Final drive (portal axle) is 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 (by using higher shaft speed). This reduces load on the axle crownwheel and differential.
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.
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).
A related development is the use of an epicyclic hub gearbox. This is mounted in-line with the halfshaft, so that there is no change in ride height. They are often used for large and heavy vehicles, where the wheel diameter already gives adequate ground clearance. The reduction gearbox allows the halfshafts to turn faster than the wheels, thus requiring less torque for the same power. This permits a smaller and lighter halfshaft and internal drivetrain.
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