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

[rallispec]

Okay - so i'm new to this AWD thing -- and just having passed the 1000 mile marker i decided to at least try a mild launch to see what exactly it could do. (didnt want to be to rough though, so no high rpm clutch dropping.)-- i just had to try it at least once, it's been 3 long weeks of driving slow.

 

so anyhow, pulls hard in first gear, i shift at around 5,800rpms and my tires chirped. I figured it would be a lot harder to chirp tires in an AWD car. So now i've been trying to figure out if it was the front tires, back tires, or all the tires that chirped.. most of the power should be going to the back when you accelerate, right?

 

 

Oh well-- it was fun. Definetly better than my Nissan. (that car would chirp going into second and third if you looked at it wrong.. didnt pull nearly as hard as the legacy though)

 

Now i've just got to keep it easy for a few more days until i get the oil changed - then i start shopping for the AP.

[godwhomismike]

It's called 250 hp and 250 lb ft of torque Enjoy!!!!

 

- Mike

[agctr]

Ok the thing is with the Subaru AWD system is, once the system detects slippage, it will move the power to the opposite side of the GT. I would say it would be yr fronts that would have churped because, while u are still accelerating, you have more lift at the front hence putting a lot more weight on the rear.

Adam.

 

AWD, have to love it.

[rallispec]

It's called 250 hp and 250 lb ft of torque Enjoy!!!!

 

- Mike

 

<duffman>oh yeah</duffman>

 

gotta love it. Beats the hell out of the 175 i was running before. My g/f isnt quiet as thrilled with it as i am though

 

 

I suppose it makes sense that the front would have slipped-- i guess i just assumed that spread out between 4 wheels, it would take a lot more to chirp the tires. Guess i understimated what 250hp can really do.

:insert evil laugh emoticon here;

[Deer Killer]

Probably took too much weight of the front tires.. It's still 50/50 even if you've shifted hard enough to take 800lbs of the front tires.

[rallispec]

Probably took too much weight of the front tires.. It's still 50/50 even if you're shifted hard enough to take 800lbs of the front tires.

 

so the limited slip is only side to side?!

 

i dont know why i hadnt realized that before.

[Deer Killer]

so the limited slip is only side to side?!

 

i dont know why i hadnt realized that before.

 

No, but the LSD takes time to kick in. A chirp is probably less than 1/8 rotation.

[carman493]

we dont have a front lsd...when i first got the car the front tires would spin when driven very aggressively in the wet and snow along with hard 1-2 shifts...now i know how to drive it better without that problem....

 

but thats mostly due to no lsd in the front diff.

[Beanboy]

The fronts will spin with a hard enough launch, same goes for the beginning of second if the conditions are right. One nice thing about an electronically controlled center diff, it could send more power to the rear while accelerating hard to prevent the unloaded front wheels from losing traction in the first place.

 

The GT is 3-wheel drive. Center viscous coupling spits power 50/50 and only deviates from that after either the fronts or rears start spinning. Power can transfer from side-to-side in the rear. The fronts can still peg-leg burnout like a FWD, especially on a curve. Been there, done that.

 

For more information about how long it takes for the viscous center diff to react and slip rates for the rear LSD, gonna have to ask a Soobie master. I'm only four months into owning a Subaru, hehe.

 

-B

[PGT]

I have gotten the front inside wheel to spin around a corner more than once, and that's without RE92's on the car.

[Xenonk]

typically, the center diff wont kick in until the fronts are slipping pretty bad (the front runs an open LSD I believe, so you'll get the inside tires to spin however much you want). The rear kicks in and slowly gets the car going. When during off-acceleration, the center diff will most likely send the torque to the rear and to the outside wheel to aid the car as much as possible to turn.. not sure what the "asymmetrical" portion comes into play... still reading up on SOA info on such technical LSD info (including the different profiles and algorithms for AVCS)

 

Keefe

[Beanboy]

Xenonk, I think the orginal poster was talking about the manual tranny, so there's no active shifting of torque based on throttle position, it will default back to 50/50 unless slip is occuring, at least that's my understanding of what a viscous coupling can do.

