Why you need a LSD if you like cornering!

[D K]

My Kaaz locker works way better than the Torsen unlimited slip

[wparsons]

Originally Posted by Ubersuber

Any diff that locks to 100%, no slip, is technically a locker. The fact that some lock immediately with a pawl (rare now) or lock progressively using clutch packs makes no difference. It still locks.

A Torsen is by definition an open differential. It never locks and cannot limit slip. It only transfers torque from a wheel with grip turning faster to a wheel with more grip turning slower. It always differentiates the wheels and never limits slip.

So based on your definitions of it either being a locker or an open diff, what exactly would you define a limited slip diff as?

[Sleepless]

Lots of great info and insights in this thread; thx!

To balance the most recent highly technical/theoretical posts, I'd like reiterate that changes/upgrades to the diff need to be qualified, otherwise it is likely to be a waste of money, time, and aggravation.

[CSG Mike]

Originally Posted by Sleepless

Lots of great info and insights in this thread; thx!

To balance the most recent highly technical/theoretical posts, I'd like reiterate that changes/upgrades to the diff need to be qualified, otherwise it is likely to be a waste of money, time, and aggravation.

Generally speaking, a clutch type LSD is almost always a performance increase. This is the rule, with very few exceptions.

The tradeoff is that you have to maintain them with a little more vigilance. You don't want to overheat a LSD, but this shouldn't ever happen under 99% of what most users will do, as long as you use the right fluid, and that includes any stress testing that we do.

That being said, the rear end grip gain under corner exit (powering out) is so massive that we're actually debating doing a test on stock suspension with stock tires, just to demonstrate it.

If you ever make it down here, please ask for a ride!

[ZDan]

Originally Posted by Ubersuber

Any diff that locks to 100%, no slip, is technically a locker. The fact that some lock immediately with a pawl (rare now) or lock progressively using clutch packs makes no difference. It still locks.

Clutch diffs may act locked under some circumstances, but they are still not "lockers". A locker is either 100% locked or totally open. A very tight clutch-type will act more like a spool than a locker. But you still wouldn't call it a spool! It's still a clutch-type limited slip.

A Torsen is by definition an open differential.

No, it is a limited slip.

It never locks

"Never locks" is not the definition of an open diff. An open diff cannot bias torque, it can only ever apply EQUAL torque to both rear wheels. A Torsen biases torque to the outside wheel during cornering.

and cannot limit slip.

Of course it can and does. If it couldn't, my 500+hp FD would always spin the inside wheel during cornering. But in fact it never does that, it *always* spins both rears when too much power is applied and rear grip is overcome.

It only transfers torque from a wheel with grip turning faster to a wheel with more grip turning slower. It always differentiates the wheels and never limits slip.

It *only* transfers torque from one wheel to the other? That's kind of the whole point of a limited slip...

Like a clutch-type, it uses friction to connect the two drive wheels, and the greater the input torque the greater the friction and the greater the effective "lockup".

Preloaded clutch type has the advantage of being able to apply the breakaway torque at one wheel even when the other is off the ground. T2R also does this to some extent, but with a clutch-type you can have as much as you want/need. In my experience with clutch-types, though, I definitely prefer MINIMAL preload and to let the ramps give increased clamping under power.


TL/DR: A Torsen is a limited slip differential. A clutch-type is not a "locker".

[Sleepless]

Originally Posted by CSG Mike

the rear end grip gain under corner exit (powering out) is so massive that we're actually debating doing a test on stock suspension with stock tires, just to demonstrate it.

That would be very interesting to see; but I think you'd have to do it using both FRS and BRZ springs.

[CSG Mike]

Originally Posted by Sleepless

That would be very interesting to see; but I think you'd have to do it using both FRS and BRZ springs.

Ideally we'd love to have a loaner stock suspension for both, so we can just hot swap at the track.

Our dampers have some springs we're testing on them, as well as top mounts and cascam plates...

[Ubersuber]

Originally Posted by SomeoneWhoIsntMe

@Ubersuber, I don't understand why you're trying to redefine this discussion in semantics which only you recognize. it's only going to confuse readers.

A welded differential or spool is locked 100% of the time.

A "locker" is a differential gear with a mechanism to change between 100% lock and 100% open. They may lock automatically or by cable or pneumatic shifter, but they cannot lock any less than fully.

A "limited slip differential" is "a type of automotive differential gear arrangement that allows for some difference in angular velocity of the output shafts, but imposes a mechanical bound on the disparity." (Wikipedia)

An open differential is a differential gear which is fully open all of the time. Four spiders in a carrier with no extra mechanisms. We all know what this looks like.

Inside the category of limited slip differential there are various types. Due to design limitations, relatively aggressive clutch-type differentials are the only differentials which can deliver power to a wheel which is lifted off the ground or on ice. Viscous and helical differentials will generally still allow you to get stuck in the extreme case of zero traction on one wheel. However, this does not mean that clutch types are "lockers" and other types of LSD are "open", it's just a finer point of the characteristics of each diff. Under normal conditions, all types of LSD bias torque, by definition.

