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Old 02-12-2021, 10:37 AM #1
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Another ATRAC explanation

This forum has helped me on my 4runner journey and I want to give a bit back. Several people have done a good job describing these systems but I will give my attempt as well since there is a lot of confusion. Driveline engineering is one of my favorite things to think about since it took me the longest to understand.

1st off, ATRAC/MTS is not the same as TRAC. TRAC is your traction control system that is interested in keeping wheels from spinning while ATRAC/MTS is concerned with keeping wheels moving at the same speed across the axle. To keep the explanation getting too complicated I will just use ATRAC with the understanding that MTS is supposedly a more advanced form of the same idea, (in my experience MTS seems like an unnecessary complication to the ATRAC system).

A LOCKED differential-Is the easiest operation to visualize in function; it is like a broomstick with wheels on either end. In other words it is impossible for one wheel to turn faster or slower than the other since they are connected. Our lockers are selectable in that they can be turned off or on.

An OPEN differential is what our 4runners have front and rear, (those with a rear locker act as an open differential unless the locker is engaged). In this system both wheels across the axle get the same amount of torque across the axle. While this sounds like the same as a locked differential it is not. While it is this open differential system that allows you to turn a corner without binding the inside wheel like a locker, it is also the reason in a low traction scenario you have a single wheel spinning while the other sits there doing nothing. The thought experiment to see why this is happening is to imagine a scenario where in 2wd one rear wheel is on solid high traction rock and the other is in the air, even by an inch or two. Because the wheel in the air has no traction it takes a relatively small amount of torque to turn, for just an example 10 lb/ft. The problem is that 10 lb/ft is probably not enough to turn the wheel on the ground with traction since it now has to move the entire weight of the vehicle by itself. You can spin the wheel in the air faster and faster but because the torque is the same across the axle you will never get the wheel on the ground to turn.

Enter ATRAC. In the above scenario the ATRAC system will begin to apply brake pressure to the wheel in the air. Because the wheel in the air now has to overcome the friction from the brake this increases the amount of torque needed to turn that wheel say from 10 lb/ft to 70 lb/ft, (just an example actual torque is variable based on available traction and other factors). Since the torque is the same across the axle, the wheel on the ground also gains more torque, ideally enough to overcome the weight of the vehicle and move the vehicle forward. Again remember the purpose of ATRAC is not to stop the wheel in the air from spinning, but only to make sure the wheel in the air and the wheel on the ground both move at close to the same speed.

Obviously the example with a wheel in the air is the extreme example and when the differential in traction is less the system still works the same way.

Because someone will ask does ATRAC work when the locker is engaged? The answer is yes but it has no practical effect on the locked axle since it is already turning both wheels at the exact same speed. What it will do is allow ATRAC to benefit your front wheels while your rear is locked.

Hope this helps someone and I will add another post with a few touchup things such as LSD and MTS.

Last edited by Nalcyon; 02-12-2021 at 10:42 AM.
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Old 02-12-2021, 10:46 AM #2
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Touch up on LSD and MTS

Just a little more info for those who are truly bored...

a LIMITED SLIP DIFFERENTIAL is not available from the factory on our 5th gen vehicles but I'll try to explain the concept. Most traditional LSD's are capable of increasing torque across the axle up to a certain amount, usually 3-4x.

In our example of an airborne wheel taking 10 ft/lb to turn this may still not be enough to turn the wheel on the ground as it would only get 30 to 40 lb/ft of torque. An LSD helps traction but is usually at its limit with a wheel in the air and it is in these situations where ATRAC or a locker really shine.

MTS-multi terrain select system found on offroad and pro models. MTS uses the same concept as ATRAC but has more adjustable tuning. The extremes are mud and sand vs rock. In mud/sand ATRAC is still doing what it usually does but the system is tuned to allow a little more speed differentiation across the axle and intervene later vs rock where it allows almost no speed differentiation. MTS also makes adjustment to throttle feel and maybe some other systems but in my experience I have found ATRAC to work just as good if not better regardless of the terrain. (I have heard people liken it to about the middle of the MTS spectrum). IMO I could totally do without MTS and just have the ATRAC button. The different modes do make a difference but usually if you are in a situation where ATRAC is going to make the difference I can't tell any benefit from using ATRAC over MTS, (other than maybe high traction off camber rocks).

A fun fact, ATRAC is only able to be activated in 4lo and MTS is the same except for the mud and sand setting which can be activated in 4hi, just be sure you are going slow are at a stop or the computer will not let mud/sand be activated. In my experience this is good for beach driving or other "higher speed" sand driving.


