Ol' Man Needs "Posi" Update

[EdK]

Since being a teen gearhead (just after covered wagons) I've always believed that, when the rear axle is off the ground, turning one wheel with the other turning the same direction meant it has "posi. " When the other wheel turned the opposite direction, the rear end was "open. "



Well, I had a friend's truck with a Dana 60 on the lift yesterday and happened to turn one rear wheel. The opposite wheel turned the other way, BUT he insisted he has a limited slip diff. Puzzled, I put my son's Z-28 Camaro SS up and, you guessed it, the wheels turned opposite ways even though I KNOW his Z-28 has limited slip. What am I missing here???



Ed

 

[Boondocker]

A limited slip is open unless there is torque differential. In other words, if one wheel is turning significantly faster than the other, then both axles will be clutched together. Limited slip differentials are designed to allow for normal differentiation, like turning a 90* corner from at stop without engaging.



Neil

 

[nps]

Since being a teen gearhead (just after covered wagons) I've always believed that, when the rear axle is off the ground, turning one wheel with the other turning the same direction meant it has "posi. " When the other wheel turned the opposite direction, the rear end was "open. "



Well, I had a friend's truck with a Dana 60 on the lift yesterday and happened to turn one rear wheel. The opposite wheel turned the other way, BUT he insisted he has a limited slip diff. Puzzled, I put my son's Z-28 Camaro SS up and, you guessed it, the wheels turned opposite ways even though I KNOW his Z-28 has limited slip. What am I missing here???



Ed

Don't know Ed. I'm with you. My Dana 80 limited slip works as expected (both wheels same direction).

 

[.44 mag]

Simply stated, both the diffs. in question need to consider a rebuild of their limited slip units. I believe that both the units you are looking at have clutch plates in them. There is a preloaded condition that allows one wheel to "slip" as the vehicle makes a turn. By design it should allow one wheel to spin faster ( as the outside wheel in a turn ); but, provide power to both wheels equally under acceleration.



A "limited slip" is not as positive and as a locking differential. They tend to wear out over time and ultimately fail to the point of acting like an open rear end. On a lift, it should take sufficiant force to be able to spin the passenger side tire by hand while holding the one on the driver side with the trans. in neutral.



Hope this helps.

 

[jkalchik]

A limited slip is open unless there is torque differential. In other words, if one wheel is turning significantly faster than the other, then both axles will be clutched together. Limited slip differentials are designed to allow for normal differentiation, like turning a 90* corner from at stop without engaging.



Neil

Uh, no. Not with clutch type limited slips. Speed differentials in corners will be enough to make the clutches slip.



What you're indicating IS true of a True Trac limited slip. The offroader's trick to get a True Trac to "hook up" is to tap the brakes.



. 44 mag has it right, I'd bet that the clutch packs in both referenced instances are shot. I don't know what a rebuild kit for D70/80 TrackLock goes for, but kits for D35Cs and Chrysler 8. 25s are only about $50.

 

[Bob V]

There are a lot of different limited slips out there, some use clutches and some are govenor type and some use gears, some clutch type do NOT have preload springs and the wheels could turn in different directions. Most of the time you can get a clutch type LS to work better by slightly setting the parking brake (rear axle)

Bobh V