Differentials: open vs limited slip

[v8440]

Through a bunch of years, I've read numerous articles about differentials, and thought about how they behave under different circumstances. (This is basically a disclaimer explaining that I can't pinpoint any specific source for what I'm about to say...)

My understanding of differentials is this:

Open differentials apply equal TORQUE to each axleshaft.

Limited slip differentials attempt to equalize the SPEED of the axleshafts.


This may not seem to make sense at first, especially the statement regarding open differentials. Lemme try to explain, and show that real-world behavior supports this.

Take a vehicle with an open differential. Let's say one wheel is on ice, and the other is on dry pavement. When you step on the accelerator, the wheel on ice spins and the wheel on pavement does little or nothing. This is the "equal torque" part of open differentials at work. It takes very little torque to spin the wheel on ice. So, the speed of the wheel on ice goes up, since it's not providing much resistance. Meanwhile, the wheel on pavement is doing nothing. Ever been told to apply the parking brake in such a case? It often works, and here's why: Applying the parking brake adds resistance to the spinning action of the wheel on ice. This resistance must be overcome, by more torque. If the drive torque to the wheel on ice increases because you applied the brake, the drive torque to the wheel on pavement also increases. It may well increase to the point that the wheel on pavement will now move the vehicle forward. It's not necessary to bring the spinning wheel to a standstill, only to provide enough resistance (and resulting torque to both wheels) to over come whatever may be trying to hold the vehicle still.

Now, you may be thinking "yeah, but the brakes are also holding the wheel on pavement back." That's true, but here's the catch: the spinning wheel is producing more friction from the brakes than the stationary wheel. So, the added torque applied due to brake resistance on the spinning wheel is greater than the brake resistance on the stationary wheel. If this difference is great enough, the vehicle can move.

Now, let's look at limited slip differentials. I said earlier that they try to equalize wheel SPEED. Go around a corner in a vehicle with a limited slip differential, and the wheels don't slide, do they? (unless it's a VERY tight limited slip unit) That's not because the unit isn't trying to hold wheel speed equal, it's because it has only a limited ability to do so, and turning a corner exceeds that ability. Hence, it lets the wheels turn at different speeds until the vehicle is pointed straight again. Ok, let's put this vehicle in the same situation as the other one. One wheel is on ice, and the other is on pavement. You step on the accelerator, and the vehicle probably moves forward. This is because the limited slip unit can apply more torque (to a limit) to the wheel with traction in it's attempt to keep speeds equal. Assuming you apply enough power to overcome whatever is trying to hold the vehicle in place (maybe just inertia) and the limited slip unit is capable of enough torque biasing in favor of the wheel with traction, the vehicle will move.

It is possible to overpower a limited slip unit, causing one wheel to rotate at a different speed than the other. Let's say you have the aforementioned vehicle in the aforementioned situation, but there's a big travel trailer with 4 flat tires hooked to the vehicle. Now there's a bunch of resistance that must be overcome to move the vehicle. To do this, the limited slip unit must be capable of biasing the torque to the wheel with traction much more than was required to move the unencumbered vehicle. If the limited slip unit is not capable of biasing enough torque to the wheel with traction, the wheel without traction will spin anyway, and the vehicle will remain stationary. Additionally, doing this will cause the limited slip unit to wear out very fast, as its clutches are slipping constantly in this situation. When it wears out, it will cease functioning as a torque biasing device, and will behave exactly as an open differential.

Note: The factory limited slip units in our dodge diesel trucks often require application of the parking brake to begin working. This is not because they are not true limited slip units, but rather is because the type of unit in these is not a simple clutch setup. It involves a worm gear that progressively biases more and more torque as the situation requires, but it apparently is designed such that a little initial resistance from the spinning wheel is needed to activate the unit. I don't know the mechanics of why this is the case, only that it is. Once activated by using the parking brake, it will then proceed to supply much more torque to the wheel with traction than an open unit would with the parking brake applied.

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'98 12v 2wd quadcab dually with 271k, BHAF, #0 plate shoved all the way forward, afc arm removed from pump, 3k gsk, timing set at 16.5 deg, 4" mandrel bent exhaust, new 3.54's and detroit locker, southbend dual disk clutch, fogger nozzle with #82 jet plumbed into air horn, htb2-64 turbo, has broken spider gears in two dana 80's so far, has run 14.65 at 98 and dynoed at 450 hp 906 ft/lb torque in 4th gear on bottle, 399 hp 814 ft/lb in 5th gear without bottle

[Mr. 12 Valve]

I put limited slip in my truck and i love it.

[Power Ram Man]

ive got posi from the factory and its awsome

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1992 Dodge W250 4x4 LE,3.54, modified automatic, 5 inch intake with AFE filter,4 inch exhaust,HX35 turbo,fuel tweaked,16cm exhaust housing,PODS,Denny T fuel pin,3200 govoner spring, timing advanced 1/8, autometer Boost,pyrometer gauges,366 spring Water/meth.Coolertubz

[5.9fan]

I dont know what mine has, but i went on some thin snow the other day to find out. I hammered it, and both tires spun, with little or no lag. Locking? Posi? any guesses?

Good article btw!

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1993 D350 dually, automatic. starwheel and afc adjusted:thumbsup
other rides: 1956 Packard patrician police car. 1955 Packard patrician sedan.

[Old 'n Rusty]

Posi........ years ago there were some Volvo's made available to the general public. They were designated 123 GT's. The European models had Overdrive. North American Models had Posi. Most ralyists paid to have the posi removed as the spinning of both wheels made the cars rather prone to doing 360's on ice. Posi will get you through stuff that standard will not but it will make both rear wheels spin and slide. I find that the posi makes it too easy to slide sideways when trying to manoever through an icy lane with parked cars nearby. It will almost always let the truck slide down hill. I will keep my posi as it helps deal with heavy loads but I am aware that it will cause sideways sliding. I guess chains would help but.......

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91 (08-90) D250

5sp 2wd 497k VE 'adjusted', 3200 spring, Pyro, boost, 3.54 ,:thumbsup KN, 1\8" timing bump, 25mpg city 30mpg hiway 20mpg towing 6k 5'er

R60/7, Volvo 245 GL, f150 4x4 5 spd, Ram 318, MPG's are in Imp gal's

[Captain Nemo]

Snow plowing will wear out the factory limited slip differential.

[DODGEZILLA W350]

I have ARB air lockers in both axles, This billy goat can climb now and I can switch them back to open diff's for a smooth highway ride, the best of both worlds!

[Alphawhiskey]

But a Locker is way cooler. Both spin as long as your on it. And when in a low traction situation. . Well lets say my foot makes sure that wheels are gettin power

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Last edited by Alphawhiskey : 05-24-2007 at 03:17 PM.

[Alphawhiskey]

For us poor folk who cant afford an ARB a Locker is way cool. As long as your on it both will spin. And when in a low traction situation my foot makes sure theys both throwin the muck around

[chuckect]

I just got a new 92 and spent $2000 to have it totaly rebuilt; all new bearings on the axles and brakes. Then $2000 on the rebuilt trans. I need some pointers on what to do with that throttle position sensor. Also, can I set the cruise control @ 80mph and just let it run - i have the 4 speed trans and 3.54 rearend

[chuckect]

that first $2000 was just the rear end and axles

[coalfed]

what size is the factory rearend on a 1991 4x2 5spd???

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2006 3500 SRW 4x4 QC,5.9, and a 97 3500 4x2 DRW both
Completely *UN-Stock
*List of mods available upon request