A-TRAC: help me out! - Toyota FJ Cruiser Forum
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post #1 of 12 (permalink) Old 03-04-2019, 05:45 PM Thread Starter
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Question A-TRAC: help me out!

I've been trying to find a simple explanation of how A-TRAC works, in what situations it would be most useful and most importantly, how it differs from LSD. So far, my search has turned up more questions than answers.

Really new to off-roading and looking to get the most out of my TT 2014. Thanks for the assistance!

JB
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post #2 of 12 (permalink) Old 03-04-2019, 07:46 PM
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Re: A-TRAC: help me out!

Go here: https://www.fjcruiserforums.com/foru...tml#post264070

Should take you to Post 71 of the thread.

Good luck,

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post #3 of 12 (permalink) Old 03-05-2019, 12:58 AM
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Re: A-TRAC: help me out!

In a 4WD FJ, the only LSD is the centre diff in the manual transmission models.
There is the "Auto LSD" in the 2WD FJ, and that works in a similar way to ATRAC I believe,

Think of ATRAC as Traction control on steroids (it uses the brakes to stop wheels spinning and to direct traction to the wheel that has grip), and is smarter than an LSD (because they typically rely on the spinning wheel having some grip), or alternatively as a diff lock that allows you to steer.
ATRAC is only available in 4 low, and some early models won't allow you to lock the rear diff AND use ATRAC at the same time (but of course there's a mod for that).
ATRAC is what you use when you're stuck or about to be stuck.
I use it in conjunction with my rear locker, and it works extremely well - just as good as a front locker for traction, but still allows you to steer (so maybe better?).

ATRAC shouldn't be confused with VSC or TRC - both will be disabled in 4L.

https://en.wikipedia.org/wiki/Torsen
"When a Torsen differential is employed, the slower-moving wheel always receives more torque than the faster-moving wheel. The Torsen T-2R RaceMaster is the only Torsen to have a preload clutch. So, even if a wheel is airborne, torque is applied to the other side. If one wheel were raised in the air, the regular Torsen units would act like an open differential, and no torque would be transferred to the other wheel. This is where the parking brake "trick" can help out. If the parking brake is applied, assuming that the parking brake applies even resistance to each side, then the drag to the airborne side is "multiplied" through the differential, and TBR times the drag torque is applied to the other side. So, the ground side would see (TBR X drag torque) minus drag torque, and that may restore motion either forward or in reverse. In Hummer/HMMWV applications, there are both front and rear Torsen differentials, so the use of the main brakes will operate this "trick" on both axles simultaneously."







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post #4 of 12 (permalink) Old 03-05-2019, 05:10 AM
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Re: A-TRAC: help me out!

in case it is helpful, here is a visual to help you with when each of the systems is in play:
Attached Files
File Type: pdf Drive Mode-Drive Mode.pdf (103.8 KB, 90 views)
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post #5 of 12 (permalink) Old 03-05-2019, 05:24 AM
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Re: A-TRAC: help me out!

Regarding the Torsen differential, I remember when Trant Jarman was the technical editor at Automobile Magazine back in the mid '80s. He and I worked together at Cars and Concepts and he heard that I'd written a paper at college about the Torsen. He grabbed me by the arm and sat me down and said, "you don't leave this table until I understand how that works!" My explanation was essentially similar to the first video linked above.

Amazing thing, the Torsen. I wish that more than just the MT FJ came with them. But, since the assumption was that most sold would be AT, and most of them would be only for grocery getting and commuting, they saved the money (Torsens are pretty expensive) and only spent it on the MT models, which would surely almost all go to hard core truckers. Also the open AT center diff allowed slightly higher mpg for the CAFE targets on the higher volume version.

At least, that was how things looked way back in '05. Today pretty much all anyone buys anymore are AT, but there you go.
Fun fact: the highest % ratio of MT equipped vehicles sold today is the Toyota FRS. Not Porsche, not Ferrari, or anything like that. No, the highest % MT is currently the FRS. Well, that and the Lotus Elise, but I think those aren't even available in anything other than MT.

Additional fun fact: the rear diff in an FRS is a Torsen.



