Portal axles not only increase ground clearance, but they also improve the robustness and reliability of an off-roader. Here’s why some of the toughest vehicles around sport this set-up.
Portal axles might not seem terribly common, but it depends where you look. Sure, precious few road-going production vehicles boast portal axles, but this set-up is popular when it comes to military, farming and construction vehicles. Portal axles are common when it comes to tractors and front-end loaders, and US Military Humvees also have them, to name a few pervasive examples. Why is this? Well, as it turns out, these vehicles have a lot of the same requirements that pukka 4×4s have. They need good ground clearance, they sport huge tyres, and the nature of the work and the environment places huge stress on their components. Portal axles are a good way to address all of these issues, which is why some serious off-road enthusiasts make use of them.
HOW PORTAL AXLES WORK
The standard axle you’ll find on a bakkie or SUV simply runs straight into the wheel. This is an efficient system for most vehicles, but for big-wheeled vehicles that operate under extreme conditions, it places too much stress on the axles and CV joints. Consider the following: if you take a 4×4, which has been fitted with large wheels (35-inch ones or bigger) and given an engine boost, for some extreme low-range driving on a tough trail, it can experience 2000% more torque/load than it would on the highway. You could, of course, fit a more robust axle and CV joints, but at a certain stage, it becomes impractical. There’s a limit to how big and heavy these components can be, and they’d need to be massive to deal with that sort of load on a regular basis. To sidestep this issue, some commercial and military vehicles are fitted with portal axles. Unlike your typical axle, the tube of a portal axle sits above the centre of the wheel hub. Connecting this tube with the wheel is a hub that contains reduction gears. Small gears are driven by the axle, and these gears in turn rotate a larger gear that turns the wheel. By transferring the torque in this way from a (relatively) small axle to a large gear in the hub, the axle torque can be increased by around 200%, while keeping the stress on the axle relatively low.
BETTER GROUND CLEARANCE
Another benefit of this set-up is the fact that it increases ground clearance substantially. As you can imagine, having the axle above the centre of the wheel means that there’s much more air between the ground and the bottom of the vehicle. The hub of a portal axle is usually at least 100mm (sometimes as much as 150mm) below the differential housing, so you could instantly see a huge benefit. A vehicle with clearance pegged at 250mm would increase to 350 or 400mm. As mentioned earlier, the reduction gears in the wheel hubs allow for the use of smaller and lighter differentials, CV joints and axles, which also improves off-road performance. Because the vehicle is lighter, it will be more responsive, and not having a massive diff housing that can get snagged off-road is a huge advantage. If you examine the diff housing on a Humvee, for example, you’ll probably be struck by how small it is. It doesn’t look like it could stand up to a lot of off-road punishment, but thanks to the portal axles, the drivetrain doesn’t need a huge differential.
Increased ground clearance is probably the main reason why serious off-roaders choose to fit portal axles to their vehicles. The second is toughness. If you’re going to be tackling grade five trails on a regular basis, breakdowns are inevitable, but they happen less often when you’re running portal axles with reduction gears. These are not the only benefits, however. There are a few others, too. As you’ve probably seen on vehicles like the Unimog and the G63 AMG 6×6, portal axles allow for the installation of central tyre-inflation systems. You can’t put an airline through a CV joint, but once the joint is above the wheel spindle, it becomes possible.
With vehicles doing hard work in the military, or in industries like forestry and construction, being able to adjust tyre pressure on the fly is immensely useful. The brakes of a vehicle benefit greatly from portal axles, too. A brake is more effective, since the gearbox in the hub multiplies the braking force. Running a 2:1 portal hub is almost like doubling the size of the disc rotor. Also, with a portal axle, your brakes are more protected from dirt and debris, which improves their operation.
CAN YOU FIT THEM?
Portal axles can be fitted as an aftermarket option. However, it’s expensive to do this. Dynatrac offers the ProRock portal axle, which moves an axle centreline up by 125mm. The hub design provides a 1.5:1 gearing reduction through a 41-spline axle shaft designed specifically to account for increased torque loads. As a cheaper option, bolt-on portal axles are available that can be bolted onto the final flange of an axle tube. In other words, you keep your standard axles in place, but modify them to become portal axles. Either way, portal-axle modifications aren’t common in South Africa, so you’d probably need to have them shipped here, and then find someone who can help you fit them. Alternatively, you could simply buy yourself a Unimog…