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Waltherone
Dodge Dakota
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8/08/2002
02:05:03

Subject: Explain open diff.
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I'm curious, I get how an LSD diff. works, but on an open diff.....is it just basically like one wheel drive?? That's what it sounds like, but that doesn't seem very effecient to me..



Bryan
Dodge Dakota
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8/08/2002
10:43:01

RE: Explain open diff.
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The open diff. is cheaper to manufacture than the " anti-spin " LSD. If you look on the window sticker on A new Dodge, you will see where they are charging for the " anti-spin " diff.
So to sell A truck cheaper, sometimes an open diff. will be installed on the truck from the factory.
Open diff.'s are ok if you are not looking for high performance where traction ( or the lack of )would become A problem.




CasuObserver
Dodge Dakota
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8/09/2002
13:50:20

RE: Explain open diff.
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Strong opinion in off-road threads is that the LSD is pretty much a weak compromise.

Basically, in an open diff, the same amount of torque is always sent to both sides of the differential. Once it gets to each of the wheels though, road & tire condition(s) determine what gets done with it at that wheel. Either it just spins the tire, or it sends the vehicle forward, or a bit of both.

So, you want this 50/50 torque distribution changed when one wheel is spinning out, so that actually more of the torque is sent to the slow wheel. However, with a LSD only about 15-20% additional torque can be transferred to the non-spinning side differential (compared to a diff locker). Supposedly the rest is just dumped to the side of the spinning wheel, like a normal open diff.

Nonetheless, LSD is sometimes useful, because this 15% extra might be just enough to get the vehicle moving again (either out of mud on some ranch, or up a wet boat ramp - especially with an empty truck box). Also (like in a normal open diff) the wheel on the slippery side has just enough traction in the side-to-side direction so that the vehicle doesn't fishtail back & forth like one with a diff locker.

My questions:
= Is this 15% torque redistribution really useful in a 'performance'/racing situation, and if so, how useful is it?

= Wouldn't diff lockers be better in a 'performance' situation, due to the limitations of LSD (more expensive, heavier, friction cones wearing out, 'special' gear oil conditioner needs to be used, limited torque redistribution)?




Texas Todd
Dodge Dakota
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8/09/2002
16:15:16

RE: Explain open diff.
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Cas,
This is also what I've heard about LSD. (not the drug)
I agree with your post.

However, It's hard to believe that only 15% power is smoking both tires, when I do the rare burnout on dry, clean, flat, cement roads. Both wheels leave the same patch. I can't really figure out why, mines the stock 3.55 LSD, 4.7, 5spd.



D
Dodge Dakota
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8/09/2002
17:06:40

RE: Explain open diff.
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if you burn out hard enough, both tires will
spin, even with an open diff setup. the only
problem is that it is not consistent, and
sometimes you get partial spin with both tires,
and most of the time only one tire spins.

lockers are great for race applications, but for
regular driving, I dont think it is the wisest
choice. if both tires lock up every time you
took a corner, you would be in trouble,
especially if it is raining or icy outside.



Bryan
Dodge Dakota
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8/09/2002
17:31:27

RE: Explain open diff.
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Every time I have laid into it both tires have broken loose. My truck has never done A peg leg burn out before. and going through A curve under hard acceleration my truck will go straight. ( that is an errie feeling )
But that's the way I got it from the factory.
Could be why I go through two sets of tires A year.



CasuObserver
Dodge Dakota
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8/09/2002
18:04:39

RE: Explain open diff.
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D - so, posi-trac/LSD on the track is just used to ensure that if one wheel gets a little bit more 'bite', the LSD will minimize the other wheel cutting loose, right?

What about the power loss inside of the LSD itself, where the braking action of the cones in the diff act to force more torque to the spinning torque? Is this power loss noticeable?

I believe that the way that lockers work though, is that the lockup mechnism senses the relative *speed* of 3 things -the driveshaft and left and right axle sides. Not the torque.

So, if you turn right, the faster left wheel actually unlocks itself and freewheels, while the right side (the inside) pulls the vehicle around. As soon as the turn is finished & you're going straight, it locks up solid again. The reference speed/rpm is the driveshaft. If either wheel finds itself going substantially faster than the other one, compared to the speed of the driveshaft, it will unlock itself.

This is great for rock crawling, dry racing, and going relatively straight in spotty slick conditions like uphill in patchy mud or small ice puddles. However, it's absolutely horrible turning corners in winter in a city, because you'd always be spinning out in every slick corner. Posi-trac, on the other hand, would actually transfer more of the torque to the outside wheel and you wouldn't sit there spinning.

Now, my personal philosophy is that the only kind of Dak I'd keep is a 4x4. I drive a mix of city, highway, a fair amount of bush country/bad roads and lots of winter snow. So, for me, I want to distribute possible torque/traction over all 4 wheels, not just 2.

If one or both rear wheels start spinning, I think there's a good chance the front axle wheels could still bite onto something and pull me through. I depend on the front axle to get me out of stuff, instead exactly metering out torque to one or the other rear axle halves. I prefer to drive in this case with a locked transfer case (so, part-time) rather than differential t.c. (would not have full-time drive).

I would never get LSD with a 4x4, due to the drawbacks of extra weight, expense, playing around with gear oil conditioners, and mechanical complexity of the axle. Also, the friction cones are a bit like brakes, and will wear out gradually leaving you with just a heavy open-diff. So, after 7 or 8 years, you'll have to end up replacing the axle or cones if you want to keep the axle working like new. If rear open diffs won't work, pop on 4x4-high. If even that's not enough and you always get one front and one rear spinning, then go to a locker. In the winter, the front axle should pull you out of a locker-corner (even though I find you have to put up with some 4x4 oversteer).





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