NEED SOME INPUT FORM SOME DIFF. EXPERTS

[JLsounds]

I have a 15 GT with the performance package which as you know has the 3.73 gears. From the day that I purchased the car new it has had some weird issues with the rear diff. For example, making a turn from a stop it would chatter and skip. I just assumed it was due to the possi style diff that the car had. A few months ago we installed a Steeda no hop package with a diff. bushing kit. I then noticed a lot of slop in the drive train. I also had a time going sideways into a driveway that the inside rear tire came off the ground due to the stiffer chassis. When this happened the outter tire did not keep pushing the car forward. I had to roll backwards out of the driveway and then go at it again. Took the car to the dealer and they said something was wrong in the diff so they ordered a new one. They also said that while the car was on the lift they could spin one tire and the other didn't spin so Ford told them to replace the diff which requires that Ford send out an entire new unit as this diff is not serviceable. That was replaced last week. It seemed fine for a couple of days and then on Sunday I had to catwalk into a driveway and the inner wheel came off the ground and the outer didn't spin again. I have also noticed some of the chattering back. The day that they replaced the diff the car felt solid in the rear. I could leave my shop and hit it hard in first and there would be two solid black marks behind me and the car would kick out in a controlled feel. Now it feels like there is a little wheel hop again and it wants to kick harder to one side or the other depending on which way I am turning. I don't do launches although I did do two launch control launches at 3500 RPM the day that I got it back with no issues. I am at a loss as to what to do. Is my car acting the way that it should or do I have a bad diff. again? The slop is still in the drivetrain and they said that is normal with a rear wheel drive manual.

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[jasonstang]

What you have is called a Torsen differential. The way it works is it has a bias ratio. One wheel will never have more torque times the ratio than the other.
For example, in a corner, the inner wheel will start to lose torque due to grip. The outer wheel will receive the inner wheel torque times the bias ratio amount of torque.
What happens when one wheel is in the air? Zero torque, and zero times the ratio is zero. That's why Torsen owners will tell you the diff will act like an open diff when one wheel is lifted off the ground like what happened to you as well as at the shop when its in the air.
Also the reason the other wheel didn't spin could be due to the transmission being in neutral. You need to put the transmission in gear to spin the other wheel when spinning one. Otherwise you are just spinning the drive shaft instead of spinning the other wheel.
If one wheel in the air is common issue, you can either apply the brakes just a little bit maybe pull up the handbrake so the spinning wheel would have some resistance to transfer torque to the outer wheel. Or you can change to a clutch type LSD.
So yes, the diff is working as designed but maybe just not for your type of application.

 

[VinnAY]

What you have is called a Torsen differential. The way it works is it has a bias ratio. One wheel will never have more torque times the ratio than the other.
For example, in a corner, the inner wheel will start to lose torque due to grip. The outer wheel will receive the inner wheel torque times the bias ratio amount of torque.
What happens when one wheel is in the air? Zero torque, and zero times the ratio is zero. That's why Torsen owners will tell you the diff will act like an open diff when one wheel is lifted off the ground like what happened to you as well as at the shop when its in the air.
Also the reason the other wheel didn't spin could be due to the transmission being in neutral. You need to put the transmission in gear to spin the other wheel when spinning one. Otherwise you are just spinning the drive shaft instead of spinning the other wheel.
If one wheel in the air is common issue, you can either apply the brakes just a little bit maybe pull up the handbrake so the spinning wheel would have some resistance to transfer torque to the outer wheel. Or you can change to a clutch type LSD.
So yes, the diff is working as designed but maybe just not for your type of application.

Informative!!!

 

[EXP Jawa]

I can probably get into a lot more detail on the operation of the Torsen differential than you really want. I'll try to keep it simple. All limited slip differentials create friction to work; friction is what resists wheelspin. But the Torsen gets its friction through helical gearing, not by way clutch plates and preload springs like the traditional Traction Lok does (Posi - or Positraction - is an Eaton trademark, BTW). So, there is no preload across the gearing to overcome in turning the gearing.

Torque Bias Ratio - or TBR - that Jason referred to is the term used to quantify how much friction the unit generates, and it actually applies to any type of LSD, not just a Torsen. The higher the TBR number, the more friction there is, and the more aggressive the performance of it. The Torsen in the GTPP has a TBR of about 2.8:1, whereas I've measured the clutch plate Traction Lok at about 1.8:1. For reference, an open differential is essentially 1:1, whereas a locked differential or spool is infinite:1. So, the relationship between amount locking and TBR is not linear, but that's beside the point.

ANY differential has the left axle geared to the right axle. If you locked up the driveshaft so the ring gear can't turn, and then turn one wheel, the other wheel has to turn the opposite direction. If it doesn't, it is broken enough that there would also be zero drive from the ring gear through to the wheels. So, if the dealer turned one wheel and the other didn't, the only explanation is that the transmission was in neutral and the propshaft turned instead. That happens because often there is less drag in turning the ring & pinion, prop and trans output than there is in the brake assembly attached to the other wheel. So, for that test to mean anything, you have to put it in gear. As I said, any diff will do that, even a preloaded clutch diff. The difference is that you'll have to put more effort into turning the tire; you have to overcome the preload torque before you can turn it.

Regarding the wheel in the air thing, any LSD needs to have a reaction torque to work against in order to support the load and transfer torque. Its a physics equal and opposite reaction thing. If the tire looses 100% of traction, an LSD cannot prevent it from spinning. To do that, you'd need an actual locking differential (like offroaders use). Now, to be fair, the preloading on a Traction Lok does allow a small amount of torque to be sent to that tire. Not enough to prevent spin, but maybe enough to keep you moving depending on circumstances. The preload would put around 30-50 lb-ft of torque to that tire. For comparison, if you were to accelerate smoothly but rapidly from a stop in first gear, your differential would see upwards of 5-6000 lb-ft of torque (assuming you have the tire traction to support it), so the preload equates to roughly 1% of the peak torque you might see under normal conditions. That's not a lot to move a 3800lb car. As an experiment, I once put an F-150 with a Traction Lok diff on a jackstand, and put the jackstand on a platform with heavy duty castors. I wanted to see if the amount of differential preload was enough to allow the truck to move across a flat, smooth concrete floor. It spun the tire and did not move...

The real function of the preload is to compress the clutches in the differential, so that they produce friction. The stiffer the preload springs, the more friction that it generates, but at the same time, the more negative influence it has on handling dynamics (induces push). So, its a balance. OTOH, the Torsen offers tunable friction characteristics without preloading, so it can often represent a more favorable compromise for chassis development. But, at the end of the day, you still need to have a reaction torque to maintain drive. You need to keep the tires on the ground; suspension compliance can still be a good thing.

That might not have been as simple as I wanted, but believe it or not, that was still a pretty condensed version...

 

[JLsounds]

thanks for all the info guys. Short story is my dealer doesn't know anything about the diffs in these cars (as I didn't really either). So they replaced my diff for no reason.

 

[lwnslw]

WoW... I learned something new today.. I have been worried for a bit that something was wrong. While installing my Steeda sub frame braces, I spun the rear tires and noticed only one would spin. All my other cars with LSD units always spun both tires at the same time so I thought I had issues with my GT... Good to know I was wrong..LOL

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