Portal axles are an offroad technology where the axle tube is above the center of the wheel hub. Compared to normal layout, this enables 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 reducing the centre of gravity for a given ground clearance.
However, as it requires a heavier and more complex hub assembly, 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.http://upload.wikimedia.org/wikipedi...PortalAxle.svg
Its essentially (from what I understand... my ex-neighbours Land Rover ran Mog axles - pictured below, before he sold it - although I have never pulled them apart), a gear reduction in the hub, with a large inner teeth gear, and a small pinion on your driveshaft.
They all have to be fully floating.
They are a great design, as they put the final reduction right next to the wheel, meaning driveshafts only take a fraction of the load they take in a normal design (i.e. Mucho more hp and bigger wheels!)
Less suspension lift required for ground clearance, better driveshaft angles
Their only downside (IMHO) is the cost, and weight.
Portal axles are pretty popular here... predominately because unimogs are popular, and we don't have the cheap access to other strong off-road axles like you guys do with Rockwells. Oh, and we are the kind of idiots that enjoy throwing 20k down on a pair of axles.
The Mog ones are stronger (but this is relative... if you can break a portal axle, you are doing a good job), but the Volvo ones (out of the C303) are lighter, and better in size for most vehicles.
What specifically did you want to know about them?
Fred I don't know much about them, so when they were mentioned in another thread I started asking questions, it was suggested I'd be better off with a new thread.
so I figured I'd ask and have people pool there Gray matter on the subject. What they know, what they've seen, and what they've read. Etc, Etc.
and like what Fishy asked, the different types with pros and cons of each.
Ok... I will call up my old neighbour, and see what info I can drag out of him (he used to run Mogs on his Defender... he runs Volvo's on his current Range Rover)
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