Traction is the sum of tire grip and wheel torque. Tire traction is highly variable according to tire design, ground conditions and ground pressure, which changes constantly according to vehicle angles and loading, not to mention tire pressure. From the axle side, the traction equation is much more predictable according to what type of differential is used. The best tire is only as useful as the amount of wheel torque it gets and the goal is to get torque to as many tires as you can... and that’s where axle traction aids come in. The trail, or at least the hard-core trail, isn’t the only spot where enhanced traction is useful. If you live in a place where the white stuff drops from the skies, have to drive a loaded truck onto a muddy jobsite or pull a behemoth boat up a slippery launching ramp, traction also has meaning. Sometimes enhanced traction comes with a price tag beyond money. Tire barking, chattering, control issues on wet or icy roads are important elements of the traction aid choice for people that spend most of their time on asphalt. What we’ll do in several parts is to give you a round up of traction aids from several manufacturers, telling you how those products work, and giving you the ammo to make the best choice for your lifestyle, driving situation and budget. First, we’ll start with telling you how the ordinary open differential works and, generically, how traction aids work. We won’t cover spools because we figure you can figure out how those work!
Open Differential
The standard, or open, diff is often the starting point in many 4x4s. On the street, it provides the baseline drivability situation we usually call “transparent” operation, because you
 The open differential’s limitations are not apparent to most of the American driving public, but those of us with four-wheel drive usually notice them soon enough. 1- Pinion (spider) gear, 2- Side Gear, 3- Pinion Shaft |
|---|
seldom feel any signs that it’s working as designed. Follow along using the nearby illustration.
First, the absolute rule. If you can get this straight in your mind, you will always understand how the differential works. The average speed of both axles will always equal the speed of the ring gear. If the ring gear is turning 100 rpm and the vehicle is going straight, both axles are turning 100 rpm. To get an average, add the two 100 rpm speeds and divide by two (100+100=200/2=100).
Engine torque enters the axle through the pinion shaft and gear and is transferred to the ring gear and differential case. The ratio of teeth between the ring and pinion provides the axle gear ratio, as well as changing the direction of torque flow. The differential case drives the pinion shaft. The pinion shaft drives the pinion gears (aka “spiders,” of which there can be two or four) and the side gears, which are splined to the axles shafts, left and right. Assuming there is equal traction from both tires, the vehicle is going in a straight line and both tires are identical diameters, the pinion and side gears are driving the axles as a solid unit, with no rotation.
If you go around a corner, the side and pinion gears allow one axle to speed up and the other to slow down, but that speed change is proportional side to side and the average speed of the two axles will still always equal ring gear speed. Torque is still being delivered to both axles but the pinion and side gears are in rotation inside the differential case.
As long as traction is equal on both tires, torque will flow equally to both. When traction is lost on one side, torque takes the path of least resistance and goes to the “loose” tire. That’s something we have all seen but still the absolute rule applies. In this situation, the pinions rotate around the stationary side gear and drive the other one. At 100 rpm ring gear speed, the high traction axle is at zero rpm while the other is running at roughly twice ring gear speed (0+200=200/2=100).
