How does ATRAC work??

[Thai]

Hey all,

Here is a question: how does ATRAC keep you moving forward when one wheel in mud and the other on solid ground?? (Let's just focus on one axle.)

How much of torque can be transferred to the wheel with traction?? (Give me %)

Any takers?? I want specific explanation.

I will come back with an answer if no one has figured it out tomorrow.

Good night.

 

[dcampen]

How much of torque can be transferred to the wheel with traction?? (Give my %)

50% of the power will go to the wheel with traction, the other 50% will be dissipated as heat in the brake of the other wheel. This is because, with the open differential, the torque is always split 50/50. To get torque (thus power) to the wheel with traction to the ground you need the same torque on the other side of the open differential to be supplied by the brake.

This is why, for extreme rock crawling, lockers are better. With a locker all of the engine power would go to traction instead of half being used in wearing down your brake pads.

 

[Thai]

Excellent response! This is commonly mistaken and misleading in a lot of manufacturer's literature, including Toyota and Mercedes (for the ML). ATRAC is there to PREVENT loss of torque to the wheel with traction.

For example, if one wheel (in mud) is spinning on an axle, then the other wheel (on dry land) LOSES it's torque because power is routed to the wheel with least resistance (the one in mud). So, power is being transferred to wheel in mud. Now, with ATRAC, the ABS sensors will sense this slippage and apply the brake to the spinning wheel. It will apply the brake UNTIL the two wheels are spinning at the same speed (in theory)...that is, it will apply the brake to slow the spinning wheel to the same speed as the wheel that is on dry land. By doing this, it tricks the differential into thinking that the slipping wheel is also on dry land...thus, preventing all power from going to the slipping wheel. It evens out the torque distribution to both wheels. This prevents the wheel with traction (on dry land) from losing torque. Thus, it will have power to move your truck forward.

Does ATRAC really allow for 50% split?? Well, in theory, yes. However, in real life, i would say that it helps prevent torque from TOTALLY going to the spinning/slipping wheel. However, because of the stress on the brakes (brakes have to go against the power flow...aka HEAT LOSS) AND powertrain (remember, ATRAC can be quite sudden in it's operation), i doubt it. I would say that it is somewhere less than 50%. That is, ATRAC will prevent most of the torque from going to the slipping wheel. So, the wheel with traction will receive less than 50% of torque, but enough to keep your 4runner moving forward.

Thus, this is the role of ATRAC...it prevents the loss of torque from the wheel with traction...to allow that wheel to get enough power to move your 4runner forward.

This is why the 4runner has big brakes with multi-pistons and 4-channels. You need good brakes in a vehicle with ATRAC for effective ATRAC operation. In addition, 4runner powertrain needs to be beefy to take the stress of ATRAC.

How good is ATRAC?? It is as good as any limited-slip differentials (LSD) on the market. It will only lose to a true locker over rocky terrain, hill climbs, and deep mud. But then again, it is less stressful on the axles than a locker in most circumstances.

With ATRAC, it's like having LSD on the front AND rear axles. With the center diff locked, the 4runner is quite capable! ATRAC is such an elegant and simple design...no extra stuff...it just uses the existing technology (4-channel ABS, ABS sensors, etc.) to create a LSD.

Hope this makes sense...and sorry about the length.

 

[Bluto]

How much torque will ATRAC actually transfer?

Not a %, but ft-lbs.

My knowledge of off roading is limited. But don't LSD differentials have a break-away torque rating? ie, How many ft-lbs they can transfer before opening. If ATRAC can't transfer more ft-lbs than an LSD, is it as good?

My current vehicle has a 4-channel ABS system with 4 wheel speed sensors and electric/hydraulic control

The dynamic stability system has throttle and 2-channel brake control (and other things).

The system is good. It works great in the corners and on most terrain. But it seems the system will not apply enough braking force to transfer a lot of torque (for going up a steep incline).

I'm guessing a master cylinder can generate about 400-600 psi of brake fluid pressure? How much can ATRAC generate?

 

[Thai]

Bluto, what car do you have now?? BMW X5??

Anyway, i was using % for torque figures to keep things simple.

Yes, ATRAC does have a limit...that is why i said that ATRAC will not achieve 50/50 split in the real world.

