A-Trac vs MTS vs rear locker?

Hey all, I've had my new '23 trd off road for about 6 months now. Currently in the middle of a Canadian winter in the rocky mountains; the truck has seen -35 temps, 0.5m of snow and winding mountain roads in blizzard conditions, and is so far it has been unstoppable. It's an absolute legend and a unit, and I love it.

I've only used 4H on several occasions, mostly during blizzard conditions (perpetual here) and for ice fishing back country trips. I have used 4L in combination with the rear lock only twice, during deep snow going up/down embankments in the remote backcountry to access some secret honey holes. It got me out of trouble instantly.

I'm an avid outdoorsman but dont know too much about 4x4ing so im trying to learn the nuances of these three systems. I dont offroad just to 'offroad' - its not my thing as a hobby or anything, I just go offroad for fishing/hunting/camping, and thats why I bought the truck. There are no 'off road parks' or the like where I live.

That being said, and the reason for this post - I'm confused about Atrac vs MTS vs stock rear locker. All three systems seem to have overlap and redundancy between each other. I've never used Atrac. Ive never used the MTS. I've only used 4L w/ locker to get me out of trouble, and it worked great.

So my question - why bother with Atrac and MTS if you have a rear locker? The rear locker seems to be superior to both these systems under almost any scenario - is there something I'm missing?

The only application I can think of would be a very slow, controlled decent or ascent - and I would engage the crawl control for that. And my understanding is that the crawl control system would engage these systems to a limited degree (i think).

What sort of situations would you want to skip the rear locker and use Atrac or MTS instead? Why three overlapping systems?

Is it possible to engage all three simultaneously? And why would someone want to do that?

Sorry if this may sound stupid to some seasoned 4x4 vets but I'm new to this tech and trying to learn as much as I can about my new truck. Thanks in advance

Edit - I understand that many people here live in warmer and more arid climates with lots of sand and different soil compositions etc. I'm in the northern boreal and here, its just snow/ice/forest/mud, so ATRAC/MTS might be more applicable to others who live farther south

Edit 2 - Its my understanding that ATrac and MTS and rear locker, you need to be in 4L to use them all anyway (aside from the 1st MTS setting which can be engaged in 4H - also dont understand why you would ever want to do this?). So that adds to my confusion. If you need to be in 4L to use all three systems - isnt the rear locker the obvious choice 99% of the time?

I wish there was more clarity in the owners manual. As a 4x4 amateur and new 4Runner owner, half a dozen different systems that all do the same thing, and all require 4L to operate... seems overengineered? Have the Toyota engineers ever actually gone in to the backcountry with these machines?

 

Ok so I could type forever to explain. This video does a great job explaining the systems. Basically, ATRAC is useful in loose material and uneven terrain where a wheel may come off the ground a bit or come close. I think it would be super helpful for you in snow for sure. It uses brakes on the wheel that is slipping to help transfer power across the axle. See video. The technique here is, plan on staying on the throttle as ATRAC needs power to the wheels to work. It's counter intuitive. It works very well.

Lockers are great but only when you need them. If you think you need them, engage them before you start your obstacle if possible. I'd refrain from using lockers for driving long distances on flat (even slick snow/ice) roads. It only works in 4L so there's that. Locked Diff's can help you climb get unstuck etc but cornering with them at any speed on slick terrain is not advised. Cornering with them on dry pavement is a definite NO. For slick snow icy roads with hills, ATRAC is good b/c it will work to send power to the wheel w/ the most traction.

MTS actually uses throttle modulation AND brakes to allow for some wheel spin based on the terrain you select. Think of it as traction control suited to different terrains. But only at low speeds.

EDIT: Can't help myself So in your climate, ATRAC is your friend, in my opinion. Check back w/ your manual as it will tell you what mode you need to be in for these systems to work. Some only work in 4L etc. I think ATRAC will work in 4H the others are only 4L. I know the Diff Lock is only 4L.

You can't engage all three per se, you can use Dif Lock and ATRAC at the same time. Super helpful actually and you kind of feel bulletproof doing it .

