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To Limited-Slip or Not???

 
  #1  
Old 03-15-2000, 10:01 PM
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To Limited-Slip or Not???

I've just ordered a F250, V10, CC, 4x4, LB with 4.30 LS diff. I live in CA, so the 3.73 LS was not an option. I'm not going to be towing anything drastically heavy, so should I have gone with the 3.73 without LS? Is LS necessary? I heard that since I'm getting a 4x4, I should have LS.

Thanks for your input.
Dan
 
  #2  
Old 03-16-2000, 10:41 PM
Roger_B
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To Limited-Slip or Not???

I think your dealer is not that knowledgable.. I thought for sure I know of folks who got the V10 + autotranny + 3.73LS..

In any case, the reason folks generally get 4x4 is to get more wheels driving. Well, the limited slip maximizes this..

4x2 w/o limited slip = 1 wheel drive (1 rear)
4x2 w/ limited slip = 2 wheel drive (both rears)
4x4 w/o limited slip = 2 wheel drive (1 front, 1 rear)
4x4 w/ limited slip = 3 wheel drive (1 front, both rears).

Just fyi though, the limited slip diff does apply torque to both rear wheels, but it's not a full locker ie, both rears do not get the same amount of torque applied. With the limited slip the rear with the least amount of grip still gets more torque, but the other wheel also gets some too.

 
  #3  
Old 03-17-2000, 12:00 AM
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To Limited-Slip or Not???

Thanks Roger.

I actually verified the 3.73LS as NOT being an option since I live in Calif.

A friend of mine did suggest I can order the 3.73 Non-LS and put the full locker which would, of course, probably void the warranty.

Something for me to think about since I figure I still have another week or two to change my mind on what diff I want on my truck.
 
  #4  
Old 03-17-2000, 03:34 AM
tjahl
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To Limited-Slip or Not???

I would stay with the 4.30 LS. My truck is an F350 V10 4x4 with the 4.30 and it has paid off (here in New England anyway) many times. I have had a 4x4 without an LS and you end up getting out to find traction for the unloaded wheel--you might as well have had a 4x2. Even in the summer on wet grass, a towing load can stick you and the 4x4 gets out without trouble.
 
  #5  
Old 03-17-2000, 06:31 PM
ROBERT_2
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To Limited-Slip or Not???

I recieved news yesterday that my F250 SD 4X4 has been pulled (finally) I opted for the 5.4 because of the EXTENDED wait for the V-10 and I stayed with the 5sp trans. My question is because I did not select the Limited Slip rear end I thought I was going to get a posi (type) rearend. Thinking that a posi (type) rearend would put equal power to the rear tires at all times. If this is not the case would someone please tell what option I need to ask for to get this type of rearend power distrubution. One last thing I heard that the 5.4 that are coming out now have 260hp is that correct??
Rob
 
  #6  
Old 03-17-2000, 10:50 PM
Roger_B
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To Limited-Slip or Not???

Yep, you got confused with old vs new terminology.
Limited slip = posi traction..

Ford only offers 2 diffs..
a. normal 'open' diff, applies torque to the one wheel that has the least traction

b. Limited slip diff: applies torque to both wheels. This is a very similar mechanism as the positraction, (using a clutchpack I believe in the rear diff).

I don't think ford ever offered a true locker for a rear diff, but I could be mistaken on this point.

Just FYI, when you get the 5.4L + 5sp, then you have to get the 4.10 ratio for LS. (they don't offer 5.4L + 5sp + 3.73LS ,only 3.73 normal)

But, no need to worry. The 4.10 is a very good gear for the 5.4L and you mileage will still be better than mosts because of the 5sp..

Take my truck for example:
Y2K F250 XLT SC 4x2 5.4l 5sp 4.10LS.
I'm averaging 15.5 mpg with mixed city/highway driving.. I get 16 if I do mostly highway..

Yep, the 5.4L is 260hp in 2K for the SD.



>I recieved news yesterday that my
>F250 SD 4X4 has been
>pulled (finally) I opted for
>the 5.4 because of the
>EXTENDED wait for the V-10
>and I stayed with the
>5sp trans. My question is
>because I did not select
>the Limited Slip rear end
>I thought I was going
>to get a posi (type)
>rearend. Thinking that a posi
>(type) rearend would put equal
>power to the rear tires
>at all times. If this
>is not the case would
>someone please tell what option
>I need to ask for
>to get this type of
>rearend power distrubution. One
>last thing I heard that
>the 5.4 that are coming
>out now have 260hp is
>that correct??
>
>
>
> Rob



 
  #7  
Old 03-19-2000, 04:37 PM
unaFORDable
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To Limited-Slip or Not???

