LSD (Limited Slip Differential) Q&A, stock and aftermarket!

[CSG Mike]

From: http://www.reddit.com/r/ft86/comment...nything_about/

I've been getting a lot of PMs about LSDs lately, so hopefully this will answer a lot of questions!

Can you briefly explain the difference between a locking differential and a Torsen differential?

A torsen differential can apply as determined by the internal TBR (torque bias ratio). A 4:1 ratio means that one side can handle up to 80% of the torque from the engine.
The Torsen itself works by attempting to slow down the wheel spinning faster (outer wheel) and speed up the wheel spinning slower (inner wheel). It can do this up to the TBR described above. If the torque to the wheels exceeds the TBR, then you get inner wheel spin.
A locking, or clutch type limited slip differential works by literally locking the outer and inner wheels when there is a speed difference and a torque being applied, forcing the wheels to turn at the same speed. You can only spin both wheels, or neither wheel, with this type of diff, under normal use. The actual locking is done by a series of clutch disks.

What would be the advantage to an aftermarket diff?

Building on my other post, we can infer a few things.

- When a Torsen (the oem type) diff's capacity is exceeded, the inside wheel will still spin, like an open differential
- A Torsen is "locked" only when you stay within the capability of the differential, the TBR
- A Torsen's behavior can be demonstrated by jacking up the rear of the the car, and then spinning one of the rear tires; the other rear tire will spin in the opposite direction

Now, from that, you can see that there are a few problems:

- If you lift a rear wheel, you effectively have an open differential
- If you are cornering hard enough under acceleration, then you will spin the inner wheel, effectively having an open differential
- If you suddenly hit a patch of low traction (hydroplaning on one side of the car, ice, snow, etc.), you effectively have an open differential

Now, a clutch/locking type differential addresses the following:

- If the inside wheel is lifted, both tires will be spinning the same speed
- If one wheel wheel suffers a loss of traction, both tires will still be spinning the same speed.
- Under cornering and acceleration, you can only spin both tires, or neither tire. You cannot spin only one tire.

The end result:

- The car will rotate more readily, but also more progressively, giving you a much larger window of opportunity to correct/compensate for the rotation. Effectively, car is **more predictable** under cornering.
- **The limit of adhesion under cornering is substantially raised.** Previously, losing traction on one rear tire resulted in a loss of control. Now you have to overcome the grip of BOTH rear tires to lose control of the rear.
- Remember, the inner rear tire is the one that spins; the inner tire is the one that is unloaded! Now you have to spin both the unloaded AND loaded (outer) tire. All that weight transfer that happened to the outer tire, giving it extra grip, also has to be overcome to lose control of the rear!
- You will not suddenly spin if you suffer loss of traction on just one side of the vehicle
- You can much more easily "throttle steer" the car (a more advanced driving technique, where the car has minimal steering wheel input, but weight transfer is being sustained horizontally, so you can make the car turn more or less by modulating the throttle)

Bonus:
- VSC Sport activates when it detects a change in rate in wheel speed between the four wheels. Because the Clutch type LSD prevents the rear wheels from spinning at different rates, you're effectively increasing the limits of the car, **substantially**.

Side note:
- VSC works by activating the brakes. If you're doing hard driving, you really should not be using VSC; it puts a lot of strain on **all** types of differentials.

[diss7]

Good info.

[Dustin]

I'm glad someone explained this in detail. After doing my 2 way people have asked me the difference and I couldn't really explain it other than it "felt" easier to control. Thanks @CSG Mike.

[CSG Mike]

Originally Posted by Dustin

I'm glad someone explained this in detail. After doing my 2 way people have asked me the difference and I couldn't really explain it other than it "felt" easier to control. Thanks @CSG Mike.

The end result is what truly matters, right?

That's an awesome DIY in your sig btw.

[Dustin]

Yes it is. One of those mods that not a lot of people will do, but now that I have done it, I would not want to go back to the Torsen. Money well spent.