Crawl control is like cruise control for 4L It will keep you at a steady pace w/ throttle and brake. I find the slowest setting to be very active and annoying. The speed is too slow for the computer to modulate well. One or two settings faster seem to be easier for the computer to modulate. Play w/ it and see. I can see it helpful in a descent. I prefer my foot. Practice makes perfect. You really have to watch the trail and know what's coming up, and going on under your tires to avoid over doing it. Esp climbing and descending rough terrain. (If that makes sense.) Too easy to come off a rock too fast and slam it up into the undercarriage. Or to hit the gas too much and crest a rock and continue down the other side doing the same thing. Trick is to pause at the top and come off slow.

If you have more questions let us know.


https://www.youtube.com/watch?v=4uM-BiJPl2I

 

Thank you for the reply. I actually just watched this video, which is one of the reasons that prompted me to write this post. The author does explain the differences between Atrac and MTS yes.. but he doesn't contrast these two vs the stock rear lock on the trd offroad (he briefly mentioned his aftermarket arb air locker on the front). It just seems to be a comparison between 2nd and 3rd place options (no1 being the stock rear locker).

For every plausible situation he presented in this video that would require implementation of atrac/mts/locker, it seems like just engaging the rear locker would have been the best option for all of them, in every situation?

Esp when you can only use the three systems while in 4L, so at quite (relatively) low speeds, the locker seems like the best choice. Maybe I just lack the understanding of the technical nuances between all 3 systems and all plausible scenarios.....seems like the locker always wins?

But I'm a fisherman at heart, not a mechanic.. so what do I know lol. Im just a classic overthinker and my wife hates it

 

I suppose if there is turning involved - even in deep snow/mud - then forgo the locker and use ATRAC instead?

 

With the rear locker only you are in three wheel drive (both rear wheels and one front wheel). With Atrac you have computer controlled four wheel drive. It is not as good as having a front locker but possibly better than just the rear locker depending on the features you are driving on.

 

Not necessarily. It's ok to use until you get clear and at low speeds like under 5 mph. So you can continue to drive w/ it engaged just fine. Just don't go fast. And don't turn too sharp when you have too much traction. It will put tremendous stress thru your axle. So just use judiciously.

 

Not necessarily one wheel drive, an open Dif will apply equal torque to both tires. So what that means is if one wheel has "less torque" because of reduced friction between rubber and road, the amount of torque available across the axle will be reduced to the lowest amount allowable. Too technical for the OP perhaps for the moment and his concerns, but that's it. I do agree w/ you on the computer controlled 4WD. It's essentially just that. It just works a touch different. Pretty brilliant system if you ask me.

DRH, for what you do, (I think) a locker will be rarely used. I got to Moab UT, climb pretty crazy things and rarely use mine. Not to say you can't because... why not I'd say for you ATRAC will be your most common ally. Don't let that stop you from playing around and learning what works well for you though. So choose 4H and 4L to suit your current need and use the tools you have available and have fun. The technical stuff is nice to know. I'm like you and love to know things. So just play around and see how things work. Just do it close to home and don't get stuck taking too many risks! If you feel better w/ a locker go for it. Your confidence will build as you get to know how your vehicle handles your environment. Honestly TIRES are probably going to help your systems work to their fullest potential. You know the saying "Where the rubber meets the road"... Your systems won't work well w/ zero traction, so buy (if you haven't already) for your climate.

 

Thanks for the information DRH and JR7. Ive been reading about the 4WD system in the 4runner and learning about different angular velocities on wheels and turning radii, open vs locked differentials, what locking the rear diff does, and how Atrac mimics a locking diff by using computer controlled breaking to shift torque to the wheel with more traction, without locking the differential. I'm starting to appreciate the nuances of these systems now, I think I get it. I can see where Atrac and MTS would be useful now.

So now.. I have a DIFFERENT question now that maybe you guys can help me with. I understand that turning with the rear locker on, in a situation where the wheels cant slip ie. on dry pavement, can cause torsion on the driveline (winding effect - "binding") and how this is bad for the drivetrain components. It makes sense to me; all 4 wheels spin at different speeds during a turn, and by forcing the rear 2 wheels to spin at the same speed while in a turn, this can wear driveline components due to torsion (provided you are on a high traction surface that doesn't allow the wheels to slip, like pavement).

