limited slip "theory" question...

[aggiedave98]

I had a debate with a friend...

here's the scenario...

I have an open rear differential. I went across a median the other day and through some mud. I went through it perpendicular so that both tires were always in mud instead of going diagonally where one tire would be in mud and one would be on grass part of the time.

My arguement is that with an open differential, it was better for me to have both tires in mud so both tires have equal torque (assuming the same traction on both tires).

He was saying that it would not matter... that in both cases:

1) that even if I had one tire in mud and one on grass
2) both tires are in mud

I would have the same amount of torque going to the tires because for #1, the tire on the grass and the tire on the mud will have the same torque as in #2.

I may not be describing it well, but let me know if that makes sense.

I thought that with an open differential, maintianing the same traction on both tires will give me a better chance of getting through mud.

[Zoomad75]

On an open diff, it is going to send the power to the wheel with the lesser amount of traction. In #1 that would be the tire in mud since the mud is slicker than the grass. On #2 with both tires in the mud it could be either that spins depending on which one has the least amount of traction.

It is possible if you hit the mud with some speed/power both tires will be spinning, but once you stop you will only spin one.

This is where Lockers and Limited slips work. Instead of "wasting" power sending it through the wheel with the least traction, it splits the torque and sends half (locker) or a %up to half (limited slip) to the wheel with traction.

So in your example with an open diff, you are just as likely to get stuck with one tire in the mud as you would with both tires in the mud. I know this because I've been stuck both ways with an open diff. Getting stuck in the mud sucks.

[aggiedave98]

But for #2, what if they both have exactly the same traction in the mud??

Also, for #1, does it send ALL the power to the wheel with less traction or potentially 80% to that wheel depending on how much traction it has?? Meaning if 1 wheel is on concrete and 1 wheel is in the air, then yes, 100% will be to the wheel in the air, but if you have a little traction, then let's say only 80% goes to that wheel??

thanks.

[Prerunner1982]

i think its all or nothing. saying that it only sends 80% makes it sound like you have a torque biasing diff that will sense the traction and apply torque accordingly.. but hell i dont know....

[special-K]

In #2,it would be which wheel had less weight or the difference in resistence in the mud from one side to the other that would determine which side will spin.

[aggiedave98]

yeah, but I'm wondering for #2 if they had exactly the same traction, weight, etc. on both sides...

[Zoomad75]

Originally Posted by aggiedave98

But for #2, what if they both have exactly the same traction in the mud??

Also, for #1, does it send ALL the power to the wheel with less traction or potentially 80% to that wheel depending on how much traction it has?? Meaning if 1 wheel is on concrete and 1 wheel is in the air, then yes, 100% will be to the wheel in the air, but if you have a little traction, then let's say only 80% goes to that wheel??

thanks.

For #1 it could be one tire on perfectly dry pavement and the other on ice. Equal wieght side to side, the tire on ice is going to spin because it has less resistance to move. Look at it this way, power will always flow to the tire with the least resistance to turn. (sounds a lot like electricity don't it?). The tire on pavement or grass has higher traction and therefor harder to move. The open diff will always work this way with 100% of the power going to the wheel with the least resistance to move.

For #2, even with equal wieghts, it's hard to predict. Mud is a slick SOB and very inconsistant. You might be able to spin both if everything is equal side to side, but it don't take much difference in traction to have the power go to one side. Chances are, you will end up with one tire spinning.

Take a look at this video and watch the spider gears when the guy holds the left axle with his hand. All the power flows to the right.

http://www.rubicon-trail.com/public/poormans_locker.mov

If open diffs always spun both tires on the slick stuff we'd have no need for limited slips and lockers...Fact is they will always spin one side. Plus knowing how different it is to drive one that is locked up in the rear on slick stuff you half to pay attention. 90% of the dirving public would end up in a ditch on a snowy road with a rear locker.

[special-K]

Originally Posted by aggiedave98

yeah, but I'm wondering for #2 if they had exactly the same traction, weight, etc. on both sides...

Yes,in a controlled laboratory situation.
No,real world.

[aggiedave98]

thanks guys, that helps and makes sense.

[special-K]

So,you`re both kinda right and kinda wrong.Shake hands and buy each other a beer.But remember,buy the second round.That way,if you drink an odd number you'll buy one less.With an even number,it`s the same.And,when there`s six ounces in the can,one`ll be half empty and the other one half full

[Stocker]

Thought I'd try to further muddy the waters on this one. Assume one tire is on a slick surface, and the other one is on dry pavement. The way I have heard it explained....

An open diff sends equal amounts of torque to each axle. So when either tire is in a poor traction situation (in mud, on wet grass, ice, your choice), it takes only a very small amount of torque to make it spin.

The diff sends only that same amount of torque to the other axle -- and because it is such a small amount, it is not nearly enough to rotate the tire, therefore it cannot move the vehicle.

[Zoomad75]

Originally Posted by Stocker

Thought I'd try to further muddy the waters on this one. Assume one tire is on a slick surface, and the other one is on dry pavement. The way I have heard it explained....

An open diff sends equal amounts of torque to each axle. So when either tire is in a poor traction situation (in mud, on wet grass, ice, your choice), it takes only a very small amount of torque to make it spin.