 

And there's no sending power to the rear outside wheel in a turn, no car can do that actively except for the new AWD system on the RL. May be some RWD German cars with a funky expensive active rear differentials that can do the same, not sure...

 

The symmetrical (asymmetrical is the antonym) aspect of the manual tranny Soobie's means that all of the drivetrain components are equal along the center line (same goes for the autos). Equal length driveshafts in front, engine is mounted longitudely, and power is divided side-to-side equally (and back-to-back, unlike the autos).

 

-B

[RobY]

All mechanical torsens or suretrack diffs can send power to an outside wheel in a bias ratio.

[Xenonk]

I guess it's time for me to go back and re-read some LSD fundamentals on http://www.howstuffworks.com again.. but from my understanding, since it's vicous, it's just a lot of fluid spinning a set of clutch plates of the LSD.. yes, the center diff on manuals are set at 50/50 for torque split (45/55 for autos).

 

Well, what if all the tires are to slip? There wont be a bias to any corner then (how do automatics respond to that)? Since we are talking about things in motion and grip, the least amount of wheel spin to happen in this case of a dry day on a straight line drag-style launch will be the rear wheels (assuming weight transfer and shifting of forces to keep the rear wheels from moving). If the grip is exceeded by the remaining torque forces to the rear, then all 4 tires can spin.

 

From my understanding of what vicous can do is like spinning an egg on a table and then forcefully stopping the egg.. the yolk inside the egg (the oil in the LSD) spins via friction against the egg shell to keep the egg moving in motion. In other words, the works of an "open" diff. The word "limited" from my understanding is when grip from one wheel/axle is lost before the opposing end of the axle reacts.

 

Someone correct me before I go any further with speculations. 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.

 

Keefe

[Beanboy]

Right... I should have been more specific with the "active" nature of the Honda system aspect versus the "passive" nature of some limited-slips. In this case, it doesn't matter with the LGT and a viscous limited slip in back.

 

-B

[rallispec]

okay so now i'm confused.. i understood how the LSD worked in my old FWD car -- but 2 more wheels is apprently too much for my brain to understand.

 

so you're saying the the backwheels have LSD, but not the front - so i'm only actually power to 3 wheels?

 

If my front tire starts to slip, rather than transfer power to the other wheel -- it just transfers it all the rear, correct?

 

 

Say you were on a sheet of ice and floored it (100% frictionless) -- would all 4 wheels spin - or just my rear, or my rear and one front tire?

[PGT]

On snow, when I give the car too much throttle, the back end comes around...leading me to believe the answer to your Q is that power is sent to the rear when the front tires slip. This, instead of all four wheels spinning at the same rate.

[RobY]

Keefe you need to look at some vacation pix

 

None of the LGT's Center diffrentials are strictly open. The MT versions come with a viscously limited diffrential that acts exactly like a normal differental until there is a speed diffrence between the front and rear wheels.

 

If one set or all sets of wheels loose traction and produce a SPEED diffrence the coupling will attempt to visc the front and rear set of wheels together. The viscous coupling will not work untill ther is a rotational diffrence between both sets of wheels.

 

None of the How Stuff works diagrams accurately describe the center differential on the LGT.

 

It is diffrent than the How Stuff Works description because there is a normal diffrential continuously connecting the front and rear sets of wheels which is LIMITED by a viscous coupling. Under normal conditions the viscous coupling does not provide the primary means of torque transfer.

 

The 5EAT Center differental is an offshoot of a WRC car differential. It is a planatary gearset differental with an electronicly controlled electromagnetic clutch pack.

 

The 5EAT system is the most advanced AWD system subaru offers. It functions in a similar fashion as the STi DCCD differential with less rear bias and no manual mode.

 

I guess thats somthing that the 5eat guys can brag about. The system works automaticly and proactively based on sensors placed throughout the car. It can theoreticly go from full open to full lock before your wheels even think about slipping.

