ATrac question

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Aznwolf118

10 Year 15 Year
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Nov 18, 2009
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hello,
I am coming from a built 2002 3rd gen 4runner with arbs and atrac/vsc, which i am keeping as my DD now :crybaby:

I am planning on trading my 07 FJ cruiser for a 100 series, don't want to but family needs come first. the FJ had Atrac system as well, but it worked with the e locker rear diff.

this leads me to believe that the Atrac systems are not the same, or at laest the control systems are not.

My question is how does the Atrac on the 100 series compare to the 4runner and FJ cruiser I have.
Is there an update in the control systems or programing in the 100 series. I have read the sticky and the wiki info about Atrac
 
you can think of ATRAC as a 'virtual' locker. or probably more accurately described as a 'virtual' LSD. when a wheel slips (any wheel) it will apply the brake to that corner to stop the wheel spin. the system works well for 90% of what you'll encounter. but if you get in some really ugly stuff, it won't provide the same as a true locked axle.
 
Aznwolf118 said:
the FJ had Atrac system as well, but it worked with the e locker rear diff.

this leads me to believe that the Atrac systems are not the same, or at laest the control systems are not.

My question is how does the Atrac on the 100 series compare to the 4runner and FJ cruiser I have.
Is there an update in the control systems or programing in the 100 series. I have read the sticky and the wiki info about Atrac
Click to expand...

Hmm.

If you read up on Atrac then you understand how it works and what it does. More info here on the details of the design: 2000 LX470 - Folder Shared from Box.net - Free Online File Storage

Excerpt:
"
A-Trac Function. During rugged offroad driving, this function controls the engine output and the brake fluid pressure that is applied to the slipping wheel, and distributes the drive force that would have been lost through the slippage to the remaining wheels in order to achieve a LSD (Limited Slip Differential) effect. As a result, the vehicle's offroad drivability and ability to free itself from the mogul have been improved."

Conceptually speaking...locker and atrac on the same axle at the same time don't make sense. iirc that's why in 2000 when LC/LX got vsc/atrac they lost the rear locker option (iirc that was in toyota design docs). since you have cdl, you have 50/50 split front and rear so even if you get rear axle in the air your front still gets 50% of drive force. if you do not have a locker in the rear obviously atrac could help there.

However in concept I could see atrac in the front helping even with locker in the rear.

Now, when you say your FJ had both, I'm curious about that. I'm not up on the latest of FJs, but according to this link for example (from '06 so maybe it's dated) -> A Trac/E locker hack (FJ Cruiser) - YotaTech Forums
"Stock you can not engage both the E locker and A Trac at the same time, the E locker will turn off the A Trac."

Apparently there was a mod to allow both together, and maybe later years and special editions enabled both from the factory? Which did you have?

Either way, conceptually I would think it's exactly the same as regular atrac and the only difference is that when the rear locker is engaged, it stops detecting slippage and reacting to slippage in the rear and it only pays attention to the front. So you're right, the software/ecu must be slightly different and if anything I would think atrac + rear locker is more effective than atrac alone except that atrac is always on whereas rear locker needs to be turned on otherwise it won't do anything for ya.

Fyi, I'm speaking from what I understand more than experience so I'm sure others will chime in as well.

Follow up another explanation and thread on atrac and rear locker:
http://www.ttora.com/forum/showthread.php?t=43236
"ATRAC works by sensing wheel spin on each axle. If it senses one wheel on an axle spining faster than the other, it applies brake to that wheel to lock it and transfer the power to the other wheel. Since the rear is locked, one wheel on that axle can't spin at a diffrent speed than the other, so the ATRAC system never activates on that axle, it ignores it. It will then only activate on the front if it senses one wheel spining up there."

For those that like to split hairs: how does this work if the rear axle is locked and in the air where you could say both rear wheels are slipping?
The excerpt above assumes slippage is detected by comparing both rear wheels speed against each other in which case both rear wheels slipping at same rate never registers as slippage. So according to that there's NO difference technically b/w atrac + locker and atrac alone - the rear slippage just never triggers atrac.
I assumed that rear wheels in the air would be recognized as slippage and in this case the ATRAC would have to actively ignore 'slippage signals' from the rear.

The toyota/lexus documentation on atrac behavior i referenced above explains the following:
"Based on the vehicle speed that has been calculated from the speeds of the wheels and the signal from the decelration sensor, the skid control ecu computes the target control speed in accordance with the transfer range.
The ecu compares the target control speed and the speeds of the wheels to determine whether or not a slippage exists. Upon detecting a slippage, the ecu controls the solenoid valve of the brake actuator to control the brake fluid pressure that is applied to the slipping wheel. When the wheel speed becomes lower than the target control speed, the ecu stops controlling the brake fluid pressure.
"

I don't think that answers anything b/c the question is where does the target control speed come from - is it from 'vehicle speed' or from comparing one wheel speed to opposite (on same axles) wheel speed?
 
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Though after running an FJC with the ATRAC and LOCKER for a couple years before selling it to get my 100 series....I think the two ATRAC systems are different in either software or functionality. The one on my 100 series seems better to me with less vibration and shuddering than that of my FJC...the changes between wheels searching for traction are more deliberate and defined according to friends that have watched me wheel both on the same trails....For what it is worth.

