pettrix

I have a Chevy 12 Bolt with a limited slip differential.

I jacked up the rear end and put the trans in neutral. I rotated one tire and the opposite side tire rotated in the same direction. Then I put the trans in park with the rear jacked up, I put a socket and wrench on a lug and when I cranked down, the tire rotated but the opposite side tire rotated in the opposite direction.

Does that mean the posi is going bad?

svnt442

That is what it's supposed to do. To know if it's going bad to need to use a torque wrench (needle type) to measure the amount of break away force needed to turn the axle while the pinion is locked (trans in park). I don't know the spec, but I'm sure MONZAS will chime in on this.

Fun71

As Randy said, that's what it's supposed to do. It's the way a differential works, whether it's a posi or open carrier.

Imagine if it didn't do that - your car would roll away if you forgot to engage the parking brake.

RetroRanger

I know i like to test my anti spin rear to be sure its functioning correctly just about every time i take the car out

pettrix

I see. So when the vehicle is in park, cranking down on the lug nut and having the other side turn the other way, doesn't test for posi issues. I would have to put a wrench on the axle somehow?

gs72

monzaz

OK well the car in neutral should spin both tire same direction IF your rear is spinning in the opposite direction with the car in NEUTRAL it has a worn tired posi or it is NOT LIMITED slip posi at all.

When you have the car in park it stops the ring gear from spinning and the spider gears are what you are spinning . .. the planetary spider will spin in opposite direction as this is what make the outside wheel capable of going a different distance ground travel than the inside tire in a turn. IT works the same as an open but the clutches are what make the LIMITED slip for the straight on performance and how much slip gets delivered in the turn.

So what you need to figure out is whether you truly have posi or not and if you do ...sounds like it will need some maintenance soon. Jim

J D Race

pettrix

It's definitely a posi (LSD).

How do they work in reverse? Do both axles get power when in reverse or just one side?

oddball

*ahem* As Retro said, the surefire way to test it is an open road or parking lot and a heavy right foot.
What it does on pavement is more important than what it does on jack stands. It'll still act like a posi on jack stands when the clutches are worn down but won't hook on the street.

Fun71

Reverse works exactly the same as in Drive.

Koda

If you have a clicker torque wrench, you can pretend you're tightening lugs. 80 ft lbs works for both the lugs and the limited slip mechanism's test.

monzaz

Posi does the same thing forward as backwards. Yes.

monzaz

Testing a posi by just spinning one wheel and having the other planted is not always the best way to tell how good it is.
Reason
Most GM units have a low spring break away pressure anyway. Limited slip works more off inertial spin on a burn out. SO the faster you free spin that rear and the tires spin the harder it locks the rear clutches/cones tight. the unit is spun and spiders are angle cut so they push outward across the spider gear teeth as it spins creating the force needed to lock the unit cones or clutches... Pre-load spring(s) are really only there for slow speed operation and to keep the clutch pack and cones ready to grip to the case so it reacts faster. Sometimes people will load the rears with 800# springs making the clutch/cones so tight they will not hardly break loose at all and really all this causes is premature wear on clutches and cones ...IE like sand paper and the more pressure you put on the sand paper the harder it cuts...Right? 400# is about the best pre-load for street and strip. If your car is really super torquy off the line and you race a FULL spool or a power trax unit would work better for your needs. Minimal street driving with a full spool ...freeway probably not real smart as the rear will track to the taper of the road making steering tough...BUT not impossible.

Jim
J D Race

Fun71

Thanks for that info. I never realized there was increasing pressure to the clutches as the carrier spins, but it certainly makes sense now that you explained it..

monzaz

Yes - look at the spider gear design. the angles they are machined at when they are forced by the pinion spiders on the cross pinion shaft as the carrier spins violently around it is the same way the POWER TRAX units work also with the serrated tooth design. IE when your turning a corner and you smash the gas BOTH tires light up and both tires spin at the same speed as the tension on both side gears are receiving equal pressure from how the over ride of the spin of the unit and tires getting less grip from the torque being delivered too quick...BUT WE LIKE IT.

Jim - J D Race

Fun71

That is exactly what I was thinking when I read your description. Again, thanks for this information!

Koda

I think I am going to disagree with your explanation of the stock unit. Considering that the gears are stopped from spinning by frictional force caused by spring pressure, one would see that the spring compression force, being the spring constant times the compression distance, will not change all that much in the very short distance that that gear could move outward in the assembly before being stopped. This is why, like you mention, people put in stiffer springs. The reason the other side hooks when you spin like a madman is because that tire is heating, and you are starting to move, which decreases the speed difference of the spinning tire to the ground, and allows for static friction coefficient to take over from the dynamic. (That's the, it's hard to start pushing a box across the floor but easier to keep it moving.) This happens both at the tire and at the limited slip spring.

monzaz

You can disagree all you want... it is what it is. pressure make s a posi work or not work. It comes from the spiders and some from the springs and some from hydraulic effect. BUT if you do the test, springs are NOT a needed item for limited slip to work.

Jim

SmirchIs#1

I do partialy agree with your explanation of how the posi works.Inertia has nothing to do with it though,where do you come up with this stuff, and yet you have convinced another person to believe incorrect info.There are two forces, 1; the pinion spinning the differential and 2; the resistance created by the tires on the ground.Then because of the angle cut of the posi internal gears they are all forced out against the inside of the diff case.That is what locks the posi,if anything when the tires spin there is less resistance to the posi,but it is already engaged.Inertia doesn't play a roll.I shouldn't be surprised because later in this thread you claim that hydraulics is also involved.Explain to all of us where anything of a hydraulic nature occures

Fun71

I assume you are referring to me. I was able to discern that the angle cut gears cause the increased lockup force, as was said similar to how a Power Trax unit works. My familiar with those, along with my knowledge of mechanics, allowed me to make the connection.

02WS6SSZ71

Back from the dead! So what IS the best way to tell if a limited slip is good vs time for a rebuild? Please do not suggest a burnout attempt....car has been parked for 30 yrs, rear is out of car for cleanup/paint/axle bearings and seals etc. If doing the test of holding one axle and spinning the other, what would be an expected torque value before the axles should free from one another, and is this a reliable test? Thanks in advance for all thoughts

JohnnyBs68S

I've always heard that if the diff can hold 100 ft-lbs of torque on one axle before it starts to spin differentially (w/ both tires in the air, trans locked in park or in gear if MT), that the posi is doing its job. An open rear will require very little torque on one axle to get the diff to spin differentially.