Category Archives: Tech Info

Customer Questions, Tech Info

Ask Torsen: Using a Mini-Spare Tire

Ed Smalley wrote into Torsen to ask:

Greetings,

My 2018 Ford Mustang GT350 has an OE Torsen® differential and I would like to know if using a mini spare tire is possible. The original tire diameter is 27.4″ and the spare tire diameter is 27.7″. What say you?

Ed Smalley

Ed,

Thanks for contacting Torsen.  Our products have been used in vehicles that have had a mini-spare tire that was as much as a 25% difference in diameter smaller than the main tire.  If kept to the operational restrictions (speed and distance), this falls within what the differential can handle.  In the case of a tire that’s only about a ¼” difference in size, it is a relative non-issue – for the Torsen differential.

I cannot say, however, if Ford has other considerations that prevented them from packaging the spare with the cars that come equipped with a Torsen.  I believe fitment of the spare wheel over the brake calipers is part of that.  So bear that in mind as well.

Regards,

JTEKT Torsen North America, Inc.

Customer Questions, Tech Info

Ask Torsen: Torque Bias Ratio Functional Questions

Today’s Ask Torsen entry comes to us by email from HMMWV owner Adam.  He asks:

Hello,

I have been reviewing how the Torsen T-1 front and rear differential functions off road. I myself have the differential in my ’94 Humvee.  My confusion in the diff comes from the Torque Biasing Ratio and what exactly is meant by a 4.5:1 ratio. I have read that it is the amount of torque that the diff can handle and still remained locked. 

I’ll [phrase] my question [as] a scenario:

One wheel is on a low traction surface I.E. a wet rock. The other wheel is on a high traction surface.  Let’s say the first wheel is getting 100 units of torque and the other wheel is getting 450 units of torque.

What exactly happens when the wheel with high traction starts to exceed 450 units of torque? Does the diff become an open diff and lose all of its locking abilities?  If the wheel that exceeded 450 units of torque is now getting 500 units of torque, what exactly is the output?  None?  OR can the diff simply not accommodate the extra torque so even though I may be sending 600 units of torque in, the diff is only sending 100 units one way and 450 units the other way? And then the diff itself is absorbing the extra 50 lb-ft?

I’m inquiring because I was talking to a person about air lockers and how technically their TBR is infinity:1 which means that the Torsen can’t compare because it can’t theoretically send infinite torque to the high traction wheel?

On a side note, how does the differential ring gear ratio interact with the TBR? The input torque is multiplied by the ring gear ratio then divided by the TBR, correct?

Sincerely,

Adam

Adam,

Thanks for contacting Torsen.  The simplest way to conceptualize torque bias ratio (TBR) is to think of it as a traction ratio.  This represents how much difference is allowed from one side to the other; if the TBR level is exceeded, the differential will “open” and allow spin or differentiation to occur.  If the ratio of traction from one side to the other is less than the TBR, the differential will remain locked up. For example, if the TBR is 4:1, differentiation (or spin) will not happen unless your low-traction tire has less than a ¼ of the traction that the high-traction tire has.

Limited slip differentials (LSDs), like Torsen, operate by generating friction internally while under load.  That friction provides resistance to the tendency for one tire to slip.  Different types of LSDs do this in different manners; traditional “posi” units do so with clutches.  Helical gear differentials (like Torsen) do so by creating friction using forces generated by their gears.  A differential’s TBR rating is a product of how much friction is created inside.

Another way to think about TBR is this: it also represents the maximum distribution ratio of torque from side to side.  This ties into your locker question; the simple formula is that the high traction tire receives (up to) whatever amount the low-traction side can sustain, multiplied by TBR.  So, T(high) = T(low) X TBR.  This does necessarily mean that the Torsen requires some amount of reaction torque on both outputs to be able to act against.  This is typically the result of tire contact with the ground.

You probably see by now that in extreme situations, such as when a tire is lifted off the ground completely, that zero X TBR is still = zero.  So, yes, in that case, the Torsen is limited.  But that is why the product is classified as a limited slip differential.  However, keep in mind that I said that the Torsen requires some reaction load on both sides to work against.  Though that typically is tire traction reaction, that is not the only means of providing a reaction torque; it can quite readily come from the brake system.  The vehicle’s brakes are quite capable of providing negative torque to the axle for the differential to work against.  In fact, the military at least used to train HMMWV drivers to use a technique called brake modulation to do exactly that (though I don’t know if that’s still the practice).

