What's the difference between a "limited slip" and a "Posi"

I just had to take out the Bronco yesterday, Still have some snow/slush on the trails.
Currently it has 3:50 "open" diff's. When stepping on it in the snow, of course it what to pull to one side and then we fishtail, fun in itself but.....

Would installing a LS or Posi help "keep it straight" on acceleration in slippery conditions?

1. So what's the difference LS vs Posi?

2. Would I have to swap out both front and rear Diff's or just do the one in the rear?
Thanks

#1 - "Posi" and limited slip are the same thing. Posi-traction is GM's brand name for it, and it kind of became universal, like "Coke" or "Kleenex".

#2 - You can run limited slip in back but not in front, no problem. That's how my truck is set up.

There are many forms of limited slip - clutch & cone, helical worm gear, torsen, detroit locker, and so on. I'm a fan of the helical worm gear or the torsens. Very tough, don't need periodic rebuilding like the clutch & cone style, and don't have the clunkiness of the detroit lockers.

There are also "part time" lockers that are activated by air or electrically, so you can turn them on and off at will.

Pete is right.  

But a bit more explanation on the front.  Ford had a limited-slip option for the front, but fortunately very few bought it.  My understanding is that with it when you are under power in slippery conditions you go forward, regardless of how you turn the wheel.  In fact, one said "You don't want to send your wife or daughter out in the snow with it."  I've not tried it, although I do have an electric locker in front on Big Blue, so if I get out in the snow some day I will just to be able to say "Yep, they were right!"

I have a Detroit TruTrack in the rear of Big Blue and really like it.  That's one of the helical-gear style units and I agree with Pete - nothing to rebuild or wear like the clutch packs in others.

However, I will say that it is always watching and will come "in" at the slightest provocation.  One day when it had just rained and the oil in the pavement had come to the top it came in as I was turning a corner.  That meant both rear tires turned, which caused the rear to step out a bit until I backed off the throttle.  Not a big deal, but something to watch out for.

You could go with a selectable locker in the rear, but then you have to know when to engage it, and then both rear wheels turn at the same speed, making cornering more difficult.  The TrueTrac takes care of that, automatically.

You should have driven my Shelby with the Detroit Locker, Shelby's choice for the cars so it could be used on the R models (SCCA and FIA rules regarding allowed changes). It drove the slowest turning wheel in a corner and the outside one unlocked and turned freely. When you straightened out and applied power, the car would shift sideways a bit, make a loud "bang" in the 9" center section. and then got straight.

When a friend and I ran the car in a series of high speed events, we learned that you needed to be sure the front of the car was pointed where you wanted to go, because particularly in 1st or 2nd, the car was going where the hood was pointed once the locker engaged. The fact that that little 289 engine could actually lift the front end enough to get the wheels off the ground in 1st was part of it.

As far as a very stout and extremely reliable unit, it was great, for someone not used to it, it usually scared the devil out of them, particularly if they were driving it.

"Posi" in another one of GM's Generic Trademark names, like "Stepside"...lol. Everybody uses the GM term Posi to describe a limited slip the same way everybody uses the word Stepside when talking about Ford's Flareside, or Dodge's Utililine, etc.

Sounds like I should just stay with the open, and put the 1000 somewhere else...
I am running the original 302, never re-built and only 20k miles on it (have the documentation on it) And also running good-ole points in the dizzy..
What do ya think of the Edelbrock (I think) true fuel injection system that comes with a new intake and 8 actual injectors?

reamer wrote

What do ya think of the Edelbrock (I think) true fuel injection system that comes with a new intake and 8 actual injectors?

I'll let you know in a few months, I'm putting a Pro Flow 4 on my 351w. There are tales of unreliability or just plain doesn't work out of the box around around these units. But then I think the same could be said of a lot of aftermarket FI units. We shall see.

well first of all let's see what axle you have as the costs and options may vary. sometimes going with a good used unit is a reasonable option.
but how they help is a big variable also. in snow they can help a lot up to the traction breaking point then when they spin, they spin together. same in mud. on dry or pavement, they can work differently. it all depends on traction of the surface. nothing is perfect. I am using the ford traction loc in my build today. the weak point is that they are clutch design, and they can wear out.
in a pinch, you could just apply a "little" parking brake, and that shared resistance will make the open behave like a traction loc. but don't run it that way as there is no heat removing lubrication at the drums as they are brakes after all.
there are three basic design theories on locking the rear wheels together
power goes to least resistance being traction lock or posi traction.
power goes to most resistance being auburn gear cone style.
locker where the stress has to override the face gear plates which are held engaged by heavy springs which reset repeatedly and reengage.

Anything that has "Locker" in it's name is going to "lock" the 2 wheels together under power. When no power they will unlock and the wheels can now turn at different rates.

I have a Detroit locker in the drag car so cant say how it is on the street but if the street car need a carrier it will go in.