My last truck, a F150 4x4 w/limited slip was a great truck. Decided to upgrade that truck and I Now have a PSD w/3.73limited slip 4x2 6 speed. Went w/powerstroke instead of 4x4 this time because with the first truck I found that the limited slip usually did the job. I guess if I'd used that 4x4 a bit more, I would have gotten it again. Unless you are planning on adding a Detroit locker (at a huge cost) then I would recomend you get the limited slip from ford. It only cost me $243 (invoice) on my 2000 F250
 
  #8  
Old 03-25-2000, 02:41 AM
AB
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To Limited-Slip or Not???

A standard "open" differential (WITHOUT limited-slip or locking capabilities) splits torque EVENLY to both wheels. For example, if one wheel is on soft sand and the other is on solid pavement, the wheel on the sand has very little traction and starts to slip with only a small amount of delivered torque, and the other wheel on solid pavement can only receive the SAME amount of delivered torque, limited by the amount of torque that causes the wheel on sand to spin. So, you end up just sitting there, spinning one wheel.

The idea behind a limited-slip differential is to be able to spit torque unevenly and deliver MORE torque to the wheel with MORE traction (NOT the wheel with LESS traction as is often believed). With a LS differential, if one wheel is on sand and the other is on solid pavement, the amount of torque delivered to the wheel on solid pavement is no longer limited to the amount of torque that causes the wheel on the sand to spin. It can now receive extra torque, hopefully enough ti propel you forward. (It would not do any good to deliver more torque to the wheel with less traction, because it would just spin at the same point, still limited by traction, meaning that you could NOT deliver more torque to it anyway.)

Getting back to your question, a limited-slip differential is a nice option, although it is not quite the panacea for traction ills that many people think it is.

The LS unit that you ordered is a clutch-type, torque-sensitive design that applies more limited-slip action as the amount of applied driveline torque increases. Unfortunately, this means that it doesn't work very effectively in extremely low-traction situations, such as when one wheel is on slick solid ice. There IS a small amount of preload on the clutch pack from a disc spring, which allows a small amount of extra torque to be delivered to the wheel with more traction, but in and of itself, it doesn't produce a huge increase of torque to the wheel with more traction. However, it is often just enough to get the vehicle moving under those circumstances. It is far MORE effective when there is enough traction on the wheel with least traction to allow more driveline torque to be applied, which applies more pressure on the clutch pack and increases the limited-slip action. This will allow an even greater increase of delivered torque to the wheel with more traction. So, in a situation where you have, for example, one wheel on sand and the other on pavement, you will get a greater "torque transfer" to the wheel with more traction than you would if one wheel were glare ice and the other on pavement.

The LS unit you ordered is often called a "soft" limited-slip differential because of the relatively small amount of preload on the clutch pack. While it may not provide as positive a torque transfer under some circumstances (like the ice situation above), it does have the advantage of being more forgiving in terms of handling ease. For example, if you had a "stiff" limited-slip unit with a lot of preload on the clutch pack, it would have a greater tendency to break tire traction as you coasted around a corner, especially on slippery surfaces, which increases the chances of the rear of the truck sliding in a turn. It also reduces the liklihood of the rear of the truck sliding sideways when you are driving with both rear wheels on an icy surface, etc.

You'll notice that with the factory LS unit, if you accelerate hard as you go around a tight corner on dry pavement, the added applied torque will compress the clutch pack enough to effectively lock up the differential (though not a true locking differential) and the tires will chirp, particularly the inside tire. Whereas, if you don't accelerate around the corner, they won't chirp (unless you're going too fast).

There are a number of other limited-slip, locking, and combo schemes, all with their pros and cons and different operational parameters and characteristics, but I think that the one that you ordered is one of the more "user-friendly" ones. For more "positive" torque transfer to the wheel with more traction, you could install something like a Detroit Locker, ARB on-demand locking differential, or a dozen other types of units. I've had the opportunity to install and/or drive nearly all of them over the last 30 years of auto and truck modification work that I've done. However, for most of the driving that most people do, the factory LS unit should be just fine.

I personally like the ARB unit a lot. If you order the truck without a locking differential, you should be able to have an ARB unit installed in the rear axle for roughly 600 dollars total price. These use an air pump and a small hose to the differential to engage the locking unit. You would only want to use this, on demand, when you need to use it and leave it disengaged or "open" the rest of the time to avoid compromising handling characteristics.

Sorry about the length. Hope that addressed a few of your questions.

Alan



 
  #9  
Old 03-25-2000, 02:56 PM
Roger_B
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To Limited-Slip or Not???

I'm not sure I agree all of the 'details' of AB's post, but I can always learn more and much of what he says is sensible..

Also, no matter how the internal workings are described the end result is the same:

w/o limited slip, only one wheel really propels you forward at a particular time..

w/ limited slip, both wheels are helping to propel you forward. Not always are both wheels giving you the same amount of forward force, but at least they're trying..

