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Old 07-26-2013, 02:10 AM   #1
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I dove into the LSD and Locker world....

Hey guys I'm hoping you can help me out in regards to understanding LSDs and Lockers. Before you tell me to search, trust me, I've done a TON of research. Here is what I'm hung up on... I have a '94 Z71 GMC with a 12 bolt rear end and what I think (or thought) has a limited slip diff. When I bought the truck years ago I jacked up the L/R tire in the air and put it in gear. The L/R rotated a bit then "clunked" and my truck lurched forward. Maybe my GMC has a locker not a LSD?

Now for my Jeep.... It is open front and rear currently and I've been considering the Trutrac LSD. Everyone seems to say that they are useless when a tire is lifted. So if a tire is off the ground, the other tire will not budge at all? Or is the Truetrac useless in the sense that it's not nearly as effective as a locker in that situation? I read in one of Jerry's posts about torque that an open diff always transmits power to each side 50/50. If that's the case why would a Truetrac be any better than an open diff?

The reason I mentioned my pickup in this thread is because I thought a Truetrac in my Jeep should perform at least the same or better than whatever is in the my truck?

Thanks for any input, I thought picking a LSD/Locker would be easy...

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Old 07-26-2013, 03:00 AM   #2
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The reason that your gm learched forward is it indeed had an LSD you are transmitting power to the wheel on the ground. But it will only do so much if that wheel was to be off of the ground and you needed to put some real power to it all it would do is slip hence the term limited slip differential. But as for a locker it will not slip at all it is a true 50/50 power distribution meaning the wheel on the ground will be able to move your vehicle forward without slipping.

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Old 07-26-2013, 03:52 AM   #3
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The fact that the GMC clunked signals to me it has an automatic locker. Once it senses a difference in speeds between the two axle shafts, it locks up.

As far as the limited slip, modern ones are much different than older ones. Traditional limited slip differentials (like those that came in many of our jeeps) used clutch pads to provide the resistance. This means once that disk slipped, there wasn't enough friction to transfer much of any power to the wheel with traction. New limited slip differentials like the Detroit trutrac are gear driven. Having it gear driven means there is no giving point like a clutch pack where it becomes useless, it should provide steady resistance. If you had one wheel in the air, you should be able to gas it and the differential should provide enough power to the wheel with traction to get you moving.

My friends 79 k10 has a trutrac in the back and it does pretty well! Keeps both tires moving offroad and leaves two big black marks off of a stop when you step on it, proving it provides a good amount of resistance to the slipping wheel.
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Old 07-26-2013, 01:50 PM   #4
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Thanks guys. Does anyone have a video of a Truetrac equipped axle with a tire in the air?

The rear end on my GMC has worked well for me so I'm trying to find something similar to put in the Wrangler. Thanks!
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Old 07-26-2013, 03:53 PM   #5
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There are a few fallacies being propogated above. First, check out a couple articles I wrote on this subject probably ten years ago. While a little wordy, they seem to be effective on explaining the differences between lockers & LSDs.

What? Why do I need a locker? I thought I had 4X4! -

4x4 & Torque answers -

Essentially, a LSD increases the torque delivered to both tires in low traction situations, and it does so in the usual 50:50 split. It increases torque by coupling the tires on an axle together so the tire with the most traction can create more resistance for the engine to work into. The engine can only deliver as much torque as it takes to get one tire spinning.

Automatic lockers don't lock when they sense poor traction, their normal mode is 100% locked. What they do automatically do is unlock (!) when the outside tire in a turn is trying to rotate faster than the inside tire, as it must do. When the tires are again turning at the same rpm, an automatic locker locks back up. An automatic locker is always locked unless you're turning left or right.

Where a LSD is concerned, which is true for both the Jeep Tracloc and the Detroit Truetrac, is that when one tire is in the air with no traction available to it, it does not "lock up". All it does is help couple the resistance of the tire still on the ground to the tire in the air so the engine sees more resistance so it can deliver more torque to both tires on the LSD-equipped axle.

An engine can develop very little torque when it is not working into a resistance. So to some extent, a LSD slightly increases the amount of resistance the engine sees so the engine can then deliver slightly more torque to both tires... equally. A LSD doesn't ever "transfer torque" to the side with more traction, it simply slightly increases the amount of torque delivered to both sides.

Where a locker is concerned, it totally locks the spinning tire to the tire with good traction so it dramatically increases the amount of torque delivered to both sides.

To my way of thinking, LSDs are not good in terrain where both tires on an axle are not usually firmly in contact with the ground, at least they are not nearly as good as a true locker is in those situations. I consider a LSD's primary benefit to be on the street where all four tires are in contact with the surface. Off-road, much less so.

So for paved road use, a LSD is best. But in my strongest possible personal opinion, a locker is by far the best traction enhancing device for offroad use..

I wish I had known this 16 years ago when I installed front/rear Detroit Truetrac LSDs into my previous TJ. In the very uneven terrain I prefer wheeling in, the Truetracs were not very effective at all. Within a couple years I replaced them both with true lockers and the offroad benefits of the lockers was nothing less than dramatic/startling/grin-producing... day and night differences.
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Old 07-26-2013, 10:51 PM   #6
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I dove into the LSD world once. What a trip
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Old 07-27-2013, 11:24 AM   #7
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Quote:
Originally Posted by harryhood View Post
I dove into the LSD world once. What a trip
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Old 07-27-2013, 11:40 AM   #8
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I was pleasantly surprised with the performance of my LSD on the trail. No where near as good as a locker but much much better than an open diff.
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Old 07-27-2013, 06:11 PM   #9
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Not to hijack, but I keep hearing not to lock the rear 35. Will an LSD be okay in there or save the money and just leave it open until the entire axle can be replaced and locked?
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Old 07-27-2013, 08:11 PM   #10
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I have a factory LSD in my 35. Limiting the slip and locking the axles are two different things.

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