I also have a lunch box locker in my 75 factory v8 Gremlin. It replaces the spider gears and lock the axles together under power. As pointed out if wet out it could be a hand full at 92" wheel base if you were not ready for it to come around. I only had to move the car once with snow down, it did not go anywhere even with 2 rear tires turning.

I have had 2 4x4's (86 K5 Blazer & 02 Durango) with factory LS or posi in the rear axle, dont know if they offered it in front? and loved them.
I would lock the front hubs (K5) at first snow and drive as far as I felt safe in 4x2 mode then just move the lever to 4x4 and keep going. The Durango you just turn the knob on the dash.
Neither truck was ever taken off road so cant say how they would do on a trail.

Only thing I can see with a LS / Posi in the short wheel base Bronco is the SWB and it wanting to come around faster like my 92" WB Gremlin other wise I dont see any issues with it.
Dave ----

I found a " mini spool" and just had to try it. it goes in the carrier replacing the spider gears and now the two wheels are bolted together. no give! I kept it in about two weeks. for a flare side wheelbase, it chattered the tires a lot. it was cool for a minute and the kid in me played with it a bit. but when I turned into my subdivision one day and it did a 180 without me trying much, I said that's enough. miss Ann would have freaked out if she were driving. so, dolly is open now.

I thought I had replied to this!

I don’t think limited slip will help keep it straight on acceleration.

My Bronco has limited slip and if it breaks loose on ice or packed snow it will go sideways, I usually have to let off to get it straightened up.

I also think the short wheelbase helps it come around quicker.

About a month ago I was driving the Bronco and locked the hubs before I left. The roads as I left town were patchy packed snow. The roads were all packed snow sooner than I thought they would be. But I still thought I had miles to go before I would need 4WD.

Then I noticed the back end moving around, engaged the transfer case and it straightened up. That’s what straightens up my Bronco, 4WD.

reamer wrote

I just had to take out the Bronco yesterday, Still have some snow/slush on the trails.
Currently it has 3:50 "open" diff's. When stepping on it in the snow, of course it what to pull to one side and then we fishtail, fun in itself but.....

Would installing a LS or Posi help "keep it straight" on acceleration in slippery conditions?

1. So what's the difference LS vs Posi?

2. Would I have to swap out both front and rear Diff's or just do the one in the rear?
Thanks

I'm late to the party here (just got back from a 2 week long cruise through the Panama canal!), and a lot of good answers have already been given, but I can't ever resist jumping in on limited slip / locker threads!

First off, would a limited slip help keep it straight in slippery conditions?  Absolutely not.  Open differentials are the best option by far when stability is the goal.  A spinning tire, like a locked up tire. isn't rolling, it's sliding on the road surface.  A sliding tire doesn't care which way it slides, so it will go sideways as happily as it will go forward.  A rolling tire will not go sideways (if it starts going sideways it's now a sliding tire) so the tire with the most traction that won't spin when you accelerate with an open diff helps you keep going straight.

Limited slips and lockers are used when you are willing to give up stability to gain more ability to push yourself forward.  There are lots of options, all of which have their pros and cons.  Here's a thread I started on a different bulletin board discussing the general categories (I started working on a guide for Gary, but kind of ran out of steam on it).  But if your biggest concern is side-slipping under acceleration, your open diffs are the best you're going to get.

Others have answered your 1 and 2 well, so I won't add anything else except to say that I've heard that a TrueTrac works well in a front axle, with little downside.  But that's just hear-say coming from me.  I have no experience with it.

grumpin wrote

....  About a month ago I was driving the Bronco and locked the hubs before I left. The roads as I left town were patchy packed snow. The roads were all packed snow sooner than I thought they would be. But I still thought I had miles to go before I would need 4WD.

Then I noticed the back end moving around, engaged the transfer case and it straightened up. That’s what straightens up my Bronco, 4WD.

This!!!  I hear so many people say that you should only use 4WD when you absolutely need it, that driving in 4WD will get you into more trouble.  Yes, you CAN drive faster in 4WD and get to where you are really over-driving your ability to stop or swerve to avoid something unexpected.  But the fact that you CAN doesn't mean you HAVE TO.  And the reason you CAN is that the vehicle is so much more stable in 4WD.  So use 4WD any time the roads are a little slippery to gain the stability, but still use your head and don't drive too fast.

85lebaront2 wrote

You should have driven my Shelby with the Detroit Locker, Shelby's choice for the cars so it could be used on the R models (SCCA and FIA rules regarding allowed changes). It drove the slowest turning wheel in a corner and the outside one unlocked and turned freely....

This is exactly how automatic lockers work.  Many think they engage under power (see below...) but they don't.  They always drive the tire that's going the slowest and they always allow the other tire to freely turn faster if it wants to.

