this post was submitted on 10 Aug 2024
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Weird News - Things that make you go 'hmmm'
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AWD has the ability to drive all four wheels, but not necessarily all of them at the same time. 4WD has the ability to drive all four at the same time.
The main difference is the additional physical gearing for 4 high and 4 low gears, both of which have different gearing than "regular" drive.
I have an older Audi with a Torsen Quattro AWD system, and an ancient Toyota 4Runner with 4WD. The 4Runner can be switched into either 4 wheel high or 4 wheel low gears to deal with different conditions. The Audi always has the same gearing, it cannot be switched.
It's like the gears on a bicycle - 4 low is the one where you barely move while standing on the pedals - maximum torque per revolution.
You can go rock crawling in a 4Runner in 4 low. You really should not go rock crawling in an Audi or Subaru without 4 low, no matter how much ground clearance the vehicle has.
The National Park Service has this rule because it doesn't want to spent time and taxpayer money rescuing people who think AWD is the same as 4WD with a low range gearbox.
Subaru has x-mode for difficult terrain, which is different gearing. They also have hill descent. https://www.sportsubaru.com/subaru-x-mode.htm
Why would you want 4wd with wasted energy when you could have symmetrical awd and get all the power to wheels that have traction, skipping those that don't have traction?
I only see awd outperform 4wd when it comes to a subie, but other awd systems from other manufacturers are probably not up to snuff.
Edit: meant to say symmetrical awd instead of slip differential.
I didn't down vote you, but you've got some misconceptions.
X-mode isn't actually different gearing in the mechanical sense. It's an electronic system that optimizes the existing drivetrain components.
It doesn't provide additional gear reduction like a low-range gearbox, it adjusts the CVT's (continuously variable transmission, which doesn't even have "gears" in the traditional sense, but is a set of chains and pulleys) behavior, traction control, and power distribution, but doesn't change the fundamental gear ratios. Hill descent control is a braking function, not a gearing one.
True 4WD systems have a physically separate low range gearbox that allows the driver to physically engage different gears to vastly reduce the gear ratio to allow the vehicle to make much more efficient use of the available engine power.
You would waste far more energy trying to get an AWD Crosstrek over a boulder with X-mode than an actual 4WD vehicle with a lever to put the vehicle into 4 Wheel Low gear.
While excellent for many things, Subaru's AWD system is essentially a fancy electronic traction control system. It cannot reduce gearing to the level of a 4WD low range gear box. And that's fine! But the incorrect assumptions of people who overestimate the capabilities of their vehicles is the precise reason for the rules the NPS has in place; Subaru Crosstreks with X-Mode are gonna need to be rescued by NPS staff far more often than 4Runners with a low range gearbox.
Subaru marketing is great, but NPS roads with AWD restrictions are not rally stages in a Finland forest, they are roads with boulders or mud or deep water or sand or many other things that a 4WD vehicle will probably be able to handle, but an AWD vehicle will probably not be able to handle. And on these roads, if you get stuck, a park ranger is going to have to rescue you, at tax payer expense, because you thought your vehicle could do something that it could not.