ATRAC is limited by the ABS system. So, that is the limiting factor.

Think of it this way...Hummer H2 can go up a 10% grade incline with one wheel traction. It has ATRAC-like system. Toyota has not published any figures, but i assume that it is probably just as good as Hummer H2's system. With all wheels having traction, a Hummer H2 can go up a 60% grade (compare to 10% with just one wheel). So, as you can see, there are limitation to ATRAC, just like LSD.

 

[Bluto]

Bluto, what car do you have now?? BMW X5??

Anyway, i was using % for torque figures to keep things simple.

Yes, ATRAC does have a limit...that is why i said that ATRAC will not achieve 50/50 split in the real world.

ATRAC is limited by the ABS system. So, that is the limiting factor.

Think of it this way...Hummer H2 can go up a 10% grade incline with one wheel traction. It has ATRAC-like system. Toyota has not published any figures, but i assume that it is probably just as good as Hummer H2's system. With all wheels having traction, a Hummer H2 can go up a 60% grade (compare to 10% with just one wheel). So, as you can see, there are limitation to ATRAC, just like LSD.

I have an Acura CL-S.

My physics is a bit rusty, but with the Hummer weight & specs, you probably could calculate torque at one wheel needed to move it up a 10% grade. Maybe even the PSI needed at a caliper.

My car has problems getting up my driveway in snow. The system modulates the brakes, but not with enough force to stop the '1 wheel peel' of an open diff. I usually had to walk up the driveway, throw some salt down, and come back later. My brother's 03 4WD Tundra drove up with no problems. I'm sure a 4Runner would do the same.

I was cross shopping with the Nissan Titan. It has an available Active Brake Limited Slip (ABLS) system. Same thing as ATRAC. (Also an optional rear locker but not available with the stability system.) On the Titan forums, owners swore up and down the ABLS system had no limitations and is miraculously better than an LSD. They just did not see the limitations. But Toyota's quality and my urban driving requirements make a 4Runner my only option. An 04 or 05 is another decision...


From what I've read on the subject, I would choose ATRAC on the front and rear axels, over a single rear LSD. Less maintainance, better traciton up front. Add some great tires, and it's all good.


I just wish manufactures would provided a bit more engineering specs on these systems. The only info available is of 'marketing quality', not for enthusiast.

 

[Hoomiester]

Thai

Good explanation and I now know how it works. But if you sit around pondering stuff like this I fear you need to find a good significant other or hobby! Too much time on your hands is not a good thing at times.

 

[Thai]

Bluto,

What's physics?? I am not smart enough to figure the exact figures...however, i don't think there is enough info to figure it out...only Toyota engineers would know it.

You Acura CLS system is a bit different. Your Acura has VSA (stability control) and traction control (over front tires). When slippage is encountered, these electronic nannies kick in. Part of it's effect is cutting your throttle (aka engine power)...so you bog down. That's the bad thing about these systems...they cut engine power to reduce wheel slippage, in addition to braking the individual wheel. So, this may explain why you cannot get up your snowy driveway. If possible, try to shut off VSA next time...you may have better luck. If Acura system is like Toyota's, then VSA and traction control are separate entity...and VSA is the part that dethrottles. So, by shutting off VSA, you won't bog down so much.

The above situation by Bluto's Acura is similar to the few complaints of Toyota's VSC function. Toyota's VSC cuts engine power when it senses slippage. That is why when you pull out onto a street from a gravel road, there's a hesitation (aka engine power being cut by VSC). Thus, when off-roading, you need to lock the center diff to shut off VSC/dethrottling function. This will leave ATRAC intact, but prevents your engine power from being cut.

 

[Thai]

I was cross shopping with the Nissan Titan. It has an available Active Brake Limited Slip (ABLS) system. Same thing as ATRAC. (Also an optional rear locker but not available with the stability system.) On the Titan forums, owners swore up and down the ABLS system had no limitations and is miraculously better than an LSD. They just did not see the limitations. But Toyota's quality and my urban driving requirements make a 4Runner my only option. An 04 or 05 is another decision...

Those Titan owners need to do more reading!

BTW, in theory, when you lock the rear axle (via locker), both wheels turn at the same speed...no speed differences between them. Thus, traction control would not turn on because ABS sensors only detect differences in wheel speed (and there is none with a locker).

