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Old 06-25-2003, 01:03 AM
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Help me understand the "diff test"

Ok, someone has to set me straight here.

I've seen the explanation before about checking for posi or lsd (not the drug, Dave, the diff) by turning one wheel and if the other follows in the same direction it's one of these types. But what I don't understand is that any type of diff., be it open or whatever, under normal conditions, should act LIKE A DIFF. And in order for there to be any type of lock-up happening there has to be some sort of force greater than just turning a wheel with your hand to activate it. In an attempt to design my own viscous diff., I've done a fair bit of research on VD's and understand how they work and in order for there to be ANY torque transfer, one of the wheels has to significantly break loose. Now, the other types are probably more touchy so the wheel doesn't have to lose as much traction as a VDiff., but come on, if both wheels were to turn together using this test, I would imagine that something must be wrong with the diff.

The only other thing I can think of is that (I'll use GM's Posi-trac diff as an example) if you leave one wheel on the ground and elevate the other and turn it, if it is Posi then you should be able to feel the pinions lock into each other, then and only then will the other wheel try to turn in the same direction. 'Cause otherwise all you are doing if both wheels are elevated is "walking" the pinions around the ring gear and unless you have a bad bearing or something impedeing the opposite wheel from turning, you are not going to transfer enough torque to create lock-up.

I just want to understand.



Marc

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Old 06-25-2003, 02:59 AM
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I think you're confused.....limited slip differentials wether it be positrac or ford is always engaged. there is no locking/unlocking to be done under load or normal street driving. They have friction plates and steel plates that are held together with springs. WHen you drive in a strait line, the two axles spin the same speed, but when you corner, the force of the car turning forces the disks to slip so you dont get chirping tires every time you go around a corner. When you punch it, they spin the same speed leaving the famous "posi marks". Lockers have the lock/unlock feature. This being said.....it takes much more force than a hand turning the tire to make the disks slip, so the turning method IS accurate. Different people have different ways to do it. What i do, is put the car IN GEAR OR PARK, and jack up both of the rear tires. Now try to turn one of the tires, if it turns and the other ones goes the opposite way, that means its open and the spider gears are just walking around each other. If you cant turn the tire at all....that means its a posi because the driveshaft is holding the pinion and ring gear steady, and the limited slip is holding the tire steady so they dont slip....does that make sence?
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Old 06-25-2003, 06:21 AM
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Re: Help me understand the "diff test"

Quote:
Originally posted by Forkliftguy
The only other thing I can think of is that (I'll use GM's Posi-trac diff as an example) if you leave one wheel on the ground and elevate the other and turn it, if it is Posi then you should be able to feel the pinions lock into each other, then and only then will the other wheel try to turn in the same direction. 'Cause otherwise all you are doing if both wheels are elevated is "walking" the pinions around the ring gear and unless you have a bad bearing or something impedeing the opposite wheel from turning, you are not going to transfer enough torque to create lock-up.

I just want to understand.

Marc
That's how I do it, actually unless your posi is toast you probably won't get the elevated tire to turn at all by hand, it will act pretty much like the car is still in gear or Park.

I think you understand very well.
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Old 06-25-2003, 08:31 AM
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ok, I think I get it now. I was under the impression that they were only in the locked state at certain times. Thanks guys....



Marc
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Old 06-25-2003, 06:53 PM
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Then of course there is the Gleason limited slip unit...
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Old 06-26-2003, 02:49 AM
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You poked my brain with that one 4Jaw, if I don't hear from you on what it is, I'll have to look in to it. But I like reading your "serious" responses, there always pretty in depth. They feed my brain



Marc
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