Chirped tires going into 2nd?! AWD what?!

[Xenonk]

Thanks RobY, that's what I was trying to say all this time about Open Diffs You have renewed my spirit.

 

Keefe

[rallispec]

RobY - yes, I mis-used power instead of Torque. I think we're both making the same point though. (you just explained it a little better than I)

 

I foudn this on open diffs

 

"The problem is an open diff always tries to balance the torque. That's a hard statement to get a grasp on, but it means that if the spider gears are pushing on both drive gears and one of them offers lots of resistance (tire sitting on pavement) and the other side offers no resistance (up in the air, or sitting on a patch of ice), then it will find a happy balance where both sides are receiving almost no torque at all. All the rotational energy is guided to the side with the least resistance. In the end, that side spins very fast and the pressure on each drive gear is the same.. Almost no torque is needed to spin one wheel, and therefore almost no torque is going to the other side as well. Anyone who's driven on snow or ice knows this trick."

 

 

 

 

 

So back the Legacy now -- Is thier an advantage to this system, or simply to keep costs down?

(edit -- keefe cleared this up with his post i think)

 

 

Okay - so how does this show itself in really tight cornering... With the rear keeping better traction than the front becuase of the rear LSD -- is the LGT going to have a tendancy towards understeering?.. Or am i thiking about this the wrong way and the LGT will tend to oversteer? I always assumed most AWD cars leaned towards understeer.

[rallispec]

HAHA @ keefe and RobY - i appreciate the input guys

 

you both are smart - Keefe don't have your confidence shot too soon -- i'm counting on you to give me some AutoX pointers this spring

[RobY]

Understeer oversteer has alot to do with weight balance too.... its a more complex matter.

 

I do know when I e-mailed MRT performance about thier heavier 20kg/100rpm Viscous LSD (Viscous Resistance mimics a locking differential according the them) they said it makes thier car understeer like miss piggy at least on the WRX.

 

Advantage compared to what? Every system has its advantages and disadvantages.

 

If you want to compare it to a jeep with part time 4wd it is advantegous because you can turn However he has better off road capability because it has locking differentials.

 

If you want to compare it to Audi it is advantegous because the LSD's work at any speed and completely unloaded.

 

If you ever burry an audi's front end in a snow bank the torsens will only bias 2.3:1 Meaning that if the front stuck wheels recieve 10 lb/ft of torque the rears will get a maximum of 23 lb/ft and hes still stuck and pissed of that a subaru had to pull him out.

 

However if you ever get in a situation in which both rear wheels and one front wheel is on ice and providing that you stay under 25mph. The audi will apply brakeforce to the spinning wheel so that the tractive wheel has more to push against. Providing that you dont overheat your brakes and you dont need traction above 25mph I guess the audi system is advantegous...

 

Oh.. It might also be advantegous because you can say your car has QUATTRO AWD!!!! Because everybody knows its l33t!!!!

[Xenonk]

As I stated above about the open differential actions in a turn, the rear does things differently. Any case, I'll try to explain this in simplier terms.

 

The rear is acting on as much grip as possible to propel the car forward, the wheels in the front guide the car where it needs to go (hence the steering). Depending on how hard you corner, the wheels will have to overcome each scenario of the front and rear's differentials. I only know of 4 different cases of cornering that are different for manual GTs:

 

-Only two outside wheels are touching the ground:

The car will try to transmit as much torque to the rear outside wheel to get the car to make the turn sharper, the front wheel will just be a pivot point based on the tire's grip capacity, if the tire's grip exceeds, the car will understeer.

 

-Two outside tires, inside front tire touches the ground (trail-braking):

the car will pull in the direction of where the tires are aimed. Any excessive torque will be sent to the rear outside tire (as the center diff is balancing the torque load of both front tires to equal one rear tire), in this case, oversteering is possible.

 

-Two outside tires, inside rear tire touches the grond (accelerating out of a corner):

since the two tires on the rear are on the ground, the logic of balancing torque for the center diff has become both rear tires much match the torque load of one front wheel. But wait, since the front is open, the excessive torque will show up from inside front wheel spinning freely in the air. This can result in understeering if the outside front tire exceeds the grip capacity in combination of the not having enough torque to transmit the front outside tire to PULL the car into the direction it needs to go. Understeering is more likely to happen

 

-all four wheels touching the ground with similar weight loads:

the rear will push the car forward (like a RWD would) and the front will spin the outside tire as much as possible to get the car to aim as fast as possible to the correct direction. Some understeering and some neutral steering happens here.

 

There's a billion other factors that make a car prone understeer or oversteer (tires, swaybars, springs, shocks, differentials, etc) as well as driver-induced moves (weight transfers, driving techniques of braking and accelerating, steering input, etc). It really depends a lot on how you drive, the car will do what it was designed to help you transmit as much torque as possible to the wheels to MOVE or accelerate the car in the direction of where the driver intended for it to go. AWD is unique on it's own (especially ones that have variable center diffs like the STi or the old 22B [as much as 10/90 torque split = bad-ass]). The Lamborghini Diablo SV I think was the first AWD Lamborghini that had a torque split of 35/65.

