Part1

As you might imagine, a very commonly asked question around these parts is: how does a Torsen® differential work?  While this might seem like something that ought to be in the FAQ (which touches on it), there really isn’t enough space there to delve too deep into the topic.  But I did say this was going to be the “simple” version, right?  That’s true, but mostly because I’m not going to get into the math behind it.  But stay with me – it’ll make sense.

The Basics

You may have assumed that understanding a helical gear differential is inherently difficult.  Fundamentally, however, the basic principle is really quite simple.  Many of the people that I’ve spoken with are so convinced that it is above their head, that they blind themselves to the simplicity.  The important thing to understand is this: it is a friction device.  There is a lot of stuff on the internet talking about worm wheels and back driving, etc.  You should ignore all of that.  In actual practice, it does not come into play.  Instead, friction is what is important.

For any type of limited slip differential (LSD), the basic intent is to generate internal friction.  This is true whether it uses a helical gear design, clutch plates, even a g-rotor pump. It is friction within the differential that limits wheel spin.  It does the actual limiting that the term “Limited Slip” refers to.  Friction is also why the differential can allow more torque to transfer to the tire with better traction.  By providing spin resistance to one tire, the other tire can receive more drive torque than it otherwise could.  Thus, more torque goes to the higher traction tire.  It’s just friction – that’s all.

The Challenge

This does lead to an interesting situation though.  It means that we’re trying to create friction – on purpose – while we’re trying to reduce it from every other part of the driveline.  An LSD is typically the only oil-lubricated part of a car that is intentionally trying to create friction.  Keep in mind that the differential exists in a system with other components, such as the ring & pinion gearing and various axle bearings.  These other parts require proper lubrication to survive.  As a result, having suitable lubrication that reduces friction for those parts is important.  So, choosing the correct lubricant for the system is important.

This adds to the challenge of generating enough friction within the differential to do what it needs to do.  Recently, vehicle manufacturers started developing even lower-friction lubricants, making it especially challenging.  That means that these sorts of lubricants will further reduce differential limited slip performance, sometimes referred to as “locking effect”.  Differential designers have to make up the difference in other ways.  In a Torsen, that usually comes from adjusting gear geometry in the design phase.  Sometimes, adjusting the thrust washers to suit helps.  But, at the end of the day, all the designer is trying to do is create and manage friction in a useful way.  That’s pretty basic, isn’t it?

Part2

So, if all limited slip differentials work by creating friction, what makes them different?  That’s a good question, and is the key to what sets Torsen® apart.  Yes, as we said before, all LSDs generate friction.  But not all do so in the same manner or to the same extent.

A typical, basic, clutch style LSD uses a set of clutch plates between the side gears and differential case to make friction.   Normally, the load on the clutches comes from a preloading spring, placed between the gears.  This type of product is fairly easy to build and is inexpensive. However, in most cases the amount of friction they create is fairly low.  Also, the amount of friction is constant, changing very little over the range of conditions the vehicle operates in.

That means at high and low speed, straight or turning, the clutch plate differential always behaves the same – for better or for worse.  This fixed friction amount – which helps reduce tire slip when accelerating – hinders turning the car into a tight parking space.  The chassis has to overcome the binding this friction causes, making maneuvering the car tougher.  In poor weather or slippery conditions, this can be especially true.

• The Torsen Advantage

On the other hand, a helical gear limited slip differential – like Torsen® – does not rely on clutch plates*.  Instead, the helical gears themselves generate friction.  When a helical gear receives a torque load, that torque creates thrust forces that want to push the gear in an axial direction.  When you mesh several of these gears together with a torque load on them, it creates mesh separation forces which try to push them apart, away from each.  If you then contain all those gears inside a fixed, rigid casing (to prevent them from sliding or pushing apart) the result is friction between those gears and casing on the surfaces where they meet.  As more torque applied, the greater the forces become, resulting in a greater the amount of friction.

