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Other Types of Differentials

What types are available?

In general, there are five types of traction adding devices (TAD's):

  1. Speed Sensing Traction Adding Devices (SSTAD) (teeth, viscous fluid, wheel sensors & brakes)
  2. Torque Sensing Traction Adding Devices (TSTAD)
  3. Friction Based Traction Adding Devices (FBTAD)
  4. Manual Operated Traction Adding Devices (MOTAD) (cable, air, solenoid/electromotor)
  5. By welding axles together (spool)
[All abbreviations hereby copywrighted if not stated otherwise and/or claimed by other persons]

Which brand is which type?

Not all TAD's are one or the other, some are mixed. I tried to classify the various brands/implementiations and names (AFAIK):

  1. SSTAD
    1. mechanical/teeth: PowerTrax Lock-Right, NoSpin Detroit Locker, Detroit Sof(t?)-Locker, Detroit EZ-Locker, Eaton/GM Gov-Lock (sp?; also partly #3)
    2. viscous: Visco-Lok and other viscous fluid units
    3. electronic: Range Rover 4.6 (rear axle only), upcoming Mercedes AAV
  2. TSTAD: TorSen, TrueTrac, Quaife (also partly #3), Powr-Trak
  3. FBTAD: Various OEM LSD's, PowerBrute, Trac-Lok, Powr-lok, Auburn (Pro), Traction Equalizer
  4. MOTAD: Various OEM diff locks, Air Locker, PowerTrax C(ommand)-Locker
  5. Spool: Not sure if they are available beyond DIY


  1. NoSpin, Detroit Locker/Soft-locker/EZ-locker and TrueTrac, sold by Tractech, are trademarks of Titan Wheel International Inc. Tractech is owned by Titan Wheel International Inc., USA.
  2. Powr-Lok and Trac-Lok are trademarks of Dana Corporation, USA.
  3. Traction Equalizer is a trademark of Rockwell International, USA.
  4. Powr-Trak and Visco-Lok are trademarks of GKN Viscodrive, Germany.
  5. Air Locker is a trademark of ARB Corporation Ltd, Australia.
  6. TorSen is a trademark of DK Gleason Inc, USA.
[For addresses and some historical background, see end of document]

So, how do they work?

1a) Speed Sensing Traction Adding Devices, mechanical/teeth:

Basically, on a straight track it functions like a spool. However, in corners, a wheel is allowed to run faster than the ring gear (outside wheel free), but never slower than the ring gear (inside wheel driven). When the inside wheel slips, the outside wheel would go slower than the ring gear, so the grip wheel is locked to the slip wheel. You can compare its function by a 'single-direction coupling', with the difference that a mechanical TAD will operate in both directions, reversing its function whenever torque reverses (I assume).

The Eaton/GM Gov-Lock works differently; a good description was posted by Christopher Hinds on the Offroad Mailinglist:

With respect to the GM TAD (Z71), Taylor asks:

> Am I missing something?  This GM type of lsd sounds like a big crock.  It 
> must have some good point other than no clutch to wear out.

Well, I have tried this before, but I will try it again.  This
explanation is going to be long, so if you don't care to know about the
GM LSD, skip this post.  Otherwise read on.  Simple explanation:  The GM
"LSD" is really more of a locker.  There are no  clutch loading springs
inside, but there are clutches, just like a clutch LSD.  There is also a
little "governor" type defice that senses relative wheel speed.  When the
relative speed exceeds approx. 100 rpm, the governor catches on a latch
and stops motion of a "ramp" gear relative to the case.  The ramp gear
spreads apart because half of it is secured to the case and half is
secured to an axle shaft which is spinning relative to the case.  This is
what loads the clutches.  Actually, the tolerences involved when new will
allow no relateve speed difference between wheels in a locked condition.
Hence, if you put one tire in the air and one on cement, then go, the one
in the air initially spins, then the thing goes klunk and suddenly both
wheels spin at the same speed.  The tire on cement will smoke if you keep
at it.  I have put one tire on ice and one on cement and left a black
streak on the cement.

