Limited Slip Differential behavior?
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Fleet Mechanic
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Limited Slip Differential behavior?
This weekend, I had the misfortune of burying my truck (07 PSD) up to the frame in near soupy mud. Both front wheels and one rear wheel were buried past the hubs in the mud, while one rear wheel was still planted on terra-firma.
I was under the impression that the limited slip rear differential would keep at least partial traction to the one wheel still on solid ground. Instead it just sat there, stationary, mocking me, while the other three wheels continued to bury themselves deeper.
Am I expecting too much from the so-called limited slip differential, or do I need to take it to the dealer and make sure it's got one, window-sticker-confirmation not withstanding?
-blaine
(p.s. we'll leave for later the discussion on how woefully inadequate the stock BFG Rugged Trail tires are in mud...)
I was under the impression that the limited slip rear differential would keep at least partial traction to the one wheel still on solid ground. Instead it just sat there, stationary, mocking me, while the other three wheels continued to bury themselves deeper.
Am I expecting too much from the so-called limited slip differential, or do I need to take it to the dealer and make sure it's got one, window-sticker-confirmation not withstanding?
-blaine
(p.s. we'll leave for later the discussion on how woefully inadequate the stock BFG Rugged Trail tires are in mud...)
Fleet Mechanic
Joined: Nov 2003
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From: West Cornwall, CT
Your LS differential is working normally. When one rear wheel has essentially no traction, it will spin freely.
If you lightly apply the emergency brake the feely spinning wheel will slow down and often share some more torque with the other rear wheel. Whether stock tires in mud would benefit much from this trick is another story....
Lou Braun
If you lightly apply the emergency brake the feely spinning wheel will slow down and often share some more torque with the other rear wheel. Whether stock tires in mud would benefit much from this trick is another story....
Lou Braun
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Thinkin you may have over estimated the abilities of a limited slip. With that other tire burried, it had very little traction, and the one that did just did not turn. The LS is working as it was designed. It can't be expected to act like a locker, or it would drive poorly on the road.
That is a very heavy truck you have, and it is going to require more than a LS unit, and stock tires to handle frame deep mud.
That is a very heavy truck you have, and it is going to require more than a LS unit, and stock tires to handle frame deep mud.
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Fleet Mechanic
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Hmmmm....
Well, we didn't know it was gonna be frame-deep mud until it was frame deep.
Feeling the truck sink like a stone was interesting, to say the least.
Perhaps I'm holding high expectations, having come from a relatively "tight" Auburn limited slip unit in my Chebby, to the (as I've read here) relatively "loose" Ford unit. The Auburn completely eliminated "1-legged burnouts" in the Chebby, while the Ford still does that with almost no effort at all.
I did try applying the parking brake to force torque to the non-spinning wheel... Apparently the ground cover was too slick to permit traction, because the tire tread immediately filled with clover leaves...
(I do have pics if anyone's interested...)
My mom and brother both offered to pay the tow bill, citing that the situation made for much-needed laughter at the time. The tow company owner did a double-take on the situation when we called for the tow truck. His first word was "Wow." It took more time to reel back the tow cable than it did to drag the truck back to solid ground.
-blaine
Well, we didn't know it was gonna be frame-deep mud until it was frame deep.
Feeling the truck sink like a stone was interesting, to say the least.Perhaps I'm holding high expectations, having come from a relatively "tight" Auburn limited slip unit in my Chebby, to the (as I've read here) relatively "loose" Ford unit. The Auburn completely eliminated "1-legged burnouts" in the Chebby, while the Ford still does that with almost no effort at all.
I did try applying the parking brake to force torque to the non-spinning wheel... Apparently the ground cover was too slick to permit traction, because the tire tread immediately filled with clover leaves...
(I do have pics if anyone's interested...)
My mom and brother both offered to pay the tow bill, citing that the situation made for much-needed laughter at the time. The tow company owner did a double-take on the situation when we called for the tow truck. His first word was "Wow." It took more time to reel back the tow cable than it did to drag the truck back to solid ground.
-blaine
Last edited by Frankenbiker; Apr 23, 2007 at 10:12 PM.