*(It is true that older versions of the T-2R product used a multiplate arrangement to enhance TBR performance.  They were not, however, typical clutches.  These models used hardened steel plates that provided more surface area for spreading the axial thrust forces over.  These plates have no friction lining, like a typical clutch plate has.)

There is a certain elegance to this; friction is generated proportionally to the magnitude of load applied.  Under high load – when accelerating out of a corner on a race track or on a highway entrance ramp – the differential offers a lot wheel spin resistance.  This allows the driver to put a lot of torque to the tires and really get the car moving.  Additionally, under very high load situations like this, the Torsen® will actually maximize the traction of the inside tire and put all extra torque to the outside tire.

This not only provides optimal traction, but also helps drive the car around the curve by inducing an understeer-canceling, positive yaw moment about the car.  You’ll actually be able to feel the Torsen help propel the car through the turn.  But, under low load – pulling into your driveway or into a parking spot on an icy lot – the differential offers little resistance and will differentiate freely.  This allows the car to maneuver easily, making for better balance behavior characteristics and a car that is more driver-friendly.  On the other hand, clutch-type LSDs can become difficult to maneuver in this situation.  These benefits allow the Torsen to be as much of handling aide as it is a traction aide.

• The New T-2R

The development of the Equvex II gear design helps set Torsen® even further apart, featured in second generation T-2R models.  Equvex II employs a technology we refer to as Split Gear.  A true helical gear design, this allows the differential’s locking characteristics to be easily reconfigured.  Previously, once a helical gear differential was designed, its properties were fixed.  Aside from playing with lubricant to affect change in the locking performance, there was nothing you could to change it if requirements changed.  You had to start over, from the gearing up because once the basic geometry changed, the casing had change as well.  If you had significant investment in tooling to produce the product, that was too bad, because you’d have to retool it all over again.

Enter the Equvex II & Split Gear.  With Split Gear technology, the side gears are comprised of two concentric pieces. A spline joins these pieces.  This allows more efficient use of the differential’s friction surfaces.  An existing design can be re-tuned without any significant retooling, just by changing the helix angle on that spline.  This allows us to offer multiple levels of performance of a common product family.  This allows costs to be spread out, since the expensive tooling is shared.  This gear design, frankly, offers an unparalleled level of design flexibility that no one else in the industry can match.  And we offer it to aftermarket customers as well as OEMs.

Equvex II gears with Split Gear Technology

All new and upcoming T-2R models will make use of Equvex II gearing.  That allows us to tailor the TBR to the needs of that specific application, instead of applying a one-size-fits-all method of enhancing performance.  Existing designs will retain the previous multiplate arrangement.  When stock is depleted for these older models, a revised design will be introduced.  And although Split Gear is just arriving on the aftermarket scene, it has become proven technology.  When it was introduced for 2012, the Ford Mustang Boss 302 was the first to use this design .  GM also adopted the Split Gear design for the late mode Camaro Z/28.  Today, the Mustang GT with Performance Pack option continues to employ a Torsen T-2R with Equvex II gearing.

An extremely common question that we hear, here at the Torsen® Cosmic Information Center, is “how much power does this handle?” To which, I like to answer: how much do you got? While a little glib, it gets to the truth of the matter, which is that we don’t really care – from the differential’s perspective – how much horsepower you have. That’s not to say that I wouldn’t find it extremely cool to know you’re making 800HP with a roots-type blower on your Camaro or that you’ve put long-tube headers and Ford Racing cams on your Boss 302 (because that’s awesome if you have!). But what the differential – and the entire drivetrain – really cares about is torque, and frankly, your “at the wheels” torque numbers from your latest dyno pull is pretty meaningless as far as that is concerned. At the same time, most people look at me like I have a third eye when I tell them how much torque our differentials are actually rated for.

