Why The National Park Service Will Toss You In Jail For Driving Your AWD Subaru On Certain Trails

Imagine you’re heading into a national park. You’re behind the wheel of your chunky Subaru Crosstrek with lots of body cladding and all-wheel-drive. You head out on the trail, enjoy your adventure, and head home. Then a letter shows up in the mail. The National Park Service (NPS) is mad at you, because your Subaru isn’t hardcore enough for their trails!

This might sound fanciful, but that’s exactly what happened to Greatbigdog69 on Reddit. They got a letter from the NPS stating they had breached regulations by driving their Subaru Crosstrek on trails restricted to “high clearance four-wheel-drive” vehicles. The Crosstrek and its all-wheel-drive system isn’t up to snuff, they were told. Even better, they were threatened with a $5,000 fine or six-month prison sentence if they did it again.

Modern automotive marketing has created a great deal of confusion about four-wheel-drive, all-wheel-drive, and just which vehicles are really suited to proper off-roading. Given that prison time is on the line, let’s see if we can clear this up for you. Dive in for a deep explainer on what these terms mean, and what the National Park Service actually wants.

PSA: All wheel drive vehicles are not considered four wheel drive by the US Park Service
byu/Greatbigdog69 inNationalPark

AWD vs. 4WD: How It Was

The difference between four-wheel-drive and all-wheel-drive has always been kind of a mess. Initially, some automakers used the terms interchangeably. After all, if you have four wheels, and you’re driving all of them… you’ve got all-wheel-drive, right? Eventually, though, popular usage differentiated the two terms when applied to typical ICE-powered vehicles.

For a time, four-wheel-drive came to refer to “part-time” systems for off-road use only. This involved the engine driving a transfer case, which sent power to the front and rear differentials. The transfer case could be shifted between different modes. It could drive just the rear wheels, which was useful for on-road use. This was normally denoted “2H” or “2HI” in off-road capable vehicles. Alternatively, the transfer case could be shifted into “4H” or “4HI” to drive all four wheels. “4L” or “4LO” would do the same, while also gearing down the output for more torque and less speed.

In the four-wheel-drive modes, the transfer case shafts leading to the front and rear differentials would spin in sync. This caused a potential problem called “drivetrain windup.” When driving, the front wheels and rear wheels tend to travel different distances, particularly when going around corners. This creates a problem if you drive in four-wheel-drive mode on a sealed road. As you drive along, the wheels on one axle will end up travelling farther than the others. But the transfer case output shafts going to each differential are locked together. How can one rotate more than the other? Well, they can’t. This creates a huge “wind up” as the front and rear differentials try to twist the transfer case shafts by varying amounts. Eventually, something will snap.

This isn’t a problem off-road. Dirt roads and other surfaces let the wheels slip enough to release any drivetrain windup before it causes problems. But when you were on proper sealed roads, you had to drive traditional four-wheel-drive systems in 2WD mode to avoid this problem. That would disconnect the front wheels from the transfer case so there would be no windup.

Eventually, though, automakers wanted to bring four-wheel-drive to road vehicles. To get around the windup problem, they figured out a simple solution that would allow driving both axles all the time, even on the road. What if there was a center differential in between the front and rear differentials? This would allow the front and rear axles to travel different distances without causing any drivetrain windup or wheel slip.

Systems with a center differential eventually became known as all-wheel-drive, and were popularized by manufacturers like Subaru and Audi. These systems were typically designed for maximizing traction in slippery on-road conditions, like rain and snow. They were applied to road-going vehicles, and weren’t really designed for off-road use.

A wide variety of designs exist, particularly varying as to whether they’re based around longitudinal rear-drive platforms or transverse front-wheel-drive platforms. Either way the basic elements are the same. An engine, a transmission, and a center, front, and rear diff. These systems don’t really have a traditional “transfer case” with high and low range as they’re not for off-road use.

Indeed, this design has a drawback if you’re trying to use it off-road. The simplest examples used an open center differential, along with open diffs front and rear. If, say, the rear left tire began to slip, the rear right tire would not provide much propulsion. That’s because an open differential can only deliver the same torque to both output shafts. Whatever torque is going to the rear left wheel will also go to the rear right. If the rear left wheel is spinning in the air, it’s not accepting much torque. So the same minimal torque will go to the rear right as well. The same is then true of the center differential. Whatever minimal torque is going to the rear axle will also go to the front, and progress will be minimal.

