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Discussion Starter #1
Up until today I was confused about what LSD in my IS300 can do. Given the nasty weather we are having here in Dallas these days, I was able to test how it behaves in different conditions. To my disappointment I realized that it behaves pretty much like a standard open differential.

I put my right wheels on the ice while the left side was on dry concrete. Shifted into the first gear and gradually increased the engine speed. The right wheel was spinning and the whole car was slowly crawling forward. Same behavior in reverse.

What is LSD supposed to do in a situation like that? I thought it should limit the slippage of the right wheel and redistribute torque more to the left.
 

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I'll be the first to admit I know nothing about LSD but what I do know tells me the torque should move to the left as you said. Are you sure your car has LSD?

for 5-speeds...open the driver's door and look at the sticker with the different codes on it. B01B indicates you have the LSD.
 

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Discussion Starter #3
acumenhokie,

I'll check the code on the door but I'm 100% positive that my car has an LSD in the list of installed options. I understand that the only reliable way to check for that is to look at the internals of the diff but still... I'm just curious whether it should behave like that if it has an LSD.

If it turns out that it does not have LSD but it is in the list of the installed equipment, what kind of benefit I can theoretically get out of it in terms of arguing with the dealership that they sold me the car, which does not have what they claimed about it?
 

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evolver, it's most likely that you have LSD in your manual car, unless you have a 2005 or late 2004 models. Remember that this is a Torsen type LSD, not a clutch type. So one wheel can spin a alot while the other can spin a little.
 

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Discussion Starter #5
ray127,

Mine is early 2004 or, more likely, late 2003 as I bought it early January 2004. But what good does that kind of a "limited slip" do to me if I cannot put torque to the ground? How is it better than a regular open diff if it bahaves the same?
 

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With an open diff, the car would not even move. One wheel will be spinning on the ice and the other would just stand still. Torsen type LSD is more street friendly while a clutch type LSD, like the TRD one for our car is the one that you want.
 

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LSD moves torque from one wheel to the other. However, LSD does require some friction even on the slipping wheel. Think of it as trying to unscrew a lightbulb while standing on a swiveling stool. Everytime you try to apply torque to the lightbulb, the stool just spins....

same thing with LSD. If one wheel is on ice, its like the stool spinning. You need to have some level of stability on that wheel to be able to apply torque to the other wheel.
 

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HoustonLex said:
LSD moves torque from one wheel to the other. However, LSD does require some friction even on the slipping wheel. Think of it as trying to unscrew a lightbulb while standing on a swiveling stool. Everytime you try to apply torque to the lightbulb, the stool just spins....

same thing with LSD. If one wheel is on ice, its like the stool spinning. You need to have some level of stability on that wheel to be able to apply torque to the other wheel.
Good description.

From "How stuff works".


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.
http://auto.howstuffworks.com/differential10.htm
 

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Discussion Starter #9
HoustonLex said:
LSD moves torque from one wheel to the other. However, LSD does require some friction even on the slipping wheel. Think of it as trying to unscrew a lightbulb while standing on a swiveling stool. Everytime you try to apply torque to the lightbulb, the stool just spins....

same thing with LSD. If one wheel is on ice, its like the stool spinning. You need to have some level of stability on that wheel to be able to apply torque to the other wheel.
Out of technical curiousity, how to determine quantitively that "some level of stability"? Ice is not enough but dry concrete is sufficient. Where is the threshold? How to define it?
 

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Discussion Starter #10
bartkat said:
Good description.

From "How stuff works".


http://auto.howstuffworks.com/differential10.htm
Thanks for the link! But it is rather vague what they mean by "completely looses traction". Obviously, if one of the wheels is in the air it probably matches their description. But what if it is on the ice, or sand on the concrete, or wet asphalt? Where is the distinction?
 

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Well if the wheel is able to spin freely with little effort... then you consider that there is no traction.
 

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evolver said:
Thanks for the link! But it is rather vague what they mean by "completely looses traction". Obviously, if one of the wheels is in the air it probably matches their description. But what if it is on the ice, or sand on the concrete, or wet asphalt? Where is the distinction?
As your original post stated, you had one wheel on ice and the other on dry pavement and there was no transfer. Therefore you could consider the wheel on ice to have completely lost traction.

Wheel spinning, no motive force attained, no grip, the inablity to attain friction. You find that out when one wheel only spins and never gets a grip on the surface it is on no matter what that surface is.

