Portal axles (or portal gear) are an offroad technology where the axle tube is above the center of the wheel hub.[1]
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Compared to normal layout, portal axles enables the vehicle to gain a higher ground clearance, as both the axle tube and differential casing are tucked up higher under the vehicle.[1][2][3]
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.[1]
As it requires 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.[4]
They are also used in railroads and low floor buses.[5][6] 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. This, the inverted portal axle, allows the floor of the bus to be lowered, thus easing access to the bus and increasing the available cabin height.
Bolt-on Portals (or drop boxes) are a housing with a set of gears which bolts on to the final flange of the axle tube. This approach allows existing vehicles to be converted to portals without modifying the axles (Volvo C303 or Unimog 404).
Off-road vehicles fitted with portal axles include:
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