Portal axle

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.

[5] They are also used in railways and low floor buses[6][7] although, in the case of the latter, the device is engineered in the opposite way to those fitted to off-road vehicles - the axle is below the center of the wheel.

Bolt-on portals (or drop boxes) are a housing with a set of gears which bolts onto the final flange of the axle tube.

If individual wheels were out of phase with their neighbours, possibly caused by cornering or slightly varying tyre diameter, this could place a considerable force on the gearbox, leading to breakages.

For this reason, it was regular practice when driving on tarmac to bump the vehicle over a kerb or other object at times, to allow this wind-up to be released.