In the case where you have extreme traction differences, (ice on wheel vs solid ground on other), limited slip has its limits.. If you use the old 'apply the brake' techinque, it seems to allow more power to be delivered to the driveline, including the wheel thats on solid ground, which can get you out of some real binds..

 
  #10  
Old 03-26-2000, 11:19 AM
AB
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To Limited-Slip or Not???

Perhaps some further discussion will clear up a few issues:

Without a limited-slip or locking differential, BOTH driving wheels "push" with the SAME amount of torque, equally divided between the wheels. This is well documented in any good automotive mechanic textbook, engineering manual, gear design text, etc. Unfortunately, it is so often misunderstood that it is frequently incorrectly stated in some "popular press" publications that are not properly researched (or which are not properly reviewed by engineers).

If anyone is unconvinced, I can dig up some quotations from engineering texts on the subject.

There are generally two issues that confuse the matter. One is observationally based and the other is technically founded.

As to the observational issue, what confuses people is that, for example, when one wheel is on ice and the other is on solid pavement, the wheel on the ice is the one that spins, which creates the impression that the wheel that is on the ice is the only one receiving the torque. However, this is absolutely NOT the case. The wheel that is on dry pavement is receiving the SAME amount of torque as the wheel on ice.

As to the technical issue, it's the term "power" that often compounds the confusion. Torque, in and of itself, is a static force, whereas power is a function of torque (force), angular rotation (distance), and time. This means that the wheel that is spinning is generating "power" while the wheel that is not spinning is not. HOWEVER (and this is a very important 'however'), BOTH wheels are receiving EQUAL torque (force) from the axle shafts, and BOTH wheels are exerting an equal amount of force in trying to PUSH or propel the vehicle forward.

There is yet another observational factor that causes confusion and helps to create the impression that one wheel is driving the car when an open differential is used. This factor is the "torque effect" on cars with solid axle housings. The torque reaction to driveline rotation causes the rear axle housing to twist around the same axis as the driveline, which has the effect of removing some weight from one wheel and transferring more weight to the other, giving that wheel more traction. This makes the wheel with less traction (which is always the same wheel on a given car) break loose and spin more easily than the other wheel, which creates the impression that that wheel receives more torque, when in actuality, they are still receiving the same amount of torque and pushing the vehicle equally.

Here's a typical scenario. Let's say that you are parked facing uphill on a very steep hill, with one wheel on dry pavement and the other on ice. You slowly apply throttle (applying torque evenly to both wheels). The car doesn't move so you apply a little more throttle. As you gradually apply throttle to try to start moving the car, the wheel that is on the ice suddenly breaks loose and starts to spin, and you just sit there, spinning the wheel on the ice. What is happening is that both wheels are receiving the same amount of torque, gradually increasing as you apply more throttle, but as soon as you apply enough torque to make the wheel with less traction break loose and start to spin, that amount of torque becomes the torque LIMIT for the wheel with less traction, with both wheels still receiving the equal torque. In a case like this, you cannot apply enough torque to the wheels to move the vehicle up the hill, so you just sit there with one wheel spinning.

With a limited-slip differential, the clutch plates simply guarantee that a certain minimum amount of torque can be delivered to the wheel with more traction (given that there is enough traction on that wheel). With the factory-supplied LS differential in a Super Duty, this minimum is variable, depending on how much torque is being applied via the driveline.

An open differential only allows as much torque to be delivered to the wheel with more traction as can be absorbed by the wheel with less traction. As soon as the wheel with less traction receives enough torque to break it loose, that is the limit of the torque that can be delivered to the wheel with more traction.

If you look at the design of the side gears and spider gears inside a standard differential carrier, you will see that the spider gears push on both side gears, not favoring one over the other. The spider gears can only push on one side gear as hard as they can push on the other. If one wheel has less traction than the other, it can only absorb a certain amount of torque before it begins to slip, and that is what determines the maximum amount of torque that can be delivered to the other wheel with more traction.

Let's say you put a vehicle with a standard, open differential up on a rack, with both drive wheels off the ground. Have someone hold the left drive wheel while you rotate the driveline. The right drive wheel will turn. Conversely, if you have someone hold the right drive wheel while you rotate the driveline, the left drive wheel will turn. There is no "favoritism."

A locking or limited-slip differential allows uneven distribution of torque to allow the wheel with more traction to receive more torque, not limited by the torque that the wheel with less traction can absorb.

By the way, Roger's suggestion about applying the parking brake when you're stuck in an unequal-traction situation is a good one, since it has an effect similar to that of the clutch plates in a limited-slip differential (although with added parasitic drag).

Again, I apologize for the length. Perhaps if anyone has questions, they could contact me offline for more detailed discussion.

[email protected]

Alan

 
  #11  
Old 03-27-2000, 02:25 PM
unaFORDable
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To Limited-Slip or Not???

 
 
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