Conversely, in order to prevent an extreme situation, such as lifting a tire, from causing a complete lack of traction, the vehicle would need to have an actual locking differential.  Lockers have, as your friend stated, essentially an infinite:1 TBR.  They are capable of sending 100% of available torque to either tire.  Instead of creating internal friction to resist slip, lockers usually have some sort of element that provides a direct, physical impediment to slip.  This can be engaging a spline or a dog clutch, or maybe use of a ratcheting one-way clutch to drive.  With that said, there are a lot of other compromises that come with a locking differential, such as compromising vehicle maneuverability.

Regarding final drive ratio, it doesn’t really have any bearing on TBR.  The ring and pinion impacts how much the prop shaft torque is multiplied by when it is applied to the differential, but that’s its only real involvement.

So, regarding your example – I’ll take you through a few scenarios so you can see the torque interaction at play.  Your differential has a TBR of 4.5:1, and we’ll say there is potentially 200 lb-ft of torque coming out of your transmission & transfer case on the rear prop shaft.  For this example, the axle has a 3:1 ring & pinion ratio.  So, that can represent where the 600 lb-ft you used comes from.

Scenario 1) Now, if both tires can sustain 300 lb-ft of torque (each) before breaking loose, the differential takes the 600 lb-ft and divides it to each side evenly, without any differentiation occurring.  So, 300 lb-ft is delivered to each tire (I’m ignoring the HMMWV’s hub reduction gearing here).  In this case, both tires are maximized in terms of traction, but are just shy of their slipping point.

Scenario 2) If, instead, both tires can handle 450 lb-ft before they slip, the torque distribution is still going to be essentially 50/50, with 300 lb-ft going to both sides.  The difference is that the tires are comfortably below their slip limits.

Scenario 3) This time, one tire (from scenario 2) suddenly encounters a drop in traction and falls off to, say, 150 lb-ft, the Torsen will immediately take that 150 and try to multiply by its TBR.  This could allow the differential to send up to 675 lb-ft to the high-traction tire – if there was enough torque going to the axle to support that.  But in this case there isn’t, since we only have 600 lb-ft total, so only 450 are available to send to the high-traction side.   Coincidentally, we said that 450 lb-ft was the tire’s traction limit, so again we’re maximizing the traction of both tires and no differentiation has occurred.

Scenario 4) Now, let’s say that farther down the trail, a tire encounters another low traction spot.  This time, the tire traction will only support 100 lb-ft of torque.  Again, the differential will immediately bias torque to the high traction side, within its TBR limits.  So, the low-traction side only receives 100-lb-ft, and the differential again multiplies that and delivers 450 lb-ft to the high-traction side.  Again, the high-traction tire is receiving its maximum torque before it would slip.  However, as you noted, 450 + 100 = 550.  So, where is the missing 50 lb-ft?

The answer to that, and what follows, is up to the driver.  If you happened to notice that the trail was slippery in spots after going through scenario #3, maybe you backed off of the throttle a little bit and allowed the torque going to the axle to drop a little.  In which case, maybe the torque is reduced down to (or below) the new 550 lb-ft threshold.  If so, again, the tires are traction maximized and the truck carries on.  No slippage or differentiation has occurred.

On the other hand, if you stayed on the throttle right through it – and this gets to the heart of your question – it becomes a different story.  Torque, like other areas of physics, is based on having an equal and opposite reaction.  The axle’s total traction limit remains 550 lb-ft.  Consequently, the torque load going into the axle cannot exceed the traction that supports it.  So, that extra 50 lb-ft disappears, or more specifically it ceases to exist.  The system torque drops, at least until traction is regained and all is right with the world.

If you continue to try to add more power at that point, you will basically turn the excess power into wheel spin.  Differentiation occurs, and if the resulting unloading is sufficient to let the engine rev up, it turns to excessive tire spin.  However, even while that is happening, the Torsen is still sending 450 lb-ft to the high-traction side, as long as the low-traction side can still manage its 100 lb-ft.  So, you still move forward.