Since there are clutches, the thing will be slightly better than an open
diff when exposed to a load.  This is because the spider gears exert some
side forces on the side gears, and this loads the clutches too.  Note
that the clutches are all steel disks and I am almost positive that
_NO_ additive is required for this diff to function.

With respect to ice and snow, _any_ lsd or locker is bad on road.
Contrary to popular belief, lsd's don't "sense wheel slip and transfer
torque accordingly".  They sence torque and as torque increases, so does
the biasing power of the diff.  They do have some initial biasing power
due to the clutch preload springs.  With respect to this situation, yes,
the GM lsd is a little worse in that all of a sudden you loose traction.
Icy roads are not what it really helps with.  Were it does work is when
you are on trails or other situations where one tire looses traction.  In
this case, the GM lsd will lock and move you.

The locking procedure is a little rough on parts if you are hard into the
gas with a big load.  Of course even in no-load situations the thing puts
some shock into the system, but not enough to cause any damage in a stock
application.  Now, if you put 35's on and sink it in a huge mud hole and
think that your locker should catch and pull you free in an attempt to
rock it out, you have anyther thing coming.  There is a built in shear
device that will render it an open diff if a certain torque load is
reached.  This is to prevent you from breaking either axle, which was only 
designed to handle slightly more than half the max. torque from the

1b) Speed Sensing Traction Adding Devices, viscous fluid:

Two sets of multiple plates are connected to each other by a viscous fluid. They allow a certain amount of rpm difference without significant 'binding'. When slip occurs, the viscous fluid gets warmer and thicker, and it will finally lock up till its maximum setting.

1c) Speed Sensing Traction Adding Devices, electronical sensors & brakes:

Whenever slip is sensed by wheel sensors (requires ABS), the brake on this wheel is engaged. This simulates grip-torque, and with an (still) open diff this means the other wheel receives an equal amount of torque.

2) Torque Sensing Traction Adding Devices:

IMO the most complicated ones, and likewise difficult to explain. I have a whole bunch of formulas from TorSen, in which its principle is described, but you will never want to drive again after reading these. Basically, it uses a set of gears inside the diff carrier, that act like a worm and roll, the worm can drive the roll, but the roll cannot drive the worm.

An example:
Suppose you have a torque of 7000Nm available on the carrier of a TorSen diff. With only 1000Nm grip available on one wheel, the diff biases 6000Nm to the other wheel (1:6; for ease of reasoning assuming that this other wheel has enough grip to 'accept' this amount of torque without slip). With an open diff in the same sitation you would have only 2000Nm grip (1000Nm on each wheel), with a TorSen 7000Nm (1000Nm on one, 6000Nm on the other wheel).

For a better understanding, I always try to imagine it as allowing speed differences between axles caused only by torque coming from the outside, not going to the outside. It is then multiplied and send to the other axle.

The Quaife mostly resembles a TrueTrac/Powr-Trac, but adds another element, friction, by which it also functions when one wheel is airborn. It functions like a lower limit of torque transfer.

3) Friction Based Traction Adding devices:

These types have some sort of clutch inside the diff, by which both wheels are always connected to some extend. It will 'bind' the wheels together with a specified amount of torque (disregarding wear in the long run), increasing when the input torque increases. Therefore there is always a certain amount of torque send to the least-grip wheel, even when this wheel is airborn.

Some brands have two sets of clutches (Powerlock), others have a cone shaped clutch instead of a flat surface.

4) Manual Operated Traction Adding Devices:

These lock by engaging a sleeve that connects both axles together, either by cable (Toyota OEM), air valve (ARB) or an solenoid (electromotor??) (C-locker). More precise: one of the axles is connected to the diff carrier, thereby connecting the other axle through the spider gears to the diff carrier as well.

For a the 'bit release' of the C-Locker from PowerTrax, see: Command Locker

5) Spools:

It is just that, welding, disallowing the entire diff inside gear train to rotate. [Note: some people claim that this can also be done by welding repair on an axle, while grounding the drive shaft ]

What are the Pro's/Con's of each type?

