The LSD Explained
03-15-2007, 12:17 PM
#1
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The LSD Explained
seems there are way to many threads about what a lsd does and if its useful or not. So ive made this thread to try to explain it to everyone and make it easy to understand.
This isnt my writing, im taking this information from the website howstuffworks.com
What is a differential?
Open Differential
Limited Slip Differential
Cliff Notes by -Jayson-
So what does this all mean? If both your tires get similar traction then you shouldnt get the one wheel peel. In an non LSD transmission both tires should peel out with the exact same amount of torque. If your getting one wheel peels, then both your wheels arent getting the same amount of traction. Because even though one wheel is spinning and the other isnt, both wheels are getting the same amount of power. This might confuse people cause they see cars spinning one tire in the snow, well yes it may appear the one wheel is getting more power, in reality its getting the same amount of power as the other wheel, just the power is so weak, it cant cause the other tire to move/spin. So if one tire requires 80 lbs/ft torque to spin, then the other tire will get 80 lbs/ft torque to use. This puts a combineb 160 lbs/ft torque to the ground. So if your on snow and it only takes 10lbs/ft torque to spin the tire then the other wheel is only getting 10 lbs/ft torque , but that might not be enough lbs/ft torque to move the wheel.
Now in a LSD based transmission each wheel has independent control of how much power it can put to the ground before it begins to slip. When one wheel starts to spin, instead of the other wheel being limited to amount of power in the spinning wheel, its free to use as much power as it can before it starts to spin. So each wheel can have different amounts of power put to the ground. One wheel might require 120lbs/ft torque to spin and the other only 80lbs/ft torque . So if both tires are spinning, one is spinning with 120lbs/ft torque and the other is spinning with 80 lbs/ft torque . Putting a combined power of 200 lbs/ft torque to the ground.
So basically in drag racing terms a limited slip differential is only helpful when traction is an issue. If your tires are good and you setup is solid, you should be able to launch the car without much wheel spin, thus negating the need for a LSD. But if your power becomes to much and launching the car easily causes wheel spin, a LSD is a helpful way to control the spinning tire and apply the power more efficiently to the ground.
In cornering aspects LSD really make a difference. FWD cars have alot of understeer, this is the tendency of a car to be subjected to centrifigul force. Centrifigul forces is that all moving objects want to travel in a straight line, so if an object is moving through a turn, at any instance of that turn, gravity its pushing to the outside of a turn. This makes it very easy to lose traction and spin a tire. Now in a NON LSD transmission, once the point of gravity causes the wheels to lose traction and get pulled to outside of the turn, the lbs/ft torque to the wheels is once again limited by the wheel that is slipping. But in a LSD based trasmission the lbs/ft torque applied to both wheels is independent, so one wheel can continue to put more power to the ground untill the point at which it loses traction.
I hope you now have a decent understand of Limited Slip Differntials
This isnt my writing, im taking this information from the website howstuffworks.com
What is a differential?
The differential has three jobs:
To aim the engine power at the wheels
To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)
In this article, you'll learn why your car needs a differential, how it works and what its shortcomings are. We'll also look at several types of positraction, also known as limited slip differentials.
Why You Need a Differential
Car wheels spin at different speeds, especially when turning. Each wheel travels a different distance through a turn, and that the inside wheels travel a shorter distance than the outside wheels. Since speed is equal to the distance traveled divided by the time it takes to go that distance, the wheels that travel a shorter distance travel at a lower speed. Also note that the front wheels travel a different distance than the rear wheels.
For the non-driven wheels on your car -- the front wheels on a rear-wheel drive car, the back wheels on a front-wheel drive car -- this is not an issue. There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. If your car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components.
To aim the engine power at the wheels
To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)
In this article, you'll learn why your car needs a differential, how it works and what its shortcomings are. We'll also look at several types of positraction, also known as limited slip differentials.
Why You Need a Differential
Car wheels spin at different speeds, especially when turning. Each wheel travels a different distance through a turn, and that the inside wheels travel a shorter distance than the outside wheels. Since speed is equal to the distance traveled divided by the time it takes to go that distance, the wheels that travel a shorter distance travel at a lower speed. Also note that the front wheels travel a different distance than the rear wheels.
