posi is just easier to say. a man only speaks so many syllables in his lifetime before he keels....i'd say LSD but people confuse it for something else too often.

 

Hmmmmm.....

 

there is nothing wrong with LSD.

 

One type of LSD lets you slip away--and the other type doesn't

 

thank you for awnsering my question. it also looks as if i started a term war, i was not aware that it was a gm term.


learn something new everyday. thank you.

 

Hey, can someone help me with my Posi in my Extended cab Step side F150?

I think it's got a mouse motor too...

 

The one that gets me is people using the term AOD to refer to any automatic overdrive transmission.

And to muddy the waters a little more.. there actually is a type of differential that is a true posi.. meaning it can shift torque beyond the fully locked 50-50 split that a tight limited slip or locker can produce, but it didn't come stock in any of these trucks. Detroit Truetrac

 

A little confusing here. A fully locked diff would mean 100% power to both axles,not 50-50.

 

No you can't send 100% to both wheels that would mean you have 200% available, with a locked diff the wheels share the total equally which means the maximum any 1 can get is 50%. An open diff sends 100% power to a wheel with zero traction.. lot of good that is, and a true posi does the exact opposite sending a higher percentage of power(greater than 50%) to the wheel with more traction.. hence the name.

 

Which is better? True posi or fully locked? On turns probably true posi right? But off road???

 

I'm pretty sure this is how the confusion starts.

I think you are referring to an Auburn differential?

Differential Product Information | Auburn Gear Aftermarket

Nowhere on their site do they claim to be a "posi" differential, let alone a "true posi".

No such thing as a "true posi", again it seems to be a corruption of the GM trademark "Positraction" differential.

 

Yep that's what I meant by "true posi"

 

Yes,you can. A locked diff means just that. If they,the axles,are locked, they are one. So torque/power is the same at both wheels.

 

An open differential sends equal torque to each wheel (50-50 if you want to think of it that way). If one wheel has no traction, zero torque is sent to that side and zero torque is sent to the other side as well - torque to each wheel is always equal (not to be confused with wheel speed).

A limited slip puts a clutch in between the two axles to limit the speed differential between the two wheels when one loses traction, allowing slightly more torque to the wheel with grip.

A torque biasing differential such as TrueTrac or Gleason Torsen uses a gear setup to bias torque to the wheel with grip. These differentials have a torque bias ratio which tells how much more torque goes to the wheel with grip. These have a TBR of 3-6, meaning they can send 3-6 times more torque to the wheel with grip. Clutch limited slips have a much lower TBR likely between 1 and 2 and open diffs have a TBR=1. In either case, if one wheel loses traction completely (i.e. in the air) there is no torque sent to that wheel, and when you multiply zero by anything, you're still stuck. That's why it helps to partially apply the brakes - to give a torque on the slipping wheel that will be multiplied to turn the other side.

A locker is capable of sending up to 100% of the torque to one wheel at the time. Again, don't confuse wheel speed with torque. If one wheel is in the air, all of the torque is sent to the wheel on the ground, even though both wheels are turning at the exact same speed. If both wheels have equal traction then they will each get 50% of the available torque. You are right that you will never have 200% available torque.

 

That is an excellent description, and there is the correct generic terminology. Torque biased differential. Quite a mouthful, which may explain why the term "posi" remains popular.

The discussion on terminology has been around for years. Here is another sample.

https://www.ford-trucks.com/forums/7...ited-slip.html

The comment on braking reminds me of an old farm tractor trick called differential braking. On a 2WD tractor with an open differential, you would have separate brake pedals for the left and right wheels. You could execute sharper turns by applying the brake on the inside wheel, or you could enhance traction by applying the brake on the slipping wheel.