1a) Speed Sensing Traction Adding Devices, mechanical/teeth type:

This type locks 100% when slip occurs. Older types do this quite aggressively, thereby causing difficult slip handling in slippery corners. Sudden changes in torque (sudden engine braking and accelerating) are also nasty, especially in corners. In corners, they send torque to the inside wheel only, thereby increasing the possibility of slip. Later versions (Sof(t?)-Locker) are claimed to be much more friendly in handling. ABS is not possible, or at least malfunctioning in a locked condition (I guess, any experiences?).

1b) Speed Sensing Traction Adding Devices, viscous type:

They will lock till their maximum setting when slip occurs (maximum torque transfer depending on design, like number of plates, size of plates and fluid characteristics). They still work when one wheel is airborn, but they also allow some speed difference at all times. Current design is claimed to need several seconds to lock up completely. Future units (Visco-Lok, OEM unit in '98 for at least two US manufacturers) claim to lock up within fractions of a second. The latter type requires a minimal speed difference of 20rpm to operate, so with a 40:1 total gear ratio, this does work in 4low at 1000rpm engine. ABS is possible by definition.

1c) Speed Sensing Traction Adding Devices, electronics & brakes:

Brake pads and disks will wear faster. This version is therefore only seen on 4x4's that won't see difficult stuff all the time. Requires wheel sensors, and therefore ABS is most likely to be present as well.

2) Torque Sensing Traction Adding Devices:

This type 'binds' before slip occurs, not afterwards as all other automatic versions do. The grip difference can be up to 1:2.5-3 (TrueTrac) or 1:6 (TorSen) before one wheel finally starts to spin.

They are claimed to have a narrow area of handling at the grip limit. Normally, you can rely on an inside wheel-spin only, still having the outside wheel for lateral grip. The chance that both wheels start to spin at the same time is larger, but the chance of slip occurring in general is lower of course lower, due to increased total traction.

With one wheel airborn, slip-torque = 0, so even a muliplication by 3 or 6 will equal a grip-torque of 0. You are stuck. However, by applying the brakes you can simulate a slip-torque larger than 0, thereby sending a multiplied torque to the grip wheel, which in turn is larger than the brake torque on that side. You probably can free yourself. (AFAIK, this procedure is also stated in the Hummer's manual, and mentioned several times in articles about other TorSen-equipped trucks)

A Quaife diff basically adds a lower limit of torque transfer to the TrueTrac principle. But although this friction is multiplied by its gears and also works with one wheel airborn, you still can get stuck, like you could with a friction TAD, only later. The brake-assist procedure is similar. It also wears out, but if it ever does completely, you would still have an ordinary TrueTrac.

Earlier TrueTracs had 2 sets of worm gears, and the manufacturer recommended not to use tires over 32". More recent versions have 3 sets of worm gears. Kevin Alcox posted his experience with the first generation model:

>>Anyone have any experience with the Detroit True Track Locker in the Front
>>Differential? Is it hard to steer, ETC?
>>                                        Thanks, Terry Koch

First of all, it is not a true locker. It is a gear driven limited slip
unit. I have ran one for about 2 years before it broke. My impression was
that it worked better with smaller tires. I noticed a difference in
operation between 32" and 33" tires.

I ran it in a Dana 30 in a CJ7 that I daily drive and it worked fine. The
steering was not adversely affected, although it did return to center 
sooner and with more force than an open diff.

On the trail with 1 front wheel in the air, application of the brake would
cause the tire on the ground to spin, but I broke a couple of axle u-joints 
doing this. The True Trac finally gave out last month when it stripped the
teeth off of the gears inside it. I have sent it off to Dyneer, who said
that they were going to warranty it because they have had this problem with 
this early design, which they have since corrected.

If you are have smaller diameter tires or do a lot of highway driving in
4WD, then it works great, but if you have big tires and/or do a lot of 1
wheel in the air 'wheeling, then I would look at something else.

Kevin Alcox  '80 CJ-7
Littleton, CO     Mile Hi Jeep Club - Patrol 16 Leader

All these types of diff's allows proper steering when mounted in the front axle. ABS is possible by definition.