Thus, four-wheel-drive with a transfer case is good off-road, and all-wheel-drive with an open center diff is bad off-road, right? Well, the world has gotten a lot more complex since then.

AWD vs. 4WD: How It Is

We used to have cars, trucks, and off-road four-wheel-drives. Now we have cars, kinda weird cars that are lifted a bit, crossovers, SUVs, hardcore four-wheel-drives, and about fifty kinds of pickup truck. They’ve also all got different drivetrains that have forever blurred the line between four-wheel-drive and all-wheel-drive.

Take the Toyota Land Cruiser. That’s a four-wheel-drive right? Well, uhhhh, I don’t know about that, Cotton! Sure, grab a classic 70 Series Land Cruiser, and you’ll get a part-time four-wheel-drive with the shiftable transfer case. Simple enough. But if you buy a modern 300 Series Land Cruiser, that’s not the case. You’ll notice it’s slathered in “FULL TIME 4WD” stickers. So what’s that about?

Toyota, along with many other automakers, realized people would get sick of shifting their transfer cases. People wanted four-wheel-drive all the time. So what did they do? They figured out how to shove a center differential into the transfer case. You still got low-range and all that, but you also got to leave four-wheel-drive on all the time.

As for the slippage problem, the solution was easy. Allow the center differential to be locked so it would act like a traditional transfer case. That gave you a four-wheel-drive system that was always on, but didn’t compromise its ability to put power down on soft surfaces.

You can argue all day whether the modern Land Cruiser has an “all-wheel-drive” or “four-wheel-drive” system. Ultimately, though, it’s largely treated as the latter. That’s because it was bred for proper off-road use, it’s tough, and it has a locking center differential.

Other cars and SUVs with all-wheel-drive systems have come a long way, too. Automakers have developed all kinds of fancy clutches and limited-slip differentials to improve their performance. The general idea is that having some degree of locking or limited slip in the center diff allows you to improve the ability to put power down in the poorest conditions. Audi and Subaru have explored various systems along these lines, particularly the DCCD system in the Subaru WRX STI. Still, no matter how sophisticated, these systems are all still typically called “all-wheel-drive” since they’re designed for use on the road, not on off-road trails.

There is also the magical Haldex AWD system, found in all kinds of Volkswagen Auto Group Products. Imagine a front-wheel-drive car, but tack on a transfer case that sends a driveshaft to the rear wheels. Then imagine the rear driveshaft has a clutch pack halfway along it that can slip to varying degrees. It’s normally left open so that just the front wheels are driven. But if the front wheels start to slip, that clutch pack locks up and sends some drive to the rear. It’s a pretty cost-effective way to do all-wheel-drive on a front-wheel-drive platform. You can also do what Lamborghini did and flip the whole thing around for a mid-engined, all-wheel-drive platform like the Aventador.

Oh, and we haven’t even discussed modern hybrids and EVs. For example, a Toyota Prius Prime drives the rear wheels with an electric motor, and the front wheels with a motor and an ICE engine. Meanwhile, other EVs have dual-motor and tri-motor systems to drive all their wheels, with varying degrees off-road capability. Few to none use a single motor driving all the wheels via a transfer case or traditional AWD setup.

How To Avoid Prison

Okay, so we’ve established that the line between AWD and 4WD is blurry at best. So when the National Park Service says you need a 4WD, what do they actually mean? And why isn’t a Subaru Crosstrek good enough? Let’s look at what the NPS actually says:

What is the difference between a 4×4 and All Wheel Drive (AWD) vehicle?

True 4×4 vehicles have a ‘transfer case’ in the drivetrain that puts full engine power to the front wheels. All Wheel Drive, common on vehicles like crossover SUVs, relies on a ‘differential’ to send variable power to each wheel. AWD is good on level roadbeds in low traction conditions like snow. It is not designed to fully power the front tires in off-pavement rugged situations. For example, if the road goes up a steep hill and there is a lot of loose rock in the road, fully powered front wheels are needed to rotate strongly and pull the vehicle up the slope. AWD cannot do that very well and may fail.