 

Just like the WRC differentials the DCCD and 5eat will declutch the center differential when the handbrake is pulled and under braking.

 

However the side to side transfer in the rear is still viscous and the front is still open.

 

On the STi there are suretrack (torsens like) differentals working on a bias ratio.

 

About torsens diffs.. (audi) Keefe is correct if all wheels become unloaded or have very little load you are SOL. Any bias ratio multiplied by zero is zero.

[Xenonk]

okay so now i'm confused.. i understood how the LSD worked in my old FWD car -- but 2 more wheels is apprently too much for my brain to understand.

 

so you're saying the the backwheels have LSD, but not the front - so i'm only actually power to 3 wheels?

 

If my front tire starts to slip, rather than transfer power to the other wheel -- it just transfers it all the rear, correct?

 

Say you were on a sheet of ice and floored it (100% frictionless) -- would all 4 wheels spin - or just my rear, or my rear and one front tire?

 

There's 3 Differentials on the GT (all are mechanical):

 

Front = Open Diff

Center = Limited Viscous

Rear = Limited Viscous

 

Typically power gets transmitted to the path of least resistance (in this case the wheels in the front would get the power first [front engine car], at least from my experience with mechanicals and knowledge of physics). The center diff will send the torque as much as 50% [that's what manual GTs are set at for the torque split] to the front before it will transmit to the rear. If the front is slipping (full 50% transmitted to both wheels), then the torque heads to the rear axle. It's a balancing act for the center diff to ensure both axles are sensing grip/slip at the same time to divide the torque by the split percentage.

 

As to how the front and rear handle their share of 50% of torque is a different issue. In the front, excessive tire spin from one side (least amount of resistance) will get the opposite side to spin as well (more resistance, think about the 'egg' example that I described earlier in this thread). That is how "open" diffs work. For the rear, once one of the rear tires (either side) begins to slip, the other side will share the power, in other words, the rear is trying to act as a whole (spin together at the same speed as much as possible) to gain grip and minimize slipping.. this is what I know about limited-slip viscous diffs. If the rear senses a lot of slippage, the center diff will send the torque back to the front axle (again, limited slips try to keep the slipping as least as possible) to move both front and rear axles with the same amount of torque (or whatever the torque split is set at).

 

At any case, the center diff gets first dibs on where the slippage is coming from based on how much slippage is going on from all the wheels combined. It will send the power accordingly to each axle first. The axle diffs will determine which tire gets grip (which depends on the type of diff controls the axle). It's designed as a heirachy system (as most AWDs are anyways unless you run two seperate engines and no center diff).

 

 

To answer your question about the 100% frictionless:

 

Manual Legacys (set at 50/50 split) will have all 4 tires spinning at the same amount of torque/force applied to each wheel. (If one was to do a launch, the front wheels would spin first because of weight transfers during acceleration).

 

Auto Legacys (set at 45/55 split [correction]) will also have all 4 tires spinning, except 55% of the torque force will be sent to the rear (meaning autos will have a more likely chance of spinning the rear tires first when doing a launch).

[rallispec]

Keefe -

 

if my understand of Open Diffs is correct (based on my experience from driving a civic vs. sentra se-r with LSD) -

 

On ice with the open diff (on a fwd car), all power is sent to the path of least resistance. I could never get both tires to spin. If i was stuck in snow, one tire would spin, and the other would not. I've always thought of it as trying to balance torque loads rather than speed.

 

 

With the LSD on the se-r, the differential attempts to keep both wheels spinning at the same speed. (like you said)

 

so given the frictionless example,-- wouldnt it be correct that the power split would be even between the front and back, however that would be split up between both rear tires in the back.. however only one tire in the front.

 

 

I'm really confused as to how the manual would end up with this kind of setup, rather than the planetary setup of the auto and STi, which seems far superior.