3) Friction Based Traction Adding Devices:

They are cheap, wear out in the long run and require special LSD oil (all??). They also are crippled by having a preset limited amount of maximum torque transfer (anyone torque numbers on this one?, '%' doesn't say much, other than for a given input torque). On the other hand, they also 'bind' at all times, due to their preload design to tackle airborn situations. This also effects near-0 grip situations on ice.

These two factors equal a stuck-in-the-middle dillema: design one with not enough friction, and they are useless; design one with too much friction, and they will induce slip (especially on ice), by acting as a spool. However, they have predictable handling characteristics. And with one wheel airborn, it will still send some torque (depending on preload setting) to the grip wheel. Not sure whether ABS will work properly (any experiences?); probably yes.

Two interesting postings on the Offroad Mailinglist, about increasing the stock performance of some units:

Date sent:        Thu, 11 Apr 1996 13:07:26 -0600
From:    (wakefield const.)
Subject:          Re: Powerlock vs TrackLock ?
Send reply to:

> Can't tell you about the Powerlock vs. the Trac Lok, except to say that
> I have a Trac Lok and it's very positive and works well...
> The Trac Lok requires an additive called "Equatorque" which is a
> friction modifier...  The instructions I got with the unit called
> for 1 tube, or 4oz.  I don't know if it is a reducer or increaser,
> but I know 80w-90 won't work correctly and you might risk damaging
> the diff if the friction modifier isn't in there....

        I use Equa-torque in my Power Brute LSD. Its supposed to decrease
friction and helps keep the LS device from chattering....also supposed to
lenghten life-span of the device.
        If you get to a gnarly trail or plan on spending a weekend in the
rocks where you need less slippage and more locker-like characteristics,
just add some power steering fluid to you diff. Be careful, a little goes a 
long way and I usually change to fresh 80-90w before I go home or
immediately upon returning.

From:             Joe Valdez - Module build coordinator
Subject:          Powerlock vs TrackLock ?
Date sent:        Thu, 11 Apr 96 11:41:21 PDT
Send reply to:

Reply From jvaldez At 11:32A On Thu Apr 11 1996:

> Is a Powerlock tighter than a Tracklock?
> I just bought a 68 scout 44 rear axle that has a Powerlock
> differential. I was wondering how it compares to other
> limited slip diffs. Also, I was told to put the friction
> modifier in. Six states distributors only had Ford m-19546-a
> friction modifier; is it the same as chevy's? It is blue, and
> chevy's was gold. I suspect it is just died blue.
> Is one 4oz. bottle enough?
> Does it reduce friction or increase it? I was thinking that I might
> try just straight 80w 90 oild first and see if it pops around
> corners.
> Thanks,
> Brent

Brent, that Powerlock of yours is a fine unit, it has 2 sets of clutches 
vs. a limited slip which has one set, you can get your powerlock rebuilt with 
extra clutch packs and torqued down tighter than stock. This will give you 
a Powerlock that locks up just like a locker.
  I run 140wt gear oil in my diffies and gear boxes, the 80-90wt stuff was 
just too thin for my tastes, especially all the desert wheelin we do out

Joe Valdez, Scouts West/CA4WDC

4) Manual Operated Traction Adding Devices:

The only type that allows full control. You can choose for the predictability, handling and steering of an open diff, and the ultimate lock of a spool. Front lock will result in a turning radius of infinity, unless one wheel is airborn.

Aftermarket installation requires some extra wiring; either airlines (ARB) and an compressor, or electrical wires (C-Locker), the first with the most added complexity and chance of malfunctioning (Murphy).

ABS is possible, but it should be turned off whenever one or both axles are locked. Some factory axle locks (Toyota 80 after '92) will only work after engaging 4low (= center diff lock), by which ABS is automatically shut off. MB Gelaendewagens have a separate ABS shut-off button, and an automatical shut-off whenever one of the three diff's is engaged.

5) Spools:

The inventor of the open diff will probably turn in its coffin, but hey, it works! Tires and gears will wear faster, cornering on sticky surfaces is awkward, risk of breaking axles. In the parking lot of the mall, everyone will think you are nuts and can't drive. But you already knew that. However, many of these spools are mounted on trail-only vehicles, and will therefore never see normal roads.

