In November 2022 Eaton launched their Truetrac differential for the Jeep Wrangler JL and Gladiator JT. This provides a new option to increase the off-road (and on-road) traction performance of current generation Wrangler and Gladiator vehicles.
So here I’ll discuss the various cross-axle differential systems and their characteristics, their relevance to use in the Jeep JL and JT vehicles, and my experiences in installing an Eaton Truetrac in a Wrangler JL Rubicon.
The Jeep Wrangler JL is perhaps one of the most capable off-road vehicles available from the showroom floor, so why would we change either of the cross-axle differentials installed from the factory? In my opinion, the factory installed standard solution using a selectable lockable open rear cross-axle differential is not the right solution for the most driving situations but rather it is only the best option for some extreme off-road situations that occur very infrequently (except in the minds of the Jeep marketing department). This discussion suggests that in almost all traction situations a Torsen cross-axle rear differential will outperform a selectable “locker” open differential.
Differential Types and Mechanics
About 5000 years ago in Mesopotamia the first wheeled Chariots appeared. These vehicles used free wheels attached to the end of a fixed (non-driven) axle. It was only with the invention of the steam-engine and locomotion in the 1820’s that the axle needed to be motor driven and later in automobile applications there was a need to allow the speed of the driven wheels to vary relative to each other, and the cross-axle differential gear was patented in 1827 by Onésiphore Pecqueur.
The cross-axle open differential allows the speed of the two half-axles to vary from 0% to 100%, and it delivers constant torque split to each half-axle. The ratio of the torque split is set by the size of the side gears relative the pinion gears. Usually (always) in automotive applications the side gears are equally sized and this splits the torque exactly half to each half-axle, i.e a 50:50 split of torque. Interestingly, the first use of an automotive cross-axle differential was by an Australian in 1897 in his steam powered car.
It is worthwhile to introduce Newton’s Third Law in terms of torque delivered to an object (half-axle or drive shaft): for every action (torque) there is an equal and opposite reaction (torque). The statement means that in every interaction, there is a pair of torque forces acting on the two interacting objects. The size of the torque on the first object equals the size of the torque on the second object. The direction of the torque on the first object is opposite to the direction of the torque on the second object.
This means that although we often talk about the engine torque delivered through the drive shaft when speaking about a cross-axle differential, what we’re really interested in is the reaction torque generated by traction from the ground to the tire (wheel) attached to the end of the axle. It must be understood that the engine torque delivered to a half-axle will always equal the reaction torque generated by the tire (wheel). If the tire (wheel) has no traction it cannot provide a reactive torque, then the engine will not deliver torque to that half-axle.
Over time the shortcomings of the open differential became obvious when different road surfaces supported unequal wheel torque, such as when one wheel is on ice, mud, or is not in contact with the road at all. In an open differential the maximum torque delivered by the engine will be twice the minimum torque reaction from one of the wheels. Therefore if one wheel is on ice supporting almost no torque reaction, then the engine will only be able to deliver torque as if both wheels were on ice. If one wheel is in the air, then similarly the engine will deliver torque as if both wheels are in the air (i.e. no torque at all).
In the 1960s high power USA “Muscle Cars” started to use limited-slip differentials to prevent the “one tire fire” as their available engine torque would be more than twice the maximum wheel reactive torque which would cause the weakest (lowest torque) tire to break traction reducing friction and its reactive torque capability to almost zero, and as a result the overall torque delivered by the engine would drop to almost zero and the car would sit “smoking” a single wheel. The limited slip differential helped to overcome this problem by using clutch packs or springs to set the minimum torque delivered to each wheel. This means that the torque delivered to each wheel is not able to go to zero, but is set to a minimum value determined by the clutch spring design, however the torque split remains exactly 50:50 as in an open differential.
The limited-slip differential goes a long way to solving the issues encountered by high powered vehicles both on-road and off-road, but it does introduce an additional maintenance as clutch packs in the differential need to be maintained, and they will wear out often if used aggressively.
For off-road and some extreme on-road applications locking differentials have become very popular. When activated, locking differentials effectively return the axle to a solid locomotive axle, and this inverts the torque and speed relationship of the open differential by holding the wheel speed relationship to be constant equal to 1:1 but allowing the torque reaction from each wheel to vary from 0% to 100% depending on what is available. Therefore if a wheel is in the air, it will produce 0% reactive torque forcing 100% reactive torque to be produced by the other wheel, but both wheels will rotate at exactly the same speed. Using a locked differential or “spool” is only useful in extreme situations, and the lock cannot be left engaged. A locked or solid axle will make it very difficult to turn the vehicle, as it will want to travel in a straight line with both inside and outside wheels rotating at the same speed, so it cannot be used in most situations. To overcome the drivability limitations the spool or solid locomotive axle produces both automatic and selectable locking differentials have been produced.
Automatic locking differentials such as the “Detroit Locker”, or the “lunchbox locker”, are constructed with each wheel connected to the drive shaft via a freewheel gear which, when the vehicle is moving in a straight line, will allow the torque to to each wheel to vary from 0% to 100% as in a solid single axle, but will allow a faster moving (outside) wheel to over-rotate freely, while providing torque the slowest moving (inside) wheel. The main disadvantage of this solution is that, when travelling around a corner, drive torque is only available to the inside (slow) wheel while the outside (fast) wheel is freewheeling, but as the inside wheel is the most lightly loaded in cornering it is usually least capable of providing reactive torque.