Just my 2cents after wheeling FJ60 with rear locker only, FJC with 33" tires, ATrac and E-Locker both (2008 model), and now 2000 100 series with 33" tires and only ATrac.
 
agaisin said:
Hmm.

If you read up on Atrac then you understand how it works and what it does. More info here on the details of the design: 2000 LX470 - Folder Shared from Box.net - Free Online File Storage

Excerpt:
"
A-Trac Function. During rugged offroad driving, this function controls the engine output and the brake fluid pressure that is applied to the slipping wheel, and distributes the drive force that would have been lost through the slippage to the remaining wheels in order to achieve a LSD (Limited Slip Differential) effect. As a result, the vehicle's offroad drivability and ability to free itself from the mogul have been improved."

Conceptually speaking...locker and atrac on the same axle at the same time don't make sense. iirc that's why in 2000 when LC/LX got vsc/atrac they lost the rear locker option (iirc that was in toyota design docs). since you have cdl, you have 50/50 split front and rear so even if you get rear axle in the air your front still gets 50% of drive force. if you do not have a locker in the rear obviously atrac could help there.

However in concept I could see atrac in the front helping even with locker in the rear.

Now, when you say your FJ had both, I'm curious about that. I'm not up on the latest of FJs, but according to this link for example (from '06 so maybe it's dated) -> A Trac/E locker hack (FJ Cruiser) - YotaTech Forums
"Stock you can not engage both the E locker and A Trac at the same time, the E locker will turn off the A Trac."

Apparently there was a mod to allow both together, and maybe later years and special editions enabled both from the factory? Which did you have?

Either way, conceptually I would think it's exactly the same as regular atrac and the only difference is that when the rear locker is engaged, it stops detecting slippage and reacting to slippage in the rear and it only pays attention to the front. So you're right, the software/ecu must be slightly different and if anything I would think atrac + rear locker is more effective than atrac alone except that atrac is always on whereas rear locker needs to be turned on otherwise it won't do anything for ya.

Fyi, I'm speaking from what I understand more than experience so I'm sure others will chime in as well.
Click to expand...

on the early FJC you could not engage them both.....at same time but could get them both. In 2008 they changed it to allow function of ATrac with ELocker engaged for ultimate traction and steering together. It was easy hack fix for those with 2007 models and many have done it...both the cut version and the no-cut HACK versions. By virtue of its setup using the locker would prevent the ATrac from thinking it was losing traction as both wheels would turn at same rate...hence no loss of traction on one wheel versus the other. The ATrac and ELocker together is a formidable combination in the FJC...and may in future put ARB in my rear end for same albeit stronger setup for the 100 series I now wheel. Either way they are great setups..and the ATrac is likely as much as 90% of the wheelers really need with a steady foot, good visual cues, and calm steering input.
 
Aznwolf118 said:
My question is how does the Atrac on the 100 series compare to the 4runner and FJ cruiser I have.
Click to expand...

A-TRAC whether it's an FJC, an LX570, a 100-Series all work alike. What is different is the "programming" or "software" and it's determined by the design and application of each vehicle.

On-trail experience always reveals to me that the 100 has tighter control compared to the FJC. Most of the time it seems to slip less. Regardless, both units keep moving and that's what counts.

Adding a locker to A-TRAC is a plus. That adds more traction in the right spots and A-TRAC will not kick in on that axle because each tire spins together. Not locking lets A-TRAC do it in the right spots for it (off camber, slippery and uneven, etc).
 
I should clarify the fj cruiser has seen limited wheel time and does not have the hack since I bought the extend warranty.

I know the atrac on my 3rd gen 4runner with f&r arbs is awesome due to the ability to select and go multi modes when wheel and pretty much got me anywhere I wanted except for the few idiot mistakes I have made after adding forced i
 
Brock said:
on the early FJC you could not engage them both.....at same time but could get them both. In 2008 they changed it to allow function of ATrac with ELocker engaged for ultimate traction and steering together. It was easy hack fix for those with 2007 models and many have done it...both the cut version and the no-cut HACK versions. By virtue of its setup using the locker would prevent the ATrac from thinking it was losing traction as both wheels would turn at same rate...hence no loss of traction on one wheel versus the other. The ATrac and ELocker together is a formidable combination in the FJC...and may in future put ARB in my rear end for same albeit stronger setup for the 100 series I now wheel. Either way they are great setups..and the ATrac is likely as much as 90% of the wheelers really need with a steady foot, good visual cues, and calm steering input.
Click to expand...

The change was made in 2007, not 08 :)

Some where around early 2007 (production date) they started allowing the a-trac and rear locker to work together. I had a 06/07 built 2007 and A-Trac and the rear locker function together.

Quick question on the 100 series with A-TRAC, is it always on when you are in 4LO? THe FJ has the ability to turn it on and off but there doesn't appear to be a button to do that in the LC/LX (2000+)
 
Layonnn said:
Quick question on the 100 series with A-TRAC, is it always on when you are in 4LO? THe FJ has the ability to turn it on and off but there doesn't appear to be a button to do that in the LC/LX (2000+)
Click to expand...

