I did not write this, but the information is very useful.

Differential Knowledge, Opinions, and Ideas
By: Dominic Orlando

Last revised 7/23/2002


Foreword:
This guide is to help you, the reader, decide on what form of a differential you will want based upon your intended automotive application. I will try my best to cover the various types of differentials in an easy to understand, well thought out manner. The reason I am writing this guide is due to the mass confusion and serious lack of information about the current types of rear-ends that are available. I was looking to upgrade the differential from an open one to a much better "limited slip" one sometime ago, and through my course of reading I found many things that were interesting along the way - such as the fact that there is more than one type of "limited slip," but I will cover that in the following paragraphs and diagrams. All technical resources can be found at the bottom of this page for your reading enjoyment, since they are what taught me most of what I know about differentials today.


"Open" Standard Differential Technology

Pros:
Cheap to build
Lasts nearly forever
Powers the outside wheel in a corner
Cheap to repair/replace

Cons:
Powers the slipperiest wheel at all times
Powers one wheel at a time in corners
Powers one wheel at a time in the slick stuff
Lowers the cornering ability limit

Positraction (Clutch/Cone-Type) Differential Technology

Pros:
Powers the outside wheel
+20% torque transfer to the wheel with traction
Old and Proven technology that works
Can handle more torque than any other differential
Internal discs are cheaply and easily replaced
Will not "break" or "shatter" on launch

Cons:
Lasts 40,000 miles, then needs servicing
Requires a friction modifier to work properly
Still allows a slipping wheel to "spin-up"
Grip is not instantaneous, requires wheel spin
Internal Discs/Cones must be replaced


Zexel Torsen (Worm Gear Driven) Differential Technology

Pros:
Never needs replacing once installed
No need for a limited-slip additive
Instantly transfers torque, no wheel spin
Is the latest technology in differentials

Cons:
Can be fragile; 500ft/lb torque limit on OEM units
Generally more expensive than other differentials
Expensive Upfront cost
Not available for all applications as of 2002

Powertrax No-Slip Differential Technology

Pros:
Can handle insane amounts of torque
Inexpensive and easy to install
Better than an open differential for rock climbing
Never needs replacing once installed
Spins wheels in sync regardless of terrain
Doesn't require additives

Cons:
Causes severe understeer in cornering
Wheelspin in adverse conditions is common
Doesn't limit the wheel slip
Torque steer in adverse conditions is common
Has the ability to break axles under launch


The Open Differential - Its faults, and its blessings

I would first like to briefly discuss the Open Differential with you, which is the most simplistic form of a differential available. Essentially a differential changes the direction of the engine's mechanical rotating force, which is known as torque. Also, a differential allows one wheel to turn faster than another in a cornering situation, and usually applies power to the outside most wheel - aiding in the travel around and through a corner. As the drive shaft spins, it turns some simple bevel gears inside the differential, which then turns an axle attached to the most convenient wheel. When I say most convenient, I mean it, because whatever wheel happens to be easiest to turn is the one it will turn! This is the great downfall of the open differential, in personified terms it is a lazy bum in the back of your car that only does the easiest job - which is turning the most convenient wheel - which might not necessarily be the one that needs to be turning.

As an example of how useless an open differential can be, say you're driving along and find a diner, and this diner has a clay parking lot, but currently everything is dry and you're hungry for some nice greasy diner food. You go inside, have a cup of coffee, eat your two day old pie, and as you go to walk back outside you notice it has started raining. The dirt you parked on is now mud which has become slick as grease. Before hopping in the car you notice the rear left wheel is on some nice grippy gravel, and the rear right wheel is sitting in a giant mud puddle. You figure, "Hey, one of my wheels is on gravel, I should be able to leave here fine right?". You crank the car, and as you ease onto the gas pedal the wheel in the mud sits there and spins mud all over your once clean car, getting you absolutely nowhere! You might try going in reverse, but this also gets you nowhere... This is where the term "one wheel drive" was coined. In drag racing situations, during the burnout, only one wheel will spin as well - during launch the maximum grip you will have will be governed to one wheel's torque limit as well.

Another interesting aspect of the open rear is that during a turn the outside wheel pushes the car, since it is moving faster it is easier to spin, and therefore it will help you right around the corner. Of course, driving straight down the road the open differential feels exactly like any other differential since it evenly applies 50% of the power to each wheel. However, in a slippery situation this same wheel can lose traction very easily when over-throttled, causing you to lose control of your vehicle, and possibly putting you into a guard rail. Nevertheless, it still spins the outside wheel, which is very important as you will see in the coming paragraphs...




