limited slip diffs explained

briggie · 24-12-2010, 08:50

Limited-Slip (or Traction Lok - the Ford name, or Posi-Traction, GM name) is a function of the differential allowing one wheel to spin only a percentage faster than the other. This has its advantages in allowing more traction, especially during hard cornering, because a normal or "open" diff will allow the inside wheel to lose traction and spin as fast as it wants, while the outer tire (with most of the traction anyway) just freewheels. This is generally opposite of what you want to have happen. A Posi unit will allow the inside wheel to spin only so much faster than the outside, at which time it transfers torque to the outer wheel, increasing power that's transmitted to the ground. This is achieved by several methods. The most common is with clutch packs in the differential that are set to a "breakaway" threshold. Another semi-common way is using viscosity (Viscous LSD's), which use a special differential fluid to control the amount of slip. A third method is a Torsen type, where there's a worm gear and pinion (the theory being the worm gear can rotate the pinion, but not vice versa). In effect, you can think of cars without Posi to be "one wheel drive" and cars with Posi to be two-wheel drive.
Picture a car rounding a corner; the inside and outside wheels have to carve differently sized circles. Because of this, they must travel at different speeds or independent of eachother. The non-drive wheels are no problem, but the drive wheels are connected to eachother via halfshafts, ring gear and driveshaft. This is where a normal OPEN differential comes into play. It allows the drive wheels to spin at different speeds when rounding a corner (the tires and drivetrain would bind and not corner as well without it). The problem with totally open differentials is that you cannot control how much and when they slip. Trying to accelerate on ice or gravel will usually result in only 1 tire spinning, producing plenty of smoke, but no go.

The limited slip differential does much the same job as the open, except (and nearly all OEM LSD's are like this...) there is a viscous fluid inside the differential that heats up and becomes VERY sticky/gooey when the two sides (either the two wheels or the two differentials - like on the Mitsubishi GTO and Eclipse AWD systems) spin at greatly different speeds (say, when one tire begins to spin wildly on ice or loose dirt...). The two sides are then locked together in order to prevent all the engine torque from going to the slipping wheel. It's basically a traction control device. High performance sports cars use them to put power down at a standing start and when powering out of corners. Trucks and minivans use it to keep the vehicle from spinning in adverse conditions (snow, ice, gravel, freak rain storms, etc.). Most AWD (as opposed to 4WD) systems emply some manner of viscous limited slip in order to transfer power to the other set of wheels when needed, but avoid it to avoid drivetrain wear and improve fuel economy. This is what Subaru means by "sending power from the wheels that slip to the wheels that grip...". Makes it sound like there's a big decision making process when you hit adverse
conditions, but not really.

zippy656 · 24-12-2010, 08:57

so....

you know dont you

got a PIC of the insides ???

briggie · 24-12-2010, 08:58

Quote:

Originally Posted by zippy656

so....

you know dont you

got a PIC of the insides ???

some piccies here

http://en.wikipedia.org/wiki/Limited_slip_differential

lacroupade · 24-12-2010, 11:26

I think you'll find that the LSDs fitted to sports cars are very different to the T2s which is a pretty unsophisticated clutch type (the first one mentioned then ignored in that write-up LOL), not the more expensive fluid, speed sensitive type units fitted on cars.

I mean can you imagine a Porker or Beemer scrabbling round corners because the LSD is thinking its on a T2? Tyres would be shagged in a week LOL

clivvy · 24-12-2010, 12:52

Deleted account DD · 24-12-2010, 17:27

So how does my quadra trac electronic lsd work then ?

clivvy · 24-12-2010, 17:29

Quote:

Originally Posted by daved

So how does my quadra trac electronic lsd work then ?

your what now?

Deleted account DD · 24-12-2010, 17:32

Quote:

Originally Posted by clivvy

your what now?

exactly

briggie · 24-12-2010, 18:30

Quote:

Originally Posted by daved

So how does my quadra trac electronic lsd work then ?

ah .... that means your thingummy is connected to your wotsit by means of a oojaflip

zippy656 · 24-12-2010, 18:34

bit thecnical there Briggie...

you lost me

Timbo_1975 · 24-12-2010, 18:39

Quote:

Originally Posted by clivvy

your what now?

In addition to the oil and clutch plates, you have a small pump, at least one electric oil control valve, and afew little pistons acting upon the clutch plates inside.

When one wheel spins, the ECU via the abs sensor notices this, and opens the valve(s) to apply the clutch in the diff locking the two wheels together. The non spinning wheel then gets drive. When the clutch is locked, its the same as a diff lock- the axle is solid.

The more current system has two valves and two clutches so that drive can go to either side rather than just lock the diff. That way we can apply drive to the outside wheel on a corner to improve handling characteristics as well as grip.

Something like an Evo has the above system on both front and rear axles, as well as a electrically switchable center diff coupling (often partially viscous-based) so that power can be directed exactly where the ecu wants to.

Does that help?