The document I linked to above has very detailed information around p. 10/17, 11/17, and 16/17 (2000 LX470 - Folder Shared from Box.net - Free Online File Storage):
On p. 16 it has a table that shows A-TRAC is always on regardless of center diff locked or free, and regardless of Transfer Case in L or H. VSC is dependant on center diff - VSC is off when CDL is locked, VSC is on when CDL is free. ABS with EBD and Brake Assist are also on all the time.
In addition to being on, the details in that document make it clear that A-Trac is smart enough to behave differently while in L than in H etc and based on other conditions.

And you're correct - no button to disable A-Trac in the LC/LX.
 
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for those of you with ATRAC and lockers, how does ATRAC respond to a spinning rear wheel with the rear locked and the center open? Does it clamp down on both rear wheels or does it get confused?
 
It can't react. Both rear wheels move at the same speed. And man...if you need to make a REAL TIGHT turn this will do it. CDL open and Rear Locked!
 
So does the logic require there to be variance in single wheel? I would have thought that if both rears were rotating at different RPM's then the front that it would inject control.
 
OregonLC said:
So does the logic require there to be variance in single wheel? I would have thought that if both rears were rotating at different RPM's then the front that it would inject control.
Click to expand...

Can't...TRAC looks at the difference between RPM on THE SAME axle. The CDL does the same...compares front to rear and adjusts...UNLESS you lock it.
 
OregonLC said:
So does the logic require there to be variance in single wheel? I would have thought that if both rears were rotating at different RPM's then the front that it would inject control.
Click to expand...

I would have thought that too. What's clear to me from toyota's own documentation is that there's a "target control speed" that the skid control ecu uses to determine when there's slippage and uses that as a baseline to determine when to apply brake pressure and/or cut engine output.

Everyone I've seen talk about this on the various forums says (translated into Toyota technical terminology) that the target control speed is calculated per axle by comparing left to right wheel speed (at each axle). Therefore in this case since the rear is locked there will never be a difference in wheel speed left to right on that axle.

In the detailed documentation I've found so far, they don't explain how they calculate the target control speed (at least not in a way I understand). I would have expected target control speed to be, as an example, the vehicle speed.

The only thing official I've found so far is this:
Toyota New Zealand - How ATC Works
It calls the system ATC, though I think it is referring to ATRAC. It says that in H4 the target control speed is the average 4 wheel speed and that in L4 it's the minimum wheel speed. You'll find the same assertion here: http://www.pressroom.com.au/pressroom/sample/presskits/99technologykit.htm#15

Anyhow - regardless of the details of how the system works (whether it ignores rear axle when it's locked or whether the slippage never registers b/c it calculates per axle) the effect is the same.
 
I'm planning a rear locker at some point but I'm actually looking forward to having ATrac up front with the rear locker. Atrac is: Simple. Easy. Works well enough for what I'll need. A rear locker will round out nicely.
 
agaisin said:
whether it ignores rear axle when it's locked or whether the slippage never registers b/c it calculates per axle) the effect is the same.
Click to expand...

Open CDL...A-TRAC cannot effect front or rear torque as the system works only within the given axle.

Other Traction Systems do add extra control where they can move power from the front to the rear axles/wheels and visa-versa via braking (not Torsen diff). For example...some Disco's can send all the power to one wheel if need be though it takes time (via system programming) for it to happen. This sounds superior though in real life one can see the Disco's slip and slip and slip as the programming sucks. The 100 CANNOT send all it's torque to one wheel via braking (A-TRAC) though again, some systems can.

Locked CDL...A-TRAC and or braking and/or slipping and/or nothing can change torque between the front and rear axles because they are LOCKED together. This is the same reason TRAC cannot operate if a given axle (front or rear) is locked. IF it could activate and operate a brake on a wheel it would stop that axle totally as they are LOCKED together.
 
lexi4darin said:
I'm planning a rear locker at some point but I'm actually looking forward to having ATrac up front with the rear locker. Atrac is: Simple. Easy. Works well enough for what I'll need. A rear locker will round out nicely.
Click to expand...

And this mode will cut it 95% of the time. Rear lock adds traction. Front TRAC adds traction (not as much) though you have full turning capability.

I will say this though...on BIG rocks and when REALLY twisted or going up steep and loose hills...that front locker helps a ton!

Here's a situation where ATRAC has the disadvantage compared to a locker:

Charouleau Gap (to see trail maintenance) 3/10 UZJ100 - John Shotts (shottscruisers)'s Photos
 
Lexi4darin - fyi, my understanding from others that have put on ARB Lockers for example is that you should disable VSC (by engaging CDL) whenever you have your rear (or front) locker engaged otherwise VSC could have issues with rear locker. This may be obvious but since it's possible to leave Center Diff open while rear axle is locked thus leaving VSC enabled, figured it's worth mentioning. And yes, I realize this point has nothing at all to do with ATRAC since that can't be disabled (at least not with a factory installed button).
 
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