Let me explain. Horsepower is really a rating of how much work can be done over a period time. In practical automotive terms, unless you’re concerned with setting a record at Bonneville, it’s a bit nebulous – it’s really just bragging rights. Conversely, torque is a measure of the engine ability to actually generate rotational force – what turns the shafts, which turn the gears, which turn the wheels. You can argue that torque and horsepower are mathematically related, and you would not be wrong. However, torque is what actually accelerates and drives the vehicle, whereas horsepower is what pulls it to its top speed. So, torque is what actually loads the drivetrain, and what can actually “shock” the differential in a severe situation. Consequently, torque capacity is a primary design criterion for developing a differential rather than horsepower.

OK, so what does that mean to me, you ask? How much torque can it handle? Now you’re getting somewhere. It is important to understand that torque is multiplicative with gearing – the amount of torque delivered to the wheels during your dyno pull when in 4th gear is not the same amount that will be delivered in 1st (or any other) gear. And that’s what you have to keep in mind. The axle, and entire drivetrain, is sized so it can handle a worst-case scenario of torque loading. That, typically, occurs at vehicle launch in first gear where the torque multiplication is greatest. To get an understanding of where we need to be in terms of torque capacity, it’s necessary to do a little math. Now, this is an extremely simplified version and it doesn’t take into account several important things (such as actual tire traction available) but it makes an interesting exercise because it shows you what sort of level torque your axle really can see. If you’re used to dyno pulls that reports 500 lb-ft of torque at the wheels, this will really open your eyes.

So, you’ll need to know your gear ratios, and you’ll need to have an idea of what to use for an impact factor. This extra factor accounts for the instantaneous shock-load that the drivetrain can see when you rev up the engine and dump the clutch, or otherwise launch your car from a standing start in a very fast, abrupt, manner. Basically, the sudden application of torque winds up all of the play and back lash in the system, then smacks down real hard with the sudden change of driveline inertia, like a sort of whip effect. This kind of launch can put a shock load on the system that is substantially higher than if you simply rolled onto the throttle while moving. For a manual transmission, this impact factor can be 3:1 or more, depending on abruptness of clutch engagement. In fact, some vehicles have limited how quickly the clutch can engage just to help protect from this. For an automatic, the factor is a bit more forgiving, more in the 1.5-2:1 range, depending on the torque convertor.

TQ_max = Maximum spike torque at ring gear
TQeng = Peak torque at engine flywheel
R1st = First gear ratio
Rfd = Final drive ratio
I = Impact factor
TQmax = TQeng * R1st * Rfd * I

Let’s assume a late model Mustang GT, with an MT-82 manual transmission and 3.73:1 axle gears. The numbers go as follows:

390 lb-ft * 3.657 x 3.73 * 3 = 15959.5 lb-ft.

Yes, that’s nearly sixteen thousand foot-pounds of torque. See what I mean? Now, one very large caveat to this comes in the form of traction. You will only ever develop as much torque as you have the ability to resist with tire traction – that’s really just fundamental physics. Once you’ve overwhelmed the tires, any additional potential torque goes up in smoke; it would be lost to wheel spin if you stay on the throttle. We know from experience that the S197 Mustang example in question can really only use around 9500 lb-ft in stock form before becoming traction limited, so that 16K lb-ft number we calculated is very theoretical. But someone could begin to approach that by making suspension and tire upgrades. We also didn’t account for any losses due to drag in the drivetrain system (as I said, this massively simplified). But, this does point out the order of magnitude that your axle has to put up with. A mere 500 lb-ft is nothing.

So, back to the original revised question: how much torque can it handle? That varies depending on the application. OEM applications are typically designed to a specification set by the vehicle manufacturer. However, due to the wide range of possible places that an aftermarket differential can find itself in, and equally wide range of uses, the best I can tell you is that each application is engineered to have the most torque capacity is as practical for that axle model. That’s vague, I know, but the actual answer would require a matrix table to sort out. The answer is also somewhat meaningless unless the person asking the question has a way to evaluate the traction limitations of their car, and that is a whole separate discussion…