How does ATRAC work?? - Page 2 - Toyota

Bluto · 08-03-2004, 02:25 PM

Quote:

Originally posted by Thai
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 · 08-03-2004, 03:30 PM

Quote:

Originally posted by Bluto
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 · 08-03-2004, 03:42 PM

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 · 08-03-2004, 03:58 PM

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 · 08-03-2004, 04:37 PM

Quote:

Originally posted by Thai
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.

Thai · 08-03-2004, 05:30 PM

Yes, rally cars need LSDs everywhere due to the nature of the race (fast off-road race). However, for track purposes, '04 STI did not perform as well as the Evo (no LSD in front) in all published comparisons. Again, i have not seen any test on the STI with the new HLSD...so i can't comment.

LSD on front drivers will lead to more understeer AND torque steer...see the new Acura TL 6-spd reviews. See CLS 6-spd reviews. All pretty much say the same thing.

Why would LSD be bad for trucks off-road?????? I know plenty of Jeepers with Torsen in the front axle. It actually makes good sense to have LSD in the front axle for off-roaders...most also have locker in the rear axle.

bulldog-yota · 08-03-2004, 10:08 PM

My 2c.

Thai you forgot to mention that TRAC turns off above certain RPM as well to prevent damage to the drivetrain. In the case of the 4th Gen it is 3000RPM I think.

LSD on front drive does not limit the turning radius of the car, as it only kicks in under higher loads. SO driving slowly will have no effect. On a FWD car an LSD is used to get the power to the road, it actually limits the torque steer in a straight line as it prevents the wheel with the short drive shaft to "run away/spin" all the power, so what is felt is the "kick" of the LSD sending power to the other wheel. That is why AUdi has limited problems in a straight line as both front driveshaft are the same length. In turns it becomes problem when you power through them as the inside wheel can get more torque that it should, and that tends to push the car wider leading to the understear effect.

Back to offroad application of ATRAC. Given the fact that the V8 4th Gen sits with 320 lb/ft at the engine and in 1st gear low range you effective multiply that 33 fold through all the gearing (gearbox 1st x transfer x axle) you have plenty of torque availible to move you forward. So loosing 50% is not a big deal in my mind. If you have to do pure physics on a proper 4WD vehicle you will find they can climb a vertical wall with torque availible and weight of vechile, it is the traction of the tires on the surface that limit the ability of the vehicle. It is however the effect of first loosing traction on a wheel before it works that is a disadvantage to a locker. Although in the 4th Gen they seem to have done pretty good job of setting it up, as it seems to kick in pretty quickly to get over obstacles. Still not a locker, but people seem to be getting around in it. I wonder what the thresholds are they programmed into the system to kick in? DOes it use the fact that one wheel has zero or very limited speed or just pure speed difference between wheels?

AS I said my 2c, now to all of you with TRAC, go test it out!!! wheeling is much more fun than talking about it ;)

Thai · 08-03-2004, 11:42 PM

Quote:

Originally posted by bulldog-yota
My 2c.

Thai you forgot to mention that TRAC turns off above certain RPM as well to prevent damage to the drivetrain. In the case of the 4th Gen it is 3000RPM I think.

LSD on front drive does not limit the turning radius of the car, as it only kicks in under higher loads. SO driving slowly will have no effect. On a FWD car an LSD is used to get the power to the road, it actually limits the torque steer in a straight line as it prevents the wheel with the short drive shaft to "run away/spin" all the power, so what is felt is the "kick" of the LSD sending power to the other wheel. That is why AUdi has limited problems in a straight line as both front driveshaft are the same length. In turns it becomes problem when you power through them as the inside wheel can get more torque that it should, and that tends to push the car wider leading to the understear effect.

DOes it use the fact that one wheel has zero or very limited speed or just pure speed difference between wheels?

Yeah, ATRAC does shutoff or decrease it's intervention above 3000 rpms. This is good because it does allow you to power up a sandy/muddy hill without interference from ATRAC.

You're right about LSD and turning radius. It does not decrease it but it does make for higher steering effort. My mistake.

As for torque steer, LSD does help in some vehicle. However, in higher-powered cars (such as the new Acura TL 6-spd and current Acura CLS 6 spd), LSD actually worsens the torque steer. C&D noted it several times on the Acura TL. They prefer TL without the LSD.

I believe that ATRAC uses the ABS sensors to detect individual wheel speed and then compares with the wheel across the axle.

Bluto · 08-04-2004, 07:33 AM

Quote:

Originally posted by Thai

Why would LSD be bad for trucks off-road?????? I know plenty of Jeepers with Torsen in the front axle. It actually makes good sense to have LSD in the front axle for off-roaders...most also have locker in the rear axle.

I did not know that. Do they have a solid front axel? Or can that be done in an IFS? I've only seen front lockers, but my knowledge is limited.

Thai · 08-04-2004, 07:59 AM

Quote:

Originally posted by Bluto
I did not know that. Do they have a solid front axel? Or can that be done in an IFS? I've only seen front lockers, but my knowledge is limited.