Now, this only applies IF Nissan allows owners to turn off stability control (as with 4runner via locking center diff). Stability control (not traction control) and locker do not mix well because stability control will kick in whenever oversteer or understeer is sensed. So, even if both rear wheels are spinning at the same rate (via locker), brakes are still applied if oversteer or understeer occur...and this goes against what a locker is trying to do. Thus, locker will not work with stability control.

Based on this, i do not think that Nissan allows switching stability control off, which sucks because stability control will kill momentum in off-roading!

 

[Thai]

From what I've read on the subject, I would choose ATRAC on the front and rear axels, over a single rear LSD. Less maintainance, better traciton up front. Add some great tires, and it's all good.

Don't forget...LSD on the front axle is not a benign thing to most people. LSD up front will alter the steering feel and turning radius...in a bad way. It will add a lot of understeer and stiffer feel. Over time, this may lead to more wear & tear to the steering and front axle.

ATRAC is good because it is totally absent in daily driving...it is only there when you need it. It does not cause more wear & tear in daily driving. It adds no weight to the vehicle or axle. Simple and elegant solution to the open diff.

 

[Thai]

Is ATRAC on all the time?? No.

ATRAC shuts off at around 40 mph (this applies to Toyota, Land Rover, and Mercedes systems). Because beyond this speed, it's not needed. Momentum is the most important thing from that point on. This is where VSC plays an important role. VSC (stability control) functions to alter momentum to prevent oversteer or understeer.

 

[Thai]

Re: Thai

But if you sit around pondering stuff like this I fear you need to find a good significant other or hobby! Too much time on your hands is not a good thing at times.

Geez, you didn't have to tell everyone my life, ya know! I have "issues."

 

[Bluto]

Bluto,
You Acura CLS system is a bit different. Your Acura has VSA (stability control) and traction control (over front tires). When slippage is encountered, these electronic nannies kick in. Part of it's effect is cutting your throttle (aka engine power)...so you bog down. That's the bad thing about these systems...they cut engine power to reduce wheel slippage, in addition to braking the individual wheel. So, this may explain why you cannot get up your snowy driveway. If possible, try to shut off VSA next time...you may have better luck. If Acura system is like Toyota's, then VSA and traction control are separate entity...and VSA is the part that dethrottles. So, by shutting off VSA, you won't bog down so much.

The above situation by Bluto's Acura is similar to the few complaints of Toyota's VSC function. Toyota's VSC cuts engine power when it senses slippage. That is why when you pull out onto a street from a gravel road, there's a hesitation (aka engine power being cut by VSC). Thus, when off-roading, you need to lock the center diff to shut off VSC/dethrottling function. This will leave ATRAC intact, but prevents your engine power from being cut.

One button turns off both VSA & Traction.

My brother and I tried to experiment. We cleared the right half of the driveway. When VSA was OFF, I did not go far. VSA ON, I would go farther up the driveway, we could hear the left brake chatter. As the driveway slope gradually increased, my forward movement would stop.

Stopping was caused by one of two things:
lack or torque or lack of traction.

My brother noted the (right) wheel with traction was not slipping, therefore throttle was not being cut by traction control. The brake could simply not apply enough torque transfer to the wheel with traction.

His truck in 2WD with the LSD had no problems, but his tires are totally different. A Tundra w/ LSD vs. Titan with ABLS would be a better comparison.

 

[Bluto]

Those Titan owners need to do more reading!

BTW, in theory, when you lock the rear axle (via locker), both wheels turn at the same speed...no speed differences between them. Thus, traction control would not turn on because ABS sensors only detect differences in wheel speed (and there is none with a locker).

Now, this only applies IF Nissan allows owners to turn off stability control (as with 4runner via locking center diff). Stability control (not traction control) and locker do not mix well because stability control will kick in whenever oversteer or understeer is sensed. So, even if both rear wheels are spinning at the same rate (via locker), brakes are still applied if oversteer or understeer occur...and this goes against what a locker is trying to do. Thus, locker will not work with stability control.

Based on this, i do not think that Nissan allows switching stability control off, which sucks because stability control will kill momentum in off-roading!

