Fo­cus on

Por­tal axles not only in­crease ground clear­ance, but they also im­prove the ro­bust­ness and re­li­a­bil­ity of an of­froader. Here’s why some of the tough­est ve­hi­cles around sport this set-up.

Leisure Wheels (South Africa) - - CONTENTS -

Por­tal axles

Por­tal axles might not seem ter­ri­bly com­mon, but it de­pends where you look. Sure, pre­cious few road­go­ing 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.

OTHER BENEFITS

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 tyre­in­fla­tion 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...

Main and bot­tom: Thanks to a pair of por­tal axles, the G-Class 4x4 Squared boasts in­cred­i­ble ground clear­ance.

Above: A Dy­na­trac ProRock por­tal axle can be fit­ted as an af­ter­mar­ket op­tion. Left, clock­wise from

top: Thanks to its por­tal axles, the Unimog is an ex­cel­lent off-roader. Re­duc­tion gears in­side the hub of a por­tal axle. The por­tal axle on the G-Class 4x4 Squared.

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