Differential question

[Dalko43]

I was watching a 4runner TRD Pro video on the youtube channel, Engineering Explained, which is a very interesting channel with some good content IMO.

https://www.youtube.com/watch?v=ih42pPHFT00

However, I noticed something was said (sometime after the 6:42 minute mark) about how the center differential on the 4runner is "open" and is "controlled by the traction control system." The Toyota rep also stated that front differential is "open" and is "controlled electronically, so it detects wheel slippage it will pulse the brakes." Please note, it was not the Youtube channel's creator (aka the author of the video) who said that, but a tech (Toyota rep I believe) who was outside of the camera view, riding shotgun.

Now, I'm fairly certain I've seen this issue brought up and answered in other posts, but I just wanted to confirm that this Toyota rep was in fact wrong on this....the 4runner (with part-time 4wd) in fact doesn't have a center diff, but rather the front and rear drive shafts lock together upon engaging 4hi or 4low, right? I thought that was the main reason why engaging 4wd (hi or low) on dry pavement was not recommended? Does the traction control system "control" the center diff, as the rep stated? Does the 4runner (part-time 4wd) even have a center diff?

Does the front diff electronically "pulse" the brakes? I thought that the traction control was a separate system and operated independently from the front diff?

Or am I confusing my terminology here and was the Toyota rep right?

 

[1engineer]

Wait... didn't you say in another thread that you need to talk to a mechanic instead of an internet forum for information? Lol just kidding. Read the link in my signature line on how our 4WD works. ATRAC is what uses the brakes to electronically slow down the spinning wheel. It works well.

 

[sandcrawler]

I was watching a 4runner TRD Pro video on the youtube channel, Engineering Explained, which is a very interesting channel with some good content IMO.

https://www.youtube.com/watch?v=ih42pPHFT00

However, I noticed something was said (sometime after the 6:42 minute mark) about how the center differential on the 4runner is "open" and is "controlled by the traction control system." The Toyota rep also stated that front differential is "open" and is "controlled electronically, so it detects wheel slippage it will pulse the brakes." Please note, it was not the Youtube channel's creator (aka the author of the video) who said that, but a tech (Toyota rep I believe) who was outside of the camera view, riding shotgun.

Now, I'm fairly certain I've seen this issue brought up and answered in other posts, but I just wanted to confirm that this Toyota rep was in fact wrong on this....the 4runner (with part-time 4wd) in fact doesn't have a center diff, but rather the front and rear drive shafts lock together upon engaging 4hi or 4low, right? I thought that was the main reason why engaging 4wd (hi or low) on dry pavement was not recommended? Does the traction control system "control" the center diff, as the rep stated? Does the 4runner (part-time 4wd) even have a center diff?

Does the front diff electronically "pulse" the brakes? I thought that the traction control was a separate system and operated independently from the front diff?

Or am I confusing my terminology here and was the Toyota rep right?

not watching that video but, yeah, no center diff on P/T 4WD 4Runners.
ATRAC uses ABS system and sensors to brake a slipping/spinning wheel and power ones with traction.

 

[Dalko43]

Wait... didn't you say in another thread that you need to talk to a mechanic instead of an internet forum for information? Lol just kidding. Read the link in my signature line on how our 4WD works. ATRAC is what uses the brakes to electronically slow down the spinning wheel. It works well.

Yes I did...but I'd feel terrible about calling my mechanic in the middle of a Sunday morning; everyone needs their "off-days."

And I was hoping my thread would start an interesting, but civil, conversation on the actual mechanical and electronic inner-workings of the 4runner trail and sr-5's part-time 4wd.

And I do understand how ATRAC works to control/modulate the brakes (I think I've read that link of yours before).

My main question was on whether or not the Toyota rep was using the correct terminology by calling the part-time 4wd 4runner's center diff "open." And do the traction control systems actually exert any type of control over the front diff or center diff, as the Toyota rep suggested?

 

[KidVermicious]

However, I noticed something was said (sometime after the 6:42 minute mark) about how the center differential on the 4runner is "open" and is "controlled by the traction control system."

The guy talking between 6:40 and 7:00 misspoke, probably was getting his models confused, there is no center differential at all on any truck with part time 4WD - that's why it's called part-time 4WD. When the transfer case is engaged, the front and rear driveshafts are tied together 1:1 and cannot slip relative to eachother, which is why you try to avoid driving a part-time 4WD truck on pavement with 4WD engaged. Full-time 4WD trucks have a differential on the transfer case (the "center diff") that allows the driveshafts to spin faster than the other (just like an axle diff allows one axle shaft shaft to spin faster than the other). If a 4WD vehicle has a center diff, it may have a locker, that when engaged gives the truck the traction potential of a part-time 4WD vehicle - if it does not have the ability to lock, it is described as "open", just like an axle diff.

Clear?

