Jeep Drivetrain Advisor: What's the Best Wrangler Differential for Off-Roading?

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Next to the tires, the axles are the most abused part of a ‘wheeling Jeep. Grenaded axle parts are “trophies” to some but the wise and thrifty jeeper would rather not have a thousand dollar paperweight. He’d rather put a check mark beside the name of the particular trail he just whipped, wash off the trail dirt and plan the next run.

If you are starting from scratch, start with scratch paper and a calculator. You can get a pretty close approximation of what you need without spending a dime or busting a knuckle. If your Jeep’s already built, you can still reap the benefits but you may have to backtrack a little on some of your modifications.

Gearing, traction and axle strength are separate issues. You need the gearing to be right no matter what, even it it’s only for the street. If all you do is cruise the highway or take very mild trails, you may not need much strength reserve or any traction improvements. From this point you need to evaluate your trail needs and factor in traction and strength needs.

Tire size is the foundational choice in making the axle triad of gearing, traction and strength work for you. If you want success and peace of mind on the trail, you can either tailor the tire choice to your axle or tailor your axle to the tire choice. Start with the tire size you’d like, do the paperwork and see if it will all work within your own Jeep’s parameters. If not, let’s figure out how to upgrade the axle the match.

Gearing choices will dictate performance and fuel economy on both the street and the trail. With the gear ratio remaining constant, increasing tire diameter makes the ratio higher (a.k.a. “taller,” numerically lower) which requires more engine effort and more fuel burned to get the vehicle moving. A lighter vehicle, or one with more power at a given weight, can successfully use higher gear ratios. That works pretty well on the street but not always on the trail. You sometimes need to be able to go slowly enough to avoid beating the Jeep into scrap and tall ratios can’t give you that. The big engine/tall gear scenario works best with mud runners and worst with rockcrawlers. The equalizer in this case could be a deeper low range in the t-case or underdrive.

There’s a “just right” axle ratio for every vehicle. Before doing a tire swap work the “Equivalent Ratio” formula in the “Math for Gearheads” sidebar using your Jeep’s existing tire size and gear ratios. This puts your rig back to the same overall gearing ballpark as stock but does not counter the other effects that come from big tires and lifts, namely increased wind and rolling resistance. The performance loss from those are not fully recoverable but by dropping a skosh lower on the ratio, you can get some of it back. The equivalent ratio formula will usually give you an odd number that doesn’t match available gear ratios, so pick the closest.

With regards to fuel economy, having a tire and gearing combo that’s too tall often costs as much fuel economy as ratios too low. The too-tall ratio will hurt you mostly in town, where more throttle is needed to get the vehicle moving. A too-low ratio will hurt you mostly at highway speeds where the engine is spinning at high rpms.

There is useful tricks for getting better results from the equivalent ratio formula. If your Jeep rig was excessively high geared at the factory, (2.76-3.07:1), even stock performance was probably not so hot and the equivalent ratio formula put you back in the same general ballpark. To counter that, use a more reasonable 3.55:1 as a starting point and you’ll often get better performance results because 3.55s with a stock Jeep gives good overall performance. The same thing applies to a Jeep that came with a four-cylinder engine and low gears, but now has a larger engine swapped in. Use 3.55:1 as a starting point and you’ll get a more usable result. You may find the gears you have are perfect for the bigger tires with a bigger engine. The formula may even indicate taller gears!

Good tires and a limber suspension that keep the wheels on the ground will go a long way in the traction department, but it’s seldom enough for anything more than lower-middle difficulty level ‘wheeling. Adding a traction aid while doing a gearing change is very cost effective because there’s little extra labor involved. The next question is, “Which traction aid?” The answers to this lie in your driving situation and budget. To help you make that choice, here is some background on the types of available traction aids, starting with the open diff.

Open Differentials

Let’s start with the old adage; The average speed of both axles is always equal to the speed of the ring gear. That applies when the Jeep is going straight with both axles and the ring gear at the same speed, in a turn with one axle a little faster and the other a little slower, or with one tire in the goo spinning at twice ring gear speed and the other tire motionless.

The trail bottom line for open diffs is that traction is essentially limited to what the tire with the least grip can offer. One tire slips… the traction story is pretty much over for that axle. Torque always takes the path of least resistance. Because of the rule above, if you apply the brake on that spinning tire, it will force the axle to transfer some torque to the opposite side. That is the foundational principle for our next discussion.