Limited Slip Differential You’re stuck with an open diff, one wheel on a high traction surface and the other on ice. One wheel spins madly and the other is stationary. If you apply the brakes on that spinning wheel and slow it down, a percentage of the torque will be transferred to the side with traction (remember the absolute rule). That’s exactly what a limited slip differential (LSD) does. There are many mechanical ways to achieve that, from clutches to gears, but the amount of “braking” in a limited slip is measured by the torque bias ratio, or simply bias ratio. Many limited slips use the same types of pinion and side gears as an open diff, with clutches outboard of the side gears to apply braking. The bias ratio expresses the ratio of how much of total driveline input torque can be sent from the low traction side to the high traction side. Using an axle with a 2:1 bias ratio as an example, if the low traction side can support 100 lbs-ft, (that’s both whatever traction exists and the “braking” force from the LSD) then 200 lbs-ft can be transferred to the high traction side (there will be a total of 300 lbs-ft being delivered via the driveline). With clutch type LSD, the amount of preload (preset braking) built into the clutches will dictate how many drivability issues there will be. A high bias ratio unit will have a lot of preload and vice versa. If the amount of preload (braking) exceeds the traction on one tire (such as the unloaded inner tire of a truck in a turn), the tire may lose grip and bark before the clutches reach their breakaway torque (the torque required to slip them). With or without preload, bias ratio is variable. As the torque load from traction increases inside the differential, gear separation forces push the side and pinion gears apart. If there is a clutch behind the side gear, those forces increase the pressure on them and the braking power. The opposite is also true. That’s why if you power around a turn with a LSD you increase the chance of barking tires and when you coast, you decrease them. Factory LSDs are usually low bias units, from about 1.6:1 to 2:1. That ensures no obnoxious drivability issues, such as chattering, tire barking in turns, etc. Aftermarket buyers usually want more traction and are willing to put up with more symptoms, so bias ratios usually start a little above 2:1 and go up to about 4:1. Some LSDs can be tuned to go higher but at that point, it’s practically like a spool and an automatic locker will probably be closer to being transparent. At 2:1 the symptoms are minimal. They start being very noticeable at around 3:1. Overall, light rigs may exhibit symptoms sooner than large, heavy rigs. Gear type LSDs (more on them in a later installment) can usually get by with a higher bias ratio without adverse symptoms than can a clutch style because they are not usually preloaded. An open diff, incidentally, has close to a 1:1 bias ratio. The generic downsides of clutch type LSDs are that the friction materials that provide the braking wear and the unit becomes less effective over time. Depending on how you drive, there can be a serious degradation in bias ratio at between 50 and 100K miles. Also, most clutch type units require a friction modifier added to the oil to avoid clutch chatter. Most clutch type LSDs can be rebuilt but some cannot. Gear type limited slips usually retain most of their factory bias ratio for their entire lifetime. Finally, when the LSD is overcome on the trail, it can be enhanced by using brake pedal modulation or partially applying the parking brake. Partial application of the parking brake (assuming is applies on the rear wheels and not on the t-case as with some older rigs), will help equalize side to side traction even more. A little footbrake can do the same thing. Generally the parking brake is the better choice, because it applies only on the rear. Both these tricks will help with an open diff too!
Locker
This is a short-n-sweet section. A locker can transfer 100 percent of the input torque from the driveshaft to either axle or both. It has an infinite bias ratio. How that is achieved varies somewhat by type (which we’ll explain in due course) and there are automatic and on-demand (driver controlled) lockers, but when the unit is locked, both axles are turning at the same speed as the ring gear. Automatic lockers have the means to release the outer (faster) axle in turns, but a locked on-demand locker is like a spool.
ARB
While ARB is synonymous with a variety of four-wheeling products, they are perhaps best known for the ARB Air Locker. It appeared in 1987, but the device upon which it 
was based goes back a few years to the early ’80s. Its inventor was an aircraft engineer name Tony Roberts and his original design was vacuum operated. Roberts sold his design to ARB in 1987 and it was soon reengineered to operate on low-pressure compressed air. The Air Locker came to the U.S. market soon after.
In the early 2000s, ARB redesigned the Air Locker to be simpler, stronger and more reliable. They implemented the changes a few applications at a time and, now, almost all of the 100+ applications have been upgraded. The new design increased case strength by reducing it from three to two pieces. The locking component was strengthened by moving it to the opposite and stronger side of the carrier, reducing complexity. This left room to upgrade the two and three pinion applications to three and four-pinion for even more strength.
ARB offers its own compressor, which can be used for airing up tires, but it’s a bit short of airflow capacity for airing up big tires quickly. If you have a compressed air system already, it needs to deliver a minimum of 85 psi and be regulated to no more than 105 psi. The Air Locker doesn’t need much air volume, so any small compressor is fine but as smallish compressors go, the ARB unit is a particularly durable one.
On the plus side of the Air Locker is the on-demand feature, which means the driver chooses when to engage or disengage it. When disengaged, it’s a totally transparent open diff. When engaged, it’s totally locked. It’s especially useful in front axle applications, where a limited slip or automatic locker can make steering difficult. The ARB is robust, most especially the newer designs.
