4 Things to Know About Portal Axles, the Secret to the Mercedes 4×4’s Badass-ness

The Mercedes G 550 4×4’s capability, most visible in its extreme ground clearance, is possible thanks to an old-school mechanical wonder: the portal axle.

The Mercedes-Benz G 550 4×4 ($227,300), or “4×4 Squared,” is seven feet, four inches tall. Even with my gangly, six-foot-tall frame, entering the cabin is a chore: I have to heave my foot higher than is naturally acceptable, grab the steering wheel and pull myself into the luxuriously-appointed driver’s chair. With a few distinct and diamond-quilted differences, this is, no doubt, what it’s like for semi-truck drivers to enter their trusty steeds. And like a semi-truck, the 4×4 is taller and wider than almost everything on Manhattan roads.

But, while this truck, which is based on the Mercedes-Benz G-Class “Geländewagen,” is weirdly, perfectly at home in the big city, it really belongs in the wilderness. The massive SUV is, after all, capable of scrambling over terrain that is completely inaccessible to 99 percent of the “off-road” SUVs on the market. This capability, most visible in its extreme ground clearance, is possible thanks to an old-school mechanical wonder: the portal axle. Below, four things you must know about the portal axle, which also happen to be required knowledge for appreciating the Mercedes-Benz 4×4 squared.

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The Axle Doesn’t Connect Directly to the Wheel Assembly

In most vehicles, axles — the shafts that run between opposing (front and front, back and back) wheels — are essentially one uninterrupted component. At the end of the conventional axle is a wheel hub, a gearing system that allows the axle’s motion to be translated at a 90-degree angle and, consequently, to turn the wheel. It’s all one mostly arrow-straight system.

In a portal axle, the axle shaft is above the wheel hub. At the ends of the axle is a more complex system of gears that translate motion downward to the wheel hub — an added step that allows the axle shaft to be higher than the center of the wheels. Essentially, the axle rotates on a higher plane than the wheels, which allows, among other things, for higher overall ground clearance. See what I mean in Eric Adams’ video below.

Mercedes-Benz-G-Wagon-4×4-Squared-gear-patrol-ambiance

Higher Ground Clearance

The obvious benefit inherent in a portal axle setup is more ground clearance. The axle — an unbroken, horizontal shaft — is raised above the mechanism that controls each wheel, so the lowest point in the center of the vehicle is higher than normal. That’s the case with the Mercedes-Benz 4×4. It affords a maximum of 17 inches of ground clearance as opposed to the current “normal” Mercedes-Benz G-Class, which has a still above-average 9.5 inches.

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Video: Eric Adams

Humvees Feature Similar Mechanisms

The now-iconic and outmoded AM General Humvee, used as the US military’s long-serving wartime vehicle, employed a similar system in its design. Its helical gear-reduction hubs allowed for top-mounted axles that provided a ground clearance of 16 additional inches. In conjunction with the Humvee’s raised central driveshaft (which, coincidentally, is at least partially responsible for the Humvee’s truly massive width), this allowed for extreme capability in off-road situations. Still, it should be noted, the Mercedes-Benz 4×4’s system is good for an extra inch overall.

Increased torque

Simply put, the portal axle setup allows for less work in the differential and axle mechanism to translate to more torque at each wheel. The axle half shafts can spin at a greater speed but generate less immediate torque, meaning less work at the outset and more productivity at the end of the machine. Differentials, then, can be smaller, thus further increasing ground clearance, which, coupled together, provides more overall capability off-road.

Mercedes-Benz G 550 4×4 Specs:

Engine: 4.0-liter twin-turbo V8
Transmission: seven-speed automatic; four-wheel drive; three locking differentials
Horsepower: 416
Torque: 450 lb-ft
0-60 MPH: 7.3 seconds
Combined fuel economy: 11 MPG
Overall height: 88 inches
Overall length: 77 inches
Curb Weight: 6,825 pounds
Towing Capacity: 7,000 pounds
MSRP: $227,300

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