4x4 Questions

[bste3]

I'm still "getting to know" the 4R I bought in October. I understand how a differential works but I'm confused by the fact that there is also a center differential.

Here is my first question... First, if I'm in 2wd and in the snow in a straight line, and I stomp on the gas pedal the VSC will take over and the truck will basically do nothing, no spinning. If I do the same thing in 4hi unlocked diff vsc on, the truck will spin like crazy. In 4hi it seems like VSC only cares about lateral sliding while in 2hi VSC corrects against any spin. Am I wrong? Why is that?

Second question.. For a Rwd vehicle with an unlocked differential and no traction control, if you have 1 tire on ice and the other on pavement, just the one on ice will spin. I understand that and why it happens. I also know we have the A-TRAC to mitigate that, but does the same apply with a center differential? Is it a true differential and not a clutch-based system like an AWD car? What I'm getting at is that if I'm in 4hi unlocked and I put the front two tires on ice, will the differential send all of the power to the front spinning tires? It's nice to be able to drive in 4wd fulltime but if the differential sends power to the slipping wheels it pretty much defeats the purpose...

I'm curious about this kind of thing and I'm one of those guys who will do donuts in the parking lot with the excuse of "getting to know" how the vehicle handles in snow (best excuse to play ever). I want to understand what is going on down there though, so let me know what you think.

-dan

 

[CJ3Flyr]

Dan, see the inserted text below:

I'm still "getting to know" the 4R I bought in October. I understand how a differential works but I'm confused by the fact that there is also a center differential. It is the 'center differential' that allows the T4R to use 4hi or 4lo on a dry surface. It allows the front wheels to travel a different overall distance than the rear wheels. It basically allows the T4R to function more like an old-school full-time 4wd or, in more modern terms, an awd.

Here is my first question... First, if I'm in 2wd and in the snow in a straight line, and I stomp on the gas pedal the VSC will take over and the truck will basically do nothing, no spinning. If I do the same thing in 4hi unlocked diff vsc on, the truck will spin like crazy. In 4hi it seems like VSC only cares about lateral sliding while in 2hi VSC corrects against any spin. Am I wrong? Why is that? Your right, sort of... In 4hi the VSC (more correctly, the TRAC function) does hold the throttle back, however you can spin a lot more in 4wd than in 2wd. The TRAC references the wheel speed sensors to work it's magic. Basically the unpowered front wheels have a tremendous influence on how the VSC/Trac manages. In 4wd the front wheels have some power too. The speed of the front wheels comes up due to the available power. The VSC basically is looking at the accelerometer (skidding sideways), the steering wheel angle, throttle position and compares the wheel speeds. Straight ahead on a slippery surface 4hi naturally spins all 4wheels. The system doesn't have a 5th wheel or GPS so it can't tell that the vehicle is moving 4mph when the tires are spinning at 30mph so it thinks it's doing a great job.

Second question.. For a Rwd vehicle with an unlocked differential and no traction control, if you have 1 tire on ice and the other on pavement, just the one on ice will spin. I understand that and why it happens. I also know we have the A-TRAC to mitigate that, but does the same apply with a center differential? Indirectly yes or partially no, however you like. There's no mechanism within the differential to "brake" the front or rear axle. The Torsen diff will throw more power to the end that's spinning, let's say the rear end. The ATRAC will pinch down on the rear equalizing the R & L tire and attempt to get more power to the front axle***. Is it a true differential and not a clutch-based system like an AWD car? It is a true mechanical differential. What I'm getting at is that if I'm in 4hi unlocked and I put the front two tires on ice, will the differential send all of the power to the front spinning tires? It's nice to be able to drive in 4wd fulltime but if the differential sends power to the slipping wheels it pretty much defeats the purpose... This is where the TRAC/ATRAC help to make sure that doesn't happen.

I'm curious about this kind of thing and I'm one of those guys who will do donuts in the parking lot with the excuse of "getting to know" how the vehicle handles in snow (best excuse to play ever). I want to understand what is going on down there though, so let me know what you think.

-dan

*** I was actually curious enough about this I jacked the rear axle with a floor jack, both wheels up in the air on my sloped driveway in 4lo. The vehicle started to roll in spite of being in P. (So I chocked it.) Next I pulled the wire on the brake master cylinder that kills the VSC/Trac. I started it up, put it in gear and the back wheels spun like I was in 2wd. I then shut it off and replugged the wire. Now the TRAC and ATRAC worked together to match all 4 wheels' speed. The throttle was held WAY back but I could creep fwd on the jacks' wheels. This confirms that it, in an indirect way, attempts to direct power to the axle with traction even when the center diff is unlocked.


