Locking, limited-slip, and positrac

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Can anyone explain the differences of these?



Let's add open differential. I THINK an open differential will supply ALL engine power to the tire that needs it the least (aka the one that spins easier). An open diff is said to be peg-legged.
 
I can explain the practical differences.

Locking = the wheels spin at exactly the same speed and get exactly the same amount of torque. Excellent for stuff like rock climbing and to some extent mudding.

Open = the wheel with the least traction gets pretty much all of the torque, generally it's not the best for icy/snowy/off road conditions but with traction control it's become a lot better. This is by far the cheapest to make and generally the most reliable. Locking differentials are usually just open diffs with a mechanical link to lock the axles together mechanically.

LSD = Somehow internally the speed differential between the wheels is limited. Supposedly good for light off roading and snow/ice, not nearly as much as locking though. I'm not sure exactly how but it must provide an advantage to sports cars intended to be used on the track too because a lot of them have LSDs

I believe posi trac is just a branded LSD system
 
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Originally Posted By: horse123
I can explain the practical differences.

Locking = the wheels spin at exactly the same speed and get exactly the same amount of torque. Excellent for stuff like rock climbing and to some extent mudding.

Open = the wheel with the least traction gets pretty much all of the torque, generally it's not the best for icy/snowy/off road conditions but with traction control it's become a lot better. This is by far the cheapest to make and generally the most reliable. Locking differentials are usually just open diffs with a mechanical link to lock the axles together mechanically.





Thanks....your posts are all 1111s. Post 1111 for you.



That is neat. An LSD will allow both wheels to be driven at different speeds so you can power out of turns and still lay down a couple of patches at the dragstrip. While, a locker (G80 for example) will (I think) just perform at lower speeds when it 'senses' one tire spinning, it willinitiate the other side so both are pulling evenly to get you out of a slick or even lay down a two mark burnout.

LSD = Somehow internally the speed differential between the wheels is limited. Supposedly good for light off roading and snow/ice, not nearly as much as locking though. I'm not sure exactly how but it must provide an advantage to sports cars intended to be used on the track too because a lot of them have LSDs

I believe posi trac is just a branded LSD system
 
Traction control is bad for racing. You are applying brake to the spinning wheel, and the power moves to the other wheel. You may be able to shift power from wheel to wheel, but never together.
Some systems simply use the brake (early-mid 90s Fords, modern RWD BMWs) but others actually reduce engine torque and or brake too.

Limited slip uses clutches in the differential to make both wheels spin together. The spinning wheel causes lots of friction in the clutches and causes it to lock to the side of the case. A special stinky fluid has to be added so the clutches don't bind if the speed difference isn't too great (making a turn for example). The amount of torque is quite limited though, maybe 30-40ft/lbs and then it becomes an open diff. Limited slip works nice if a muscle car starts out on dry pavement, leaving two marks. Almost acts like a locker there. There is always traction for the other wheel to grab. In snowy weather, it helps, but you can see things getting tricky.

However if both wheels have no traction (ice for example) the limited slip turns more into an open diff(I guess this is in the name of safety, no joke hitting an ice patch at 60 mph with both wheels spinning). Lockers are not street friendly, and if it from the OEM it works only at very low speed. Turning becomes an understeer nightmare because the wheels can't move at different speeds in the arcs they follow. One tire starts to slip. Not much different from Locked 4x4, which is normally used offroad.

In all these cases though, you need good friction between the rubber and road. Snow tires could boost that friction.
 
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Yeah, never had a problem with my open differentials. Are there times I wish I had a locker maybe? Sure. And 4 more inches of ground clearance. But for a $17,000 car AWD with opens is good for me.
 
I'll take a stab at this...

An open differential will allow one tire to turn slower than the drivetrain but never faster, so in a corner it's actually powering the outer wheel. When one tire starts to spin it doesn't get all the torque, the other tire gets exactly the same torque applied to it. For example one tire on ice the other tire on dry pavement. The tire on the ice spins and the tire on the pavement gets exactly the same torque as it takes to spin the tire on the ice.

Limited slip uses many different methods to accomplish the task of limiting the differential action between one axle and the other. It does this through clutches (either cone or plate) or via helical gears. When one tire starts to turn faster than the other the force of the spider gears turning aided by spring preload helps transfer torque to the wheel with more traction. In a helical gear differential the bias ratio does the same thing without clutches.

Mechanical locking differential will allow a tire to turn faster than the drivetrain but never slower. Going around a corner it will always be powering the inside tire. These usually use some sort of locking tooth design with springs to disengage when torque isn't applied to the axle.
 
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Sorry hit post too fast.

The positraction was a name used by GM. Mopar called theirs SureGrip. Just trade names.

Limited slips are not disengaged at speed with the exception of the Gov-Lock. True mechanical lockers are also not disengaged at speed.

Factory selectable lockers electric or air physically lock the two axles together removing all differential action and force the two axle shafts to turn at the same speed. Like a spool.
 
Originally Posted By: horse123
...
Locking = the wheels spin at exactly the same speed and get exactly the same amount of torque...
Open = the wheel with the least traction gets pretty much all of the torque, ...