What does NOT make sense to me - is why using 4H (with no locker engaged) is bad on pavement. If both front and rear differentials are open-style like in the 4Runner TRD Offroad (with no rear locker engaged), and 4H is engaged - the front and rear driveshafts are locked together, causing the front and rear axles to spin at the same speed. I understand this.

So what I dont understand is - even if the front and rear driveshafts are locked together in 4H, causing both the front and rear axles to spin at the same speed - both the front and rear differentials are still open; neither are locked. So in theory - shouldnt all 4 wheels still be able to spin at different speeds? I get that the DRIVESHAFTS are locked together, causing 50/50 torque split between front and rear. But the front and rear DIFS are still both open (neither are locked) - so shouldn't this still let the 4 wheels all spin at different speeds? Isnt this the point of having an open differential on an axle - it allows the left and right wheel to spin at different speeds? If this is the case, why is driving 4H on dry pavement still bad? Both differentials are open, which should allow all 4 wheels to spin at different speeds, even while in a turn, even on a high friction surface... what am I getting wrong here?

 

https://www.youtube.com/watch?v=1EHcrjIlkHk&t=72s

 

Thanks Backpacker, I learned a lot from this video. How do open vs locked differentials impact windup in these examples - shouldn't open differentials allow the windup to dissipate bc the wheels can move at different speeds ?

 

Rear locker is more likely to spin you sideways into an obstacle. It may give you the most traction but it also will guarantee you lose both wheels if it starts spinning. Use with care.

As an aside, I learned a similar lesson with sheet ice this winter. Getting started without mts on extremely low traction ice and the wheels would spin a little then traction control kicked in and I’d start moving. With mts on, I’d give some throttle and all wheels would spin long enough to send me sideways toward the ditch. Off-road, mts/attack is great. On hard pack snow and ice, it’s a detriment.

 

Well open diffs are for allowing Right Left wheels to travel at different speeds. But your Transfer case is locked front to back. During a corner the front axle takes a different path than the rear. So the tighter the corner the bigger the delta is from front to rear and you will experience more and more binding the tighter the corner. Think of a compass drawing a circle. The center doesn’t move at all.
AWD vehicles have an open transfer case some have clutches, allowing for differential speeds front to rear allowing for full time AWD on all surfaces. 4wD is a locked, 50/50 split. Some older AWD 4Runners have a transfer case lock button to go to a 50/50 locked mode. My buddy’s 06 has this I think. Oh I think the Limited 4Rs are still like this too.

Any time you lock left right or front back you’ll have this binding issue. All wheels travel their own speed and take their own path.

 

There isn't just a difference side-to-side in a turn, both axles overall will turn at different speeds when turning and locking them together will result in the same overall issue of two things wanting to turn at different speeds that won't be relieved until a tire slips or something mechanical breaks.

See 14:16 for a demo of open front/rear differentials but a locked center: https://www.youtube.com/watch?v=B6H6BA6W1Ws

(The last-gen Sequoia has a limited-slip center differential that can be locked)

 

Since there's no limited slip in the transfer case, there's nothing to relieve the stress in the transfer case caused by the front and rear drive shafts having rotated by a greater difference than gearing tolerates.

 

I think where I'm having difficulty is, Im not understanding how the stress is building up in the first place... front and rear drive shafts are locked together, delivering equal power to front and rear axles... so if the wheels weren't allowed to spin at different speeds, I can see how stress would accumulate in the system.

But both front and rear difs are open, and the wheels CAN spin at different speeds... so how would stress buildup in the transfer case in the first place? Wouldnt the ability of the wheels to spin at different speeds relative to eachother allow for any stress to dissipate naturally?

Or is the fact that the wheels can all spin at different speeds not enough - we also need a sufficiently low traction surface to allow the stress to disperse through wheel slippage ?