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post #6 of 12 (permalink) Old 03-05-2019, 05:44 AM
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Re: A-TRAC: help me out!

Quote:
norm356 previously said: View Post
Regarding the Torsen differential, I remember when Trant Jarman was the technical editor at Automobile Magazine back in the mid '80s. He and I worked together at Cars and Concepts and he heard that I'd written a paper at college about the Torsen. He grabbed me by the arm and sat me down and said, "you don't leave this table until I understand how that works!" My explanation was essentially similar to the first video linked above.

Amazing thing, the Torsen. I wish that more than just the MT FJ came with them. But, since the assumption was that most sold would be AT, and most of them would be only for grocery getting and commuting, they saved the money (Torsens are pretty expensive) and only spent it on the MT models, which would surely almost all go to hard core truckers. Also the open AT center diff allowed slightly higher mpg for the CAFE targets on the higher volume version.

At least, that was how things looked way back in '05. Today pretty much all anyone buys anymore are AT, but there you go.
Fun fact: the highest % ratio of MT equipped vehicles sold today is the Toyota FRS. Not Porsche, not Ferrari, or anything like that. No, the highest % MT is currently the FRS. Well, that and the Lotus Elise, but I think those aren't even available in anything other than MT.

Additional fun fact: the rear diff in an FRS is a Torsen.



Norm "manual forever" Kerr
Good to know.
Also - the LC70 is only available with a 5 speed manual.

The AT FJ doesn't have a centre diff - it's just a straight transfer case - if it's in 4WD, it's locked. This was a fuel economy measure when combined with the ADD on the front passenger axle, and is why the 6MT has worse fuel economy than the 5AT - not because the 5AT has an amazing transmission, it just sucks less power than the FT 4WD on the 6MT.

Just as a side rant - I hate when people say the 6MT isn't a proper 4WD because it's AWD. That's just as stupid as saying a Humvee or a LC150 / LC200 / 4Runner isn't a real 4WD because they're all AWD as well by their definition. If it meets the below criteria, it's a 4WD:

1) the centre diff/transfer case manually locks
2) it has low range

If it has a lockable centre diff, and it has low range, then it's FT 4WD and is naturally better than a part time 4WD.

Rant over
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post #7 of 12 (permalink) Old 03-05-2019, 09:14 AM
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Re: A-TRAC: help me out!

@amaclach @norm356 Interesting. I now know what the ADD system is (and everything else )
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Re: A-TRAC: help me out!

" the LC70 is only available with a 5 speed manual"

Sadly, tragically, LC70 was never available in the US (the largest, but arguably the boringest auto market in the world).

I love it when I travel overseas and am able to rent a MT rental car (not a possibility in the US for may decades).


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post #9 of 12 (permalink) Old 03-05-2019, 09:41 PM Thread Starter
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Re: A-TRAC: help me out!

Now it makes sense! Really appreciate all the great insights, links and commentary. Love the forum!

Thanks all!!
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post #10 of 12 (permalink) Old 03-06-2019, 07:32 AM
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Re: A-TRAC: help me out!

Quote:
jbadolato previously said: View Post
I've been trying to find a simple explanation of how A-TRAC works, in what situations it would be most useful and most importantly, how it differs from LSD. So far, my search has turned up more questions than answers.

Really new to off-roading and looking to get the most out of my TT 2014. Thanks for the assistance!

JB
First you need to understand how a regular open differential works (it allows the wheels to turn at different rates, such as while going around corners and curves, while still providing power to the wheels), and understand that by it's nature a differential tends to direct power to the wheel with the least grip (traction) on that axle.

Not a big deal on the road where both tires generally have good traction, but a problem off-road when you might have a tire or two entirely off the ground at times. You can see there will be times when you will only have one wheel on the ground from each axle, and you will be dead in the water because the wheels in the air will get all of the power and just spin uselessly.

The ATRAC simply uses the braking system to stop the wheel with the most spin (or least traction), thereby making the other wheel the one with the least traction, relatively speaking, so the differential will now send power to the wheel that initially had the most traction. Clear as mud?
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