(RIP) PLANK · 24-12-2010, 18:55

Quote:

Originally Posted by briggie

ah .... that means your thingummy is connected to your wotsit by means of a oojaflip

no mate it's not an oojaflip on that model its a doofa !

Deleted account DD · 24-12-2010, 19:16

Quote:

Originally Posted by PLANK

no mate it's not an oojaflip on that model its a doofa !

and there was me thinking the watchacallit was the main bit

(RIP) PLANK · 24-12-2010, 19:40

Quote:

Originally Posted by daved

and there was me thinking the watchacallit was the main bit

no mate that was on the old model, the latest one has a thingamejig!

Deleted account DD · 24-12-2010, 19:45

Quote:

Originally Posted by PLANK

no mate that was on the old model, the latest one has a thingamejig!

ok , im a numpyty

banghead thank you

24-12-2010, 08:50	#1
briggie Senior Member Join Date: Mar 2010 Location: peoples democratic republic of west yorkshire Vehicle: " alice " Posts: 10,473	limited slip diffs explained Limited-Slip (or Traction Lok - the Ford name, or Posi-Traction, GM name) is a function of the differential allowing one wheel to spin only a percentage faster than the other. This has its advantages in allowing more traction, especially during hard cornering, because a normal or "open" diff will allow the inside wheel to lose traction and spin as fast as it wants, while the outer tire (with most of the traction anyway) just freewheels. This is generally opposite of what you want to have happen. A Posi unit will allow the inside wheel to spin only so much faster than the outside, at which time it transfers torque to the outer wheel, increasing power that's transmitted to the ground. This is achieved by several methods. The most common is with clutch packs in the differential that are set to a "breakaway" threshold. Another semi-common way is using viscosity (Viscous LSD's), which use a special differential fluid to control the amount of slip. A third method is a Torsen type, where there's a worm gear and pinion (the theory being the worm gear can rotate the pinion, but not vice versa). In effect, you can think of cars without Posi to be "one wheel drive" and cars with Posi to be two-wheel drive. Picture a car rounding a corner; the inside and outside wheels have to carve differently sized circles. Because of this, they must travel at different speeds or independent of eachother. The non-drive wheels are no problem, but the drive wheels are connected to eachother via halfshafts, ring gear and driveshaft. This is where a normal OPEN differential comes into play. It allows the drive wheels to spin at different speeds when rounding a corner (the tires and drivetrain would bind and not corner as well without it). The problem with totally open differentials is that you cannot control how much and when they slip. Trying to accelerate on ice or gravel will usually result in only 1 tire spinning, producing plenty of smoke, but no go. The limited slip differential does much the same job as the open, except (and nearly all OEM LSD's are like this...) there is a viscous fluid inside the differential that heats up and becomes VERY sticky/gooey when the two sides (either the two wheels or the two differentials - like on the Mitsubishi GTO and Eclipse AWD systems) spin at greatly different speeds (say, when one tire begins to spin wildly on ice or loose dirt...). The two sides are then locked together in order to prevent all the engine torque from going to the slipping wheel. It's basically a traction control device. High performance sports cars use them to put power down at a standing start and when powering out of corners. Trucks and minivans use it to keep the vehicle from spinning in adverse conditions (snow, ice, gravel, freak rain storms, etc.). Most AWD (as opposed to 4WD) systems emply some manner of viscous limited slip in order to transfer power to the other set of wheels when needed, but avoid it to avoid drivetrain wear and improve fuel economy. This is what Subaru means by "sending power from the wheels that slip to the wheels that grip...". Makes it sound like there's a big decision making process when you hit adverse conditions, but not really.

24-12-2010, 08:57	#2
zippy656 Senior Member Join Date: Aug 2008 Location: Devizes Wiltshire Vehicle: Nissan Note Ntec 1.5 Posts: 14,138	so.... you know dont you got a PIC of the insides ???

24-12-2010, 11:26	#4
lacroupade Senior Member Join Date: Jan 2009 Location: All hail to the Glove of Love... Posts: 9,212	I think you'll find that the LSDs fitted to sports cars are very different to the T2s which is a pretty unsophisticated clutch type (the first one mentioned then ignored in that write-up LOL), not the more expensive fluid, speed sensitive type units fitted on cars. I mean can you imagine a Porker or Beemer scrabbling round corners because the LSD is thinking its on a T2? Tyres would be shagged in a week LOL

24-12-2010, 12:52	#5
clivvy Senior Member Join Date: Mar 2009 Location: Huddersfield Vehicle: Terrano 03 LWB Posts: 6,947

24-12-2010, 17:27	#6
Deleted account DD Senior Member Join Date: Sep 2008 Posts: 4,705	So how does my quadra trac electronic lsd work then ?

24-12-2010, 18:34	#10
zippy656 Senior Member Join Date: Aug 2008 Location: Devizes Wiltshire Vehicle: Nissan Note Ntec 1.5 Posts: 14,138	bit thecnical there Briggie... you lost me