Whats the difference, LS vs Posi?

[reamer]

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?

 

[bmc69]

Both are friction limited slip differentials. "posi" or "positraction" is the GM term for it. ;-)

Adding LS to front or both makes tracking on slippery road surfaces worse.

 

[Yeller]

A LS or Posi are the same. LS is generic and Posi was a term licensed by GM in the 60’s.

As for “staying Straight” in slippery conditions they will not help, intact will aggravate that condition by making both rear spin together making it easier to slide around. Gets worse again with front and rear by allowing all 4 tires to do the same. Where to do help is adding traction in dry road conditions and help traction off road.

 

[Rustytruck]

they will help you go further just not straight especially if there is any kind of camber in the road. I did a 360 once on a mountain shelf road with a 300 foot drop on the side. the only thing that kept me up there was the snow ruts from the trucks in front of me. It happened so fast I couldn't react fast enough, other than freak about the turn of events. yes I was driving too fast.
but I have a detroit in the rear and a tru trac in the front. also been on a cambered shelf road covered in Ice. crawling along going 1 foot forward and 1 inch sideway do to the Detroit locker. had to stop many times to use a high lift and push the truck back over to stay on the road. that was a very nervous 100 yards. so.

 

[Broncobowsher]

As mentioned, Posi is a trademarked name for limited slip by General Motors.

Beyond that limited slip differentials come in many versions. The most common by the auto makers are the clutch plate style. There is a small stack of clutch plates very much like a clutch pack in an automatic transmission. But where an automatic transmission applies the clutch with hydraulic pressure, there is a spring to pre-load the clutches in the differential. The clutches in the differential are generally a lot smaller and in a much higher torque load than a transmission as well, so they slip. Very often there is some feature where axle loading will increase the clutch pressure. But they are clutch frictions, slipping, and do wear out.
There are other versions that use cone clutches, and probably the best of the limited slips use helical gears. Think of a worm gear drive. You can put power to the worm and it moves, but try and drive the worm and it doesn't move. That principle is used with gears that have a high helical twist to them inside the differential to make a very smooth limited slip that does a very good job of driving power to both wheels. More than that you are getting into a true locking differential, something that has locking teeth that don't allow slip. Either mechanical that puts a pin through the differential that just makes it solid, or a mechanical locker that is a ratchet that allows one wheel to spin faster than the one being driven. A mechanical locker (Detroit, lunchbox, etc.) has the engine driving the slowest tire and if one wants to go faster it is free to freewheel faster.

 

[jamesroney]

Limited Slip is a description for a differential that provides unlimited slippage.
Positraction is a contraction of the two words positive and traction. The GM posi-traction usually does not provide positive traction.

I cannot see where the term Positraction, or Posi, or Positive Traction are registered BY GM in the USPTO database. (Although Trac-Lok is still a registered trademark of the Spicer Corporation.) And even if it were currently registered, it would only have a generic descriptor in Automotive Differential applications.

So I have no idea what you mean when you say "posi." I know what GM meant in the late 50's and early 60's. But to say that "posi" is a GM term for a cluthc type Limited Slip Differential would be a gross oversimplification. The Gen 5 Camaro Z28 is factory equipped with a Gen2 Torsen gear driven differential. It has no clutches. Is that still a "posi?" The G80 Gov-Lok by Eaton is also available in the GM corporate axles. They use a flying weight to engage a locking mechanism in a clutch type limited slip differential. The Gov-Lok is also a posi. But it is both "limited slip" and it is actually positive engagement and a true locker. (once it decides to lock up...) Mercifully, the Gov-lok is such a horrid design that most blow up when actually needed.

It appears that GM has abandoned the term posi-traction in their literature. Such is how it goes with trademarks. They only mean what the trademark owner says that they mean at a particular time.

So I have no idea what "Posi" means.

 

[DirtDonk]

But I think you just proved (in print even!) that you do in fact know!
Or at least are cognizant of a few of its potential meanings…

GM can abandon it if it wants. The name lives on because a whole new generation of auto enthusiasts seems determined to keep it at the top of the list and are still using it as the generic term for Limited-Slip. I wonder if that had anything to do with GM's decision?
Probably not...

Paul

 

[DirtDonk]

Funny coincidence? Given the state of the internet, probably not. Just got a new Jeg's ad for 10-bolt and 12-bolt "Posi-Traction" differentials.
The picture shows a carrier with open sides with coil springs showing. Is that the original design?

paul