A portal axle is a component used on off-road vehicles that allows for greater ground clearance between the vehicle's undercarriage and any obstacles the vehicle must drive over. The axle connects two wheels together in many cases, and it is usually responsible for holding some or all of the vehicle's weight. Most axles are lined up with the center of the wheels, but a portal axle is raised up higher than the center of the wheels, allowing for better ground clearance. The hubs on each end of the axle will contain gears that will help drive the wheels.
The gears on either end of the portal axle also reduce the amount of torque being placed on the components of the hub and axle, meaning the casings can be smaller and thinner. This means the entire axle assembly can be lighter, which can help improve the vehicle's center of gravity. The problem with having gears at these locations, however, is that they can overheat after an extended period of use, and excess heat can lead to the failure of internal components. This is an important consideration since the portal axle is commonly used on military vehicles or other vehicles that operate over harsh terrain or in unforgiving conditions.
Another distinct disadvantage of the portal axle is the cost. These components tend to cost quite a bit more than a standard axle, and installation can be difficult or impossible on some vehicles. Most vehicles that feature such components are specially designed to accept them, and they may come stock with the axles already installed. Conversion kits do exist, however, but they may be difficult to install. The designers of the vehicle will need to account for the extra weight at the hubs, and steering and stability elements will need to be overbuilt to compensate for the added weight.
Sometimes buses and other vehicles will be equipped with a portal axle as well, though in a distinctly different configuration. On off-road vehicles, the axle will be situated above the wheel's center; on a bus, the axle will instead be situated below the wheel's center, effectively dropping the body of the bus lower to the ground. This is usually done to allow easier access to the bus entrances; lower entrances mean easier access for people who cannot climb stairs well or step up safely.