Scenario 5) This time, you actually lift a tire off of the ground, maybe while traversing a ditch.  As we said above, a tire in the air can support zero torque, and zero multiplied by 4.5 still equals zero.  But here, you can apply a little bit of pressure to the brake pedal, so you generate a brake load of, say, 100 lb-ft.  The Torsen works off of the brake load just like it would a traction load, and then Scenario 4 is essentially repeated.  This time, though, because you have your foot on the brake, you’re not as likely to spin up the low-traction tire.

Scenario 6) In one last situation, let’s say that scenario #3 is repeated, except this time, the high-traction tire has a limit of only 400 lb-ft before it slips.  Again, the Torsen would want to send up to 450 lb-ft to it, but that exceeds the tire’s traction ability.  This time, if you stay on the gas, you run the risk of causing both tires to break traction and spin.  In this case, both tires are potentially going to be overpowered.  The differential is still working to try to balance the torque load to the tires’ traction limitations, but unless you reduce driveline torque, you cause both to saturate and break free.

Ultimately, everything comes down to tire traction.  Torsen works to try to maximize the tires’ effectiveness up to the point where slip begins, though it does have some limitations.  I know that’s a fairly lengthy answer to what probably seems like a basic question, but it’s important to understand the different situations that could occur in order to explain what happens in one specific one.

Customer Questions, Tech Info

Ask Torsen: More about Lubrication

Today’s question was sent by Bastiaan:

Hello,
I have a question for my car Honda s2000. It has a torsen rear differential, rear wheel drive.
There is much discussion online about which oil to use.

Options are: 75w90, 80w90, 75w140. Some can be found in mineral and some in synthetic.

I dont know if you are just a company that produces torsen differentials, or if you guys are THE torsen manufacturer and original.

Anyway, can you recoomend me between the different oils for my car? It’s a daily driver, sporty drive style, not tracking.

Please let me know what would be the best option.

thanks much appreciated!

kind regards
 Bastiaan

Bastiaan,

Thank you for contacting Torsen.  Note that we are Torsen with a capital “T”; Torsen is our brand name and our trademark.  No other companies can use the name “Torsen” legally.

With regards to you lubrication question – the short answer is it doesn’t make much difference to the differential.  Ultimately, it’s a question for the axle manufacturer.  What I mean is, the differential operates as just one component in a larger system, a system that shares lubricant.  The axle assembly that contains the differential also contains other significant components to which proper lubrication is equally (or more) important.  Helical gear differentials like the Torsen that came in your car can actually operate in wide range of fluids and are not particularly fussy about it, so long as the lubricant is of a good quality.  They will operate just fine in all of the oil weights that you’ve mentioned.  On the other hand, the ring gear and the pinion gear that drives it are extremely fussy about correct lubrication.  So are the bearings and oil seals, though most people don’t consider them.

A lot of people ask the same basic question – what lube does the Torsen need?  Basically, I have to point out that this is not the correct question to ask, for the reasons mentioned before.  What you need to ask is “what oil does my ring & pinion gearing need?”  That is what you should be concerned with, as the differential itself doesn’t really care.  You’ll need to ask the gear manufacturer that.  If you are using the production ring & pinion gears, that information should be included in your owner’s manual, or available from the dealer.

Regards,

JTEKT Torsen North America, Inc.

Tech Info, Uncategorized

How it Works – the “Simple” Version (Part 1)

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?

Tech Info, Uncategorized

How it Works, Simplified – Part 2

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.

WP_20160824_13_08_45_Rich

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.

Tech Info, Uncategorized

How Much HP Can it Handle?

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.

TQmax = 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…

 

Tech Info, Maintenance

More about TBR, Lube, Friction Modifier & You

Torque Bias Ratio (TBR) – also referred to as Locking Effect – is a term used to quantify how much resistance to wheel spin a differential has. This is true of any type of LSD, plate or a helical design like Torsen®. These devices work by creating internal friction; friction in the diff is what allows it resist wheel spin.

By the same token, that resistance is also what allows a differential of the non-open variety to distribute more torque to the side that can best use it. The higher the TBR setting is, the greater the resistance will be to the start of slippage. That means more torque can be sent to whichever tire has better traction. In essence, by building resistance to spin, the differential can hold back the wheel with lower traction from spinning, allowing the higher traction wheel make use of more torque.