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Fleet Mechanic
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Here are 3 pics, I think... (I'm not sure how FTE's image attachments work, but I'm posting direct links to the pics)
http://www.powerbear.com/images/fordmud1.jpg
http://www.powerbear.com/images/fordmud2.jpg
http://www.powerbear.com/images/fordmud3.jpg
-blaine
http://www.powerbear.com/images/fordmud1.jpg
http://www.powerbear.com/images/fordmud2.jpg
http://www.powerbear.com/images/fordmud3.jpg
-blaine
Postmaster
Joined: Dec 2003
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From: Arizona
Originally Posted by Frankenbiker
Perhaps I'm holding high expectations, having come from a relatively "tight" Auburn limited slip unit in my Chebby, to the (as I've read here) relatively "loose" Ford unit. The Auburn completely eliminated "1-legged burnouts" in the Chebby, while the Ford still does that with almost no effort at all.-blaine
Hope this helps
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Fleet Mechanic
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Ah... kinda suspected that... The Auburn was a little TOO tight, but never gave me any problems... just didn't want to have to duplicate effort from the factory unless I really had to... Looks like like I really have to. Is it possible to adjust the Ford unit "tighter" or need I just replace the whole thing?
-blaine
-blaine
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Oh, that wasn't so bad. Probably should have stopped spinning the tires once you were actually stuck, but oh well.
The problem lies with those little street tires. Once one side started to loose traction, the other side had enough traction to make the limited slip, well slip.
The LS has to slip when you make turns, and with those little tires, the unit just thought it was doing just that. If you had slightly better tires with a little more traction, you may have had better results.
I ran 40 inch boggers on my SD with a stock LS unit with decent results until I finally changed everything out for ARB's front and rear. The units work a little better with tires that get decent traction.
The problem lies with those little street tires. Once one side started to loose traction, the other side had enough traction to make the limited slip, well slip.
The LS has to slip when you make turns, and with those little tires, the unit just thought it was doing just that. If you had slightly better tires with a little more traction, you may have had better results.
I ran 40 inch boggers on my SD with a stock LS unit with decent results until I finally changed everything out for ARB's front and rear. The units work a little better with tires that get decent traction.
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From: Arizona
Originally Posted by Frankenbiker
Ah... kinda suspected that... The Auburn was a little TOO tight, but never gave me any problems... just didn't want to have to duplicate effort from the factory unless I really had to... Looks like like I really have to. Is it possible to adjust the Ford unit "tighter" or need I just replace the whole thing?-blaine
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Fleet Mechanic
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I'll have to try draining and refilling... that looks to be the easiest for now.
I hadn't planned on testing the truck in this particular manner... I hadn't realized just how soft, "soft" was, and how heavy the truck was... I'm still working on the idea of doubling the vehicle weight from my last truck. I guess I'll more or less be forced into buying better tires before I give it it's "official" mud bath... hadn't wanted to fork up for tires when the factory ones were still so new, but.................... *shrug*
Thanks for all the input, in any case.
-blaine
I hadn't planned on testing the truck in this particular manner... I hadn't realized just how soft, "soft" was, and how heavy the truck was... I'm still working on the idea of doubling the vehicle weight from my last truck. I guess I'll more or less be forced into buying better tires before I give it it's "official" mud bath... hadn't wanted to fork up for tires when the factory ones were still so new, but.................... *shrug*
Thanks for all the input, in any case.
-blaine
FTE Leadership Emeritus
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From: Stephensville WI
The main problem is that the Ford plant uses too much friction modifier when the trucks are assembled. I suspect they don't want any chatter in the differential, that would mean warranty work. I'll put up with chatter if it means I get a working LS differential on my truck.
Yes, there are better units available. If you're going to make this mud fetish a habit you may even want to invest in some lockers, after you get some decent tires.
Yes, there are better units available. If you're going to make this mud fetish a habit you may even want to invest in some lockers, after you get some decent tires.
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Joined: Jul 2002
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From: Long Island USA
Everyone's spot on with this one 
The Ford LS unit needs SOME preload to get it to lock-up (or at least, attempt to). The emergency (parking) brake trick helps, but not always.
I went down to 4 oz friction-modifier, and while I feel the chatter a LOT more, it really didn't help overall in low-traction situations. On the street, it works better, but on dirt/mud/snow? Nope... Long time ago, there was a thread around here about shoving an extra clutch plate (or shim) in the clutch pack and it helped a LOT. But they just wear out ever faster.
Suggestion? Aftermarket.
The Ford LS unit needs SOME preload to get it to lock-up (or at least, attempt to). The emergency (parking) brake trick helps, but not always.
I went down to 4 oz friction-modifier, and while I feel the chatter a LOT more, it really didn't help overall in low-traction situations. On the street, it works better, but on dirt/mud/snow? Nope... Long time ago, there was a thread around here about shoving an extra clutch plate (or shim) in the clutch pack and it helped a LOT. But they just wear out ever faster.
Suggestion? Aftermarket.