Honestly, that’s kind of confusing. The line about “fully-powered front wheels” makes it sound like the average Toyota Corolla could do the job, but we know that’s not the case.

Really, what they want is for you to bring a proper off-road vehicle when you’re tackling the “4WD Only” trails. Think a Toyota Land Cruiser, or a Jeep Wrangler. Or, say, a truck like the Ford F-150 with 4WD. If your truck has low-range, that’s a good clue that it’s properly equipped for off-road duties. Don’t forget to use your center diff lock when the going gets rough.

Similarly, the NPS defines certain areas as only suitable for “High Clearance” vehicles. That has a more straightforward definition:

You Will Need A High Clearance Four-Wheel-Drive (4WD) Vehicle

A high clearance 4WD vehicle is defined as a SUV or truck type vehicle, with at least 15 inch tire rims or more, with a low gear transfer case, designed for heavier type use than a standard passenger vehicle, with at least 8 inches of clearance or more from the lowest point of the frame, body, suspension, or differential, to the ground, also including a means to mechanically power both, front and real wheels at the same time.

Easy enough, right? Check your ground clearance, make sure you’ve got a proper drivetrain with a locking center diff, and you’re good to go.

Indeed, it appears this rule is what Greatbigdog69 fell afoul of. As per their letter from the NPS, “High clearance four-wheel-drive is required on motor vehicles travelling on the following roads: Elephant Hill, Salt Creek, Horse Canyon, Lavendar Canyon, Lockhart Canyon, and Colorado Overlook road.” Their orange Subaru Crosstrek was deemed an “all-wheel-drive vehicle,” and this put our Reddit poster in violation of regulation 36 CFR §1.5 (f): Closures and Public Use Limits.

Ultimately, Subaru Crosstrek models have had well over 8 inches of ground clearance since the first-gen models dropped in 2013. Between that and the wording of the letter, we can assume it was targeted for its “all-wheel-drive” status. That’s understandable to a degree, given that a Crosstrek certainly isn’t up to the hardcore standards of something like a Land Rover Defender or Toyota Tundra. Still, it’s easy to see how someone could fall victim to this confusion.

You don’t want to fall afoul of these restrictions. The NPS letter notes that “future violations may incur a collateral forfeiture, a fine up to $5,000, up to six months imprisonment, and/or other penalties.” Imagine going to prison for driving the wrong SUV down a trail. Sure, it’s probably unlikely, but they wouldn’t put it in the letter if it wasn’t true.

Given the blurry definition of all-wheel-drive and four-wheel-drive, I’ve written to the National Park Service for clarification. Is the NPS simply relying on whether a vehicle has “4WD” or “AWD” plastered on the back, or does it rule based on the presence of low range, a transfer case, or a locking center diff?

The fine people at the NPS wrote back to us and directed us to the superintendent’s determination, which is fairly straightforward. The NPS explains the requirements and its reasoning:

A four wheel drive vehicle is defined as a sport utility vehicle (SUV) or truck with at least 15-inch tire rims and at least eight inches of clearance from the lowest point of the frame, body, suspension, or differential to the ground. Four wheel drive vehicles have a transfer case between the front and rear axles that locks the front and rear drive shafts together when four wheel drive is engaged. All wheel drive (AWD) vehicles do not meet this definition.

Many of the backcountry roads in the SEUG are affected by flooding, snow, ice, quicksand, deep sand, ledges and rock fall. Road conditions and obstacles can be permanent, occur regularly, and/or change unexpectedly, especially during the winter and summer monsoon seasons.

Four wheel drive vehicles provide the minimum necessary mechanical advantage to negotiate these types of road conditions and obstacles.

Speaking to The Autopian, NPS Acting Public Affairs Specialist Karen Garthwait also highlighted the value of proper low-range, too:

In summary, a true “4-LOW” gear that drivers can access deliberately affords an additional level of vehicular control that may be necessary to safely negotiate sections of Canyonlands’ backcountry roads, particularly after a rockfall, heavy rain event, or other situation that temporarily or permanently degrades the road’s condition. The presence or absence of a 4-LOW gear, locking differentials, and the minimum clearance required are the primary ways Canyonlands’ rangers determine whether a vehicle meets the requirement for a use on designated 4WD roads in the park.

Image credits: NPS, Subaru, Toyota, Lewin Day