 

[3wbdriver]

My main question was on whether or not the Toyota rep was using the correct terminology by calling the part-time 4wd 4runner's center diff "open." And do the traction control systems actually exert any type of control over the front diff or center diff, as the Toyota rep suggested?

Yes, the traction control system helps somewhat in controlling how power is distributed through the front open differential. By the judicious application of brakes on the spinning wheel power is redistributed to the wheel with traction on that axle.

I don't think the same applies to the open center diff of the Limited. It is there to allow "all wheel drive" for better overall traction in slippery on road conditions (wet roads, snow/ice, light mud), but it also allows slippage between front and rear diffs to prevent transfer case binding. If it also provided traction control there would be almost no reason to have the ability to lock it in. It is typically sold in versions where driving off road is an afterthought and provides a little better control in slick road conditions for less experienced drivers (soccer moms).

No insult intended toward those Limited owners who do enjoy getting dirty. But it's da roof..... :wink2:

Just watched the video. The Toyota rep couldn't even state WHAT actual speeds the CRAWL control uses, so I would say he was less informed than he should have been. Yes, he was wrong about the open diff being traction controlled.

 

[Jeremy556]

Didn't watch the video, but the SR5 and TE do not have a center differential, at all. The limited has a Torsen T3 LSD with locking feature (which creates the same affect as a part time 4wd without a center differential).

The TRAC and ATRAC systems are tied into the ABS system and pulse the brakes, they don't do anything to the differentials.

 

[Bilbo]

...

I don't think the same applies to the open center diff of the Limited. It is there to allow "all wheel drive" for better overall traction in slippery on road conditions (wet roads, snow/ice, light mud), but it also allows slippage between front and rear diffs to prevent transfer case binding. If it also provided traction control there would be almost no reason to have the ability to lock it in. It is typically sold in versions where driving off road is an afterthought and provides a little better control in slick road conditions for less experienced drivers (soccer moms).

No insult intended toward those Limited owners who do enjoy getting dirty. But it's da roof..... :wink2: ...

Almost right. But the LE does have TRAC and VSC when in 4W-high (unlocked) in addition to the center diff. So maybe there is no reason to lock the center diff except under very unusual circumstances. Toyota no longer allows 4W-low with an unlocked center diff in the 4Runner (although it could), so locked it is when low.

It also provides a little better control for experienced drivers too, lol.

 

[wfo9]

Didn't watch the video, but the SR5 and TE do not have a center differential, at all. The limited has a Torsen T3 LSD with locking feature (which creates the same affect as a part time 4wd without a center differential).

The TRAC and ATRAC systems are tied into the ABS system and pulse the brakes, they don't do anything to the differentials.

^^^ what he said, but TRAC an ATRAC do have an indirect effect on the the function of the differentials.

An open differential is designed to output consistent TQ to both output shafts. So, When one wheel is off the ground or on a very slick surface, it ends up getting all the wheel speed since very little tq can be applied.

When ATRAC senses the spinning wheel (via comparing wheel speeds sensors) and sends ABS brake input, the diff is able to distribute TQ to the wheel with traction (or at least the one moving at a slower wheel speed). This results in much better traction off-road and the ability to get through some very difficult terrain. It's still not the same as a true locker in the diff. As stated before, the diff is still very much open. Overall, ATRAC is pretty darn good and is all that is needed for most situations a 4runner will see.

 

[KidVermicious]

So other than being a bunch of electronics that can go wrong at precisely the wrong time, how is ATRAC inferior to a locker? Does the braking it does not lead to a precisely 50/50 split?

 

[nb rob]

I can't comment on that, but I will say-- The 4runner's ATRAC is much better than any other anti-lock limited slip I've ever used. I think simply because it's much more aggressive, and doesn't tickle the brakes, but comes on to them hard initially, and keeps that spin in check for good.

My Nissan's ABLS's was a joke. And I understand that it's not well-regarded in the jeep community (neither are 35" tires, so YMMV)

With the rear locked and ATRAC taking care of the bow-- well, cripes. I'm not a rock-crawler, but it suits my needs VERY well.

 

[jwkilgore]

So other than being a bunch of electronics that can go wrong at precisely the wrong time, how is ATRAC inferior to a locker? Does the braking it does not lead to a precisely 50/50 split?

Because you are applying braking force to aid in forward progress. In most situations this works great, but not all. One situation would be a steep incline on a slippery surface where you need as much forward force as possible to climb. Say you hit a point where diagonal wheels are slightly lifted, but still in contact with the ground. If they start to slip the ATRAC will apply brakes to those wheel, which means those wheels are no longer pulling and may even impede progress. With a locked diff all four wheels are always pulling and they can't slip with respect to each other.

This is a sales pitch for ARB lockers, so obviously they found a specific place where they knew electronic traction control would fail, but it does show how full lockers is better than ATRAC: https://www.youtube.com/watch?v=C2wkW05Pc6c

The first 6 mins is the sales pitch; they start the climbing demonstration at 6:00.