The diff sends only that same amount of torque to the other axle -- and because it is such a small amount, it is not nearly enough to rotate the tire, therefore it cannot move the vehicle.

You are right that an open diff will split equal amounts when both tires have equal traction. But, when one tire is on ice and the other on dry pavement it gets NO torque, it all goes to the one will less resistance (less traction) to move.

Look at it this way...(assuming equal traction) with the pinion gear spinning the ring gear/carrier, power flows from the Carrier to the cross shaft and spider gears (spiders don't turn with equal traction). Power then goes to both side gears and then to the axles/wheels. Equal amounts of power are sent to each wheel.

Now change the example by having one tire with 100% traction and the other with 0 traction. Power still will flow from the pinion to the ring gear/carrier as it does in the other example. However, with one tire essentially stopped, the side gear splined to that axle is stationary. The carrier still rotates around the side gear since they are not splined together. The spider gears (on the cross shaft) rotate and still are able to turn the side gear (connected to the axle with 0 traction) with 100% of the power since the other side is stopped.

What is important to note here is that the side gears that are splined to the axle are not connected to the carrier in any way on an open diff. This is how you can get the difference in speed between wheels when turning. Without the differential action the inside tire would chirp or skid when turning (rotating at the same speed of the outside tire over a lesser distance). The only way power flows to the side gears is via the spider gears which rotate with the carrier via the cross shaft. IF the side gears were connected to the spider gears (welding the gears, locker or spool) each axle would always get 50% of the torque/power. Limited slip diffs use clutches instead of physically locking both axles together, depending on how tight the clutches are set up, they can send 50% of the power or less to each tire. The tighter the clutch, the more power split (no more than 50% though). Looser or worn out limited slips fall off the 50% mark.

Here's a real world example. A few years back I attended an event for the then new Chevy and GMC HD pickups. It was when they launched the then new duramax diesel. This was one of those Ride and drive type events where they had trucks from Ford and Dodge to compare against the new GM stuff. One drive they had demonstrated 2 items on the trucks, articulation and the locking diff (AKA G80, gov lock). The course was series of stacked railroad ties that would force one tire in the air in the rear while the other was still on the ground. Of course when the ford or dodge went over and got one tire in the air, that tire would just spin and forward progress stopped. Then they drove a GM truck to the same point. Due to how a gov-lock works, when power was applied, the tire in the air spun for a couple of seconds then the gov-lock snapped in (locking both axles together) and the truck proceeded to move forward and off the railroad ties. If you were watching the tire that was in the air, once locked it spun at the same rate as the tire on the ground did. Had it been a Detroit locker or other Lunchbox type locker it would not have spun the tire in the air, it would have proceeded forward.

Its not hard to understand if you can picture it in you mind or watch one spin with the cover off. Next time you change your diff fluid is a good opportunity! All you got to do is spin the pinion by hand while one wheel is stopped. Watch what the spiders and side gears do.

[Stocker]

Rob, I think we are saying virtually the same thing, but are saying it in different ways. Your in-depth explanations are very helpful. I disagree with you on one point, though, about equal torque being (or not) transmitted to each axle.

when one tire is on ice and the other on dry pavement it gets NO torque, it all goes to the one will less resistance (less traction) to move.

No, you get just enought torque to the "ice" tire to spin it. The "dry" tire gets the same amount of torque, which is not enough to spin it (or move it, and the vehicle, at all). Said another way, though the torque is equal to each axle, it is enough to spin the "ice" tire, but not enough to spin the "dry" tire.

How Stuff Works has a pretty good section on differentials and how they work, complete with some animated illustrations for those of us who don't have a diff cover off at the moment. http://auto.howstuffworks.com/differential.htm

[Zoomad75]

I can understand your thought. Bottom line, with an open diff with one tire on ice and one on dry pavement, you are not going anywhere.

I guess I was looking at it from pure power perspective. you start with 100% power on the input, since minimal power is lost through the simple gear train, you would end up with almost 100% (easy figure for arguements sake) output at the wheels. Equal traction has 50% at each wheel. The ice/pavement senario would still not loose power, so 100% of the power would hit the tire on ice. So if the still splits the power evenly, but if it only takes 20% of the torque to turn the tire on ice, 20% of the power would also go to dry tire. That would mean 60% of the power input is "lost". I'm wondering where it went...I'm not trying to be a smart arse, just curious.

I'm thinking of the zillion "1 tire fire" burnouts I've made with my 69 Nova. I can spin that right rear up to above 70 mph with the left tire going nowhere. It is an extreme example, but that's what got me thinking that all the torque/power goes to the tire with less traction. Just confusing to think about.

[Chevrolet4x4s]

Yeah open diffs have a tendency to spin the right rear. I suppose that is due to the torque reaction unloading the right rear and adding weight to the left rear.
Shane

[Chevrolet4x4s]

Originally Posted by Zoomad75

..........

http://www.rubicon-trail.com/public/poormans_locker.mov

If open diffs always spun both tires on the slick stuff we'd have no need for limited slips and lockers...Fact is they will always spin one side. Plus knowing how different it is to drive one that is locked up in the rear on slick stuff you half to pay attention. 90% of the dirving public would end up in a ditch on a snowy road with a rear locker.

As the animation shows > lockers/limited slips can and eventually will kick you sideways which has and will continue to put lots of people in ditches.
Shane