Limited Slip Question

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Gary Kendall

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Hi Guys. Took my son hiking today and parked on a little grade off of the road. When we got back to the ST, I had to back up into a ditch a little way as a car had parked close to me and there wasn't room to pull out. Anyway, my driver's side tire came up off the ground a bit because of the angle. I put my truck in drive and the drivers side wheel just started spinning. At first I thought there was something wrong with the tranny, but then realized that the wheel was indeed just spinning and not getting traction. Then I thought, wait a minute I have a limited slip rear end with the 4.10 rear end! Shouldn't the passenger's side tire have kicked in to give me traction? I have done a little research on this site and seen that others have had similiar things happen with theirs at a low mileage. My ST only has 38,500 miles on it. I saw where some people of added some of the Ford friction modifier to the rear end. I have not heard any noises coming from the rear end to indicate a problem. My question is the filler hole for the rear end easy to access and is it ok just to add some to mine, or could I do some harm? I have 4wd, so it is not really a problem, but I like things to work like they are supposed to. Any thoughts or opinions on what the best way would be to handle this will be appreciated.

Gary
 
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Yes. I have the vehicle ticket that was on the window when the vehicle was new. It was listed in the optional equipment section. "4.10 Limitied Slip Axle"... $355.00
 
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Well, I don't know much about them, but it seems everyone has a life tonight except me. My only comment, limited slip doesn't mean no slip. I'd look into changing the rear end fluid and making sure you add the friction stuff to it. Maybe your clutches have worn too much.
 
Very often with a limited-slip, if one wheel is off the ground or barely touching, the other wheel will not kick in. It's what is called "bias" in the differential. If I understand it correctly, and I may not, if you have say a 70/30 bias, then when the wheel with less traction gets below 30% of the total available traction, you get nothing.

This is why rockcrawlers who often have a wheel or two or three off the ground use lockers.
 
What happened to you is normal. The clutch pack (term from years gone by) is slipping and is designed to. This is for going around corners and not creating a loaded or torqued condition. With Limited Slip if you get a tire spinning it will keep on spinning. It is more responsive if both tires are down and you get slippage from the road such as in the snow. This is when the working load will transfer back and forth until you really get one tire spinning again.
 
IT only takes 40 ft lbs of differential torque to over power the clutches and they will slip...



What happened was normal..



Todd Z
 
As others more knowledgeable in mechanics have stated... it's normal. Though, there is a possible remedy which can help your L/S to engage more easily. I and others have found that when you are under no or a little bit of throttle, tap your brakes to bring the rear speed down and then try again. On a sidebar note; I have had some interesting conversations with a master tech mechanic bud that has noted issues with his rear-end (albeit his is locked, but nonetheless) when the temperatures are low. He is EXTREMELY knowledgeable with the 8.8 and L/S and locked applications... just food for thought for what it's worth.
 
Yep, Normal, and the brake tapping technique Becker referred to is often used by guys who off-road with LSDs.



As a side note: it seems like my '07 with AdvanceTrac & Rollover Stability Control (RSC) does a great job engaging the LSD (better than my '04 with 4.10 &LSD).
 
Wait....you can have a LSD and have 4 wheel drive? I thought those 2 things were mutually exclusive...or am I just missing something in my very limited knowledge of drivetrains?



BTW, I just found that I have the 4.10 LSD...didn't think my Carmax wonder came with that luxury, but it did :)



I don't know if it works any better than a normal rear differential for my driving, as my awesome ST is the only car I've ever driven with RWD. Minivans, baby SUVs, LeSabres, and even a baby sports car like the Escort ZX2 are all front wheel (or 4 wheel, but it's at the computer's discretion, so that doesn't count)
 
Wait....you can have a LSD and have 4 wheel drive? I thought those 2 things were mutually exclusive...or am I just missing something in my very limited knowledge of drivetrains?
Click to expand...



Nope, LSD is just supposed to keep one rear wheel from spinning by allowing power to also go to the non-spinning wheel on the rear axle. 4x4 puts 50% power to the front and 50% power to the rear axle. If you have a 4x4 with open diffs you really have a 2x2 as one wheel in the front and one wheel in the back will be getting the power. With 4x4 and LSD you have power supposedly going to both rear wheel plus to the front axle. (that is my limited knowledge non technical explanation)
 
Wait....you can have a LSD and have 4 wheel drive? I thought those 2 things were mutually exclusive...or am I just missing something in my very limited knowledge of drivetrains?
Click to expand...



Yes your are missing something, many 4x4 vehicles are built and shipped with rear LSDs.



If 4 wheel drive was actually 4 wheel drive they would be mutually exclusive. However, as stated above, what is frequently referred to as 4 wheel drive is often actually 2 wheel drive (a drive wheel on each axle, aka: open diffs). True 4 wheel drive is used by off-road enthusiasts by installing locking differentials front and back (Jeep Wrangler Rubicon comes standard with locking front and rear axles). Others will leave their LSDs in the rear and put a locker in the front.
 
And now I know...the Sport Trac manual states that a LSD provides more traction than a normal differential, but less than 4 wheel drive, which had me thinking of a chain (this is better than that, but worse than this) relationship, which was clearly wrong.



The manual let me down :(
 
If you use the emergency brake when this happens you will usually get favorable results. It will slow down the spinning wheel & transfer power to the other side while leaving your front wheels free to turn so you can free yourself.
 
Wow .... makes me glad that I got one with an open differential & was able to put in a lunch-box locker

'Math-you'
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You can put a "lunchbox" locker in a limited slip rear end as well.



Yes your are missing something, many 4x4 vehicles are built and shipped with rear LSDs.



If 4 wheel drive was actually 4 wheel drive they would be mutually exclusive. However, as stated above, what is frequently referred to as 4 wheel drive is often actually 2 wheel drive (a drive wheel on each axle, aka: open diffs). True 4 wheel drive is used by off-road enthusiasts by installing locking differentials front and back (Jeep Wrangler Rubicon comes standard with locking front and rear axles). Others will leave their LSDs in the rear and put a locker in the front.
Click to expand...



If you have a 4x4 ST with the 4.10 L/S option, then it is L/S both front and rear. Also would not suggest a locker in the front, as it will help destroy the CV's.



Chad
 
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