 

Yes, this. The stress is built up in the transfer case, the front and rear drive shafts, and the gears that the drive shafts engage in the front and rear differentials.

 

Ok I think I understand this now. Thank you al for your responses.

Thats why we need low traction surfaces during 4H/4L operation (ESPECIALLY with a differential lock engaged): to allow for stress that builds within the system to be dispersed. Wheel slippage allows this - if wheel slippage cant occur (ie high traction surfaces like pavement) then the stress dispersion occurs by way of mechanical damage to the drivetrain components.

Now I can annoy my wife by talking about this for hours tonight after work. Thanks guys

 

Another way to look at it is that with a non-slip center differential and no wheel slip, you would need either both rear wheels to slip relative to the rear drive shaft or both front wheels to slip relative to the front drive shaft to relieve that stress.

 

The AH HA Moment! 100% spot on. We did leave out the wheel slippage part allowing for the stresses to be relieved. It's cringy when you corner on dry pavement and you feel the binding... Something will definitely go bang pop eventually.

 

Man I LOVE those AH HA moments! Thats what learning is all about!

Yeah haven't had that experience in the new 4runner yet. But I can recall when I was younger doing more field work and operating company trucks.... definitely felt some uncomfortable binding back then.

So question then - if the '23 TRD off road had a differential within the transfer case, that would allow 4H operation under all circumstances bc the front and rear driveshafts could spin at different speeds - would this just be the same system as the Limited ? Is this what they refer to as a center/middle differential on the limited ?

 

Yes, the limited-slip center differential on the Limited is what enables the full-time 4WD without binding.

 

Correct. The Limited is basically AWD until you lock the center diff. If I recall. I think they have just 4H (Unlocked center and thus AWD) and 4L (which auto locks the center diff) and you lock the center in 4H allowing for 4H (50/50 split). I don't think they have a 2H.

 

Ah ok. So basically full time AWD on any surface.

And so locking that center diff in 4H/4L on the Limited is obviously only something you would only do on low traction surfaces as well then.

Interesting.

 

Sounds like the question has been answered, but TFL just dropped a new 4Runner video this morning that has a great demonstration of traction control vs ATRAC vs locker. The whole video is good, but to see the 4Runner on the rollers skipped to the 6:00. mark. Keep watching and they get it in some flexy stuff in the dirt.

(The Mercedes in the video is actually quite impressive for what it is, looks like it would be fun to build)

https://youtube.com/watch?v=2HVBjEHKHbc&si=zP8DhpPV2q_ckASw

 

I think that is James' 4Runner.

 

Yes it is. He's in the video too, he's been on TFL quite a few times now.

 

I can't stand the noises that ATRAC makes so I don't use it. I only use my rear locker occasionally and don't personally have the need to add a front locker. If I wanted a rock crawler again I would have something with a solid front axle like an 80 series.

 

I specifically got the ORP because it comes with a rear locker. The ATRAC and MTS are brilliant implementations and Toyota deserves much credit. I also know, there are situations where each of these three traction technologies are useful.

 

I get it but, it works well. It’s just hydraulic solenoids actuating. It does sound odd. I’d think if it did kick in it would be for a few seconds. If it keeps you moving, I’d just deal with it as the alternative of getting stuck is way more annoying. Agreed on the front locker. Not necessary. Then again you may really never need the atrac. Especially if you have good tires.

When I have the wife and kids and we’re all alone in the back country, I use all the tools just to keep us from breaking things, getting stuck, and maintaining control. I don’t need a technical challenge with precious cargo lol. I want a happy warm wife and kids.

 

I think they're both useful, under different situations. I've tried ATRAC, it is pretty impressive, and I like that. It works on the front wheels as well. Definitely better than having a totally open front differential. I also like that it's pretty much set and forget.

I like the locker because you can activate it when you think you're going to need it, so you already have the traction when you do need it without waiting for wheel spin to cause it to kick in. I also don't think that in low traction situations such as sand, snow, or mud, you necessarily want at traction device that's going to be grabbing brakes. You want BOTH wheels spinning, propelling you forward.