page_title: Portal axle
section_title: Description
hierarchical_section_title: Portal axle / Description
caption_attribution_description: English: Side view showing the changes in approach and departure angles and under body clearance, in comparison with a standard Land Rover Defender 130CC, after fitting bolt-on portal drop boxes to the original axle
context_page_description: Portal axles are an off-road vehicle suspension and drive technology where the axle tube or the half-shaft is off-set from – usually above – the center of the wheel hub and where driving power is transferred to each wheel via a simple gearbox, built onto each hub. This gives two advantages: ground clearance is increased, particularly beneath the low-slung differential housing of the main axles — and secondly, any hub reduction gearing allows the axle halfshafts to drive the same power but at reduced torque. This reduces load on the axle crownwheel and differential.
  The portal gear configuration is also sometimes called a drop gear or drop gearset configuration.
context_section_description: 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. Where a vehicle also requires a reduced top speed, in at least one gear, the gear ratio can be chosen to reduce this. The military Kübelwagen of WWII used a ratio of 1.4:1 to provide a 2.5 mph walking speed in first gear, as well as a useful lift of 50 mm.
  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 railways 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.
   
  Bolt-on Portals (or drop boxes) are a housing with a set of gears which bolts onto 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).
caption: A Land Rover Defender fitted with bolt-on portals
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