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This FAQ concerns the Hummer differentials, introducing other differential technologies only for comparison purposes. The intent is not to say which is best, only to illustrate uses.


A differential is a device to allow two things to spin at different rates while being driven by a single source. Take just one axle on a car or truck as an example. Each wheel is connected to a halfshaft (HS). As you know, as you turn, the outside wheel must turn faster than the inside wheel. Hence, the outside HS must turn faster than the inside. To handle this, the driveshaft supplies torque to the HS via a differential. The diff allows the torque to be split between the two HS, but allows them to turn at different rates if needed.

A differential also provides a gear reduction from the HS rotational speed and the driveshaft rotational speed.

How a differential behaves is what separates all the diffs. In the following discussion, we will not go into the exact workings of the devices. Instead we will explain the net result, or effective workings, of the devices.

Differential Types:

I find it convenient to think of torque division as the distinguishing feature among differentials. That is, how much torque enters the diff and how much goes to each HS. The goal is to direct the input torque to each HS (wheel) based on the available traction at each wheel.

  1. Open differential:

    This is the most common type of differential used in most cars and trucks. Each HS is not directly connected to the other. Instead, a set of side gears (bevel gears) allows them to spin independently of each other. The driveshaft from the engine turns these side gears.

    The problem with an open diff is that it cannot unequally divide torque to one HS over another. That is, each HS ends up with just about the same amount of torque going to it. We can say that the torque bias ratio between the HS is 1:1.

    When one wheel has very little traction, not much torque can be given to it. It just spins. The other wheel (with traction) sees the same amount of torque (1:1 ratio), so we end up with very little torque going to the ground to get us moving. Note however that there can be a large rotational speed difference between the two HS.

  2. Limited slip differential:

    As mentioned above, the open diff allows a large rotational speed difference between each wheel. When two things rotate at different speeds when they really should be rotating at the same speed, we say that we have a slip condition. Note that we need some slip in order to turn w/o dragging/ overspinning a wheel.

    A limited slip (LS) diff adds some coupling to between the two HS. Inside the diff, the two HS are connected together via some friction device (say like a clutch disc) which tries to keep the two HS turning at the same rate. In other words, there is some resistance to slip between the HS. When both are turning at the same rate, the friction device does not absorb any energy, i.e. it does nothing.

    When turning on a high traction surface, the traction between the wheels and the ground forces one HS to turn faster/slower. The friction device is then overcome, allowing some slip between the two HS.

    When one wheel has low traction, it can accept very little torque before spinning, say X lbs-ft. The (open) diff directs equal torque to each HS. When we reach X lbs-ft to each HS, a non-LS open diff would start to allow the low traction side to spin, limiting the torque input to 2*X. In the LS diff, the friction device must be overcome before the wheel can overspin. Let's say it takes Y lbs-ft. So, at the verge of spinning, we have X+Y lbs-ft going to each side. Hence, the high traction wheel gets a little more torque.

    The amount of torque needed to overcome the friction device is called the preload. As the preload increases, the diff becomes "stiffer". If the preload is high enough, we can start to skid tires during turns.

    LS units actually work a little more complicated than this. However, for OR driving, this is probably the most interesting case (X lbs-ft is very low). Under higher torque inputs, they actually work a little better but is less common for OR.

    The result is that LS units allow some biasing of torque, but not very much for OR driving situations. For street use, they can be very effective b/c pavement allows for higher torque inputs.

  3. Locking differentials:

    A locking differential locks the two HS together to elimintate single wheel spin. The two wheels combine to form a single axle. Input torque goes to the entire axle, trasmitted via a rigid member. Hence, torque can go to either side in any amount. In our example, the low traction wheel cannot turn faster than the other. Hence, our high traction wheel gets all of the input torque.

    When locked, both wheels must turn at the same rate, precluding turning on high traction surfaces. If a front diff is locked, turning is just about impossible even in OR situations.

    Lockers are divided into two categories based on how they lock up.

    1. Manual locking

      The user controls when the diff should be locked up. The most common manufacturer is ARB. When the locker is activated from the cab, a compressor directs air pressure to the appropriate differential(s), causing the diff to lock up.

      Manual lockers allow the driver to determine when the extra traction is needed. When turned off, the diff behaves just like an open diff. There are no artifacts left during street driving. Of course, this is also a disadvantage b/c the driver must anticipate when to engage the lockers, i.e. not automatic.

    2. Automatic locking

      Detroit Lockers and Lock-Right are two examples of auto lockers. The basic principle is that they unlock, allowing a wheel to overspeed to make a turn. The important principle is that the ground can cause a wheel to overspeed, engine power cannot. No HS can turn slower than the input driveshaft (taking into account gearing of course). So when turning, the outside wheel is forced to turn faster, overspeeding the inside HS and driveshaft. This is ok and torque goes to the inside wheel only. When a wheel hits a low traction surface, the torque going to it causes the HS and wheel to spin. The driveshaft speeds up also. However, the other HS is not turning any faster yet. The device locks up, connecting the two HS together, preventing the wheelspin.

      The engagement of the locker can be quite sudden and abrubt. Subtle torque changes can cause the locker to actuate. For OR driving, this is usually not a problem. For steet driving, this can be dangerous. Hence, many people do not recommend auto lockers for steet use, especially not on the front axle.

      Note that lockers are based on HS speeds and not on traction availability. For example, take turing with equal traction on both wheels. An auto locker directs torque to the inside only and an ARB drags/overspeeds a wheel (or is an open diff). While they do allow the most torque to the ground in extreme situations (rock crawling, etc), they do have disadvantages.