Yeah, most have solid axle in front. However, if your front differential (on IFS or solid axle) is strong enough, then LSD can be placed anywhere...just see Subaru STi. Of course, if you buy the 4runner, then you won't need to consider this option at all.

Iowa4Runner · 08-08-2004, 01:08 PM

Quote:

Originally posted by Thai

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.

How are oversteer and understeer related to momentum?

Doesn't VSC mostly try to control wheel slippage? There are circumstances where VSC can do it's job perfectly without changing the vehicle's momentum.

Thai · 08-08-2004, 04:24 PM

Quote:

Originally posted by Iowa4Runner
How are oversteer and understeer related to momentum?

Doesn't VSC mostly try to control wheel slippage? There are circumstances where VSC can do it's job perfectly without changing the vehicle's momentum.

VSC functions to correct oversteer and understeer. Let's take an example...you're taking an exit from a highway too fast in light rain. Your steering wheel will be turned to take the ramp off the highway. However, because of some slippage (and going too fast), your vehicle just want to go straight (despite your steering angle). VSC compares your steering wheel angle, momentum (via g-force sensor), slippage at wheel(s) (via ABS sensors), and a few other things beyond my knowledge. From all this info, it can detect your understeer (aka car plowing straight instead of turning to go down the ramp). VSC then responds by pulsing the inside brakes on the front and rear wheels to hopefully correct the understeer (producing controlled oversteer). VSC may also cut your engine power (dethrottling) to help better control the slide. Thus, your car will take the ramp safely.

Sometimes, this whole thing may take place BEFORE you even realize the event. VSC functions quickly. The only time that you will know that it's working is the VSC light in the dash.

So, as you can see, VSC funtions to keep you on the intended path. VSC usually does not function if you're going straight (no oversteer or understeer).

Iowa4Runner · 08-08-2004, 05:34 PM

OK so the VSC looks at steering angle, yaw rate, acceleration (g force), and wheel speed to apply left/right differential braking when needed. That's pretty cool. My question is what's all this got to do with momentum? Earlier you said that momentum is the most important thing past 40 MPH and that the VSC alters momentum. I'm not seeing the connection.

As for straight line conditions the VSC will dethrottle the engine if it detects wheel slippage, right?

Thai · 08-08-2004, 06:28 PM

Quote:

Originally posted by Iowa4Runner
OK so the VSC looks at steering angle, yaw rate, acceleration (g force), and wheel speed to apply left/right differential braking when needed. That's pretty cool. My question is what's all this got to do with momentum? Earlier you said that momentum is the most important thing past 40 MPH and that the VSC alters momentum. I'm not seeing the connection.

As for straight line conditions the VSC will dethrottle the engine if it detects wheel slippage, right?

If you're going on a straight path and one wheel slips, then is it going to matter whether ATRAC brakes that wheel?? Probably not (assuming that your 4runner has not lost control yet); your momentum is carrying you straight. Therefore, ATRAC does not do much at high speed.

Now, if you are slipping (momentum carrying you straight), and you turn your steering wheel to the right (or left), then VSC will come into effect because the g-sensor is sensing you going straight (momentum), but your steering wheel angle is to the right. So, VSC will act (via braking individual wheel[s] +/- dethrottling) to make your 4runner go in the direction of the steering wheel (aka intended path)...aka correcting understeer. Thus, momemtum is altered.

Disclaimer: i may be using "momentum" in a wrong way...i suck at physics!

Anyway, i hope that you know what i mean.

Iowa4Runner · 08-08-2004, 08:20 PM

If Toyota's engineers have more than 3 brain cells between them (and I'd imagine they do) then something will try to keep wheelspin under control even if you're not experiencing understeer / oversteer. If Larry Leadfoot is cruising along an icy road at 40MPH and starts to spin the tires, the systems should get control right away instead of waiting until the vehicle starts fishtailing all over the place. I seem to remember people complaining about the VSC dethrottling the engine when they were stuck in the snow, where understeer / oversteer was not a factor. I haven't had the chance to drive my 4Runner in the snow yet, but I'll be pretty shocked if it allows me to spin the tires all I want when I'm not in an understeer / oversteer situation.

As for momentum (Mass x Speed) it sounds like you're speaking of a vehicle's resistance to a change in speed or direction. I think inertia would best fit the bill if that's your intent. As for what's being altered it's not inertia, and momentum really shouldn't be mentioned unless you want to make a roundabout reference to speed. What's being altered is the vehicle's yawing motion and it's speed in the X and Y directions.

