In powered axles, gears that deliver torque 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 (thus the term "differential"), so that neither wheel has to scuff. Conventional "open" differentials tend to equalize the torque 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 (power = torque x RPM, so if torque goes to 0, so does power). The other wheel will deliver only the same very torque as the spinning wheel and if this is not enough to overcome the inertia and static friction holding the verhicle in place, you are stuck. Traction aiding differentials allow the wheel with traction to deliver more torque than the wheel without traction. Often this is enough to keep the vehicle moving.
In an open differential, the following rule applies:
The average speed of the two axles is equal to the speed of the ring gear.
This means is one wheel is on glare ice and the other on the dry ground, the wheel on the ice can spin twice as fast as the ring gear, or as indicated by your speedometer. So, if you are stuck like this and floor it to 60 MPH, that one tire will be spinning up to 120 MPH and may be damaged.
Because of this feature of one tire losing traction and causing you to get stuck, several other differential designs have been used to get around this limitation. Since there are two variables involved in getting power from the engine to the ground, namely torque and speed, you'll find the different designs make use of one or the other in operation. They can all be classified as traction aiding differentials and there are three main types:
Each type has advantages for specific types of vehicles and driving conditions.
Open differentials use spider gears to transmit torque equally to both axles while allowing a speed difference between the axles. This results in smooth operation on high traction surfaces and is probably used in 99.9% of all vehicles. The governing rules of operation are that the same torque is applied to each axle, and the avearage speed of the two axles is equal to the speed as the ring gear. Note the 1 and 1 average to 1 as does 0 and 2. So, with one tire in the air and the other one on the ground, the tire in the air may spin as twice as fast as the indicated on your speedometer. This is why you should not over-rev your engine when stuck.
Spool type differentials can either be permanently locked (i.e. not really a differential at all, a.k.a. a Lincoln Locker - named after the popular brand of welder used to weld the differential) 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, i.e. each axle turns at exactly the same speed as the ring gear.
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 Locker, below. The governing rule is that at least one axle must turn at the same speed as the ring gear, the other is either locked or coasting, depending on the direction of the applied torque.
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 torque 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.
George Couyant has an excellent article on differential 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.
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.
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?
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 torque is quite smooth, but still perceptible due to understeer/oversteer.
Check out the following patents; the scanned patent images are no longer freely available :-(
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 ft.lb.) 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 ft.lb. of drag, allowing up to 150 ft.lb. (3x50) of torque to transfer to the other axle, but it also has 50 ft.lb. of brake drag on it, reducing the net torque to 100 ft.lb. 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 biasing the differential.
In my 4-cylinder Toyota, I find brake biasing only works at crawl ratios of 75:1 or (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.
The brake 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 (below) and notice how the driver's side tire is in the air and is turning just as fast as the other tires as evidenced by the slight motion blur in the photo:
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.
So, after about 3 years running the front TrueTrac (and rear Detroit) I decided it was time to move on. I sold both 3rd members and instead went with completely new 3rd members set up with 4.88 gears and ARB RD23 air lockers. In back I went with a V6 3rd member for the added strength and up front an FJ-80 reverse-rotation/high-pinion 3rd member for added strength and clearance. As I already had on-board air, the ait lockers were an obvious choice. So now after wheeling with ARBs for a while, I can offer some comparisons:
On road, I do not miss the rear Detroit at all. I never was too happy with the on-road behavior of the locker. It was relatively quiet, didn't chirp the tires too much, but what was most irritating was the on and off nature in turns. If you accelerated it would try to straighten the truck out, let off the gas and it would try to turn sharper. Wasn't so bad when I did the first 3.5" lift and had fairly stiff springs and shocks, but as I lifted the truck a bit more and went to way softer springs, this self-steering behavior got worse and worse. With the ARB, its open on the road and I almost forgot what driving an open diff felt like, its wonderful. Off-road, its about a wash, I never worried about the Detroit out back, it was just there and if did its job. There were situations where it would have been nicer to not be locked, but you had no choice. Now, I'm finding all sorts of situations where I don't need even a rear locker. I can try the obstacle open, if I get into trouble, flip the switch and try it again. But in snow, mud, off-camber situations, I can turn it off and be open.
The front TrueTrac is a different story. Most of the time I actually miss it off-road as it was just there all the time and mostly just did its job without getting in the way (i.e. no steering feedback, etc.). However, when the going gets really tough, the TrueTrac did take some finesse to make it lock up and pull with a front wheel in the air, for example. Now, when I see an extreme obstacle coming, I just flip both switches on, grab a low gear and focus 100% on driving, no worries about working brakes and gas pedals, just steering. Also, there is a bit less axle hop with the ARB, as it has no play to it as compared to the TrueTrac (or Detroit for that matter). I can now get all 4 wheels turning slowly, work the steering side to side, find some traction and keep going. Before, in similar situations, the truck would start hopping as you lost traction, which then meant reversing and trying a different line. However, the TrueTrac would almost never interfere with steering, but if you lock the front ARB and have to turn, it can be very difficult. Also, if the driveline is bound up, it may not unlock right away, so you really have to plan where to unlock, etc.
I did wire my ARBs up in a non-stock manner. Normally, the rear ARB must be on before the front can be engaged. I chose not to use the ARB switches and wiring harness, rather just hooked each air solenoid to a separate switch. Sometimes I find having just the front locked is better than both ends locked. Anyway, the ARBs take a lot more planning on when the engage and disengage (still working on this). Overall, I think I like the ARBs better than my old setup. I've not done a lot of snow driving with the ARBs, that's one time I'll probably miss the TrueTrac, as it really shiines on slippery roads.
[Last updated: 09.February.2011]