What is the benefit of a G86 Limited Slip Differential?

[Jkust]

Hi all, I meant to post this when I first got my 9-7 a few weeks back. The 9-7x both the 5.3 version and the 6.0 version came with the G86 like the Trailblazer SS did. The G86 is the Limited Slip versus the Locking Differential as in the G80. My question is what is the benefit of the limited slip? All the youtube videos bash the Limited Slip showing it fail to perform in all the torture tests. I haven't had it long enough to really appreciate it or not yet since there has been no snow and I haven't towed anything.

The G86 seems to be the odd duck of the bunch.

 

[HARDTRAILZ]

One tire in the air or on ice and all power goes to it leaving you stuck. There are some cheats to help this though

 

[Jkust]

One tire in the air or on ice and all power goes to it leaving you stuck. There are some cheats to help this though

Sounds as bad as the open differential. Why put it on the 9-7 and the SS? Why not an open diff or a G80? Must be some rational??

 

[gmac310]

An LSD will provide traction to both wheels whether they're both on the ground or not just like a locker will. The main difference between an LSD and a locker is that a locker sends the traction to both wheels 50/50 but an LSD will send only a portion to the non-spinning wheel.

 

[The_Roadie]

An LSD also engages with a bit of softness, which is good for the high HP wheel-spinning takeoffs the SS folks can do. For 4WD offroad use, the locker is better, but you have to tickle the throttle to get it engaged without having a lot of wheel-spinning energy for it to absorb when it engages abruptly. G80s are prone to breaking under extreme overload shock.

 

[RayVoy]

The biggest difference between the G86 and the G80 is the mechanical locking capability of the G80.

Both are considered "limited slip" diffs, both have clutch packs that provide power transfer to the axle with the highest traction.

The G80 has the ability to "lock" the left and right axles (using a centrifugal weight activated mechanism) if the limited slip clutches need additional help.
The G80 LSD clutches can get you out of a low traction situation, without the locker, if you don't spin a wheel.


The G86 is a high performance LSD. Because of the clutch action, the power is transfered "gently" to the wheel with the greatest traction, ideal for high hp applications.


The G80 uses a locking pawl to "hard" engage the wheel with the greatest traction, exposing the diff to a lot of stress.


The end result is the same, power is transferred to the wheel with the greatest traction, the mechanics are just a little different.

Personally, I like the G86.

 

[The_Roadie]

The G80 LSD clutches can get you out of a low traction situation, without the locker, if you don't spin a wheel.

That's the way the shop manual says the clutch pack works, supplying a small amount of torque to the low-traction tire. But whatever the torque is, I've never found it useful in a hill climb. When the delta between the rear tires gets to 100 RPM and the pawls engage hard, that's when I start moving again.

My best demo video (very old now) of a location where the locker was essential:

[video=youtube;24SwmSN0-bI]http://www.youtube.com/watch?v=24SwmSN0-bI[/video]

 

[RayVoy]

But whatever the torque is, I've never found it useful in a hill climb.

Yep, an other key difference. The G86 is for street use, the G80 is better in the off-road world.

 

[Jkust]

An LSD will provide traction to both wheels whether they're both on the ground or not just like a locker will. The main difference between an LSD and a locker is that a locker sends the traction to both wheels 50/50 but an LSD will send only a portion to the non-spinning wheel.

That makes sense but what is a portion? Up to 49/51?. I can see the benefit of not simply sending it out 50/50 as the only ratio and adjusting as needed. I see the LSD on the Eaton site but it doesn't give much info about it.

In this first video, the second truck, the diesel with the LSD, fails while the Locker does the best.
[video=youtube;-S2nDDvKHoA]http://www.youtube.com/watch?v=-S2nDDvKHoA[/video]

In this second one, suprisingly the GM G80 equipped vehicles do poorly and at the GM testing grounds no less.
[video=youtube;LFJI0tLMVpI]http://www.youtube.com/watch?NR=1&feature=fvwp&v=LFJI0tLMVpI[/video]

 

[RayVoy]

In this second one, suprisingly the GM G80 equipped vehicles do poorly and at the GM testing grounds no less.

Hmmm, I had to watch it twice to see what was going on.

In all cases, the trucks were in 2WD, the hill was steep enough that traction under one wheel (the dry one) was not enough to overcome gravity and the weight of the truck, both wheels were spinning.