Limited Slips

The limited slip (LS) differential can transfer some torque from the low traction tire to the high traction tire by automatically applying “braking” to the low traction side from inside the diff. This is commonly accomplished in two ways, via clutches (plate or cone types) or helical gears.

Clutch type limited slip diffs have a set of normal differential gears but also clutches outside the side gears to provide the “braking” we talked about above. On the mildest LS, gear separation forces of the side and pinion gears alone provide the force needed apply the clutches. Most often, clutches are also preloaded with springs of various types to increase that braking action. How much preload will determine the traction and drivability characteristics of the unit.

The common way to express the amount of torque a limited slip can transfer from side to side is bias ratio. This indicates in a ratio (X:1) how much torque the differential can transfer from the low traction wheel to the high traction wheel, relative to the amount of traction torque the low traction tire can support. A 3:1 bias ratio will transfer three times the amount of torque to be supplied to the high traction side than the low traction side. If the low traction tire can support 100 lbs.-ft (including the preload in the clutches), then the differential can deliver up to 400 lbs.-ft to the opposite side.

A mild (or “loose”) bias ratio would be 2:1 or under. Most stock LS units are in that general area. A moderate bias LS is in the 2-3:1 range and most aftermarket LS are in this range. A high bias (“tight”) LS is above 3:1. Few over-the-counter LS are offered this way, except for race applications, but many low or mid-bias LS can be tuned higher.

The loose/tight classifications also indicate the street drivability characteristics (“manners”) of the unit. In turns, the outside tire is turning faster than the inside. With a preloaded LS, the clutches have to release for there to be a speed differential between the tires and you may feel that release in the form of a mild shudder. With a lot of preload, a tire may actually break loose before the clutches. The higher the bias ratio, the worse the manners. Overall, if you need a high bias limited slip for the trail, you should be thinking about a locker. Many lockers have better manners than high bias LS and better traction to boot.

LS, especially low and moderate bias units, are most effective when the tire grip side to side is relatively even. They can be overcome when the differences in grip side to side are great and the rolling resistance is also great. Even a high bias unit can be overcome if only one tire has grip and it’s trying to push the Jeep over a tough obstacle. Clutch LS performance tends to decrease with miles due to clutch wear, so they may need rebuilding at some point to maintain performance. Clutch LS are good for a daily drivers into the upper range of the mid bias LS, though the middle-bias unit is so-so for snow country.

Gear type LS achieve the same result as the clutch units, but get there quite differently. They use small helical, or “worm” gears around side gears splined to each axle. They use from two to four pinion gears around the side gear, each side in mesh with the pinion gears on the opposite side. The worm gears will rotate freely only in one direction. In turns, the faster outside tire can freely speed up but is still being powered at the same time and the inside tire can slow down… also still under power. With unequal traction side to side, the one way action of the worm gears resist letting the low traction side simply spin. The number and tooth characteristics of the gears dictate the bias ratio. As with clutch LS, enough traction difference side to side will overcome the resistance. In general, given equal bias ratios, a gear type limited slip will have better manners than a clutch type. They are a great setup for a daily driver and generally good in snow country.

Locking Differentials

All sorts of traction aiding differentials are often put under the heading of “locker” A true locking differential is one that will supply all the torque to either axle, or both, regardless of tire grip side to side. That means all the torque can go to one tire if the other is in the air or lightly loaded.

There are two general types of lockers, automatic and on-demand (or on-command, driver controlled). Automatic lockers are wheel speed sensing devices. They try to keep both wheel turning at the same speed under a torque load. In order to provide speed differentiation for turns, they will allow the outside wheel to speed up under light torque loads. Typically, it will “ratchet” ahead by various means. If you add power, however, the unit will lock up and tires will start barking.

Automatic lockers can be divided amongst those that replace the carrier and the “plug-in” units that use the OE carrier but replace the spider and side gears. The latter setup has the advantage of being easy to install and inexpensive. On the downside, it relies on the OE carrier for its basic strength. A good carrier combined with the plug-in locker ends up being a stout unit, but when combined with a weak or badly worn carrier it can be a weak unit overall. The lockers with replacement carriers are stronger overall but more expensive and difficult to install.

