Many EVs Aren’t Legal on Park Service 4WD Roads Leave a comment

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A recent story at Jalopnik tells us the story of a Subaru driver who got in trouble for going on the wrong trail. Why? Because the National Park Service only allows four-wheel-drive vehicles on some roads, and the Subaru model in question was all-wheel-drive. This may sound like a silly distinction if you’re not familiar with automotive drivetrain technologies, but it’s a distinction that could get many EV drivers in trouble.

What’s The Difference Between 4WD & AWD?

If you’re unfamiliar with four-wheel-drive and all-wheel-drive, it might sound like the National Park Service is arguing semantics. After all, if a car has four wheels and it’s got an all-wheel-drive system, then that must mean that the vehicle is four-wheel-drive, right? All of 4 is 4, isn’t it? But, the difference between an all-wheel-drive system and a four-wheel-drive system goes a lot deeper than words.

It’s true that both systems can provide power to all four wheels. In the case of an ICE vehicle, the power from the engine is split out to all four wheels with a transfer case of some kind and sent to all four wheels. In the case of EVs, there is usually (but not always) multiple motors that power one or two wheels via a differential system.

Where four-wheel-drive systems differ from all-wheel-drive systems is how they distribute the power. An all-wheel-drive system varies the amount of power that’s sent to each wheel. In some cases, open differentials are used, and the vehicle’s computer puts a brake on an individual wheel that slips, which ends up sending the power to the other wheel or wheels. In other cases, a limited slip differential is used to redirect power between either two or all four wheels to keep power from going to wheels that slip.

But, a four-wheel-drive system has locking differentials that can be used to force the system to send equal power to all four wheels at all times. You wouldn’t want to lock differentials on dry pavement, as the tires would squeal and shred themselves when going around corners. But, on loose surfaces, locked differentials make for a proactive way to keep power going to wheels that aren’t slipping. This is better than all-wheel-drive systems in extreme conditions, as it acts proactively instead of reactively.

Why NPS Designates Some Roads As 4WD Only

With a skilled driver, an all-wheel-drive system is almost as good as a 4WD system. By carefully selecting a path through obstacles and making wise choices about tire placement, it’s possible to go many places that people think that only a 4WD vehicle can go. And, people are proving this every day. But, not all trails are forgiving.

Because 4WD systems with locking differentials proactively keep energy from going to the wrong wheels, there really are some trails and some obstacles where most drivers won’t be able to get through without it. In remote areas of national parks, monuments, and such, it can be a life-threatening problem if a vehicle gets stuck. Without cellular service, many people wouldn’t be able to get help, and might find themselves stuck in dangerously cold or hot conditions. Even when there’s no threat of death, getting a stuck car out of some trails could be a serious hassle that costs thousands of dollars.

For this reason, the Park Service prohibits vehicles that can’t lock their differentials from such trails. Some people (like the one mentioned in the Jalopnik article who got a warning from NPS) might get through, but the service considers these trails too risky for AWD vehicles.

There Aren’t Very Many 4WD EVs

Where this becomes a problem for EV owners is that there aren’t very many that can send equal power to all four wheels to get out of a bind. But, unlike ICE vehicles, some EVs can achieve equal power without a locking differential, so it doesn’t come down to that in all cases.

One good example of a full-capable 4WD system is the Tesla Cybertruck, provided you get the “Cyberbeast” tri-motor configuration. It has a single electric motor on the front wheel with a locking differential and two motors in the rear (one for each wheel) that can put out equal power upon request from the computer. Dual motor versions have locking diffs front and rear.

The quad-motor versions of the Rivian R1T don’t need locking differentials to achieve equal power, as each motor can be commanded to do equal power, but the F-150 lightning can only lock the rear differential, which would make it ineligible for these trails.

Sadly, most other EVs don’t have any mix of locking differentials or a motor for each wheel. They still have all-wheel-drive systems, but no way to give equal power to all four wheels. The ability to either use a limited-slip differential or brakes to redistribute power still makes them very capable, but doesn’t make them eligible to use 4WD-only trails in the National Park System. This may also be true for other trails on public lands, such as those found on Bureau of Land Management (BLM) or U.S. Forest Service land, so be sure to check ahead before you go wheeling!

Possible Solutions

The sad fact is that many vehicles simply weren’t designed to go off road in any serious way. With some suspension spacers, skid plating, and other modifications, something like a Tesla Model Y can still be quite capable, and with a skilled driver, can probably tackle most NPS 4WD roads. But, with the Service apparently combing through internet videos and otherwise looking out for violators, it’s a ticket you’d probably want to avoid.

One possible solution would be for aftermarket parts suppliers to figure out how to crack the drive units and put in lockers. A complete aftermarket drive unit of some kind might even be needed. This would be a very costly product to develop, and probably wouldn’t be supported well with OTA updates, so this might not be a very realistic option.

Another options could be to get the National Park Service to update their rules and allow at least some all-wheel-drive vehicles to take these trails on. Other criteria, like all-terrain tires (or better), clearance, recovery equipment (winches and boards) and the experience of the driver could be used to issue permits. Requiring a satellite communicator and arrangements to be made with an off-road recovery company or use of the “buddy system” with a 4WD vehicle could be another way to let AWD vehicles onto these trails.

Either way, it’s probably not completely impossible to get more EVs legal access to these trails!

Featured image by Kyle Field.