Next I performed the same test with the center diff open and my VSC off switch installed (link in sig), it the ATRAC did the same thing but with no throttle intervention. I could've stomped on it and pulled it off of the jack, plowed through the garage etc, you know, the stuff YouTube videos are made of I have done some off-roading with the center-diff unlocked and the VSC off. (The '08/'09 came with a VSC off switch from the factory.) It's amazingly capable and in some circumstances locking the center diff only improves traction/pull minimally. Open center diff and VSC off is also great on fairly level ground (or going downhill) with very low traction like wet clay. I'm talking about slick, hard surfaces. The vehicle steers better on tight turns with less of a tendency to understeer. Now, if there's a good bit of 'uphill' or deeper mud you're better off with center locked.


The final test was center diff locked, of course it moved along just fine.


Hope that helps

:cheers:

 

[bste3]

Hell of an answer, thanks!

 

[CJ3Flyr]

Hell of an answer, thanks!

Yeh, but did it make sense?:lmao: I understand some things much better than I can write them down...

 

[JB.]

Yeh, but did it make sense?:lmao: I understand some things much better than I can write them down...

Great post and experimentation!

With humility, as your posts are of unequalled quality, may I ask: did you mean to say "The Torsen diff will throw more power to the end that's spinning"? That was a lot of writing and I'm assuming it was a typo.

 

[CJ3Flyr]

"The Torsen diff will throw more power to the end that's spinning"? That was a lot of writing and I'm assuming it was a typo.

J.B., 100% correct, a typo of sorts. More correctly a brain fart... The Torsen in t-case will actually send more power to the slower axle assuming the is some resistance on the axle that's slipping. That's what makes this diff so ideal for center diff applications. In the case of pure, slick ice (or with the back end jacked up the air) the wheels with zero resistance will get all the power. That's what I was writing about but failed to alert the readers' to:bash: Thank-you J.B. for keeping me straight



My understanding of the 'threshold' for the power going the wrong way (low traction wheels) is traction. The Torsen can send up to 3x the torque applied to good-traction axle. If one axle is on ice and 3 times zero is, well, zero; that's when the ice wheels spin rather than the vehicle moving. If you have even a little traction then 3 x whatever torque value the slipping axle can handle will be sent to the other axle.



Please straighten me out if I'm missing something...:cheers:

 

[KAM]

I have similar question about the VSC... I have 4x4 and during the snow last week I came out of a left turn and kind a went little fast to see what would happen whether vsc would take over or not but truck spinned like crazy and did 180. I thought that vsc would prevent that and would take over.

 

[CJ3Flyr]

I have similar question about the VSC... I have 4x4 and during the snow last week I came out of a left turn and kind a went little fast to see what would happen whether vsc would take over or not but truck spinned like crazy and did 180. I thought that vsc would prevent that and would take over.

There's limits to everything...

Were you powering up through the left turn or coasting?

 

[scottm]

My understanding of the 'threshold' for the power going the wrong way (low traction wheels) is traction. The Torsen can send up to 3x the torque applied to good-traction axle. If one axle is on ice and 3 times zero is nothing; that's when the ice wheels spin rather than the vehicle moving. If you have even a little traction then 3 x whatever torque value the slipping axle can handle will be sent to the other axle.

As I understand it, the Torsen uses the initial difference in torque to create internal mechanical binding, that locks up the differential, which equalizes the torque to the two output shafts somewhat (up to 3:1 or 5:1 or however it's designed). If one output shaft is going to a wheel on ice, there's no internal torque to start the internal binding, so it acts as an open differential. The tire on ice will spin freely and the tire with traction will get little torque to get you moving.

This situation is perfect for a vehicle with traction control, which will use the brakes to slow the wheel on ice. That output shaft now is applying torque against the brake, so the Torsen will begin to lock up and send torque to the opposite wheel with traction.

If the Torsen diff has a 5:1 capability, it can send five times the torque of the low traction wheel to the high traction wheel. If the low traction wheel is on ice, it's getting no torque, and five times nothing is nothing. If the brake applies moderate resistance to the spinning wheel, the wheel with traction can get five times that amount of torque, which can be substantial.

 

[CJ3Flyr]

Yep... That's it.


The ATRAC is why an aftermarket Torsen rear (and front) diff is so cool on a 4Runner. A guy over at T120 has done it and says it's spectacular. I had a Titan prior to this. Many of the guys went to a Torsen diff on the Titan with great results, again because there was the Nissan equivalent to ATRAC to 'nudge' the Torsen when necessary.

Most claim performance that's very, very close to a locker with none of the drawbacks of the locker.