 

Does Subaru offer an vehicle with an advanced power distribution stability or traction control? (such as MB's 4matic or audi's Quattro system)

 

 

(not that i'm complaining about the LGT's setup in any way -- in fact i've been very impressed with how well it handles and how good it does in the snow --- i'm just interested to learn all I can about a car and company that i've had very little experience with in the past)

[RobY]

An open diff never sends all the "power" to one wheel it ALWAYS splits 50/50 percent of the TORQUE to the tires. IRREGARDLESS OF the situation.

 

Before you jump on me the reason why I dont use the term POWER is because it is inaccurate. A diffrential spits TORQUE.

 

Lets use your snow example... say you have one wheel stuck in a snow bank and the other in perfectly dry pavement. The OPEN diffrential will still split torque 50/50. Heres where it gets confusing.

 

In the snow bank the tractive capability of the stuck tire is say 10 lb/ft. The tire on dry pavement will also recieve 10 lb/ft. It splits torque 50/50

 

However If you rev up your engine the tire in the snow bank will spin up because you are exceeding the 10/lb ft traction capability. Your engine is producing 100 lb/ft

 

Eventhough you are spinning the tire up into smoke the TORQUE split is still 50/50. Both tires will still recieve a maximum of 10 lb/ft. The tire on the pavement wont move for the reason that its tractive capability exceeds 10 lb/ft. But the dry pavement tire STILL receives 10/lb ft irregardless.

 

You get what im saying.

 

Audi's and the 4 matic system are not all that advanced. The differentials are basicly open differentials that apply brakeforce to limit the slip of the spinning wheels. Sounds great but the sytem dosent work over 25mph. Over that speed you basicly riding on open diffs (center torsens for audi).

[05LegacyGT330Ci]

There's 3 LSDs on the GT (all are mechanical):

 

Front = Open Diff

Center = Limited Viscous

Rear = Limited Viscous

/

 

 

So you mean we don't have 3 LSDs then......

 

 

Song of Post- Led Zeppelin Live on DVD the only way they can be truly appreciated besides for the lucky ones who got to see them live.

[Xenonk]

Open differentials send power to both ends of the axles with equal amount of torque, if the one tire has least amount of resistance then , yes, that tire will spin more so than the other side, since that is the capacticy of the torque can the two tires can handle together. Remember that torque and wheel speed are two different things (one is a force multiplied by distance, while the speed is a component of mass and acceleration and derivative of acceleration). From what I remember about open differentials is this "it's like leaving the door 'open' for all the excessive torque to escape."

 

If a car with open differentials made a turn with similar resistance on each side of the car, the torque is sent out to the outside tire (path of least resistance because it's easier to move the outer wheels of the car than the inside wheels since the inside wheels are acting like a pivot point). If the car is rolling or leaning soo hard that the inside tire is lifting (and it does happen with smaller cars and VW Rabbits) the inside wheel would spin, because the inside wheel has become the path of least resistance.

 

I am guessing the manual cars ended up with this set up is to make sure that us typical drivers have an outlet for mistakes of allowing the excessive torque (from adding too much throttle or steering input) to 'escape' out of the drivetrain system while keeping the car under control. You can say this is a loophole of the manual Subarus and thus making the other setups a little more superior or better performing.

 

I hope that made sense as I am just throwing all this bland and possibly incorrect information out.

 

Someone shoot me as I am beginning to feel inadequate about this matter on drivetrains.

 

 

Keefe

[Xenonk]

There's 3 LSDs on the GT (all are mechanical):

 

Front = Open Diff

Center = Limited Viscous

Rear = Limited Viscous

 

 

So you mean we don't have 3 LSDs then......

 

 

Err, yea, I meant to just type "3 Differentials", not "3 LSDs", sorry.

 

Keefe

[RobY]

"For every action there is an equal and opposite reaction" Newtonian Physics. The perfect conceptual example of an open differential.

 

In order for the right wheel to receive torque the left wheel must push back equally and vice versa.

 

You in order for the right wheel to receive 10 lb/ft the left wheel must push back 10/lb/ft.