A spool in front will equal a turning radius of infinity, unless one wheel is airborn. ABS and spools are mutually exclusive. (anyone ever tried, or even did this to a truck new enough to have ABS?)

Good luck, and don't get stuck!

Historical notes:

DK Gleason & TorSen

Diesel KIKI Co. Ltd, was founded in 1939 as a joint venture involving almost every engine manufacturer in Japan, and began production of fuel injection equipment under license from Robert Bosch GmbH, Germany. A world leader in fuel injection systems and automotive airconditioners, Diesel Kiki has expanded into many other high-technolgy fields including hydraulics, pneumatics, electronics, robotics, and now, with the acquisition of the Torsen differential business, traction management. Having over one and one half billion dollars in annual sales, Diesel Kiki brings to the Torsen business the necessary resources to achieve world class status for this technology.

DK Gleason Inc. has been formed for the purpose of further developing and promoting the use of Torsen differentials for the world automotive markets and has sales offices in by virtually every major world automotive manufacturer and has been accepted for production in the complete line of Audi Quattros (center and/or rear differential), the Lancia Delta HF 4WD Evoluzione (rear differential), the Peugeot 505 and 405 Mil6x4 (rear differential), the Citroen BX 4WD (rear differential), the IATO 4WD (rear differential) and the US Army's High Mobility Multi- purpose Wheeled Vehicle (front and rear differential).

[note WJ: since this text was written (probably not later than 1988!), it can at least be added: the Swiss Army's Bucho 4x4 and 6x6 (rear, center and front differential), the Toyota Mega Cruiser (front and rear differential, both combined *with* differential locks!) and the Toyota RAV4/FunCruiser (rear differential).]

Manufacturer addresses:

ARB Corporation Ltd.
Maroondah Highway
Victoria 3136
Voice: +61 3 726-7166
  Fax: +61 3 726-4053

ARB Air Locker Inc.
20 S. Spokane st.
Seattle WA 98134

Auburn Gear Incorporated
400 Auburn Drive, Auburn, IN 46706
(219) 925-3200

DK Gleason Inc.
Two Jetview Drive
P.O. Box 23246
14692 New York
Voice: +1 716 464-5000
  Fax: +1 716 328-5477

Detroit, Michigan:
Voice: +1 313 524-1562
  Fax: +1 313 524-4914

Stuttgart, Germany:
Voice: +49 711 79 70 98
  Fax: +49 711 79 11 48

Coventry, UK:
Voice: +44 20 369-4684
  Fax: +44 20 369-4255

Milano, Italy:
Voice: +39 245 26443
  Fax: +39 245 21519

Saitama, Japan:
Voice: +81 485 36-1121
  Fax: +81 485 36-6686

GKN Viscodrive GmbH
Auelsweg 29
P.O. Box 1152
D-5204 Lohmar 1
Voice: +49 2241 30 15 34
  Fax: +49 2241 30 14 07

A Titan Wheel Company
11445 Stephens Drive
P.O. Box 882
48090 Michigan
Voice: +1 810 759-3850
  Fax: +1 810 759-1645

The differential case is a Dana (AMC) 20, hypoid (top entry), same as in some older Jeeps (AFAIR). The differential has a ratio of 2.73:1. Hummers use Zexel-Gleason Torsen torque sensing/biasing differentials. These are not a limited slip design or locking design. The torsen diffs have a 3.6:1 torque bias. That means they split the power 50/50 left and right until one wheel exceeds the 3.6:1 ratio. Then the power just follws the path of least resistance, and the free wheel spins. By appling the inboard mounted brakes (would it make a difference if they were outboard?), you trick the diff into thinking it's inside the 3.6:1 ratio, it then splits the power 50/50 left and right, and away you go.

I learned to today that even the detroit (NO SPIN) locker and most others still slips and may give only 15-50% locking.

Some tractors ect have hydraulic activation of the locking mechanism.

Need more info still, such as which tractors, trucks and which models have purely mechanical Activation of the R and L output shafts.

Step one is the locking mechanism. Step 2 is adapting the mechanism to fit the DANA (AMC) 20 subcarriage.

