05 Legacy GT stuck in dry conditions

Wondering if you guys might explain to me how this is possible.

Situation: Driving in the mountains, while pulling a u-turn I drove over a small ditch. In the process I unweighted the front passenger wheel at a near stand still speed. When I gave the car a throttle input the car didn't move. The front passenger side wheel spun. It had just enough contact to burn a little rubber.

No lights flashed on the dash and no movement of the car. It actually came to a stop. I was probably doing 1-2mph. All 3 other tires were loaded and in contact with the ground - a visual check confirm.

Ground condition: Combination of paved road (front tire contact) and hard packed dirt (rear tire contact)

How I recovered: Put it in reverse and backed out the way I came.

Questions:

1. Why did the front wheel spin? I thought we had traction control.

2. Dont we have limited slip front diffs?

Thoughts?

You should have bought a Honda

No stock Legacy has front LSD.

And in a situation like that, you're going to want to turn traction control off.

Where is the button for that?

What transmission?

Auto 5 speed.

the button to turn TC off is under your mirror switch

perhaps your Honda Racing air freshener was blocking the switch

I am wondering why you'd need to turn off transaction control when 3 of 4 wheels had traction. This is an AWD car right? It's acting like a fwd

Should the traction control system not apply break to the spinning wheel and direct power rear?

What year car do you have ? My 05 Gt Wagon 5mt doesn't have the button. But my 09 Spec does.

OP put more info in your profile

Were you going uphill? Front diff is open, so you were spinning off 50% of your torque to the front, the rear does have a LSD, so you're getting 25% to each rear, maybe you just didn't give it enough go to push from the back with only half the power available?

Ground was flat. Speedo hit 40mph and I burned rubber. Should I have floored it?

What year car do you have ? My 05 Gt Wagon 5mt doesn't have the button. But my 09 Spec does.

 

OP put more info in your profile

05 limited GT auto tranny 5speed. Sorry thought the subject line provided all info required.

Sorry as we get down in the thread, some info get forgotten.

I think that button has something to do with the VDC I have played with it on my 09 and you can read about it in the manual.

I turn it off when I want to slide the car on wet pavement.

Er, speedo hit 40, what gear were you in?

The 5EAT is VTD, so it can split torque pretty severely (up to like 70/30 or something?). It's possible I guess for some reason it decided the front was the right place to go. Hard to say really.

I wonder if there is a angle sensor that thought the car was in danger of going over if it kept driving knowing there was nothing under that front tire ?

I'm sure someone who's more tech savy can explain.

Er, speedo hit 40, what gear were you in?

 

The 5EAT is VTD, so it can split torque pretty severely (up to like 70/30 or something?). It's possible I guess for some reason it decided the front was the right place to go. Hard to say really.

In auto, I would imagine in 2nd or 3rd by the time I hit 40.

I wonder if there is a angle sensor that thought the car was in danger of going over if it kept driving knowing there was nothing under that front tire ?

 

I'm sure someone who's more tech savy can explain.

There might be, but the car wasn't far off level. Slightly nose down.

I think you're giving the AWD system a lot more complexity than it really has. Are there any angle sensors at all in the 5EAT?

AWD should drive power to the wheels that have grip or at a minimum direct front to rear power to give an axle with grip some power. All my reviews of the subi system suggest this is what to happen and my experience shows me otherwise I'm trying to figure out why.

I don't know, is there much in the way of documentation of the logic of the VTD center? I agree with the should. We know that the front open diff will spin all the torque given to the front off with a loose wheel. We know the rear is a viscous LSD, though I'm not 100% on its behavior, is it just 50/50 torque split? What the VTD center is doing :iiam:

If you dropped it into manumatic 1st and hit the gas, I wonder what would've happened.

I don't know, is there much in the way of documentation of the logic of the VTD center? I agree with the should. We know that the front open diff will spin all the torque given to the front off with a loose wheel. We know the rear is a viscous LSD, though I'm not 100% on its behavior, is it just 50/50 torque split? What the VTD center is doing :iiam:

 

If you dropped it into manumatic 1st and hit the gas, I wonder what would've happened.

I'll be out there next weekend and can duplicate the situation again.

So far uncertain why rear wheels got no power and it appears 100% of the power when to the spinning wheel.

here is why it happened.

you assume that the center diff can transfer power to the rear if the front wheels are off the ground(even if one is off the ground, its the same as both being off since the stock front diff is open). the truth of the matter is the 5eat VTD center and the 5mt VLSD center are not strong enough to transfer full engine torque/power to one end if the other is open. its only gonna be able to do a fraction of it. and if that amount is not enough to move the car due to the situation the car is, then the car wont move. remember the bias ratios (50/50 for the 5mt or 36/65 for the 5eat) are just that, bias ratios. they do not lock up 100%. they only work like the bias indicates if all the wheels have some traction.

lets use the 5mt for example cause i know the numbers. the center VLSD in is is rated at 4kgf-m/100rpm(according to rallispec.com). this is the amount of force the viscous clutch can transmit from one end to the other in the case one end of the diff is totally open. notice it is based on rpm. this rating is independent of gearing or engine power output. it is only based on the difference in rpm from the front wheels to the back. so regardless of what your doing, if one of the front wheels if off the ground, the center diff will only be able to send 4kgf-m/100rpm to the back wheels to move the car. now that rpm is the prm of the output shaft of the transmission, not the engine, since the diff is on the output shaft. so, in first gear (with a ratio of 3.08 of engine rpm, just using the number as an example) even at 6000 engine rpm(you would never do that int he real world), the output shaft of the trans is spinning at about 1950rpm which means the diff will only transfer 78kgf-m to the rear wheels if the front had no traction. that may not be enough to move the car, and this is only if the center diff has no wear at all. odds are your diff is not as strong as it was new.

the VTD unit may not have type of partial lock up at all and my act like a completely open diff if one of the ends has no traction. from the looks of it here - http://opposedforces.com/parts/legacy/us_b13/type_8/automatic_transmission/at_transfer_and_extension/

it may use a AT style clutch pack to aid TQ transfer, but we dont know how much that clutch pack can actually transfer in the case of one end of the diff being open. clearly it wasnt enough to get you over the ditch

Also, VDC did not show up on the Legacy until the 07 model I believe.

even so, VDC would NOT help the situation if one wheel was totally off the ground. the only time it would is if the car had helical/torsen diffs in all three spots.

Sorry about that, wasn't saying that VDC would have helped. It would have done the

opposite by cutting power from the engine. With having the open diff in the front and the

amount of power it it can deliver to the rear questionable, this is one way to possibly

defeat the Subaru AWD system on a 5EAT.