Differentials - What Are They?

In powered axles, gears that deliver power from the drive shaft to the left and right axle half - shafts separately. They allow the left and right wheels to turn at different road speeds when turning, so that neither wheel has to scuff. Conventional "open" differentials tend to equalize the power delivered through both wheels. Thus if one wheel loses traction - "spins out" on snow, mud, sand, or gravel - it delivers very little power to the ground. The other wheel will deliver only the same very little power. Often this is not enough to keep the vehicle moving on - it's stuck. Traction control differentials allow the wheel with traction to deliver more power than the wheel without traction. Often this is enough to keep the vehicle moving.

There are three basic types of traction control differentials:

Each type has advantages for specific types of vehicles and driving conditions.

Differentials - Spool Type

Spool type differentials can either be permanently locked (i.e. not really a differential at all) or manually locked and unlocked, such as the ARB AirLocker or the Toyota electric locker. When locked, a spool allows no difference in speed between the two wheels on a given axle.

Differentials - Locking (speed - sensitive) Type

Locking differentials such as the Tractech NoSPIN® and Detroit Locker® brands (same product, different market segments), the Detroit E-Z Locker[tm] and Detroit Gearless Locker[tm] brands. They keep the wheels locked together (except when turning) so that together the left and right wheels always deliver maximum traction to the ground; neither wheel can spin out. They allow different wheel speeds in a turn by disconnecting the faster - moving wheel (usually the outside wheel which is ground - driven faster throughout the turn), driving the vehicle with the other (inside) wheel. See

Differentials - Limited Slip (torque - sensitive) Type

Tractech's limited slip differentials are the Detroit TrueTrac® brand, and the SureTrac® and the Detroit LSD[tm] brands (same product, different market segments). They provide a controlled amount of resistance to a one-wheel spin-out, so that the other wheel (with traction) receives sufficient power to keep the vehicle moving. The Detroit Truetrac® uses gears only - no clutch packs. It is ideal for 4WD front axles. It features torque bias ratios from 2.5 to 3.5:1 range.
There are several other manufacturers of geared-LSDs including Torsen and Quaiffe. Other LSD designs employ clutch plates to affect a similar operation, albeit at much lower torque bias ratios. Also, clutch-type LSDs require special additives in the gear oil to operate and are subject to wear and periodic maintenance.

TrueTrac® Operation:

Truetrac® differentials are unique in that they increase traction but do not affect steering or wear out prematurely; these problems are common with limited-slip differentials that use clutch plates and springs. TrueTrac performs like a conventional (open) differential, until there is a loss of traction. Only then will the torque transfer occur - when it is needed.

TrueTrac Differential : Internal Components

A typical TrueTrac differential is shown above. As with an open differential, the TrueTrac side (or spider) gears are interconnected by pinion gears, which allow one wheel to slow down or speed up as required. TrueTrac gears have spiral teeth and the pinions are mounted in pockets in the case.

If one wheel begins to lose traction , the pinions separate slightly from the side gear and wedge in the pockets. As torque increases, the separating force increases, thus slowing or stopping the spin-out. This allows torque to be distributed to the wheel with the best traction.


Automatic Locking Differential Operation:


In case you use different names for the parts, using the Lockright as the model for discussion:

Salient points:

The arrangement of stop pins and holes are engineered so that the center gears can only compress together slightly; just enough for only one axle to uncouple. When the pinion shaft is bearing on the elliptical hole in the center gear pair it forces the center gears apart. This happens any time there is torque transferred to the wheels. There has to be a difference in torque between the two axles for the two driver halves (center gears) to turn relative to one another so the pins can slip into their respective holes and the center gears can compress together to uncouple an axle. This happens when one wheel is turning faster than the other, no matter whether you're accelerating or engine braking?

