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Mopar Portal Axles: the good, the bad, “what on Earth is it?”

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Mopar Portal Axles: the good, the bad, "what on Earth is it?"

Mopar's new Portal Axle set of offset axles provides a major increase in ground clearance without excessive suspension lift. Mopar's Front Portal Axle (8-lug, part number P5155670) has a list price of $12,500; the Rear Portal Axle (8-lug, part number P5155671) is $11,000.

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In the Mopar - Jeep FC concept, the axles are controlled with King coil-over assemblies along with beefy Teraflex control arms and track bars. Traction is handled by massive 40-inch tires mounted on custom Hutchinson 17-inch beadlock wheels.

Aftermarket business owner Norm Layton wrote about some shortcomings in production vehicles:

Ever ride/drive a race car? Everything is tight and crude. The gear whine from the portal reduction gear design is loud! There are changes in steering geometry that take some getting used to. They do allow some interesting potential for CTIS (central tire inflation system).

Suspension engineer Bob Sheaves added:

The steering effects are most pronounced when the intersection of the KPI (KingPin Inclination angle) intersects the ground contact patch outside of the vertical centerline of the tire, as shown in the illustration below (on left). This leads to tire instability in a forward motion (further exacerbating the effects of "death wobble" of this type of suspension) and a lack of steering "feel" and "lane wander." This is an issue with the previously shown kits because they must mate to an existing steering yoke for the Dana 60 front and Dana 60 rear axles used in the FC show car.

The steering linkage is also raised, relative to the KPI, so the geometric motion is changed into an oversteering location, adding even more "death wobble" to the design. (See last diagram).

These effects could be minimized by a product specific design and casting/machining of the reduction hub knuckle for the applicable vehicle, but this also means it would no longer be a kit available to the aftermarket, so the costs would increase even further.

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The "death wobble" problem

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As a side note, "portal" is not the complete description of these style hubs when applied to an axle; the proper name is "double reduction hubs fitted to a beam axle."

... another issue with these kits is an 8" track width increase using a standard suspension spring location and shock location and standard offset wheels. Because the springs sit outside the frame rail, combined with the distance between the KPI and the wheel mounting flange, you end up pushing the tires outboard considerably. This also requires changes in the steering linkage, due to the extra distance between the tierod ball joint mounting, eliminating the Haltenbarger drag link. Overall, not good for a daily driver vehicle. Now if you do it right and use M998 12 bolt wheels, that track width issue goes away, among other problems.

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