On-demand lockers achieve 100% lockup by various means but the commonality is that they are driver controlled. They can be actuated hydraulically, via cable, air, vacuum or even electrically. The beauty is that they operate as a normal open diff until actuated. No side effects when unlocked but 100% traction when you need it.

Traction Aids in Front Axles

There are some complications to using LS and lockers in front axles. With an engaged locker or tight limited slip up front, you may find it difficult to steer or maneuver. Many people think an on-demand locker is the ideal front axle traction aid. The on demand locker is off when you don’t need it but full on when you do.

Historically, Jeeps were offered with low bias limited slips up front and that’s still a viable alternative. It provides an extra margin of traction with few bad side effects (except one, read below) and at a low cost. Moderate bias LS are a bit more problematic and high bias units pretty much a nightmare unless you can shift into 2-low or step out to unlock a hub. Gear type limited slips work pretty well up front, combining a relatively high bias ratio with enough “give” to still maneuver. Automatic lockers are much the same, but they actually are a little more friendly than a tight LS.

The thing that bites many YJ, TJ and JK owners in the butt is that a limited slip or automatic locker up front will spin the front driveshaft going down the road, even with a CAD (Center Axle Disconnect). It’s very difficult to achieve a front driveshaft angle with a lift that does not vibrate a little… or a lot. The only sure cure is a locking hub conversion of some type.

Making the Traction Choice

The first choice involves evaluating your driving situation. If your Jeep is a daily driver… especially if you have to drive on ice and snow covered roads… you need a traction aid with good manners. If your Jeep is primarily a trail machine, then manners can take a back seat to traction and strength.

The factory carefully calculates how small an axle they can use in a vehicle application. Among the most important parameters they consider are projected use, tire size, vehicle weight and driveline torque (engine torque multiplied by the gears). When you change the important elements in that equation, you will get a different answer and find the stock axle is no longer adequate for the job.

Tire size is the most important element in the strength equation on three levels. The first is the increased weight of the rotating mass. The second is traction. A bigger, stickier tire offers more traction torque (how many foot-pounds of drivetrain torque the tire will support before it slips). The increased radius, or “torque arm” effect, probably changes the axle strength equation the most.

Draw an imaginary line from the center of the hub to the contact point of the tire on the ground. That is your radius and the length of that line is the torque arm. A longer arm increases the load on the axle by multiplying the torque more. The axle is the “fuse” between engine torque, multiplied by trans and t-case gearing (called drivetrain torque) and tire grip.

Even though modern tires are pretty good at delivering grip, the moments when traction torque is high are relatively few but maximum drivetrain torque is almost always more than the capacity of some part in the drivetrain. Tire grip is the variable. If the available traction torque is higher than the strength of an axle part, something breaks. None of this takes into account dynamic situations like the torque spikes and shock loads that comes from a spinning tire suddenly getting grip.

So, what are the answers to these problems? They range from axle mods to complete swaps. Axles have many weak links, the most easily evaluated of these are the axle shafts and, in the case of front axles, the axle u-joints.

Starting off with axle shafts, bigger is always better but the steel it’s made from counts for a lot. Most Dana axles use induction hardened SAE 1040 carbon steel. Some OE and aftermarket replacement shafts are of a better 1050 carbon steel, which is stronger. Better aftermarket shafts are of a 1541H high-silicon alloy carbon steel and the best shafts are of a 4340 chrome-moly steel alloy, though it isn’t possible to use 4340 in every application. It’s difficult to forge the flange on a semi-float rear axle, for example.

We could spend a lot of time talking about the steel groups, like 1040, 1541H, etc., but let’s not. It really boils down to that, based on industry standard yield strength values, 1050 is 25 percent stronger than 1040, 1541 is about 35 percent stronger than 1040 and 4340 is about 50 percent stronger than 1040. The percentages may vary up or down according to the exact “recipe” of the steel and other design qualities of the shaft. By installing alloy shafts, you can reap the benefits of a stronger shafts without major alterations to the axle. The price ranges follow the strength.