Because there is a lot of misunderstanding about how these devices work, much of which comes from the standard terminology. That misunderstanding translates to owners buying the wrong type. The standard Torsen fitted to these cars is adequate for most people. With a car this modestly powered I don't know why anyone would bother with a mechanical lsd and a two way or 1.5 way makes no sense at all. Spend the money on a supercharger or a turbocharger first, but then you might not want to even bother with upgrading the diff.

All the mechanical type lock at 100% quite quickly which makes them indistinguishable from true lockers under those conditions. By "lock" I mean the drive axles turn at the same speed which occurs whenever sufficient torque is applied to the pinion input to activate the clutches (preload aside), they are "progressive" only in an extremely limited sense. Most drivers will experience these devices as locked most of the time the car is being accelerated (and of course with 2 way or 1.5 way, the same will occur on overrun). The clutches only slip when torque inputs are relatively low relative to tire grip. Torsen never lock and cannot lock, they can always differentiate regardless of traction levels. A viscous coupling should provide drive to the axle with grip even if the other axle is broken. A viscous coupling replaces a gearbox and works like a torque converter with special fluid that becomes more viscous when heated than when cooler. Any differential action heats the fluid and locks the coupling.

A mechanical clutch type lsd with no spring preload is an open differential until torque is applied. I don't know why anyone would bother to fit such an lsd nowadays because a Torsen is superior to that design.

With spring preload the mechanical type is a genuine lsd and will limit wheel slip always, which a Torsen will not. This is pretty much only desirable if the roll stiffness is altered significantly from stock, which you should of course figure out first. Even then, for anything but track work a mechanical lsd on this car is either going to be useless or worse than useless.

That's why.

[ZDan]

Originally Posted by Ubersuber

All the mechanical type lock at 100% quite quickly which makes them indistinguishable from true lockers under those conditions.

Not unless they have high preload. Ideally, they won't.
Acting like a "true locker" is not what you want. Big time understeer, for no good reason.

Torsen never lock and cannot lock, they can always differentiate regardless of traction levels.

This is a good thing. Locked or welded diff is not what you want for good handling!

A viscous coupling should provide drive to the axle with grip even if the other axle is broken. A viscous coupling replaces a gearbox and works like a torque converter with special fluid that becomes more viscous when heated than when cooler. Any differential action heats the fluid and locks the coupling.

Viscous doesn't "replace a gearbox", and viscous doesn't LOCK. Viscous LSD requires differential wheel speeds. Unlike a clutch-type or a gear-type (torsen, quaife), a viscous type requires one wheel to start spinning before it biases torque. Definitely inferior for performance applications.

A mechanical clutch type lsd with no spring preload is an open differential until torque is applied. I don't know why anyone would bother to fit such an lsd nowadays because a Torsen is superior to that design.

Low preload is the way to go IMO. As low as you can get away with. A rwd car with front-biased weight distribution like the FR-S/BRZ will require more preload than one with 50/50 or more rearward weight distribution.

With spring preload the mechanical type is a genuine lsd and will limit wheel slip always, which a Torsen will not.

"Mechanical type" doesn't specify much. Gear-types and clutch-types and lockers are all "mechanical". A clutch type with zero preload is still a "genuine lsd", and of course so are Torsens (T1, T2, and T2R). On a car with decent weight distribution, on most tracks lifting a rear wheel never happens, limiting wheelspin with a wheel in the air is usually not a requirement for track work.

This is pretty much only desirable if the roll stiffness is altered significantly from stock, which you should of course figure out first. Even then, for anything but track work a mechanical lsd on this car is either going to be useless or worse than useless.

Assuming you mean clutch-type here, I agree it's unnecessary, but it won't be *worse* than useless. An appropriately set up clutch-type shouldn't have a ton of preload. A lightly-preloaded clutch type is not going to have undesirable effects on handling and street driveability.

[Ubersuber]

This is worth one more attempt at clarification.

A Torsen limits slip at the tire without limiting differentiation by actually pushing torque across the differential to the wheel with better grip. It does not absorb any torque and it is not wheel speed sensitive, only torque sensitive. The Torsen never reaches any form of lockup. A Torsen never hinders any required differentiation, these are very cleverly designed devices and very difficult to understand if you are not an engineer.

The other mechanical lsd only limit slip inside the differential. They do nothing about the real problem which is limited grip at the tire. They do not and cannot transfer the excess torque as a Torsen does. Very rapidly after wheelspin begins the clutch type lsd locks up 100% and both axles turn at exactly the same speed, at least one tire is abandoned to traction robbing wheelspin (it has to because these diffs cease differentiation and lock up) and often both begin to spin. These devices are axle speed sensistive, not torque differential sensitive. All input torque is immediately divided 50/50 and the only difference in axle speeds tolerated results from tire grip. If the less loaded tire can develop enough grip some differentiation can occur in these lsd. They can only do so by physically dragging the tire that is to turn more slowly against the friction in the clutch, the opposite of what is usually meant by "progressive". In other words, the lsd locks up unless the inside tire can be dragged down to a slower speed by forcing the clutch on that side of the diff to slip...not often in practice.