The Titan has an button to turn off stability conrtol, VDC (Vehicle Dynamic Control).

But Nissan does not offer the VDC and rear E-locker together, a common compliant from owners. All Nissan would need to do is have the E-locker button automatically turn off the VDC.

 

[Thai]

The Titan has an button to turn off stability conrtol, VDC (Vehicle Dynamic Control).

But Nissan does not offer the VDC and rear E-locker together, a common compliant from owners. All Nissan would need to do is have the E-locker button automatically turn off the VDC.

Unfortunately, this is not always the case. In Mercedes, if you turn off ESP (stability control), then it will only increase the threshold before ESP kicks in. Therefore, it is never totally off. BMW stability system can be turned off completely. Toyota 4runner system can be turned off completely by locking center diff. So, Nissan's system may be similar to Mercedes.

Of course, turning off VDC for E-locker may make Nissan more susceptible to dumbass lawsuits by some idiot who ran his Titan over a cliff because he was testing the locker.

 

[Bluto]

Don't forget...LSD on the front axle is not a benign thing to most people. LSD up front will alter the steering feel and turning radius...in a bad way. It will add a lot of understeer and stiffer feel. Over time, this may lead to more wear & tear to the steering and front axle.

ATRAC is good because it is totally absent in daily driving...it is only there when you need it. It does not cause more wear & tear in daily driving. It adds no weight to the vehicle or axle. Simple and elegant solution to the open diff.

An LSD/HLSD up front on a FWD or AWD car is good for performance and handling. : The 6 speed CLS, Subaru STI, and Nissan Maxima have them.

Not on a truck though, that would be :awais:

 

[Thai]

An LSD/HLSD up front on a FWD or AWD car is good for performance and handling. : The 6 speed CLS, Subaru STI, and Nissan Maxima have them.

Yup, that's why BOTH of them have been criticized for understeering more than competitors.

 

[jj8111]

Thai I always enjoy your insights.


Does the Atrac system execute independently for each axle?

Does the Atrac system execute independently for each wheel?

If the front left wheel and rear right wheel are both slipping. Will it brake just those wheels?

Also lets say the front left wheel is slipping and both the rear wheels have traction. Will the system brake both wheels on the drivers side or just the one?

Last question...

What would adding a rear and/or front locker do to the system?

 

[Thai]

Thanks.

1. Power routing to each axle is handled by the center differential (Torsen). Front-rear power distribution is by Torsen.

2. ATRAC handles the side-to-side power transfer. So, yes, ATRAC independently controls each wheel, based on the action of the opposite wheel at the other end of the axle. Thus, all 4runners from 2001-2004 have 4-channel ABS system and ATRAC & VSC.

3 & 4. Yes, ATRAC will brake the front left wheel and rear right wheel but for different reason. If the left front wheel is in mud and the right front wheel is on dry land, then ATRAC will brake the left front wheel...it does not care what the rear wheels are doing. The same applies to the right rear wheel. ATRAC compares the right rear wheel to the left rear wheel...independent of what the front wheels are doing. Remember, ATRAC compares one wheel versus the other wheel at the other end of the SAME axle. It does not compare front axle to rear axle...that's the center diff's job.

5. Locker, in theory, should not affect ATRAC's job. However, it will conflict with VSC (stability control). However, in the 4runner, this is a non-issue because you can shut off VSC (by locking center diff). There have been a few owners who have added lockers to their 4runners with success.

Lockers lock the two wheels on the same axle together...thus, no speed differences between them. ATRAC works by detecting wheel speed differences (via ABS sensors)...so, if there is none, then it won't care.

 

[Bluto]

Yup, that's why BOTH of them have been criticized for understeering more than competitors.

Just about any FWD car understeers. Are there any that don't?

Subaru updated the 05 STI front to an HLSD in 05, Suretrac is gone.

Mitsu added an front HLSD to the EVO RS, I don't think it will add understeer or steering problems.

The Integra Type-R is another example of a great handling FWD car with an LSD.


(EDIT)

Got a little off topic. I'm not sure what I'm discussing or replying to at this point.... but I'll summarize.

Adding a front LSD to a FWD/AWD car improves performance and handling.

It would be detrimental for a truck or SUV off-road, with no benefit on pavement.