The actual amount of friction that the differential develops will be different depending on if the internal gears are static (not moving in relation to the diff case) or if differentiation is taking place. Think of trying to push a crate across a floor – you need more force to get it to move initially than you need to keep it moving. The coefficient of friction is higher when things are not moving compared when to when they’re sliding.  The same is true for the gearing in the differential.

So, there is more resistance to wheelspin when the diff gearing is not moving inside the case, which is the case when driving in a straight line. But when you turn, and one wheel speeds up while the other slows down, the differential gears want to move. Once there is enough force is on the axle shafts to overcome the friction, the gearing will start to move. In doing that, the amount of friction generated is reduced. However, this is true for clutches as well as for helical gears.

Now, that transition from static to dynamic friction is often kind of abrupt; torque gets wound into the gearing until the friction is overcome, then it pops free and moves. But when it does that, the movement slows and the coefficient goes up, and so it stops moving. At least, until the friction is again overcome and it pops free – again. In that transition, sometimes you find that stick/slip/stick/slip behavior occurs a lot. This stick/slip characteristic is also what causes clutch plates to chatter in a tradition limited slip device.

Bringing this all back around to friction modifier – the actual function of modifier (the “modification” it makes to the friction) is that it brings the static coefficient of friction down to a similar level as the dynamic coefficient of friction. This helps smooth out the transition from static to dynamic friction, getting rid of the tendency for abrupt stick/slip. This, as a result, largely eliminates the chatter or squeak . However, it does this at a price – the price of lowering the coefficient of friction for the whole system.

So, if we return to the notion that the differential is a friction device, it then stands to reason that the friction the differential generates is directly influenced by the lubricity of the oil blend that is used in the system. If the oil’s friction properties are reduced, the amount of friction that the diff produces is proportionally reduced.  That, in turn, has the same affect on TBR. Usually, it is a small but measurable amount, maybe up to a 10% loss of TBR.

This begs the question of whether or not friction modifier is actually necessary in a Torsen.  Like I said earlier, the modifier is beneficial in smoothing out the operations of friction devices.  However, because our friction properties are different than that of a clutch plate differential, you might not see the same symptoms of stick-slip behavior that those would.  As noted, clutch plate differentials have a distinct tendency to chatter without friction modifier in the oil, and that chattering can become very objectionable.   So, the modifier is essentially a requirement.

In a helical gear diff, however, the stick-slip tends to show up as an audible noise, described by some as a “rusty screw” noise, or as a squeak or moan.  You tend to hear it in very low-speed turns, when the steering is at or near full-lock.  So, you might hear it while maneuvering in a parking lot, or turning the car around in your driveway.  This noise is, as I said, just a characteristic of the friction properties, and is normal.  But, if you hear it a lot and it annoys you, adding a few ounces of friction modifier to the axle lubricant will almost always eliminate it.  So, the bottom line is it’s up to you.

Customer Questions, Tech Info, Maintenance

Ask Torsen: Differential Contribution to Axle Temp

This question comes by way of www.mustang6g.com forum user Brent Dalton.  This came from a discussion regarding managing axle temperature on the S550 Mustang and preventing overheating of the axle during track use.  For reference, 2016 and newer Mustang GT models with the Performance Pack (PP) option (which includes a Torsen® differential) have a temperature sensor on the rear axle cover to monitor axle temperature and warn the driver if temps become high.

Quote:

Originally Posted by Brent Dalton

…So it’s only really on 3.73 torsen diffs… so it makes some of us wonder… is it overheating due to the torsen and the regular type diff’s don’t suffer the same? A few guys … are working on the answer.

 

This is a worthwhile question, especially as more people take these cars on track day events.  A couple of comments – all limited slip differentials (LSD) are friction devices. The higher the TBR/locking effect is, the more friction it generates.  So, the Torsen® in this car – with a TBR of around 2.8:1 (versus 1.8:1 for base clutch plate LSD) – does create more friction than the base differential does.  However, with all of that said, friction only creates heat when you force slippage of the friction surfaces under high (torque) load.  I think that if you could actually watch wheel speeds during track events, you’d be surprised by how little differentiation occurs, at least under load.  At that TBR level, the Torsen® isn’t allowing the wheels to change speed much, even if it continues to bias torque side to side as warranted by traction conditions.  In general, in track use under high torque loads, the TBR level is sufficient to prevent most differentiation.  If you have a plug-in device that’s interacting with the vehicle CAN by way of the OBD2 port, see if you can monitor the rear wheel speed sensors and see what actually goes on during a hot lap.  You might find it interesting.