If you read the link you should see why. It's not a totally laboratory scientific term, but close enought. It there is a 5:1 ratio of transfer through the differential, and the one wheel is essentially zero, essentially, mind you, there is no transfer as the net result is zero.
 

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so what does the TRD clutch LSD do different from torson?
 

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I got caught in a snowstorm here in Chicago yesterday. Usually, I drive my all-trac Celica in such conditions, so I've got summer performance compounds on my IS. Traction was horrendous in fresh snow and slush. However, several times I came to areas in the street where the right or left groove in the snow was down to bare street while the other track was full of slush. Every time I hit these areas I gave the car some gas to test my LSD, and each time, instead of wheelspin I got a smooth push from the wheel that had street traction, regardless of whether it was the right or left side. So, the LSD does work where there is limited grip, but just not when there is no grip (like on a sheet of ice with summer tires).
 

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evolver said:
Out of technical curiousity, how to determine quantitively that "some level of stability"? Ice is not enough but dry concrete is sufficient. Where is the threshold? How to define it?

You probably want to spend some time reading up on the coefficient of friction. A quick link that I found using Google is, http://www.school-for-champions.com/science/frictioncoeff.htm

A rubber tire on dry asphalt has a very high coefficient of friction (~.8), whereas a tire on ice has a very low coefficient (~0). Wet ashphalt may only have a coefficient of .25. Obviously there is an infinite level in between.

The fact that the genteman's car was slowly creeping forward says that there was some traction, however small... maybe .05 coefficient.
 

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Discussion Starter #17 (Edited)
HoustonLex said:
You probably want to spend some time reading up on the coefficient of friction. A quick link that I found using Google is, http://www.school-for-champions.com/science/frictioncoeff.htm
Thanks for the hint but having a PhD degree in Material Science I think I know what the friction coefficient and the nature of friction are :)

I probably should rephrase my question though. In its original form it sounds too theoretical, I guess. Let me put it this way: "What are the driving conditions that the stock LSD is designed for?"

We have already excluded icy pavement and other flavors of loosing traction by one or both wheels. In such conditions LSD behaves as if was an open diff. Is it correct?

Ideally, it would be very interesting to know how the torque distribution changes depending on the traction ratio between the wheels. Two scenarios are the most interesting -- when the ratio is gradually decresing (one wheel is loosing traction), and when the ratio increases (one of the wheels regains traction). Is there a threshold at which the torque distribution has a change in behavior in both direction? Or does it have a hysteresis of some sort?

"Howstuffworks" only has an explanation that a housewife can understand... I could not find any other, more scientific, description anywhere.

+rep for willingness to help and spending time to do so :)
 

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Stock LSD is not meant to be a all-weather device. It is designed for spirited driving in near ideal conditions. The whole purpose of LSD is to put the torque on the outside wheel during cornering since that wheel has the larger contact patch and therefore more traction. Also, that allows the car to push itself through the corner at a faster rate of speed.

However, LSD acts like LSD in all conditions. An open diff would behave differently in icy conditions, although I did mean to ask the original poster whether traction control was on or off when he conducted this experiment?

For example, I would expect an open diff to put power to the wheel with the ice first (path of least resistance), and then I would expect traction control to apply the brake on that wheel. The new path of least resistance should be the wheel with traction, and away you go.

I have never conducted this experiement, but have driven my open diff IS on icy parking lots with no problems, so I assume this is what was happening behind the scenes.

I would expect the same outcome of the LSD too, provided traction control was on.

regarding your last question, that is well above my understanding of LSD, although it would make for one hell of a project.
 

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Discussion Starter #19
HoustonLex said:
I would expect the same outcome of the LSD too, provided traction control was on.

regarding your last question, that is well above my understanding of LSD, although it would make for one hell of a project.
I had traction control turned off for that particular experiment. I did not want automatic braking and throttle control to interfere with LSD bahavior.

Yeah, that could be a very interesting project. Too bad I do not have time and resources for that...
 

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evolver said:
I had traction control turned off for that particular experiment. I did not want automatic braking and throttle control to interfere with LSD bahavior.

Yeah, that could be a very interesting project. Too bad I do not have time and resources for that...
You're expriment just verifies that with a Torsen, when one wheel has zero traction, you're SOL.
 
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