Hope this stuff helps someone.
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Old 02-12-2021, 11:15 AM #3
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Quote:
Originally Posted by Nalcyon View Post
Just a little more info for those who are truly bored...

a LIMITED SLIP DIFFERENTIAL is not available from the factory on our 5th gen vehicles but I'll try to explain the concept. Most traditional LSD's are capable of increasing torque across the axle up to a certain amount, usually 3-4x.
The 4Runner Limited comes with a Torsen center differential. The Torsen is a kind of limited slip differential, albeit, a center differential. All other 4Runners with 4X4 do not have center differentials, i.e. the front and rear axles are forced to turn at the same speed.
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Old 02-12-2021, 11:30 AM #4
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I thought 2WD models had auto LSD?
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Old 02-12-2021, 12:18 PM #5
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Old 02-12-2021, 12:58 PM #6
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Quote:
Originally Posted by fkheath View Post
The 4Runner Limited comes with a Torsen center differential. The Torsen is a kind of limited slip differential, albeit, a center differential. All other 4Runners with 4X4 do not have center differentials, i.e. the front and rear axles are forced to turn at the same speed.
Good point.

For those wondering the information above pertains mostly to the differentials front and rear. For non limited models think of a center transfer case kind of like a locker that can have the front wheels disengaged completely but when in 4hi or 4lo the driveshafts are forced to turn the same speed front and back like a locker. When in 2wd the front wheels are still turning the front half shafts, the differential is still turning, and the front driveshaft is still turning, but the front driveshaft is not receiving power from the transfer case because it is not locked in as it is in 4hi or lo.

(On older vehicles and some current HD pickup trucks you can manually disengage the front hubs which disconnects the front wheels from the half shafts thereby letting the half shafts, differential, and front driveshaft not have movement during 2wd operation. This has the benefit of less wear and tear on the components.)

Just like a locker ATRAC can do nothing for front to back traction in 4hi or lo because the front and rear driveshafts are moving at the exact same speed like a locker. Plus I don't believe the sensors are set up to worry about front to rear speed as that would be pointless. (Don't know if the Limited uses any front to back traction adjustment other than the mechanical adjustment of the Torsen Diff).

For the limited with full time 4wd it is indeed a Torsen style limited slip and I don't think I have the ability to explain exactly how it works, (watch a video or two they're pretty cool), but in practice it operates similar to a traditional friction LSD allowing some slip front to back which allows you to drive it on the road without damaging or binding the driveline. When the limited is in 4hi or 4lo the front and rear are locked together just like the other 4wd models.
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Old 02-12-2021, 04:42 PM #7
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MTS is essentially just variable levels of ATRAC, allowing for different levels of "slop" between wheel speed on different terrain types, and how quickly ATRAC responds to low traction events.
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Old 02-12-2021, 05:35 PM #8
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Originally Posted by LandCruiser View Post
I thought 2WD models had auto LSD?
"Auto LSD" is not a mechanical limited slip differential, rather it is a fancy name for traction control. Traction control applies the brakes to a slipping wheel so more power goes to the wheel with better grip.
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Old 02-12-2021, 06:14 PM #9
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I like MTS in the M+S setting for sand and snow. It can be used in 4HI whereas ATRAC only in 4LO.
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Old 02-12-2021, 06:23 PM #10
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We had an ice storm last night, reading all this made me have to play in the driveway. Mostly 2wd, tried with trac off and on and was a little work on a small incline to get up. 4Hi with M/T was no problem. 2wd on level was OK with gentle throttle, but easy to break loose.

Time to go back in the garage, had a long icy straight section. 4Hi with M/T set to snow. Good take off, got moving quickly with all 4 wheels spinning. Neighbor was watching and was wondering what I was doing. I wanted to tell him to try that in his Forester.
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Old 02-13-2021, 09:05 PM #11
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alright, here is a total noob question.

playing in deep snow today, how do you force power to the front wheels?

I was sitting in place, in 4low and only the rear wheels were spinning.

thx
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Old 02-13-2021, 09:11 PM #12
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4WD Low
Atrac turned on
Locked rear dif

GOES LIKE HELL in just about every off road condition.

That's all you really need to know.
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Old 02-15-2021, 01:47 PM #13
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Quote:
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alright, here is a total noob question.

playing in deep snow today, how do you force power to the front wheels?

I was sitting in place, in 4low and only the rear wheels were spinning.

thx
Sounds like you weren't actually in 4wd. The dash light has to be solid illuminated, not blinking.
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Old 02-15-2021, 03:00 PM #14
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Quote:
Originally Posted by Nalcyon View Post

For the limited with full time 4wd it is indeed a Torsen style limited slip and I don't think I have the ability to explain exactly how it works, (watch a video or two they're pretty cool), but in practice it operates similar to a traditional friction LSD allowing some slip front to back which allows you to drive it on the road without damaging or binding the driveline. When the limited is in 4hi or 4lo the front and rear are locked together just like the other 4wd models.
I've read several times on this forum that the Limited Full Time 4WD is simply AWD.

I can't argue except to say, if it is indeed AWD the Toyota strategy is much different (better) than other AWD systems.

There's a spot on a certain gravel road I'm consistently standing and watching folks make a slow turn and up a small incline. Every single AWD car spins a tire making that maneuver. My Limited never has spun a tire.
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Old 02-15-2021, 03:18 PM #15
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Every single AWD car spins a tire making that maneuver. My Limited never has spun a tire.
Well, our Limited's probably have at least 1000lbs on the cars too, so that maybe be part of the "why".
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