic gallon jugs for 14 bucks, and drain plugs can be installed to make the ATF change a yearly event that’s stayed on top of. Even better, because people who tow inline secondary transmission coolers are plentiful and cheap!

Just Spinning Our Wheels – Main Takeaways

This has been a discussion of simply how the wheels get spinning between the back of the engine and the hub. In all, we have discussed suspension archetypes, a few sets of gears, and the gearbox itself.

And honestly, we’ve held back on complexity. But what should be becoming apparent, is the anatomy of the best 4×4 is fast shaping up to be a very complex topic!

But it’s also showing itself to be very important! As our open differential may be dangerous in snow or absolutely no help. We may be stuck in a certain way of thinking or not considering options that actually would be way more fun!

Thanks for reading and stay dirty