Bad Posi? How to test?

Old September 6th, 2018, 10:13 PM
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Bad Posi? How to test?

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?

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Old September 7th, 2018, 02:34 AM
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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.
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Old September 7th, 2018, 11:40 AM
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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.

Last edited by Fun71; September 7th, 2018 at 11:42 AM.
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Old September 7th, 2018, 01:54 PM
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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
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Old September 7th, 2018, 08:13 PM
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Originally Posted by Fun71 View Post
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.
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?

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Old September 7th, 2018, 09:00 PM
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I know its an eaton but you get the idea
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Old September 9th, 2018, 05:17 AM
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posi unit

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

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Old September 9th, 2018, 11:41 PM
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It's definitely a posi (LSD).

How do they work in reverse? Do both axles get power when in reverse or just one side?
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Old September 10th, 2018, 06:28 AM
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*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.
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Old September 10th, 2018, 11:35 AM
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Originally Posted by pettrix View Post
It's definitely a posi (LSD).

How do they work in reverse? Do both axles get power when in reverse or just one side?
Reverse works exactly the same as in Drive.
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Old September 10th, 2018, 11:39 AM
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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.
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Old September 10th, 2018, 06:04 PM
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posi

Originally Posted by pettrix View Post
It's definitely a posi (LSD).

How do they work in reverse? Do both axles get power when in reverse or just one side?
Posi does the same thing forward as backwards. Yes.
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Old September 10th, 2018, 06:22 PM
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Posi test

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
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Old September 11th, 2018, 10:29 AM
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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..
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Old September 12th, 2018, 08:40 PM
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Posi workings

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
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Old September 13th, 2018, 12:41 PM
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Originally Posted by monzaz View Post
it is the same way the POWER TRAX units work
That is exactly what I was thinking when I read your description. Again, thanks for this information!
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Old September 13th, 2018, 10:05 PM
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Originally Posted by monzaz View Post
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
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.
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Old September 14th, 2018, 04:31 PM
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posi

Originally Posted by Koda View Post
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.
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

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Old October 21st, 2018, 05:59 PM
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Originally Posted by monzaz View Post
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
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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
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Old October 22nd, 2018, 08:10 PM
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Originally Posted by SmirchIs#1 View Post
and yet you have convinced another person to believe incorrect info.
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.
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