Anyway, the differentiation level is low, and even when it does, the differential rate (referred to as delta-N) is also quite low – on the order of 15-20 RPM difference from side to side.  So the frictional heating component that the differential contributes to the axle oil is pretty low, in relation to the system as a whole.  You get a lot more heat generated by the ring and pinion due to the hypoid mesh, which is running constantly, and at much higher speeds than the differential gears operate at.  Past that, most of the heat probably comes from external sources – like the proximity of the exhaust, which passes quite close to the axle carrier on this car.  Combine that with the fairly small volume of oil available to absorb heat, and you get the situation you’re in.

In my opinion, likely the biggest reason that Ford monitors axle temp on Performance Pack cars isn’t because the Torsen® creates more heat, but rather because they see a greater need to pay attention to axle temp on those cars, due to the perceived (track) usage.  I expect that in their minds, the non-PP GT models are much less likely to be on the track, so they (Ford) doesn’t need to spend the extra couple of bucks to fit those cars with the sensor.  It isn’t that the non-PP axle is likely to run cooler, but instead they are less likely to be in a situation where excessive heat is generated.

Customer Questions, Tech Info

Ask Torsen: Type-2 vs. T-2R

Today’s Ask Torsen question comes to us by email from James Stapleton.  He asks:

(I) Have seen where you’ve stated that a T-2 might be better suited for open tracking than the T-2R.  I do both open track and autocross currently on Michelin Pilot Super Sport tires, and maybe changing to a 200 treadwear type tire in the near future.  Thoughts?

Well James, that’s a great question – and one we’ve been asked a lot over the years.  However, it is highly dependent on a number of things: chassis set up, track conditions and degree of technicality, even driver preference, so there is no singular hard and fast answer.  The basic essence of the answer, though, is that it’s a matter of what trade-off is best for the situation in question.

As a limited slip differential becomes more aggressive – with a greater degree of lockup (or torque bias ratio) – it has greater resistance to differentiation.  This has two influences on the car in the context of what we’re talking about here.  First, it affords better resistance to wheel spin, which of course means that the driver can apply more throttle and accelerate faster.  Second, it has greater resistance to steer input, at least during turn-in and initial corner exit, because by resisting differentiation, it is also resisting the car’s need to rotate and change direction.

This initial understeer is somewhat mitigated, at corner exit, by resisting inside wheelspin.  In so doing, more torque is allowed to go to the outside tire once the inside tire reaches its traction limit than would otherwise be the case.  That, in turn, has the effect of generating an understeer-canceling yaw moment on the chassis, serving to actually drive out of the corner with greater authority.  But you have to overcome initial understeer to get to that point.  This effect is described in more detail elsewhere on this blog.

To boil that all down to the nuts and bolts of the matter, higher TBR can make the car “push” some in a corner, but allows a great deal of corner-exit traction.  Lower TBR, on the other hand, can be more neutral-feeling, but potentially lack sufficient traction to prevent inside spin.  So, as with any aspect of chassis set-up, it’s a trade-off.  Cars with lower torque output (such as a Miata or BRZ) can be comfortable with a lower TBR because they favor nimble response and don’t have the torque to overwhelm the tires.  Big-bore cars with big torque output may need the higher TBR of a T-2R model.

However, as noted above, venue plays into it as well.  A track that is fast and flowing will probably favor the lower TBR of the standard Type-2.  With less dynamic weight transfer in a corner, you don’t need to compromise handling balance based on corner-exit traction.  The lower dynamic weight transfer means the inside tire doesn’t become unloaded and loose traction as readily.  But a smaller, more technical course – especially in autocross – may force you into a more aggressive differential due to tight corners and high weight transfer.  There is a much greater potential to lift an inside tire off the pavement.  Frankly, the T-2R owes its existence to autocross.  It was due to the complaints of wheelspin and Type-2 not being aggressive enough in that environment that we created the T-2R product line.  But again, it comes down to what compromise makes most sense to you.

In your specific case, because you participate in both open track events and autocross, I would probably prefer using the T-2R.  It is easier to make some chassis tweaks to compensate for a little extra initial understeer than it is to cope with insufficient traction at corner exit.  At least, that would be the approach I would take.  Best of luck!