The most valid downsides are the relatively intense installation and the extra complexity. The complexity of air lines and finding space to mount a compressor is a factor. If you already have air, or want it, it’s a minor issue. Generally, the air system is reliable but they do have points where they are vulnerable to damage, so contemplate carefully the best locations for lines and wiring.In use, there is very little to nitpick. There are times when something less than 100 percent locked is desirable. Sometimes, too many controls can cause driver overload, so it’s possible to encounter driver error. Because the ARB uses meshing gears for lockup, the unit doesn’t always engage or disengage instantly. It can be held engaged by torque or not engage because it’s between teeth. The driver learns how to deal with these operational issues and a good driver can usually outthink an automatic device.
 How the ARB Works |
|---|
Auburn Gear
Auburn Gear was once a part of Borg Warner Corporation, but was split off on its own in 1982. Since then, they have been supplying the OEM and the aftermarket with a 
variety of traction aids. Auburn has three main products of interest here, the ECTED (Electronically Controlled Traction Enhancing Differential), and two limited slips; the medium bias High Performance Series, and the higher bias Pro Series.
Pro Series/High Performance Series Limited Slips
These units are very similar in design, with one being tuned for a higher bias ratio. The designs of both go back to about 1965 and the Borg Warner Spin resistant differential, which was offered in a number of OE applications. The High Performance delivers about a 2.5:1 bias ratio, which is positive but still pretty street friendly. The Pro Series is tuned to about a 3.5:1 bias ratio and is designed for high performance applications. For a mix of street and trail, the High Performance Series is the best choice. That’s especially true for a lighter vehicle with smaller tires. A heavier rig or one with very grippy tires, can deal with the higher bias Pro Series a little better day to day. Otherwise, expect to experience some tire barking in turns and squirreliness on slippery roads if you aren’t paying attention. Both units have a reputation for strength and durability. To some degree they can be “tuned” by changing the amount of preload, though the parts to do so may not be readily available. Beyond the generic downsides of a clutch type limited slip, the Auburn units are generally not rebuildable once they are worn out but, compared to a stock plate clutch LSD, their effective working life can be longer. Coupling that to a fairly low buy-in, the non-rebuildable aspects often balance out.
 How The Auburn Limited Slips Work The Auburn limited slips use cone clutches to provide the braking. A tapered cone (2) is combined with the side gears and they fit into tapered pockets in the case (1). The preload springs (3) provide the initial preload to force the cones into the case and create friction. That force is augmented by the gear separation forces of the side and pinion gears. |
|---|
ECTED
Introduced in 2003, the ECTED fits between the common definitions of “locker” and “limited slip.” Many think that provides a useful “missing link” between the two. It uses an
electronically activated clutch pack to connect the axles instead of a mechanical. When disengaged, it operates as a low-to-medium-bias LSD, about 1.5-2.0:1. The bias range is variable because it’s not preloaded and relies on gear separation force alone to apply the clutches.
The ECTED is a great choice for those who want factory level limited slip capability every day, with good manners, but want to combine that with locker, or near-locker, performance on the trail at the push of a button. The ECTED can be used in front or rear axles, though in the front axle of a full-time 4wd rig, you may (but not always) notice some adverse symptoms in the form of steering wheel bucking in hard turns. Because the ECTED is engaged electrically, installation is simple, though it does require a hole to be made in either the housing or axle cover for the power wire.
Downsides are almost non-existent for the mild to moderate off-roaders that run at or below the 33 inch tire realm. That’s where its mild manners and broad range of capability will stand out. In hard-core situations, the realm of big-tire, built-up rigs with lots of torque multiplication from low gearing, it’s possible to slip the clutches when there is a large difference in traction side to side (one tire in the air and another well mashed into a rock and pushing hard). Also, with only two pinions, the upper limit for tire size (variable according to application) must be carefully noted.
 How the ECTED Works The ECTED has a clutch pack (1) behind one side gear and gear separation forces between the side (2) and pinion (3) gears offers mild to moderate limited slip capability before the unit is energized. When full voltage is applied to the electromagnetic coil (4), which is stationary and mounted outside the carrier on a ball bearing (5), the tapered pilot cone (6) is pulled tightly into a corresponding recess in the case. This allows the ball bearings (7) to ride up onto ramps in the side gear and push it inboard. The center block (8) then pushes against the opposite side gear, compressing the clutch pack against the case and stopping side gear movement. |
|---|