Sure, my tractor has this pedal. If you have one rear spinning, you can lock them together by standing on the pedal. I also have split brakes, a separate pedal for left and right that are locked/unlocked with a lever. I don't think this tractor locker works well enough for a Hummer though. You have to be careful and getting it unlocked sometimes does not happen if I get on hard pavement and the rear is wadded up. I can go awhile and pop it unlocks and the lever wacks the back of my leg. John

Gerald Luiz wrote:
> By large trucks you mean dual rear axle?  The locking
> diff on those is the interdifferential does not lock the left to
> right diffs AFAIK.  Anyway, I don't know of any manual lockers beyond
> the ARB that is widely available for our diffs.  Gerald

Yes some of the larger dual axle trucks used a mechanical device to lock
the two rear axles together, but there are some that used a cable to
actually lock the diff, just like some of the other mechanical locks we
have been talking about.  I think that this is something we should all
think about, any heavy truck-tractor mechanics out there with some
manuals we could look at?  Any body who has taken one of these apart?  I
know they exist, it is just finding somebody with some knowledge in this
area...anything.  Mark

Old landcruisers used a cable setup to lock the diffs.  It is an open
diff that gets locked.  The newer ones use a solenoid to activate the
device instead.  By large trucks you mean dual rear axle?  The locking
diff on those is the interdifferential does not lock the left to
right diffs AFAIK.   Gerald

I took a look at the 97 and 86 Landcruiser diff lock schematics.  They use
a locking collar and a shfting fork to move the collar.  This appears 
slightly external to the diff on the straight axle.

The John Deere uses a similar method but the fork and collars are internal.

John Deere models:655,755,855,650,750,850,950,1050,1250,1450,1650

New Holland: 4635,4835,5635,6635,7635,L60,L65,L75,L85,L95

I called a few places,  said that there is not enough space in the
Dana diff  to put a locking cam or collars as the big tractors that
have huge diffs.  The carriers also will not be simple to set up.

        I was reading my brothers Rover Magazine yesterday and came upon an
add for Great Basin Rovers in South Salt Lake City, Utah.  Their phone
number is (801) 486-5049.  They say they are differential specialists.  
They sell KAM Diffs from the UK and Jack McNamara diffs from Australia. 
They both claim to be full hypoid-vacuum Locking differentials.  They say 
their much stronger than standard rover diffs and come in two different axle
diameters and 9 gear ratios.  Vacuum actuated, no air compressor.  I have
no idea how they work but I hear that they are really good.  I doubt they 
will ever make HUMMER diffs, but it might give someone here an idea of how to 
go about making some.  Instead of the tractor diffs.  Tim Stinson

The M-B Gelendwagon ("G-Wagon") came from the factory with manual lockers.  
Also, Spector Off Road offers "cable" lockers for older Toyota Land Cruiser 
diffs.  Alan

I have a good size Russian (actually Belarussian) tractor which is built
for rugged use and for  field maintainance.  The 4WD and Dif Locks are
strictly mechanical.    The 4WD has three positions:  Off, Part-time, and 
Full-time.  When in the "Part-time"  position, it will disengage when the 
turn exceeds 10 degrees.  Pete.

> What is a TruTrac? How is it different from a Detroit. I have
> noticed that people put a TruTrac in the front and Detroit in the
> rear, why?

The TruTrac is a limited slip differential, meaning that it doesn't begin to act as a locker until wheel slip is detected. Even when it IS acting as a locker it does NOT fully lock. If you lift a wheel in the air, the TruTrac will not send power to the opposite wheel. It is this inability to lock fully and the ability to differentiate between turning a corner and slipping in mud or ice makes it ideal for the front wheels.

The Detroit locker is a TRUE locker. It WILL lock the axles together as though they were welded. This means if you lift a wheel, the other WILL drive the car. A true automatically locker should NOT be used in the front. It sets up very dangerous steering anomalies. If you want to get a full locker in the front, use something like an ARB or KAM that can be locked and unlocked "on-demand".

If you'd like more information check out They make the Detroit lockers and the TruTrac.


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