Tech Question: Center Locking Dif on a Lexus GX470

[WhiteH2O]

I decided to test out the GX on some easy trails yesterday, when I ran into a 4x4 club with 5 Jeeps. They invited me along, and I quickly found myself on some trails that I never thought were possible in a Lexus.

The Jeeps were all modified with lifts and at least 33" tires; some of them were very modified. My GX is stock other than 2.5" lift and 31" tires (and a few non relevant things (I have a 2006 non KDSS model)).

At the end of the day, they admitted that they were all impressed with how well I drove (for being a 4x4 newb) and how well the Lexus did. They even took turns following me to see how the traction control worked, and they said it was fun to watch a Lexus with the wheels so far in the air. Since most of the people in their club has jeeps, they were used to having much more travel and a shorter wheel base, so the GX was a different animal than what they were used to running trails with.

I had it in 4Lo with the center dif locked and in 1st gear. We were running on trails that had bumps where opposing tires would be in the air (like the weight of the vehicle would be on front driver and rear pax wheels, and the others would be dangling in the air). With a tire in the air, they said that the tire in the air would spin for a split second, and then it would jerk spin-stop-spin-stop a few times a second, where it was obviously also sending power to the wheel on the ground. I could hear and feel this happening, and as long as I had two wheels on the ground, I could move forward. Often times, the front would drop, and I would have three wheels on the ground with just one rear tire in the air, but it would work just as well with two in the air.

So apparently I don't understand how a center locking dif works. The GX has VSC that is disabled when the center dif is locked. My understanding was that locking the center dif would send the same amount of torque to both front and rear (50-50 split) but since I don't have a locking front or rear dif, I thought they would still act as an open dif, where one tire could spin and take all the torque to that axle. This obviously isn't the case because of what I described above.

I am hoping that someone here can help me understand what is going on here. I understand how locking differentials work for a front and rear, but it seems like a center locking dif wouldn't be much help with an open front and rear dif. The Jeep guys decided that it was some type of computerized system that would apply the brakes on the spinning wheel that was in the air, pushing torque to the wheel with traction. I don't think this is right, but I don't have a better theory either. My searching on here and on Google has provided a little info on what it does, but not how, and I am obviously still confused.

 

[p nut]

The system is called ATRAC. Works well, as you already found out. Basically uses brakes to send the power to the tire on the ground.

http://forum.ih8mud.com/threads/a-trac-101.537946/

 

[WhiteH2O]

Ah, thanks! That clears it up quite well.

 

[brianjwilson]

If any of those jeeps were JKs, they had a similar feature that jeep calls BLD or brake lock differential.

For not having lockers, it works pretty in most situations like the a trac. As long as there is decent traction and only one tire is up, it's usually not bad. But on slippery surfaces (hill climbs in snow, sand, soft dirt) it doesn't work so well. The braking action kills momentum until you're basically doing a brake stand and come to a stop. It can be self defeating, and for me is especially annoying in the snow. The other downside is that it requires wheel spin before it works, where lockers help keep the momentum and prevent wheel spin. Personally I have a hard time telling myself to keep spinning my 35s with a manual trans and waiting for the brakes to do something.

Still, it works pretty well for stock without lockers.

Here is a video showing different levels of performance during a hill climb, between lockers and a trac.
http://youtu.be/EtlOegpRu6s

 

[Mrknowitall]

With your GX in HI-range, the center diff is a helical gear LSD, meaning it helps to deliver torque (rather well) even if the front or rear axle loses traction. Your open front and rear differentials actually transmit torque equally to both sides (don't call me names just yet). If you lift a tire (and it spins freely, no torque is transmitted at all. Once the ATRAC takes over, The brakes are applied at the spinning wheel, causing torque to be transmitted to both sides (of course, only the torque going to the wheel on the ground actually moves you along). Of course, this robs a lot of power, because half of the available torque is just turning into heat (in the brake rotor), but with a V8 and stock-ish tires, your GX has plenty to spare. In low range, with the center diff locked, no extra power is burned sending power front/rear, making an even better go of it.

 

[WhiteH2O]

Thanks everyone for the clarification. I am still reading a lot about atrac to make sure I understand it so that I can use it appropriately. I'm learning a lot, but have a ways to go still.

Is the center diff a helical LSD in 4LO when the center diff is open also?

Also, the IH8Mud link says to unlock the center diff when on a downhill; is this correct? How long or technical of a downhill would it be that I would want to unlock? What is the advantage of unlocking for a downhill obsticle?

 

[Ryanmb21]

I think most folks leave the center diff locked up when off road. Never heard of any good reason not too and no issues from doing so.

 

[AKmoney]

Not that I'm a huge expert on the subject but here are a couple videos that sort of illustrate the effect of a locking center differential.

Here's my V8 4Runner climbing a short, steep hill on the Fins and Things trail in Moab, UT. The 4Runner has more or less the same hardware as the GX470 (V8, full-time 4WD w/ lockable Torsen center diff, open front and rear diffs). The truck is in 4Lo and the Center Diff Lock is turned on. You'll note that right about 0:45 into the video, the passenger side front wheel gets airborne and starts to spin, albeit not quickly. The center diff lock forces the front axle to spin at the same rate as the rear, and with good traction on the rear axle, this limits the speed at which the wheel spins. You can see the effect of ATRAC grabbing the brakes on this wheel which transfers torque over to the front driver side and helps pull the truck up and over.


Now watch a 2003 Land Rover Discovery attempt the same climb. The '03 Disco *doesn't* have a locking center diff; all three diffs are open and the truck is completely reliant on brake-based Traction Control. When the Disco reaches the same point in the climb and the front passenger wheel lifts off the ground, you'll see it spin at very high speed. Without a center diff lock, once this wheel goes airborne, *all* of the engine's torque gets sent to this wheel, causing it to spin rapidly before the brake-based traction control kicks in and grabs it. Ultimately the truck makes it up the hill without a lot of difficulty so the overall benefit of a center diff lock is perhaps reduced, but to me it seems like having the center diff lock at least makes the Traction Control's job a little bit easier.


BTW, for most of eternity, your traditional part-time 4wd system found in most domestic pickups don't even have a center diff. When you put it in 4wd, the front and rear axles are locked together, the same way your GX470 is with the center diff lock turned on. For this reason, it's never recommended to drive these trucks in 4wd mode unless you're on dirt or some other low-traction surface (allowing slippage and preventing the axles from binding).

 

[AKmoney]

BTW, one other thing my Disco-owning friend pointed out to me: If you blow a half-shaft in a Disco with three open diffs, you're walking home. Do the same thing in the 4Runner/GX470, and at least in theory, the center diff lock will turn your truck into a 2wd vehicle, hopefully allowing you to limp it somewhere within range of a tow truck.

 

[Mrknowitall]

Is the center diff a helical LSD in 4LO when the center diff is open also?

AFAIK, yes. Making very tight turns off road may work better with the center diff unlocked. Going up steep climbs, the ATRAC will have to do less work with the center diff locked, however. I think the idea with unlocking the diff on a descent is so that the VSC can do its thing- not that important to most people, but I suppose it could be nice going down a steep sand dune.