 

Remember, all the suspension and drivetrain in the world is useless if the tires dont get the resistance of torque it needs to propel you in the direction that you want to go. You need to get in contact with pavement before the tires can grip and go anywhere, so afterall, I think it's a driver's job to know about weight transfers to create resistance that determines if the car oversteers or understeers.

 

Man, I am tired, I wrote at least 5000 word essay in just this one thread. I am going to leave work

 

Keefe

[Deer Killer]

The viscous coupling will not work untill ther is a rotational diffrence between both sets of wheels.

No, viscous coupling/viscous limiters can have preload on them.. i.e. It takes a certain amount of torque to hold one end and spin the other, when cold. From emperical data it seems our rear LSD has more preload than the center. It's quite a bit actually, if you try and spin them by hand.

[RobY]

The service manual makes no mention of a preload.

 

From my understanding it is a differential in which the diff "carrier" rotaional movement is resisted by a viscous fluid.

 

If there is no roational diffrence between the wheels the carrier is stationary relative to one another and so is the viscous coupling they rotate as a unit. If there is a rotational diffrence ie a turn or ie slippage the viscous coupling starts coupling.

 

Im not sure what you are saying...

 

If you hold one end stationary and attempt to spin the other you will be turning the engine. It IS linked by a traditional diff modified by a viscous coupling.

 

Please elaborate.... Maybe theres somthing I am missing in your post.

[rallispec]

Keefe, nice write up man

You're 3rd scenario - acc. out of a corner - is what i'm most used too. And that is what i had the hardest time with in my nissan, It would understeer a lot. (i'll be the first to admit that a good portion of that was driver caused)

 

 

I guess it just comes down to the fact that this drivetrain is nothing like what i'm used to in FWD cars and the only way i'll ever really grasp what its doing is by 1)doing some more reading; and 2) getting out there and tossing the car around some turns in a parking lot to see how it actually responds.

 

 

 

Man, I love physics. (ha) Im' enjoying reading this though -- learning quiet a bit.

[Deer Killer]

If you hold one end stationary and attempt to spin the other you will be turning the engine. It IS linked by a traditional diff modified by a viscous coupling.

Turning the engine in neutral? No. Try putting it in gear, with both rear wheels of the ground try spinning one of them.. in an open diff situation it would be relatively easy and the other wheel would spin in the opposite direction, with the driveshaft still. However you will find that it is quite difficult. If you try the same sort of experiment with the center diff there isn't nearly as much resistance. i.e. car in neutral, one front wheel of the ground, try spinning it. (Of course there are reasons for it to be less, just not as much as you will feel). Now with both front wheels off the ground spin one front wheel.. Huge difference, no LSD pushing back at you.

 

In both of these situations the excess torque you feel is from the LSD in a cold state. This is the preload. My original example was meant as if you pulled out a viscous coupler, put it in your lap, held once side in your hand and turned the other. According to some of the wisdom floating around, you should be able to turn it without resistance until the fluid starts to heat up. Not the case.

[Deer Killer]

And while we are at it, someone please give a detail explanation of how 1-way, 1.5-way and 2-way differentials work and give examples

"1-Way, 2-Way, 1.5-Way.......... What Does It Mean?

 

Some manufacturers make LSD's in different configurations and are commonly classified as 1 way, 2way, and 1.5 way. This designation reflects the design of the cam groove which enables the LSD to function differently under different loads. A 1 way differential means that the cam is shaped in such way as to have positive lock only when accelerating. The 2 way is constructed in a way to have positive lock motion in either acceleration or deceleration. The 1.5 way is a new term used to describe the 2 way cam which enables different lock up rates during the two directional forces. The 1.5 distribute positive lock stronger under acceleration than when decelerating. The 1.5 way can provide more forgiving balance when braking than a full 2 way setup, although it is less effective for true racing applications, it provides easier operation for beginners in throttle off conditions. It is also effective for front drive cars which need extra stability during braking."

 

We have 2-way diffs.

[Beanboy]

Hmm, I really didn't think regular ol' viscous diffs lock under load, only when a certain % of slip between axles occurs is how I understood it...

 

Many mechanical diffs certainly can, one reason why many performance cars have them. Mash the throttle and they partially lock to prevent wheelspin before it starts.

 

Be neat to have a viscous diff that used "magnetorheological (MR) fluid, which is a complex, synthetic - hydrocarbon - based liquid having somewhere between 20% and 40% (by volume) Carbony Iron Particles (97% - 99% iron) to offer excellent electromagnetic properties. The flow characteristics (rheology) change when subjucted to a direct-current - magnetic field stregth."

 

GM is using it in high-end struts, can change 1000 times a second all controlled by a computer. Let a computer adjust front/center/rear diffs, or fiddle with it yourself. A more elegant, lighter and "real" AWD solution than many computer-controlled systems on the market.

 

-B