It’s the first rule of off-roading: get away from the ground. When it comes to steep approach angles, big holes, and obstacles, ground clearance is everything. Bigger tires, lift kits, high lift jacks, and even the front and rear bumper 12,000 lb electric winches on this project Land Cruiser; most of the components and accessories of an off-roading vehicle are designed to get you or your buddies through situations where the ground wants to catch your rig.
Clearance, Power, and Stress
The low points on most vehicles are the axles. No matter how big your tires and wheels are, in most cases your axles are going to be stuck halfway up from the ground. Allpar.com explains that the Mopar Portal Axle “provides a major increase in ground clearance without excessive suspension lift.” The drive axis in a portal axle system is vertically offset from the center of the wheel. Power is transferred from the axle to the wheel via a reduction gearbox.
Not only do portal axles add significant ground clearance, they also provide additional robustness and reliability to vehicles with large wheels and tires and the need to perform well in low speed, high torque conditions. Leisure Wheels explains how portal axles are common on farm and industry vehicles because they work so much in off-road, high torque conditions. A standard drivetrain will feature an axle feeding into a CV joint. This system won’t stand the vertical stresses of big wheels and the power demands of granny gearing. Leisure Wheels states that the reduction gearbox in a portal axle end can increase the axle torque by 200% over a standard design while significantly reducing the axle stress.
Portal Axles in Action
While portal axles are not common in street use production vehicles, they are found on a lot of burly work and play vehicles. This Mercedes-Benz G500 utilizes a portal axle in combination with 37-inch tires to be able to drive through over 3 feet of water. Being a Mercedes, it uses a dual strut system on each wheel to allow for street driving too. One of the struts is electronically adjustable so you can switch from sport mode into a more comfortable suspension setting.
The German automaker also has a more utilitarian side. The Unimog has roots in the European workforce and military but has traveled as far as Burning Man. Unimog portal axle mods are one of the classic, and most affordable, ways to get a custom portal axle onto your personal rig. This FourWheeler Network article delves into the evolution of portal axle design by comparing 74Weld‘s portal axle components to the original Unimog design.
While there are exceptions like some of the Mercedes-Benz G Class, portal axles aren’t often a standard option, even on vehicles built with lots of off-roading in mind. For Jeeps, Toyotas and other rigs, there are plenty of other aftermarket options. Along with 74Weld, ProRock Dynatrac Portal Axles combine AxleTech axle ends with Mopar axles to custom fit various rigs with a system that provides 5 inches of extra ground clearance and a 1.5:1 reduction gear ratio for reduced stress and increased torque.
Tibus Engineering and AxleTech both make a bolt-on portal end that attaches to the vehicle’s standard axle. These systems are approved for street use. They can add 4 to 5 inches of lift and are designed to allow off-roaders to meet the demands of both high ground clearance and larger wheels and tires.
Portal axles offer all of the benefits described above: additional ground clearance and the ability to run bigger tires, combined with reduced stress on the downstream drivetrain components. But there are a couple of reasons why you don’t see them on every off-road vehicle: they require additional maintenance – regular gearbox oil changes at four wheels – and they don’t come cheap.