  4. Torque sensing differentials:

    Torque sensing diffs direct torque to each HS based on available traction. In other words, there is some feedback from the wheels to the differential to guide the torque distribution. The Hummer uses a Torsen, made by Zexel-Gleason. The Torsen name comes from TORque SENsing. The Torsen is not the only torque sensing diff around.

    The Torsen design is based on worm gears, rotating on different axes. It is extremely difficult to describe w/o a picture. Each side gear is a worm gear splined to the HS. There are 3 sets of planetary worm gears (called element gears), perpendicular to the side gear axis. Each set consists of two worm gears, connected via spur gears, and mated with each side gear. Thus, the two side gears are interconnected via the element worm gears.

    As traction at each wheel changes, the pressure between the element worm and side gears change, causing the element pairs to counter-rotate, biasing the torque to the other side. Unlike the other desings, torque sensors (TS) work in just about all conditions. Even if the wheels are turning at different rates (turing, going over bumps, etc), they still shuffle the torque around based on traction. LS units and lockers try to maintain rotational speed which is not the same thing.

    Going back to the torque bias approach, Torsens can be designed have a ratio of 2:1 to 6:1. I think the Hummer's are around 4:1. Hence, we can get 4x the amount of torque to the traction wheel ALWAYS. Lockers are beneficial in a very narrow window of operation. Torsens are also very smooth in operation, quite suitable (if not the best) for street/ foul weather driving.

    The limitation is that they are not lockers. If one wheel is in the air it takes very little torque to spin. 4x of very little is still not enough to get moving. Hence, you can get stuck where a locker would keep you going. To overcome this limitation, we can apply a technique similar to a LS unit. By applying the brakes, we increase the torque input to the low traction side from say 0 to X lbs-ft. The other side now sees 4*X lbs-ft. by applying enough throttle and brake, we can get going always. This is how we can "lock" the differential.

    For Hummers, note that this 4x torque is HS torque, before the hubs. since the hubs are gear reducing about 2:1, we actually get about 8x torque to the wheel. The brake/throttle works for just about any TS unit, however, it works extremely well for Hummers.

    When we do get wheel spin, you can feel a wiggling sensation. This is the signal that you have exceeded the traction at a wheel and the Torsen cannot bias enough power to the other wheel. The wiggling comes from the shuffling of torque from one side to the other as the worm gears bind up and then release. This can be detrimental to the life of the diffs and hence, you should limit wheel spin speed. Time to modulate to "lock" the diffs.

    Of course, the brake throttle technique is not as effective at a speed b/c we are creating a lot of drag. Lockers excel at crawling (like Torsens) but are also good for mud, sand hill climbs, etc. Torsens also have a lot of internal friction (inherent to the design) causing drag.

    Note that Torsens are not limited slip differentials. They are torque sensors. They are totally different beasts. LS units try to limit slip between HS in an effort to improve power to the ground. Torsens bias torque between HS to improve power to the ground. In almost all situations, torque sensors put the most power to the ground compared to any other design.


As everybody knows, there is not a single best differential for everybody. It really depends on the type of use. If you only drive on street, opens are the best. No drag, no strange behavior, simple. If you are a heavy footed OR driver (required for mud), lockers are the best b/c you lose no power and is passive (after activiation for a manual locker). If you drive varying conditions, slick roads, OR trail driving, even crawling, Torsens are a good choice. For what the Hummer was designed to do, the Torsen is a great unit. Thanks Gerald.


HS Halfshafts. Connects the differential side gears to the wheel hubs, transmitting the engine torque.
LS Limited slip differential.
OR Off-road.
TS Torque sensor or torque sensing differential.

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 bias ratio is around 4:1.

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.

If anyone has some schematics or can take some digital pics of these diff assemblies, maybe they can be sent to Bob or me.

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 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 2155,2355,4045T,6068T

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

There are 4 basic types of differentials to discuss:

  1. Convential or Open - This allows good differential at the cost of traction. The torque is supplied to the side with the least resistance or traction.

  2. Limited Slip - Torque is supplied to both axles through clutches or friction cones which allow some slippage. The limited slip will act like a locker until enough torque is applied to break the grip of the clutch or cone. A fresh limited slip in the front will cause steering loss in many situations. Since the HUMMER is a full time 4WD, the clutches or cones will wear constantly on hard surfaces, reducing the useful like substantially.

  3. Locking Differential or Lockers - The purest form of a locker is called a "spool" where both axle sides are driven at the same RPM. The ARB Air Locker works like a spool except you can decide when you want it "spooled" and when you want it "open". The Detroit Locker and Lockright (there are probably others) allow some differential but will not allow one wheel to turn slower than the other. All locker schemes wiil drive both sides of the axles equally.

  4. Zexel Torsen - This differential is an interesting cross between an open and locked differential. When both tires have approximately equal traction (within 3.6:1), you will have a differential that can distribute torque to both tires equally or unequally. To use a simplified example of 3:1 ratio, if you input 400 lbs of torque, the torque can be split 100 lbs to one side and 300 lbs to the other with no wheel spin. Beyond this point, brake/throttle modulation will retain that ratio. The resistance of the brakes will simulate traction on both sides, but the cost is torque and horsepower on the ground.

Life is a compromise. The Zexel Torsen is a good compromise for most situations and for most HUMMER drivers. There have been times that I knew a certain obstacle would be easier with a mechanical or Air Locker, but I can't think of an obstacle that stopped me because I didn't have it.

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