[CSG Mike]

Originally Posted by Dustin

Yes it is. One of those mods that not a lot of people will do, but now that I have done it, I would not want to go back to the Torsen. Money well spent.

Precisely my sentiment.

You have no idea how much I want a clutch type in my s2k

[DougW]

Take a look at Wavetrac http://www.wavetrac.net/ this might be a good choice for the street/track car keeping the gas milege the low drag torsen offers. Track only or high hp I agree with Mike LCD or spool is the only choice.

[zooki]

I think a locking differential and a limited slip differential are two different things. A Detroit Locker type diff is way different than a clutch type. A locking differential will drive both of the wheels equally, unless one is moving at a different speed. For example, when turning, the inner tire is being driven, but since the outer tire has to travel farther it is turning faster. The gears that engage the axles are cut in such a way that the outer gear will "ratchet" on the gear that it's engaged to. This produces that "clicking" noise you will hear with these types of diffs. I have a locking type diff in both my Mustang and my 4Runner, and they take some getting used to as far as street driving.

[DougW]

Originally Posted by zooki

I think a locking differential and a limited slip differential are two different things. A Detroit Locker type diff is way different than a clutch type. A locking differential will drive both of the wheels equally, unless one is moving at a different speed. For example, when turning, the inner tire is being driven, but since the outer tire has to travel farther it is turning faster. The gears that engage the axles are cut in such a way that the outer gear will "ratchet" on the gear that it's engaged to. This produces that "clicking" noise you will hear with these types of diffs. I have a locking type diff in both my Mustang and my 4Runner, and they take some getting used to as far as street driving.

WaveTrac is a torsen with a clutch pack, I talked to them at SEMA if they can get an order for 10 they would make them for the SCION. Maybe a choice?
DougW

[CSG Mike]

Originally Posted by DougW

Take a look at Wavetrac http://www.wavetrac.net/ this might be a good choice for the street/track car keeping the gas milege the low drag torsen offers. Track only or high hp I agree with Mike LCD or spool is the only choice.

A Clutch type LSD doesn't add drag; it's not adding any load when you're going in a straight line.

[Poodles]

Having driven both, a clutch type feels like it's fighting you when you're in a corner while the torsen is a bit "spooky" and almost feels like it's making you corner harder. This spookiness transitions a bit to when you powerslide as well...


Any OEM clutch type will essentially do essentially nothing if you unload a tire though, as they just don't lock up enough. Aftermarket stuff or a rebuild with stiffer springs and better clutches can act much like a spool and have skipping in corners.


A true locker type is a whole different animal. I'd never use one on anything other than an offroad vehicle or maybe a serious drag car that I wanted to drive on the street (drag only would run a spool).


There's other types out there, like the a viscous, but they're generally not seen as very good.


Also, last I knew, torsens were quite a bit more expensive to make than a a clutch style, so there must be a reason for using it if the clutch style is "better."

[CSG Mike]

Originally Posted by Poodles

Having driven both, a clutch type feels like it's fighting you when you're in a corner while the torsen is a bit "spooky" and almost feels like it's making you corner harder.

It sounds like you had a badly matched LSD for whatever car you drove. A properly matched and tuned LSD will feel exactly how you described a Torsen, but more progressive, and more effective.

As with suspension tuning, you can't just blindly throw on parts, and expect it to improve your car.

[zooki]

Originally Posted by Poodles

A true locker type is a whole different animal. I'd never use one on anything other than an offroad vehicle or maybe a serious drag car that I wanted to drive on the street (drag only would run a spool).

Yea, running a locker on the street is entertaining at times...especially in the rain. I had one in a Samurai also, and a short wheelbase plus a manual transmission made for an interesting ride. Good thing it didn't have any HP to speak of.

[SubieNate]

I know lotus refuses to put any kind of LSD into their cars as they believe they interfere with cornering. What's with that?