08-03-2004, 03:58 PM	#19 (permalink)
Thai Snowflake, Guns, & Texas! Join Date: Nov 2002 Location: Texas Posts: 11,905	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. __________________ 2010 Toyota Prius V with ATP (daily driver) 2008 Toyota Sienna Salsa Red Pearl 2wd Limited (urban family "land cruiser") 2009 Acura TSX Palladium (wife's car) Off-roading Videos of My G I AM FAMOUS!! "Everyday is Halloween on the Internet"

08-03-2004, 05:30 PM	#21 (permalink)
Thai Snowflake, Guns, & Texas! Join Date: Nov 2002 Location: Texas Posts: 11,905	Yes, rally cars need LSDs everywhere due to the nature of the race (fast off-road race). However, for track purposes, '04 STI did not perform as well as the Evo (no LSD in front) in all published comparisons. Again, i have not seen any test on the STI with the new HLSD...so i can't comment. LSD on front drivers will lead to more understeer AND torque steer...see the new Acura TL 6-spd reviews. See CLS 6-spd reviews. All pretty much say the same thing. Why would LSD be bad for trucks off-road?????? I know plenty of Jeepers with Torsen in the front axle. It actually makes good sense to have LSD in the front axle for off-roaders...most also have locker in the rear axle. __________________ 2010 Toyota Prius V with ATP (daily driver) 2008 Toyota Sienna Salsa Red Pearl 2wd Limited (urban family "land cruiser") 2009 Acura TSX Palladium (wife's car) Off-roading Videos of My G I AM FAMOUS!! "Everyday is Halloween on the Internet"

08-03-2004, 10:08 PM	#22 (permalink)
bulldog-yota Senior Member Join Date: Mar 2004 Location: Morgan Hill, CA Posts: 1,013	My 2c. Thai you forgot to mention that TRAC turns off above certain RPM as well to prevent damage to the drivetrain. In the case of the 4th Gen it is 3000RPM I think. LSD on front drive does not limit the turning radius of the car, as it only kicks in under higher loads. SO driving slowly will have no effect. On a FWD car an LSD is used to get the power to the road, it actually limits the torque steer in a straight line as it prevents the wheel with the short drive shaft to "run away/spin" all the power, so what is felt is the "kick" of the LSD sending power to the other wheel. That is why AUdi has limited problems in a straight line as both front driveshaft are the same length. In turns it becomes problem when you power through them as the inside wheel can get more torque that it should, and that tends to push the car wider leading to the understear effect. Back to offroad application of ATRAC. Given the fact that the V8 4th Gen sits with 320 lb/ft at the engine and in 1st gear low range you effective multiply that 33 fold through all the gearing (gearbox 1st x transfer x axle) you have plenty of torque availible to move you forward. So loosing 50% is not a big deal in my mind. If you have to do pure physics on a proper 4WD vehicle you will find they can climb a vertical wall with torque availible and weight of vechile, it is the traction of the tires on the surface that limit the ability of the vehicle. It is however the effect of first loosing traction on a wheel before it works that is a disadvantage to a locker. Although in the 4th Gen they seem to have done pretty good job of setting it up, as it seems to kick in pretty quickly to get over obstacles. Still not a locker, but people seem to be getting around in it. I wonder what the thresholds are they programmed into the system to kick in? DOes it use the fact that one wheel has zero or very limited speed or just pure speed difference between wheels? AS I said my 2c, now to all of you with TRAC, go test it out!!! wheeling is much more fun than talking about it ;) __________________ --------------------------------------------- 03 4R 05 GX www.toyota120.com www.yotatech.com www.ih8mud.com Last edited by bulldog-yota; 08-03-2004 at 11:27 PM.

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08-03-2004, 03:42 PM	#18 (permalink)
jj8111 Senior Member Join Date: Jan 2004 Location: NY Posts: 128	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?

08-08-2004, 05:34 PM	#28 (permalink)
Iowa4Runner Registered User Join Date: Apr 2004 Posts: 465	OK so the VSC looks at steering angle, yaw rate, acceleration (g force), and wheel speed to apply left/right differential braking when needed. That's pretty cool. My question is what's all this got to do with momentum? Earlier you said that momentum is the most important thing past 40 MPH and that the VSC alters momentum. I'm not seeing the connection. As for straight line conditions the VSC will dethrottle the engine if it detects wheel slippage, right?

08-08-2004, 08:20 PM	#30 (permalink)
Iowa4Runner Registered User Join Date: Apr 2004 Posts: 465	If Toyota's engineers have more than 3 brain cells between them (and I'd imagine they do) then something will try to keep wheelspin under control even if you're not experiencing understeer / oversteer. If Larry Leadfoot is cruising along an icy road at 40MPH and starts to spin the tires, the systems should get control right away instead of waiting until the vehicle starts fishtailing all over the place. I seem to remember people complaining about the VSC dethrottling the engine when they were stuck in the snow, where understeer / oversteer was not a factor. I haven't had the chance to drive my 4Runner in the snow yet, but I'll be pretty shocked if it allows me to spin the tires all I want when I'm not in an understeer / oversteer situation. As for momentum (Mass x Speed) it sounds like you're speaking of a vehicle's resistance to a change in speed or direction. I think inertia would best fit the bill if that's your intent. As for what's being altered it's not inertia, and momentum really shouldn't be mentioned unless you want to make a roundabout reference to speed. What's being altered is the vehicle's yawing motion and it's speed in the X and Y directions.

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