 

[Mike07SE]

Man that was a great disertation CJ. Now if I had $ to put TruTracs in my rear and front diffs. I've been saying this for quite awhile now, but I'd love to wheel this pig in 4HI/unlocked and the nanny controls turned off. Should be real capable, and very capable with the ATRAC complementing it.

 

[Will_4R]

It almost sounds like you get better traction with the VSC off and the centre unlocked - if the torsen sends torque to the axle with more traction. ATRAC is definetly cool in and of itself. I feel it working when the centre is locked (no VSC), and it works in situations where I am pretty sure open diffs would fail to maintain forward movement.

 

[CJ3Flyr]

Man that was a great disertation CJ. Now if I had $ to put TruTracs in my rear and front diffs. I've been saying this for quite awhile now, but I'd love to wheel this pig in 4HI/unlocked and the nanny controls turned off. Should be real capable, and very capable with the ATRAC complementing it.

Me too... I drive too much now. I had no idea how much I'd like this vehicle. I think the plan is to look for a junker and an '06-'09 GX470. The 470 will get a full Toytec and a TruTrac (in the back at least). For me my V6 T4R is great as an only vehicle. I will enjoy 2 used vehicles more than a new vehicle next time

It almost sounds like you get better traction with the VSC off and the centre unlocked - if the torsen sends torque to the axle with more traction. ATRAC is definetly cool in and of itself. I feel it working when the centre is locked (no VSC), and it works in situations where I am pretty sure open diffs would fail to maintain forward movement.

That's my take... What puts more power down to slippery surfaces than a Mitsi or a Suby (aka the Rally Race cars)? What handles better? NOTHING! How do they do it?:biggrin2: Pretty much like a 4Runner with the VSC off and a couple of Torsen diffs added....

 

[Mike07SE]

Just had a the big blizzard here in B'more two weeks ago and I took my 2000 Scoob 2.5RS with Mich Pilot Alpin's out in the deep. Car did awesome! Almost like 2 feet of snow didn't matter to it at all. All diffs are Torsen. Luv it!

 

[JB.]

J.B......

Thanks for the reply. My post was sort of a lazy way of absorbing your meaning by continuing the dialogue. It's clear to me now what you were saying even though it probably should have been before.

My understanding of the 'threshold' for the power going the wrong way (low traction wheels) is traction. The Torsen can send up to 3x the torque applied to good-traction axle. If one axle is on ice and 3 times zero is, well, zero; that's when the ice wheels spin rather than the vehicle moving. If you have even a little traction then 3 x whatever torque value the slipping axle can handle will be sent to the other axle.

Please straighten me out if I'm missing something...:cheers:

Sounds good. As for the 3x number, that's pretty close for the rear axle, but it's about 1 to 1 for the front so it's an average. It's a little tricky because of the unequal 60/40 torque split of the Torsen-C. Assuming my logic is sound, in the case of the Torsen-C which has a tighter range than previous Torsens, I figure it (the bias ratio) is about 1.8x -- 71/29 max rear=2.45, 53/47 max front=1.13 so 2.45 + 1.13/2 = 1.8

 

[JB.]

It almost sounds like you get better traction with the VSC off and the centre unlocked - if the torsen sends torque to the axle with more traction.

Yeah, it can seem that way, but here's one way to think of it. The Torsen is limited in the amount of torque it can transfer, a locked diff is not. Either end can recieve from 0 to 100 percent (nearly) depending on available traction .

Another way to look at it is that even though the Torsen amplifies ATRACs effect, thus closing the gap with a locked center, a locked center will require less ATRAC intervention in the first place.

A Torsen center is awesome, though. I really enjoy having it.

 

[CJ3Flyr]

Sounds good. As for the 3x number, that's pretty close for the rear axle, but it's about 1 to 1 for the front so it's an average. It's a little tricky because of the unequal 60/40 torque split of the Torsen-C. Assuming my logic is sound, in the case of the Torsen-C which has a tighter range than previous Torsens, I figure it (the bias ratio) is about 1.8x -- 71/29 max rear=2.45, 53/47 max front=1.13 so 2.45 + 1.13/2 = 1.8

:trophy::bravo::nicethread:

I am bookmarking this thread.This question, 'beeping in 4lo' and 'will 265/70/17's fit?' comes up so frequently... This way I can continue my post-whoring ways without having to type so much:lmao:
:thanks:

 

[scottm]

Just had a the big blizzard here in B'more two weeks ago and I took my 2000 Scoob 2.5RS with Mich Pilot Alpin's out in the deep. Car did awesome! Almost like 2 feet of snow didn't matter to it at all. All diffs are Torsen. Luv it!

Really? I just helped my teenager buy a '00 RS sedan, didn't know it had three Torsens in it. I may have to borrow it...