However, the LSD trucks utilized traction control to reduce engine power, thus reducing rear wheel torque and letting the small traction, along with the application of the brakes, to get the trucks moving.

The GM trucks had both wheels spinning, no traction; but also, no mention of traction control, therefore, no auto application of brakes to slow down the wheels. A good driver, using "heel and toe" could have worked those trucks up that hill without using 4WD.

 

[The_Roadie]

Torque Bias Ratio is the typical number to compare LSDs. 3.5-to-1 to 6-to-1 are in the typical range for clutch-based units. I don't have a number for the pre-locked-up G80 unit.

On pavement, with one wheel spinning, that wheel STILL has significant traction because it's not in the air (where it has ZERO traction). So a 4-to-1 LSD could send 4 times as much torque to the higher-traction wheel than it would leak away into the slipping wheel. Think of torque as a fluid. You can't build up pressure in a hose if there are holes in it.

If the spinning low traction wheel was on ice, it may only provide 50 ft-pounds of torque. Less if you're spinning it to the point the ice is melting and you have a wet tire sliding on the ice. And friction is always less when the two surfaces are already moving compared to stopped and just starting up (high school physics: static friction > moving friction)

So a 4-to-1 LSD when the low traction tire is providing 50 ft-pounds of torque, will be capable of providing 200 ft-pounds of torque to the higher traction tire. That might not be enough to get your vehicle moving. That's why the "left foot braking" trick might help. Google that for more reading.

People sometimes describe lockers as having 50-50 torque ratio, which isn't exactly the best way to describe it. The RPM will be the same on both sides of a locked diff, which can be described as 50-50, but the torque equation is different. If the spinning tire stops, and acquires the same RPM as the one it's locked to, 100% of the torque is going to go to the tire with traction. So it's essentially a 100-0 torque sharing ratio. Or an infinity-to-1 TBR.

 

[Jkust]

Hmmm, I had to watch it twice to see what was going on.

In all cases, the trucks were in 2WD, the hill was steep enough that traction under one wheel (the dry one) was not enough to overcome gravity and the weight of the truck, both wheels were spinning.

However, the LSD trucks utilized traction control to reduce engine power, thus reducing rear wheel torque and letting the small traction, along with the application of the brakes, to get the trucks moving.

The GM trucks had both wheels spinning, no traction; but also, no mention of traction control, therefore, no auto application of brakes to slow down the wheels. A good driver, using "heel and toe" could have worked those trucks up that hill without using 4WD.

Our trucks have the traction control standard for 06 and later if it had A4wd or 4x4 and so I've got to assume those trucks have it as well but you're right they would have mentioned it. Really for me, since I obviously don't off road in the SAAb, I only need something to manage traction at the steep boat accesses, when they are slippery. Really then, it looks like the G86 won't do a ton for me but instead will likely feel benefits from the AWD and I suppose the traction control. I suppose a little help on the snow and ice too.

 

[HARDTRAILZ]

Traction control is inactive in 4lo as well.

 

[RayVoy]

Our trucks have the traction control standard for 06 and later if it had A4wd or 4x4 and so I've got to assume those trucks have it as well but you're right they would have mentioned it.

Just looked it up, the big trucks got traction control in '07. That was an '08 truck, it should have it.

 

[Jkust]

Just looked it up, the big trucks got traction control in '07. That was an '08 truck, it should have it.

That sounds about right. Can't see why the GM's with G80 did poorly.

 

[Cloudsurfer]

The torque split in an LSD depends upon many things: clutch pre-load, clutch plate material, ramp angle, etc.

Locking diffs lock 100% once enough RPM difference has been ached between the wheels to lock the diff up, and their engagement is very abrupt.

Locking diffs are preferable in off road situations, whereas a good LSD is preferable in a high performance car situation.

One thing not discussed here, which has zero real application in an off road vehicle, is the torque biasing diff, of which Quaife is the most well known aftermarket brand.

These use Gleason gear packs to "bias" torque across the axle, and work very well and very smooth up to one critical point: if a wheel lifts, or has zero traction (i.e. if one wheel is on ice with minimal forward movement), it behaves like an open diff. TBD's also do not provide any locking upon deceleration (which is critical in a race car, hence why LSD's are the defacto).

My guess is that since the powers that be figured the SAAB owners would be even less likely to take their SUV's off road, the SAAB's got the LSD, versus the locker.