For the non-driven wheels on your car -- the front wheels on a rear-wheel drive car, the back wheels on a front-wheel drive car -- this is not an issue. There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. If your car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components.
Open Differential
The open differential always applies the same amount of torque to each wheel. There are two factors that determine how much torque can be applied to the wheels: equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; in a low traction situation, such as when driving on ice, the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions. So, even though a car may be able to produce more torque, there needs to be enough traction to transmit that torque to the ground. If you give the car more gas after the wheels start to slip, the wheels will just spin faster.
On Thin Ice
If you've ever driven on ice, you may know of a trick that makes acceleration easier: If you start out in second gear, or even third gear, instead of first, because of the gearing in the transmission you will have less torque available to the wheels. This will make it easier to accelerate without spinning the wheels.
Now what happens if one of the drive wheels has good traction, and the other one is on ice? This is where the problem with open differentials comes in.
Remember that the open differential always applies the same torque to both wheels, and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip. It doesn't take much torque to make a tire slip on ice. And when the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction, your car isn't going to move very much.
On Thin Ice
If you've ever driven on ice, you may know of a trick that makes acceleration easier: If you start out in second gear, or even third gear, instead of first, because of the gearing in the transmission you will have less torque available to the wheels. This will make it easier to accelerate without spinning the wheels.
Now what happens if one of the drive wheels has good traction, and the other one is on ice? This is where the problem with open differentials comes in.
Remember that the open differential always applies the same torque to both wheels, and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip. It doesn't take much torque to make a tire slip on ice. And when the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction, your car isn't going to move very much.
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 determine 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.
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 determine 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.
So what does this all mean? If both your tires get similar traction then you shouldnt get the one wheel peel. In an non LSD transmission both tires should peel out with the exact same amount of torque. If your getting one wheel peels, then both your wheels arent getting the same amount of traction. Because even though one wheel is spinning and the other isnt, both wheels are getting the same amount of power. This might confuse people cause they see cars spinning one tire in the snow, well yes it may appear the one wheel is getting more power, in reality its getting the same amount of power as the other wheel, just the power is so weak, it cant cause the other tire to move/spin. So if one tire requires 80 lbs/ft torque to spin, then the other tire will get 80 lbs/ft torque to use. This puts a combineb 160 lbs/ft torque to the ground. So if your on snow and it only takes 10lbs/ft torque to spin the tire then the other wheel is only getting 10 lbs/ft torque , but that might not be enough lbs/ft torque to move the wheel.
Now in a LSD based transmission each wheel has independent control of how much power it can put to the ground before it begins to slip. When one wheel starts to spin, instead of the other wheel being limited to amount of power in the spinning wheel, its free to use as much power as it can before it starts to spin. So each wheel can have different amounts of power put to the ground. One wheel might require 120lbs/ft torque to spin and the other only 80lbs/ft torque . So if both tires are spinning, one is spinning with 120lbs/ft torque and the other is spinning with 80 lbs/ft torque . Putting a combined power of 200 lbs/ft torque to the ground.
So basically in drag racing terms a limited slip differential is only helpful when traction is an issue. If your tires are good and you setup is solid, you should be able to launch the car without much wheel spin, thus negating the need for a LSD. But if your power becomes to much and launching the car easily causes wheel spin, a LSD is a helpful way to control the spinning tire and apply the power more efficiently to the ground.
In cornering aspects LSD really make a difference. FWD cars have alot of understeer, this is the tendency of a car to be subjected to centrifigul force. Centrifigul forces is that all moving objects want to travel in a straight line, so if an object is moving through a turn, at any instance of that turn, gravity its pushing to the outside of a turn. This makes it very easy to lose traction and spin a tire. Now in a NON LSD transmission, once the point of gravity causes the wheels to lose traction and get pulled to outside of the turn, the lbs/ft torque to the wheels is once again limited by the wheel that is slipping. But in a LSD based trasmission the lbs/ft torque applied to both wheels is independent, so one wheel can continue to put more power to the ground untill the point at which it loses traction.
I hope you now have a decent understand of Limited Slip Differntials
03-16-2007, 02:35 PM
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maybe we could dig up some info on the specific kinds of LSDs and other diffs, like the helical, viscous, posi, etc. i've personally always wondered how those differentials work differently than the usual types.
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