This is a fundamental difference you need to consider, as the engineers at Toyota and Subaru must have done, when choosing which type of diff to fit. Nobody is suggesting an open diff would be preferable to the Torsen. What is being suggested is a mechanical clutch type diff will be superior to the Torsen under the conditions discussed in this thread. My purpose was to underline what you will be giving up if you decide to fit such a diff to your car for the benefit you seek on the track. My idea is that in return for that benefit you will sacrifice a huge benefit the Torsen gives you everywhere else, which is why Subaru fitted the Torsen type.

Just BTW, the technical product support article on the Cusco website supports my use of the terminology. I recommend reading what they have to say before making this decision.

[wparsons]

You do realize that a clutch LSD can (and does) lock less than 100%, right?

You also realize that every word you say just confuses people worse because you try to sound smart, but in reality just go on paragraph long diatribes where a single sentence would've been more than sufficient?

[ZDan]

Originally Posted by Ubersuber

A Torsen limits slip at the tire without limiting differentiation by actually pushing torque across the differential to the wheel with better grip.

Both torsens and clutch-types limit the differential in wheel speeds, transferring torque from the wheel trying to spin to the slower wheel via friction (between clutch plates in the clutch-type, in the worm gears in the Torsen).

It does not absorb any torque

Torsen does "absorb torque" via friction, and they do get quite hot.

and it is not wheel speed sensitive, only torque sensitive. The Torsen never reaches any form of lockup. A Torsen never hinders any required differentiation, these are very cleverly designed devices and very difficult to understand if you are not an engineer.

Every day I feel and hear the Torsen in my s2000 hinder differentiation as I make the 90 degree corner out of my driveway.

The other mechanical lsd only limit slip inside the differential. They do nothing about the real problem which is limited grip at the tire. They do not and cannot transfer the excess torque as a Torsen does.

Of course the clutch type DOES transfer torque, same as the Torsen.

Very rapidly after wheelspin begins the clutch type lsd locks up 100%

No, it binds the two output shafts BEFORE wheelspin, and can bias torque prior to one wheel spinning. It does not necessarily lock up 100%, and for normal tarmac track driving it shouldn't have to to ensure getting all the power to both wheels. The can be set up to give 100% lock under maximum acceleration, but IMO for a well set up and well-balanced car this should not be necessary.

and both axles turn at exactly the same speed, at least one tire is abandoned to traction robbing wheelspin (it has to because these diffs cease differentiation and lock up) and often both begin to spin.

Again, they don't always fully lock, and they don't need to to achieve both drive wheels spinning.

These devices are axle speed sensistive, not torque differential sensitive.

No, they aren't speed sensitive, and they ARE torque sensitive! They lock up more with more applied torque via engagement ramps.

All input torque is immediately divided 50/50

WRONG! That is in fact what an OPEN diff does! A limited slip works because it allows the wheel with more grip to take MORE torque. In a 4:1 Torsen, the bias ratio limits the torque difference to 80/20. Even a 2.5:1 Torsen can deliver 71.5/28.5 bias. A tight enough clutch-type can go 100/0, but again, IMO this isn't usually necessary.

and the only difference in axle speeds tolerated results from tire grip. If the less loaded tire can develop enough grip some differentiation can occur in these lsd. They can only do so by physically dragging the tire that is to turn more slowly against the friction in the clutch, the opposite of what is usually meant by "progressive". In other words, the lsd locks up unless the inside tire can be dragged down to a slower speed by forcing the clutch on that side of the diff to slip...not often in practice.

This does happen often in practice, most of the time in fact. Both the Torsen and the clutch-type allow differentiation at the same time they are providing more torque to the outside wheel with more grip. The clutches slip most of the time in a clutch type, and this is a GOOD thing. Only on a tightish clutch type under high torque load will the outputs be "locked".

What is being suggested is a mechanical clutch type diff will be superior to the Torsen under the conditions discussed in this thread.

I would dispute the idea that a clutch-type is always superior. There's a reason the Boss 302 LS came with the T2R Torsen while lesser V8 Mustangs got the clutch-type, and it's not because the T2R was inferior.
That said, there are aftermarket clutch-types that are better for performance than an OEM Torsen, but there are also plenty of aftermarket clutch-types that are no better.

[Sleepless]

Can we split this thread into "why" and "how they work"?

[Ubersuber]

Originally Posted by wparsons

You do realize that a clutch LSD can (and does) lock less than 100%, right?

Oxymoron