Portal axle

Portal axles (or portal gear) are an offroad technology where the axle tube is above the center of the wheel hub and where there is a reduction gearbox in the hub. This gives two advantages: ground clearance is increased, particularly beneath the low-slung differential housing of the main axles; and secondly the hub gearing allows the axle halfshafts to drive the same power but at reduced torque (by using higher shaft speed). This reduces load on the axle crownwheel and differential.

The portal gear configuration is also sometimes called a drop gear or drop gearset configuration (which, despite its similarity to the term dropped axle, produces the opposite effect).

Compared to normal layout, portal axles enable the vehicle to gain a higher ground clearance, as both the axle tube and differential casing are tucked up higher under the vehicle.

Due to the gear reduction at the wheel which lessens the torque on all the other drivetrain components, the size of the differential casing can be reduced to gain even more ground clearance. Additionally, all drivetrain elements, in particular the transfer gearbox and driveshafts, can be built lighter. This can be of use in lowering the center of gravity for a given ground clearance.

As they require a heavier and more complex hub assembly, however, these systems can result in an increased unsprung weight and require robust axle-control elements to give predictable handling. In addition, at higher speeds the hub assembly can overheat.

They are also used in railroads and low floor buses although, in the case of buses, the device is engineered in the opposite way to those fitted to off-road vehicles - the axle is below the center of the wheel. Thus, the inverted portal axle allows the floor of the bus to be lowered, easing access to the bus and increasing the available cabin height.

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Wikipedia