Positraction - what on earth is positraction?
Ahh yes Positraction, most commonly referred to as the Slip-Plate Style Limited Slip Differential by engineers of Eaton, who actually designed the thing in the mid 20th century for muscle cars of the era. This is essentially an open differential, with a set of clutch plates hooked to some springs. These springs have a set amount of force which they can apply to the clutch-plates, helping to transfer torque. Take the mud puddle story above for example, with an Auburn or Eaton Slip-Plate style differential, as one wheel slips and spins up the differential now will start pressing together two Kevlar or steel plates together, creating friction, and hopefully moving the car right out of the hole - sounds good doesn't it? Its a very simple device, but it too has its downfalls. When one wheel slips, and it must slip for this to work, the springs slap the plates together and begin to transfer torque to the other side of the car... Depending on the differential this could be anywhere from 5% to 20% of your total torque capacity - which is luckily enough to get you out of that mud puddle, but will still get a heck of a lot of mud on your car as that other wheel keeps spinning up!

Also, while you're spinning your tire, and slowly moving the other one with a mere 20% of your torque capacity, you'll be grinding down the steel or Kevlar plates inside the differential. Usually the lifespan of an authentic positraction differential, 60+ year old technology, is around 40,000 miles; if you drag race, you'll be lucky to see 20,000 miles out of this type of limited slip. As soon as these clutches wear out, you're back at square one with a simple open differential.

The other problem with these differentials, is that for them to actually last 40,000 miles you must put a limited slip additive in the differential, which is essentially a simple friction modifier, which allows the clutches to slip past each other without "chattering." Also, this additive can sometimes be harmful to the lifespan of the ring and pinion drive gears.

Note: The only difference between an Eaton positraction unit and an Aurburn positraction unit is the style of the clutches. While Eaton makes use of several flat discs, Auburn uses cone shaped clutches instead. The positraction principle is the same though, and so both can be just as effective - although many prefer the newer Auburn units.




The Zexel Torsen Limited Slip Differential
Boy that's a mouthful isn't it? Say it with me, "ZEXEL TORSEN LIMITED SLIP DIFFERENTIAL". Long words, and it sounds really cool, but what is it? Well Torsen stands for "Torque Sensing" and Zexel is the type of gears used in the differential (syncronous worm gears to be specific). The Torsen differential is a miraculous improvement over the standard positraction unit since it essentially does the same thing, only better, and it is entirely gear based! No clutch packs, no springs, no limited slip additive, and no wearing out in 40,000 miles. The Torsen differential's primary downfall is the fact can be fragile; OEM units often won't hold up to cars producing 500ftlbs of torque or more - but thats okay since you can buy far stronger ones for dragrace/roadrace applications! The Torsen company is now owned by Bosch, and was founded in 1984.

Here's the cool scoop on the Torsen differential, it is used in every production Hummer made, including all military applications! Much less the fact most Firebirds, Camaros, Corvettes, Miatas, IS300s, and some General Motors trucks come with them OEM. When I first discovered the Zexel Torsen, I was really enchanted by how it worked, but I didn't understand how much better it was than a standard posi until recently.

Lets return to our car in the mud incident, sitting with our worn out limited slip from Auburn we churn away at the mud and sling it all over our car because our clutch plates are worn out and now its acting just like an open differential. Lets backup a second, install the Torsen, and apply the throttle a bit to see what happens!

The wheel in the gravel should turn immediately - requiring no wheel-spin of anykind from the mud stuck wheel. Both wheels will spin simultaneously and the torque will be transferred to the wheel with grip, pushing us cleanly out of the mudhole without spinning the muddy wheel hardly any - just rolling it out cleanly.

However, had this been a situation on ice where there would've been nearly 0% friction, the wheels would've spun like an open until some form of friction was applied. A common hummer trick in these situations is to lightly apply the brakes, which then allows both wheels to work as needed to get the vehicle out of the sticky situation.




The Road Course -- Zexel Torsen vs. Positraction vs. Open Differential
The open differential on a road course is not a good thing, and its an even worse thing in lowspeed cornering situations. Often when rounding a right-hand corner in a RWD car, the inner wheel will spin and the outer wheel will just roll - this causes a lack of traction exiting a corner and will slow you down considerably. However, making a left-hand turn this slip won't occur and everything will work perfectly. Most open differentials are wheel-biased, and this is determined by which way driveshaft spins. On most vehicles this is the right rear, but occasionally its the left. I guess this is why Nascar never turns right

Both the Positraction clutch-type and Torsen type differentials solve this problem by diverting as much power as possible to the outside wheel regardless of the turning situation. The more torque-bias a differential has, the better it will be able to do this. Many road racers in firebirds and camaros adore the Torsen T2R for its high bias, eliminating any chance of inner wheel spin during a corner - however this differential is not cheap, usually $500-850.




