So what is the Big Diff? (pun intended sorry)

[Erik @ GBox]

Hey there everyone. So I was recently doing some searching around here on Limited Slip Differentials. I was curious to see what everyone’s opinion has been so far on the stock Torsen unit that comes with these great new cars, and found that there seems to be a bit of misunderstanding regarding the difference between a Torsen style differential and a Plate-Type (Salisbury) Limited Slip Differential.

The best description I have found to easily explain the differences, of how these differentials actually work, was written by Karim Nice and is taken from the website How Stuff Works. In it he explains the following:

The Torsen differential is a purely mechanical device; it has no electronics, clutches or viscous fluids.

The Torsen (from Torque Sensing) works as an open differential when the amount of torque going to each wheel is equal. As soon as one wheel starts to lose traction, the difference in torque causes the gears in the Torsen differential to bind together. The design of the gears in the differential determines the torque bias ratio. For instance, if a particular Torsen differential is designed with a 5:1 bias ratio, it is capable of applying up to five times more torque to the wheel that has good traction.

These devices are often used in high-performance all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and rear wheels. In this application, the Torsen is superior to the viscous coupling because it transfers torque to the stable wheels before the actual slipping occurs.

However, if one set of wheels loses traction completely, the Torsen differential will be unable to supply any torque to the other set of wheels. The bias ratio determines how much torque can be transferred, and five times zero is zero.


Clutch-type Limited Slip Differential (Salisbury)
The clutch-type LSD is probably the most common version of the limited slip differential.

This type of LSD has all of the same components as an open differential, but it adds a spring pack and a set of clutches. Some of these have a cone clutch that is just like the synchronizers in a manual transmission.

The spring pack pushes the side gears against the clutches, which are attached to the cage. Both side gears spin with the cage when both wheels are moving at the same speed, and the clutches aren't really needed -- the only time the clutches step in is when something happens to make one wheel spin faster than the other, as in a turn. The clutches fight this behavior, wanting both wheels to go the same speed. If one wheel wants to spin faster than the other, it must first overpower the clutch. The stiffness of the springs combined with the friction of the clutch determines how much torque it takes to overpower it.

Getting back to the situation in which one drive wheel is on the ice and the other one has good traction: With this limited slip differential, even though the wheel on the ice is not able to transmit much torque to the ground, the other wheel will still get the torque it needs to move. The torque supplied to the wheel not on the ice is equal to the amount of torque it takes to overpower the clutches. The result is that you can move forward, although still not with the full power of your car.

I also found a rather interesting factoid out as well. The first Limited Slip differential was commissioned to ZF in 1932…………wait for it……….by Ferdinand Porsche.

Anyways, I thought that this would be a rather easy was to set the record straight for some of the people here. Personally I tend to favor the Plate Type Differentials in any car that is going to be driven hard, or taken to the track, but there are many who very much enjoy their Torsen differentials, as well.

Enjoy all.
Erik Johnson

[Mobius357]

I'm very happy with the torsen unit, It's much better than the locker in my truck. That split second or so of one wheel spin before the locker engages. I think for street use a torsen is hard to beat, and if you somehow find yourself stuck with a wheel off the ground, light use of the brakes will give enough resistance for the diff to work.

[TheSlickOne17]

Os giken that's what I'm running.

Makes sliding. THAT MUCH MORE FUN!!!

[Erik @ GBox]

We are also a big fan of the OS Giken Limited Slips. I am thinking of putting a article together for the club here to help explain what makes it work so well, and have the durability that it does, do you think that would be helpful to everyone here as well?

Erik

[SkullWorks]

Erik,

Thank you for tackling this subject it will help numerous people to understand the changes they are making or may want to make to their cars.

Am i correct in my understanding that an OS giken LSD combines the torsen and a small clutch type diff in the same unit?

[Erik @ GBox]

Hi Skullworks,

Nope not so much in this case. The OS Giken is a plate type (salisbury) differential with a few new tricks added to it to really make it very durable and usable not only for the track but also for the street.

I will put something together here either later today or tomorrow to explain how it works and is slightly different from other style differentials. I think that it will help to really clear things up on this subject a bit and help everyone understand what makes this one of the better options out there, even though they are slightly more expensive in the short term.

Erik Johnson

[fuddbutter]

After reading this things now make more sense,
I have noticed a few times going up some below average drive way's on an angle where one of the wheels comes off the ground the traction control kicks in, even though it is switched off, also does it in the wet when the wheels are spinning.

They must have set it up for situations like that, when one wheel comes off the ground and cannot get any traction at all, the brake starts to grab, gives it "traction" so it can send power to the other wheel and get you out of a bad situation.

[fuddbutter]

And feel free to add in a review on the good ol' back yard, welded diff

[GenkiElite]

Originally Posted by TheSlickOne17

Os giken that's what I'm running.

Makes sliding. THAT MUCH MORE FUN!!!

Which one?

[BerkTechnology]

If you want to know how a diff works, watch this video. It is older than dirt but it is the best teaching video I've seen to date.

[ame="http://www.youtube.com/watch?v=K4JhruinbWc"]How Differential Gear works (BEST Tutorial) - YouTube[/ame]

That how stuff works article has some things that I don't agree with.

"The Torsen differential is a purely mechanical device; it has no electronics, clutches or viscous fluids."

Both a gear type of clutch type LSD are purely mechanical. Center diffs and locking diffs on trucks are often hydraulically or pneumatically controlled via electronics. But for this discussion is about RWD sport cars like the FRS they are all going to be purely mechanical.

"The clutch-type LSD is probably the most common version of the limited slip differential."

False. Most diffs are gear type or viscous coupling. Why? OEM's don't want customers complaining about clicking diffs, the added maintenance of worn disks, and too many people crashing cars. Gear type diffs are only 1 way diffs, meaning that they only work on accel and are open diffs on decel. And in the hands of a newer driver this is a recipe for disaster.

[BerkTechnology]

Originally Posted by GenkiElite

Which one?

He said he's running an OS Giken.

[Dimman]

Big headache on this subject is a communication/translation issue between English and Japanese. There is a video of a Toyota rep (maybe even Tada himself?) saying they may offer a 'mechanical' LSD in the future. Some of us were all WTF? A torsen IS mechanical, but I'm pretty sure that's what they are calling the clutch-pack LSDs.

Part of the confusion comes from that, I think.

[TheSlickOne17]

Originally Posted by BerkTechnology

He said he's running an OS Giken.

Lol.

[Erik @ GBox]

I would have to agree the terminology used in differentials has lead many to believe that to adequately understand their operation requires some sort of mechanical engineering degree which, while helpful to have, certainly is not needed.

One of the main differences between the Salisbury differential and the Torsen, a bit simplified here so bear with me, would be the Torsen does not offer any lock up on the deceleration side of the differential. This is a very useful feature of the Salisbury as it allow you to use the differential as a both a source of braking, as well as a component in stabilizing the car during breaking, as well.

Another difference would be most visible on the track under really aggressive driving. Should you actually lift one of the rear wheels the Torsen differential will for a moment send 100% of the power it is distributing to the wheel NOT on the ground. A plate type differential will still maintain drive to both (rear in this case) wheels. This can really make a difference in the handling characteristics and track behavior of the car.

This is why you will typically see the Salisbury style differentials in nearly all racing applications when the series rules allow for it.

I appreciate the comments keep them coming.

Erik Johnson