Corrections:

Locking = both wheels spin at the same speed, which means torque can be unequal, but also means the inner wheel is forced to turn the same speed as the outer one in turns

Open = both wheels always get equal torque, which means torque at the wheel with the best traction can't exceed torque at the one with least traction
 
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Then you also Toyota's weird implementation of using an open diff and ABS system to create a limited slip diff effect. Dubbed A-LSD that was introduced in the 2007 2nd gen Tundra it uses the brakes on the spinning wheel so power can be transferred to the stuck wheel.

I'd wish they would stick a real LSD or locker in their fullsize trucks. I rather take my 02 Silverado than the 01 Tundra if I ever encounter an off-road situation.
 
Originally Posted By: FlyNavyP3
True mechanical lockers are also not disengaged at speed.


They can only be engaged at low speed, but will stay engaged at any speed. Engaging a locker with one wheel spinning madly is going to cause some damage.
 
Originally Posted By: Kibitoshin
Then you also Toyota's weird implementation of using an open diff and ABS system to create a limited slip diff effect. Dubbed A-LSD that was introduced in the 2007 2nd gen Tundra it uses the brakes on the spinning wheel so power can be transferred to the stuck wheel.


Chevvy used that on a Corvette in the '80s...the one with reverse cooling had that as well according to the mags at the time.
 
Originally Posted By: FlyNavyP3
Mechanical locking differential will allow a tire to turn faster than the drivetrain but never slower. Going around a corner it will always be powering the inside tire. These usually use some sort of locking tooth design with springs to disengage when torque isn't applied to the axle.


If the torque is backwards (e.g. on over-run), the slower of the wheels will be the freewheeler.

That's why the lockers can be such fun (not), they power the inside wheel out of a turn, until you spin it enough to catch the outside, then they both turn at equal speed. On overrun, the outer wheel is powered, while the inner freewheels...power on again, and the roles are (clunk) reversed, which upsets balance.
 
mjoeking, a couple of other points.

Single wheeler diffs have simple gear trains inside them that apply equal torque to both wheels (as has been previously described by others). If one wheel has no torque available (e.g. on ice), than that's ALL the tractive effort that's available...that wheel will spin, and power = torque x rpm, more POWER will go to the spinner, and you won't go very far/fast. Easily identified by jacking both wheels off the ground, and spinning a wheel...the other one will go backwards.

Limited Slip has (as has been described by others), a clutch of some sort....many have packs of clutches, they used often to be metallic cones (cone type LSD). These clutches are usually between the differential side gears and the rotating diff centre. They CAN be one side only, but usually symetrical. Being between the side gears and centre casing, the more load that is applied, the more squeeze is put on the clutch packs (or cones), making the diff tighter. Identifiable by jacking both wheels up, and turning one...the other will go the same direction...can also be identified by squeels and chirping as owners drive around concrete carparks, or painted lines.

Lockers...aren't truly a differential, but a clutch arrangement. A true differential, the centre will turn at the average speed of the wheels, a locker, the centre turnes only at the speed of the driven wheel. Due to (dog type, not frictional) clutching arrangements, the unloaded wheel will over-run the speed of the centre, and only provide lock up when the drive wheel speed matches the over-running speed, i.e the wheel slips...clunky, but effective, can transfer nearly 100% power to the wheel whose axle you are about to break.

Spool (or CIG/welded locker), fixed, so that there's no differentialising action...can't do U turns.

Viscous LSD uses a viscous clutch instead of a traditional clutch...relied on the unloaded wheel spinning, and the difference in speed to apply torque to the planted wheel (not a fan, but they are pretty OK in a FWD as I found when my parents owned one).

Torsen...these are really, really clever. Think of a worm gear, and how you can easily drive a ring gear with a worm, and can't drive the worm gear via the ring gear. That is called "non backdriving", you can't reverse the power flow. get the angles right, and you can create a "backdriving worm gear"...couple a backdriving and a non backdriving gear in a differential centre, and you can create a true differential where you can turn one wheel and have the other go backwards, but will not allow a wheel to massively increase in speed, and so biases torque to the loaded wheel, all through these worm gears. Through angle design, you can vary the torque split.

There's others these days that use electromagnetic fluids controlled electronically to provide torque split, internal electronic clutches, and lord knows what.
 
Originally Posted By: Shannow
Spool (or CIG/welded locker), fixed, so that there's no differentialising action...can't do U turns.


Ahh, the good old CIG locker - whose name probably makes no sense outside of Oz - as kids, we had so much fun learning to do doughnuts in our paddock basher Holdens and Falcons after my cousins had welded up the diffs!
 
LSD doesn't always have a clutch. Many cars have mechanical units, usually Torsens. The downside to a Torsen is that both wheels have to have some traction for it to work; if you lift a wheel, it acts as an open diff. They also can't be tuned, really. The upside is they don't wear out.

robert
 
Shannow,

Good point about mechanical lockers and the over run or coast aspect, I left it out in terms of simplicity of description. However the inherent coast drive gremlins is what ultimately caused me to switch from a Detroit Locker in my Toyota to a TrueTrac limited slip.

Also left off the list is the hydraulic actuated Dana limited slip in which a hydraulic pump actuated by differential action in the spider gears pumps fluid to apply pressure to a piston increasing preload to the clutches. I had one in a Jeep and I really liked how it worked, smooth and seamless and it would still transmit a great deal of torque even with a wheel off the ground.
 
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