Here's the scenario:

We are engine braking. The pinion shaft is pushing hard on the trailing edge of the 'elliptic' hole through the center gears. It's pushing on the trailing edge because the wheels are pushing on the drive shaft, not vice versa. We start a turn to the right. The right hand tire starts to turn slower, which means the right half of the center gear pair turns backwards (relative to the left half) so that the pinion shaft is now roughly in the center of the trough on the right-hand center gear, but still hard on the trailing edge of the trough on the left-hand center gear. Now the stop pins *can* fit into their holes (once there's force to overcome the bias springs) and it's possible for the center gears to get closer to one another so an axle can uncouple. At this point, the pinion shaft is still hard on the trailing end of the left hand center gear's half of the elliptical hole, but slack on the right hand center gear. This means that it is pushing OUT on the left hand center gear, but *not* pushing on the right hand center gear. Since the left hand center gear *can't* push in, it stays coupled. As soon as the torque built up in the right hand center gear is enough to push away from the right hand side gear using the ramped sides of the teeth, and overcome the force of the bias springs, the pins will slip into their holes and the right side will uncouple. It *has* to. The left side center gear is still being held out by the pinion shaft, so the right side center gear must uncouple. The right side is the slow tire; the inside tire.

Now what happens when you add some throttle while still in the turn? The torque changes direction. Both the left and right halves of the center gear rotate backwards (relative to the carrier). They must rotate together because they are still held by the pins between them. As they rotate backwards the pinion shaft is no longer pushing on the trailing edge of the elliptical hole on the left side center gear. It can now move inwards (if it wants). The center gears continue to rotate backwards (relative to the carrier) until the pinion shaft hits the *leading edge* of the elliptical hole on the right hand center gear (remember it's rotated slightly backwards of the left hand center gear). Now the right hand gear is forced outward by the pressure on the leading edge of the elliptical hole. As soon as the teeth match the right center gear is forced out and the right hand axle (inside) couples. Now, since there is torque on the outside tire (it is currently driven, right?) yet there is no force on the leading or trailing edge of the left center gear to force it outward, it happily disengages (due to the ramped teeth and the fact that the center gears are already compressed together) so that the transfer of power is quite smooth, but still perceptible due to understeer/oversteer.

Check out the following patents (Click on "View Images" to see the entire scanned patent):

Driving Impressions:

Hydraulic Locker:

As mentioned above, the TrueTrac differential, like all limited slip designs, will only work if there is some load on each axle. If one wheel loses all traction (i.e. a wheel in the air), that wheel will spin and no torque will be transferred to the wheel with traction. This is where brake biasing (or as I like to call it: "hydraulically actuated locker") the spinning wheel in order to transfer that torque to the opposite wheel. It can take a fair amount of brake drag to actually make this work. Let's assume a 3:1 torque bias ratio and one wheel in the air (i.e. no load on it) and the other one on the ground. In order to transfer a given amount of torque to the wheel on the ground (say 100 you need to apply ~1/2 that amount of brake drag to both wheels on that axle (unless you have custom turning brakes). So now the spinning wheel (and axle) "sees" 50 of drag, allowing up to 150 (3x50) of torque to transfer to the other axle, but it also has 50 of brake drag on it, reducing the net torque to 100 applied to the wheel on the ground.
But the "extra" torque to overcome the brake drag must come from somewhere, namely the engine, so you've got to give it some extra throttle at the same time. So basically, the harder you brake, the more torque you put into the ground. Trust me, it is a totally unique experience.If you have a TrueTrac in your rear axle, the parking brakes are another mechanism for pre-loading the differential.
In my 4-cylinder Toyota, I find brake biasing only works at crawl ratios of 75 (numerically) greater (since I find I usually need this on a steep hill where the engine is already heavily loaded to begin with). Below that gear ratio, I end up killing the engine before the TrueTrac locks up. At ratios above 200:1, I find my engine can overpower the brakes w/o additional throttle. This biasing technique also works better when used *before* you get stuck rather than after the fact. Learn to recognize terrain that will cause a TrueTrac-driven tire to lift, get in a low enough gear before hand and start riding the brakes *before* you hit the obstacle and you'll crawl right over it like you had a true locker. For photographic evidence of this, check out the 2nd Annual Day After Turkey Day write-up on the ORC web site
Note the left front tire is not spinning

On Road Behavior:

For a week, while my rear drive shaft was getting re-balanced, I drove my truck in front-wheel drive mode, getting the chance to experience the front TrueTrac on pavement in normal traffic. It is not as transparent as I had assumed from off-road use in 4WD mode. You do know it is there, but it is much less noticeable than a locker. There are certain characteristics of an LSD and locker that are similar. Likewise there are certain characteristics of an LSD and an open differential that are similar.

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Last updated: 16.JUN.1999