Selectable locking differentials, such as the Jeep elockers fitted to the Wrangler JL Rubicon, allow the driver to choose whether the differential is in open mode (the usual case) or set to locked mode when needed. The driver is forced to select between the two options, locked torque and open speed (the open differential) or locked speed and open torque (the locked differential), depending on the expected traction. So then the question is when should the differential be locked?
And the answer is: it depends. Essentially, the driver is trying to guess the vehicle traction dynamics, which usually change rapidly. Usually in technical off-road driving the driver will first attempt an obstacle with an open differential, and only when they have failed the obstacle will they will engage the differential lock and try again. A trial and error solution.
Whilst it is possible for the driver to actively select open or locked differentials in low speed technical off-road situations, it is not practical for the driver to undertake this decision every time they face a different real world low traction or differentiated traction situation. In every day situations of low traction, such as accelerating from a country dirt roadside (one wheel on dirt, one wheel on tar), or crossing a curb, or snow or slush on the roadside, ice patches, or any other impossible to predict situation it is certain that the vehicle will be running with an open differential and will therefore face the limits of unbalanced wheel traction.
Ideally the vehicle cross-axle differential would immediately sense which wheel can provide the greatest reaction torque and adjust (or bias) itself to deliver the available engine torque to that wheel without any conscious input from the driver.
In 1958 Vernon Gleasman patented the torque sensing or Torsen differential, which uses the gear separation forces and friction generated by the sun planet worm gear to provide cross-axle torque biasing. The Torque Bias Ratio (TBR) is a measure of how strongly the Torsen differential will lock. The higher the TBR setting, the more aggressive the traction performance locking. Typical TBRs in production Torsen differentials range from 3.5:1 in the Eaton Truetrac, to 2:1 to 6:1 in the Quaife ATB depending on the application for front or rear cross-axle.
The TBR represents the ratio of high traction to low traction that the Torsen differential can allow while remaining locked. When input torque is applied to a helical gear, it creates a series of thrust forces that push that gearing into the differential casing. When these forces push against the wall of the differential casing, that contact creates friction. As the torque load increases so do the forces and so friction increases in proportion to the amount of torque applied. That gives the Torsen differential the ability to support a lot of reactive torque (traction) imbalance when under heavy throttle conditions, yet it still can differentiate freely and smoothly at low engine torque levels, so the car is docile and easy to drive and manoeuvre.
It is worth noting that the HMMVW, perhaps the most iconic modern off-road vehicle, uses two cross-axle Torsen differentials. Although the Torsen differentials provide the best traction for the military cross country application, the fact that the military KISS principle applies and there is no operator input required (unlike a selectable locking differential) and no additional maintenance procedures are needed (unlike a clutch based limited-slip differential) probably also contributes to the US military’s decision to use Torsen differentials in the HMMVW.
Installation for Wrangler JL Rubicon
The new Eaton Truetrac 917A736 for the Wrangler JL and Gladiator JT is specified for all Dana 44 M210 and M220 axles, being for both front and rear cross-axle differentials. This joins the existing availability of the Nitro Helix also available for the Wrangler JL or Gladiator JT.
For reference, the Eaton Truetrac 917A736 is available in Australia from Harrop Engineering, though I used another source to obtain my example.
Leigh Conlan, Aftermarket Sales & Service Coordinator
Harrop Engineering Australia Pty Ltd
It is noted that the Eaton 917A736 is not specified to be compatible with the Rubicon trim level. This is because the Rubicon is already fitted with an electrically selectable locking differential. So by removing the existing differential the engine management system would believe that there is a system fault, and would generate system errors.
However there is a product from Z Automotive called Z locker OEM to resolve this issue. This part is designed to spoof or fool the Rubicon electronics to overcome the fault inherent in the Rubicon selectable locking differential whereby when the locking sensor fails it causes the vehicle electronics to believe that the locker is inactive and therefore register a system fault, preventing the use of both front and rear differential lockers.
The Z locker OEM can be installed to the Rubicon locker wiring, but not connected to the rear differential housing, and the entire axle wiring loom is then zip-tied up out of the way under the vehicle body. The Rubicon electronics believes the rear selectable locker is functioning normally, which also allows the front selectable locker to function normally and the rear selectable locker to be replaced with the Eaton Truetrac differential.
For other Wrangler JL or Gladiator GT trim levels such as Sahara, Night Eagle, or Overland, it is fully supported to install the Eaton Truetrac as normal.
Overall 4WD System
Note that this discussion is relevant to the Wrangler JL rear cross-axle differential only. Whilst Torsen differentials can be used very successfully with high power front wheel drive vehicles and in the front axle of 4WD vehicles, in the specific case of the Wrangler JL Rubicon with an existing selectable locking open differential there are good reasons to retain that capability in the front axle. For other Wrangler JL trim levels should use either a Torsen front cross-axle differential, similar to the HMMWV, or to use a selectable locking differential, similar to the Rubicon.
The Wrangler 4WD system has a locked centre transfer case when used in 4WD Part Time mode which will allow 0% to 100% torque split between front and rear cross-axles, and using a locked front cross-axle differential from 0% up to 100% of the available torque can be directed to either of the front wheels. This allows the entire engine torque to be delivered through one front wheel, should it be able to provide sufficient traction. Several real world tests suggest that if one cross-axle can be locked, then it should be the front axle.
By using a Torsen cross-axle differential in the rear we’re optimising traction in 2WD and 4WD on all differentiated friction on-road and off-road surfaces, whilst adhering to the KISS Principle, so we’re generating the best available traction for most situations (99% of the time). And by retaining the front cross-axle lockable open differential we have the traction option for any remaining extreme situations (one wheel on each axle off the ground).
Check back late June 2023.