85lebaront2 wrote

.... When a friend and I ran the car in a series of high speed events, we learned that you needed to be sure the front of the car was pointed where you wanted to go, because particularly in 1st or 2nd, the car was going where the hood was pointed once the locker engaged....

... but this is what gets people thinking wrong about automatic lockers (the name doesn't help either, because they never actually "lock").  In the car Bill is talking about here the "going where the hood is pointed" comes from two things.  One is that the UNLOCKED locker is only driving the inside tire in a turn.  That's like pushing on one end of a grocery cart handle.  It will try to turn away from the side that's being pushed, so it tends to straighten out.  But the biggest thing is, as he points out, that car would actually lift the front tires off the ground.  You won't get good steering response like that no matter what differential you have!

FuzzFace2 wrote

Anything that has "Locker" in it's name is going to "lock" the 2 wheels together under power. When no power they will unlock and the wheels can now turn at different rates....

As noted above, this is not strictly true.  The truth isn't much different, but the difference is significant.  As Bill said above, automatic lockers always allow the outside tire to turn faster if it wants to.  This is why I say that they never technically lock.  However, if you get on the power you will spin the inside tire (which is the only tire that is being driven).  Once you do that it catches up to the speed of the outside tire, so the outside tire no longer wants to go faster.  The locker then reengages, so people say that it locks under power.

But there are two things wrong with that statement.  One is that it's not really locked at that point.  Yes, both sides are engaged, but if one of the tires wanted to go faster the "locker" would freely let it.  Second is that it's not the fact that it's under power that made both sides engage, it's the fact that neither tire is trying to turn faster.

OK, it's the fact that you are overpowering the inside tire that makes the outside tire not want to turn faster, and if you get off the power the inside tire will slow down and the diff will "unlock".  But if the inside tire gets enough traction it will hook up, slow down and "unlock" the diff even if you don't get off the power.

So you can see how close to right it is to say that "it locks under power."  But when people think that they start feeling like automatic lockers are these unpredictable things that will suddenly lock and throw you out of control if you get on the power in a turn.  If you think about it the right way, it becomes a lot more predictable:  it's only driving the inside tire, so if you get on the power too much you could break that tire loose.  If you do that the inside tire will catch up to the outside tire with some momentum, possbly hammering the outside tire enough that it will break loose too.  The end result might be the same, spinning out.  But if you understand how it happens it's a lot easier to prevent it.

mat in tn wrote

....  in a pinch, you could just apply a "little" parking brake, and that shared resistance will make the open behave like a traction loc....

This might work, and if you are stuck it's worth a try.  But the physics say that it's not a good bet.  Yes, adding drag to the tire that's spinning will send more torque to the tire that's not spinning.  But then again, you are also adding drag to the tire that's not spinning, and essentially all of the extra torque only goes into overcoming that extra drag.  ("Cutting brakes" that can be applied on one side only are a great option here.  Tractors and dune buggies use those to great effect, but they are not at all common on trucks.)

The exceptions to that are TrueTracs and PowrLoks (and MAYBE other "positraction" type limited slips).  TrueTracs are gear-type limited slips and use the gear pressure angles to send 3.5 times as much torque to the non-spinning tire.  So applying the parking brake will send 3.5x as much more torque to the non-spinning tire as the brake torque.  That means that after overcoming the brake on that tire you still have 2.5x the extra torque to get you moving.

I don't understand PowrLoks completely, but they are a friction-type limited slip where the clutches get loaded up more heavily if there is more load on the spinning tire.  So applying the parking brake will help to the extent that the additional clutch loading tightens up the limited slip.  It may be that other friction-type limited slips do this as well, but I don't know if they do or not.

certainly, I'm not suggesting anyone take off on an offroad excursion planning only to use the parking brake. more of "when one aint workin". add a little brake and that will spread the load around a bit. it's something we use to do a lot taking off in snow. snow removal has not always been so great here. then it gets hard packed and watching people try to learn on the fly on the one or two days a year is hilarious.

generally, I use ford traction loc diffs. I know there are improved designs and I know the clutches need replacing periodically. but there are many factors that play into that. shorter wheelbases, wider tires, tight turning vs mostly highway strait line etc. cause more working of the clutches. years ago, my friend installed an auburn gear diff in his f150, and that unit had cones and sent torque to the greatest resistance not the least and he didn't really like it as we were more "street racers" some thirty years ago. he did not like that it would not get sideways like other pos diffs would. I loved that it would stick and move! to each his own I say. on trails I don't care if one pulls more or less than the other if it crawls in and crawls out. every specialty sport in this arena has a unit better suited for it.

I would say that you would benefit from a limited slip. I've been stuck many times on hilariously small obstacles where 4WD and open diffs just wouldn't cut it. I decided on the Eaton Detroit TrueTrac for my Bronco because of this. IMO selectable lockers would be overkill for just some cruising out in the woods, but they are always nice to have just in case. I've heard automatic lockers will wear out your tires on the street.