Here's a second one. Here the ATRAC works, but the lockers allows a slower and safer ascent: https://www.youtube.com/watch?v=YNFacsKnswM

 

[DesertCanyons]

So other than being a bunch of electronics that can go wrong at precisely the wrong time, how is ATRAC inferior to a locker? Does the braking it does not lead to a precisely 50/50 split?

No, it does not. The ATRAC brakes the wheel with less traction so it doesn't move. The torque is applied to the wheel that has traction. That's very different from a locking differential that distributes the same torque and turning revolutions/rotations to both wheels on the axle. The locking diff' supplies 50/50 traction to both rear wheels.

The locking diff' is much more useful, IMHO. But it can only be used briefly, according to the manual.

 

[KidVermicious]

Say you hit a point where diagonal wheels are slightly lifted, but still in contact with the ground. If they start to slip the ATRAC will apply brakes to those wheel, which means those wheels are no longer pulling and may even impede progress. With a locked diff all four wheels are always pulling and they can't slip with respect to each other.

Got it, thanks.

 

[ProspectiveBuyer]

Because you are applying braking force to aid in forward progress. In most situations this works great, but not all. One situation would be a steep incline on a slippery surface where you need as much forward force as possible to climb. Say you hit a point where diagonal wheels are slightly lifted, but still in contact with the ground. If they start to slip the ATRAC will apply brakes to those wheel, which means those wheels are no longer pulling and may even impede progress. With a locked diff all four wheels are always pulling and they can't slip with respect to each other.

This is a sales pitch for ARB lockers, so obviously they found a specific place where they knew electronic traction control would fail, but it does show how full lockers is better than ATRAC: https://www.youtube.com/watch?v=C2wkW05Pc6c

The first 6 mins is the sales pitch; they start the climbing demonstration at 6:00.

Here's a second one. Here the ATRAC works, but the lockers allows a slower and safer ascent: https://www.youtube.com/watch?v=YNFacsKnswM

Very nice videos. It seems to me that, if you dont have a front locker, it sometimes would make more sense to lock the rear differential and keep the vehicle in RWD instead of putting it in 4WD and have the open front differential waste half the power of the engine when one of the the front wheels slip.

 

[Jeremy556]

Very nice videos. It seems to me that, if you dont have a front locker, it sometimes would make more sense to lock the rear differential and keep the vehicle in RWD instead of putting it in 4WD and have the open front differential waste half the power of the engine when one of the the front wheels slip.

Traction is going to be the limiting factor, not power. 4lo also gives you low gearing with almost 3 times as much torque as in 2wd.

 

[ProspectiveBuyer]

Traction is going to be the limiting factor, not power. 4lo also gives you low gearing with almost 3 times as much torque as in 2wd.

Cant you use 4lo in RWD?

 

[wfo9]

There is a big difference between ATRAC and lockers. ATRAC will usually get the job done, but it is a bit unpredictable and jerky at times. The power more or less pulses between wheels, which can be a little problematic in rock crawling sitations.

Lockers are 100% predictable, the wheels all spin at the exact same speed and the driver is in full control. It can be very graceful vs jerky bursts of power.

There are situations where ATRAC just fails miserably. Mainly a loose steep climb that needs momentum or deep mud that needs lots of wheel spin.

However there are also cases where ATRAC is better than a locker. In very slick off camber situations lockers will make the vehicle crab and pitch the rear end to go up hill. ATRAC tends to not cause that as much.

 

[wfo9]

Cant you use 4lo in RWD?

No. The transfer case has to be engaged to get the reduction gear.

 

[jwkilgore]

Very nice videos. It seems to me that, if you dont have a front locker, it sometimes would make more sense to lock the rear differential and keep the vehicle in RWD instead of putting it in 4WD and have the open front differential waste half the power of the engine when one of the the front wheels slip.

Last summer I spent several weeks in a Polaris Ranger in the Sierra Nevadas navigating rough service roads. The options for the transmission were RWD Open -> RWD Lock -> 4WD Lock. Quite often if a wheel started slipping while climbing up a steel hill (usually dirt or gravel), all we had to do to keep moving is go to the second position (RWD Lock). We usually only did 4WD Lock in mud or rough terrain where all four wheels needed traction. So what you said makes sense.

However, the Ranger is the only vehicle I know of with that option from the factory. There may be more, but I don't know of one. Unless you're talking about installing an aftermarket rear locker (ARB, Harrup)... I believe those may be activated in 2WD.


All this said, understand that the 4Runner's ATRAC is a substantial improvement over open differentials. I finally got around to really using mine a few weeks ago and I was seriously impressed. A rear locker is better, and front &rear lockers are even better.

IMO, in order of improving offroading ability, it goes:
2WD -> decent jump to
4WD open -> huge jump to
4WD ATRAC -> smaller jump to
4WD rear lock + front ATRAC -> even smaller jump to
4WD both lock