The other option is to use larger shafts. That option can be economically combined with an improvement in materials to increase strength even more. In the Jeep line, only four axles offer size upgrades, the Dana 30 (from 27 to 30 spline), the Dana 35 (27 to 30 spline), the Dana 44 (30 to 33 spline) and the early Chrysler 8.25 (27 to 29 spline). There are options for other axles commonly swapped into Jeeps as well, but they are beyond the scope of this discussion.

Size is measured several ways, via spline count, spline diameter (the diameter over the splines, or major spline diameter), minor spline diameter (the diameter at the bottom of the spines) and minimum diameter (the part of the axle with the smallest diameter, which is usually, but not always, the minor spline diameter). Because the smallest diameter is the weakest part, minimum diameter is the most accurate measurement but the hardest to find.

Front Axle Universal Joints

There are two primary sizes of axle joints used in Jeep front axles the 260, found in pre-’95 Dana 30s and the 297/760 found in ’95.5 and later D30 and D44s. These numbers are from Spicer, the “760” being the new cold forged u-joints that are approximately 15 percent stronger than the older 297 size.

Even better are the u-joints with alloy crosses made of 8620, 300M or 4340 alloy steel. They are available with needle bearing or bushed caps. The bushed cap joints are stronger but are not suitable for continuous operation, such as full-time four-wheel drive or even an axle that is freewheeling. The needle bearing joints are somewhat weaker, though significantly stronger than any standard u-joint, but are suitable for long term operation like a standard joint. The bushed caps need frequent lubrication. The key element to remember in using these “Super” u-joints is that they must be paired with alloy shafts. They are way stronger than a factory shaft. Most are even stronger than an alloy shaft.

Ultimately, it may become more practical and effective to perform a swap rather than trying to beef up a weak axle. In many cases, you can increase the torque capacity of an axle by installing larger shafts or stronger ones, but then the next weakest link is then exposed and that’s often something that can’t be easily changed, such as the ring and pinion, carrier, carrier caps and bolts, bearings or even the housing itself. In some cases there are direct, bolt in swaps. In others, modification and fabrication is involved.

The best idea for approaching the axle strength issue is to try to keep a reserve of strength while staying within the basic strength limits the axle housing and other parts, can handle. Our opinion is that the following list represents those limits on a Jeep that will be subjected to moderately difficult four-wheeling. For hardcore jeepers, go one size lower on the tire size to retain a safely margin. Shown are for the stock axle and with an axle modified with the array of the bolt-on parts available to beef them up.

Maximum Tire Diameters for OE Jeep Axles

(Diameters in Inches)

Jeep Axle Stock Modified

Dana 30 Front, SR, SN 31 33

Dana 30 Front, RR, SN 31 35

Dana 30 Front, RR, LN 33 35

Dana 44 Front 35 37

Dana 35 Rear 31 33

Chrysler 8.25 Rear (29 spline) 33 35

AMC-20 Rear 32 35

Dana 44 Rear 35 37

Key: SR= Standard rotation, low pinion. RR= Reverse rotation. SN= Small knuckle, 260 U-joint. LN= Large knuckle, 297/760 size U-joint.

If you stay within the limits above and modify by adding items like alloy shafts and other improvements, you are still maintaining a safety margin of strength. The OE Dana 44s are not rated higher because they use rather thin axle tubes. A custom built D44 with thick wall tubes could go higher.

Math for Gearheads

Equivalent Ratio Formula:

New Tire Diameter/Old Tire Diameter X Original Gear Ratio = New Ratio

Example:

A Wrangler YJ with 225/75R-15 tires (28-in. mounted diameter) with 3.55:1 axle ratios. The owner wants to run 33x12.50R-15 BFG MTs (32.5-in. mounted diameter).

32.5/28 X 3.55= 4.12

The nearest available ratio is 4.10. With a Jeep with adequate power, this would be fine. On a low power Jeep, or a heavy one, better results would come by going lower, to 4.56:1.

Tire Size vs Axle Strength Chart

Here are some general guidelines for matching tire diameter to axle strength. Find out the number of axle splines that came stock with your axle and match that up to the maximum recommended tire size. If you have upgraded to larger axles and/or alloy shafts, these rules do not apply. These general rules assume axles in good condition and moderate trail use. For really hard work, drop at least one tire size.

Axle Splines/Major Diameter Maximum Tire Diameter

10/1.00 31

19/1.25 33

27/1.13 33

29/1.25 35

30/1.31 35