Powertrax - The ultimate differential, or is it?
The last type, and most oddball of the types of differentials I would like to cover is the No-Slip Traction system made by a company known as Powertrax. The No-Slip is commonly referred to as a locker, or locking differential, only it doesn't make a *clack, clack, clack, clack* noise when you go around corners. This company advertises their differentials as "The ultimate in differentials," boasting them over and above the likes of any open or limited slip differential, and making it seem as though everyone should own one. After reading their site, I found everything they said to be true, even in the technical article they said everything that should be said, but I noticed one minor flaw in their marketing. THEY NEVER MENTIONED CORNERING!!

Here is the good, the bad, and the pure ugly of the Powertrax unit. When you're going straight ahead, it locks the rear axles together creating a perfectly solid axle - like a spool. The axles are in everyway locked up, and they will not lose traction no matter what. Both wheels will spin regardless of what is underneath them, and in that simplistic notion it is the ultimate in traction.

The problem occurs when they claim their system is better than a limited slip. As I said before, the Torsen unit never wears out, its gear driven, and allows power transfer to occur from one, to both, and to the other wheel seamlessly and efficiently. When one wheel slips, it doesn't spin it, thereby conserving your energy and keeping your car going in the direction you intended it to for the most part.

Say you're driving along, and the Powertrax unit is locked up as usual, you're going straight ahead, and you hit a puddle with your right rear wheel. The puddle causes you to hydroplane, and dand your car darts off to the right because your differential kept applying power to both wheels evenly. You might possibly hit another car, or a nice cement wall, neither of which are comfortable thoughts in my mind.

Well, how would a limited slip have fixed this problem, you ask? A limited slip would have immediately allowed the free-spinning wheel in the puddle to coast through, instantly transferring power to the wheel with grip keeping you going mostly straight ahead and probably not affecting your direction at all. In this scenario, the limited slip (either kind) would be a lot better than a locked differential.

Okay, but what about turning? Well, remember how I said earlier that the best differential on the planet powers the outside wheel, allowing for zero inner wheel spin, which allows for more control in the corners? Guess what, the Powertrax works completely backwards! During a corner, when the differential differentiates (separates and unlocks) it sends all the power to the inside wheel trying to force you to go straight ahead! This means when you press the throttle your car will go into an understeer condition, something that normally never occurs in a high powered high performance rear wheel drive vehicle.

A brief lesson in understeer and oversteer:

Understeer = When you go to turn your car, and the front end slips around the corner, effectively making your steering "less" than what you intended... hence the term understeer.

Oversteer = When you go to turn your car, and the rear end slips around the corner while the front end maintains grip, effectively making your steering "more" than what you intended... hence the term oversteer.

Most racecar drivers prefer oversteer, since it is easily controllable with the throttle, and even though it is a slide it is very easy to pull out of by counter-steering. Understeer on the other hand is very bad, since you essentially instruct your car to turn and it doesn't . This is definitely not a good thing, since a car is supposed to go where you tell it right? The only way to reduce understeer is to decelerate and pray, which is not good for a race car, and is not expected from a high performance vehicle.

So the locking differential really bites in cornering, and during bad weather it can cause you to go sliding around all over the place... What in the hell was it invented for then?! Well, see, there's this little issue with limited slips that the locking differential and only the locking differential can get around. Pretty much, if you don't have any traction to push on a limited slip and let it know which wheel needs traction, no wheel gets traction, and you're back where you started with an open differential. The easiest way out of this condition is to just lightly apply the brakes, and is recommended in all off-road vehicles like the GM Hummer equipped with a Torsen style differential.

A locking differential, once locked, remains locked so long the vehicle drives straight ahead, and so if one wheel comes off the ground both wheels spin in sync and you continue to climb the hill with ease. About the only application for something like this I have found is a Jeep which is equipped to do rock crawling - often encountering areas where one wheel might come off the ground.

Okay, so after all that, what is the best differential? Well that depends on the application...

I personally recommend lockers only for serious off-roading where a locking differential is the only thing that can get you through the slick stuff.

I recommend limited slips for all on road applications as well as light duty off road. Torsen is better than anything else unless you've got a car that can only accept an Eaton posi in the housing, or is capable of shattering the internal gears of a torsen.

Open differentials are almost worthless in extreme situations, but they are better than nothing and will get you down the highway just fine 80-90% of the time. If you have an open there is a limited slip replacement for your vehicle somewhere, and if you're having problems cornering or in slippery conditions I recommend you invest in one or buy a vehicle with one. For more information - see the links below! (http://www.angelfire.com/my/fastcar/diffey.html)