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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 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.
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.
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:
- Mercedes-Benz Unimog
- Mercedes G63 AMG 6x6 truck
- Steyr Puch Haflinger
- Steyr-Daimler-Puch Pinzgauer
- Volkswagen Kübelwagen
- Volvo C303
- Land Rover Defender 130CC-R(hino)
- AM General HMMWV
- AM General Hummer H1
- International FTTS
- Toyota Mega Cruiser (Civilian model and JGSDF's High Mobility Vehicle)
- Tatra T 810 medium truck
Agco Spra Coupes
Media related to Portal axles at Wikimedia Commons
- New Spicer portal axles for compact utility tractors - introduced by Off-Highway Systems Group of Dana Corp - Brief Article | Diesel Progress North American Edition | Find Articles at BNET.com
- Portal-Axles, Getting your car off the ground
- RockThis Off-Road!
- Exaxt 4WD
- Railway Technology - Portal axle with SAB V60 rubber-sprung wheels and sound absorbers
- ArvinMeritor Reveals New Second Generation Inverted Portal Axle
- The Unimog Principle
- An Expedition Truck on Portals
- 4WD Glossary & FAQ