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2019 Jeep Wrangler in depth: Transmissions and transfer cases

by Robert W. Sheaves (edited). Part 8 of a series. Written in November 2014.

Bob Sheaves was responsible for 4x4 suspension design at the Jeep/Truck Engineering's PreProgram Engineering Department from the AMC days until 1993.

Allpar's members asked what we are likely to see as the next-generation Jeep Wrangler. This article is based on my engineering evaluation of various public sources, plus private discussions.

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A co-worker from Jeep/Truck Engineering pointed out,

The front of the transmission is the hardest of hard points (which means that changing it costs a great deal in tooling and assembly-line changes). That means all powertrain combinations will start from the same point. Differences in length between manual, automatic, or transfer case are made up by propshaft length.

The engines start from the front of the transmission, and are packaged forward. Believe it or not, the vehicle is designed around the front of the transmission and the driver's heel point. In my long experience on the Ram team, that relationship never wavered.
As it exists today, the "Torqueflite Eight" transmission does not fit into the short wheelbase Wrangler, due to the lack of room between the transfer case output and the rear axle input yoke.

That being said, there are several items you all need to be aware of. First, in the last paragraph, I deliberately gave the Chrysler name for this transmission - "TorqueFlite Eight," or 845RFE. Chrysler builds this transmission under license, and may make design changes without getting ZF's permission (indeed, they have already made numerous changes to fit their methods and machines). This is the first key.

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The next key is that they still own the rights to the RockTrac transfer case from New Venture Gear (which is the largest physical transfer case to package in the 2019 Wrangler, or "JL").

The problem in the short-wheelbase Wrangler is the assembled length of the drivetrain, and suspension travel not allowing a propshaft to fit. This means that a single cardan joint (desired for low cost) at each end of the propshaft has a critical length it must stay below, while changing angle from jounce to rebound as the rear axle moves.

You may think that this is just movement up and down, but you'd be wrong. Horribly so. Due to all the various arcs scribed at each control arm, the axle input actually dances around like a drunk belly dancer. The pinion tilts up and down, left and right, and fore and aft, depending on where the axle is at in its travel. Then you also have to consider a one-wheel jounce and rebound from the left and right tires that impart a corkscrew motion to this 300 pound barbell in space.

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As a rule of thumb, a single cardan joint is only capable of 2.5° of off-axis angularity at critical speed, for a total of 5° maximum between both ends (since we are only going to talk about conventional drivelines and not a "broken back" design).

As you lower the rpm of the propshaft, you may increase this angularity proportionally, to where you "hit the stops" or approach the maximum travel of the joint at its slowest rotational speed.

The Rock Trac has the 4:1 low range and the planetary gearset is huge in diameter, and long front to rear. It is about twice the diameter of the ZF planetary in this image, and about an extra inch in length.

There are two places in the transmission assembly that can be modified for (relatively) low cost and gain about 6 inches. The first place is at the split line between the transmission main case and the adapter housing of the transfer case. By shortening the space between the spud shaft and transmission output, you can pull the main case of the transfer case forward a couple of inches.

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The second place is a bit more complex- that of the slip shaft and joint of the rear propshaft. By using what the aftermarket calls a slip yoke eliminator ("SYE"), you gain about four more inches, at the cost of a slightly more expensive driveline and a more complex assembly procedure. Then, you move the slip yoke to allow the shaft to change length, instead of allowing the yoke to plunge into the tailshaft housing of the transfer case.

You must still ensure that the joint angles are balanced, to prevent vibration of the driveline.

In short, the package of the driveline has to be shortened to fit into the wheelbase of the two-door Wrangler. Today, the driveline is too long to fit. You do not screw with the transmission. You do not alter it in any manner. You do not change it, you do not change any characteristic.

You leave the transmission alone.

You do change the length of the transfer case. That changes the length of the drivetrain.

Since the manual is a shorter package than the 8 speed, the 6 speed manual is an easy install.

How to build it?
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Steel or aluminum?
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New suspension

Unit-body, frame...?
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Open or fixed top?
2019 Wrangler
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Weight and strength
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True Jeep
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Original Jeep
More to come?

The 2018 Jeep Wrangler JL: suspension aluminum vs steelopen or fixed roof pickup
body engineering
weight, strength, and safetytransmissionsengines


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