How does ATRAC work??

[Thai]

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]

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]

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]

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]

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]

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]

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]

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]

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 +/- 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]

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.

 

[Thai]

Iowa,

Details, details...yeah, thanks for the physics lesson. Like i said, i hope that everyone else found my explanation to be adequate. :rolleye2:

Don't forget, when you're stuck in mud/snow, VSC will activate because you're sliding around, which will be interpreted by the computer as oversteer/understeer. Also, the effects of ATRAC can be interpreted by some as cutting engine power (even though it doesn't when center diff is locked). If you're gentle on the gas pedal, it will feel like you're not getting power to the ground. That's why you need to really press on the gas pedal to overcome ATRAC when going up a sandy hill (or getting out of a mud hole). Like posted above, when the rpms go past about 3000 rpms, ATRAC intervention decreases.

Now, i know for a FACT that ATRAC shuts off after about 40 mph in Land Rovers and Mercedes systems. I can't imagine that Toyota's system would be any different. Both Mercedes and LR started using ATRAC before Toyota did. I think that Mercedes designed the first system. It may also be that at high speed, brake pulsing a wheel may create even more dynamic instability, especially in situation where a wheel loses traction only a split second (ex: hitting an oil spot on the road). Thus, any braking situation at high speed is reserved to bail you out of trouble (e.g. oversteer and understeer).

From my experience off-road, ATRAC is NOT instantaneous in it's activation...it does require a little bit of wheel spin before coming on. So, there's a "reaction time." You can imagine what kind of problem this will be when one tire hit a small oil spot on the road...by the time ATRAC kicks in, your tire has regained traction...this can create a lot of problem (and lawsuits!).

Or, if you don't believe me, then READ up on Toyota's definition of VSC or any other manufacturer's system.

 

[Thai]

Here is an article on VSC's:

Vehicle Stability Control Systems
by Jim Kerr

First, there was anti-lock braking. Then came traction control systems. Stability control is the third step in electronic systems that help us control our vehicles. Almost all automobile manufacturers offer a stability control system on select models. Most of them are on cars, although trucks are quickly adding stability control to the option list.

While all systems are similar in concept, one system has important market significance. AdvanceTrac, the stability control option on the 2001 Ford Focus is the first system offered on an economy car, and it brings vehicle stability controls within the financial grasp of the majority of the buying public. AdvanceTrac is available on two of the Focus models, the economical and sporty ZX3 three-door, and the top of the line ZTS four-door sedan.

AdvanceTrac came from Ford's involvement with Formula One racing. Although active vehicle controls like AdvanceTrac are no longer used in F1 racing, their benefits are very real in everyday driving. Ford first offered AdvanceTrac in the Lincoln LS. This system, on the Focus, brings a new level of safety to small cars.

Stability control systems are especially beneficial on wet, snowy, or icy conditions, although they do offer handling benefits during emergency maneuvers on dry pavement. The design intent of stability control is to keep the vehicle going in the direction the driver is steering the car. To do this, the brakes are applied on one wheel to help steer the car in the correct direction. For example, if poor traction causes the front end of the car to slip sideways when you are going around a corner, the computer will apply the wheel brake on the inside of the corner causing the car to turn and slow down. If the back end of the car slips sideways, the brake on the outside of the corner is applied to bring the car back into line. The system works when the car starts to slide on a straight road the same as it does when turning corners.

Not all stability control systems feel the same. The difference is mainly in the computer programming. I find systems on Mercedes and Lexus cars react aggressively by reducing engine power and vehicle speed at the first instance of vehicle instability. Their emphasis is on always keeping the car in total control. Other systems, such as the Subaru Outback Vehicle Dynamic Control system, allow the vehicle to move around a bit for a more sporty feel, but as soon as the movement is too much, control is smoothly engaged. The Subaru system operation is my personal favourite. Most systems, including AdvanceTrac, fall somewhere between these two. Control is smooth and seamless.

Several inputs are required for stability control operation. The wheel speed sensors for the anti-lock brakes are used to detect wheel spin. A steering wheel position sensor is used to determine where the driver is wishing the car to go. A yaw rate sensor measures the speed at which the car is rotating around its centre, while a lateral acceleration sensor measures sideways force on the car. The AdvanceTrac computer monitors these sensors 150 times per second and can react within milliseconds to briefly apply individual brakes, retard ignition timing, and cut back on fuel delivery to keep the vehicle under control.

Stability control systems are a fantastic safety system. According to Johnny Unser, race car driver and performance driving consultant, "the yaw control is phenomenal in avoiding an accident by keeping the car pointing in the right direction, instead of getting out of control."

There is one caution about stability control. It can't overcome the laws of physics. Trying to corner at excessive speeds or driving beyond the limits of tire traction can cause the car to lose control. The stability system will help, but there is only so much it can do.

I have enjoyed driving many cars in the $35,000 and up price range that were equipped with stability control. The systems work even better than they sound. Now that stability control is available on a Ford Focus at under $22,000, I look forward to the safety benefits on even more cars.

 

[Thai]

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.

Larry Leadfoot is not very smart (and i am using kind words)! In your situation, no system in the world will save him. Remember, to RE-EMPHASIZE, VSC cannot alter physics...it will only work to an extent...praying will help the rest of the way!

 

[Iowa4Runner]

Larry Leadfoot is not very smart (and i am using kind words)! In your situation, no system in the world will save him. Remember, to RE-EMPHASIZE, VSC cannot alter physics...it will only work to an extent...praying will help the rest of the way!

Alter physics??? Don't be ridiculous. It's not like Larry is heading into an icy corner at 120 MPH. All that's required is some dethrottling and/or brake modulation to get the wheelspin under control. It'd be a piece of cake for today's vehicle control systems.

 

[Thai]

It'd be a piece of cake for today's vehicle control systems.

Only if your tires have traction....

 

[Iowa4Runner]

Only if your tires have traction....

No. The computer doesn't need traction to dethrottle the engine and modulate the brakes.

 

[Thai]

No. The computer doesn't need traction to dethrottle the engine and modulate the brakes.

All those actions ain't gonna work if you have no traction at the tires...in fact, you can even shut off the engine if you like!

On a more serious note, there's no need to further discuss this topic unless you have some additional info.

I hope that you found the above info helpful in reading up on VSC's.

 

[Iowa4Runner]

All those actions ain't gonna work if you have no traction at the tires...in fact, you can even shut off the engine if you like!

Ain't gonna work??? I've done it 100's of times with my right foot!!! If your wheels start to slip you simply reduce or close the throttle and your tires regain grip. The idea is to minimize the wheelspin before it leads to fishtailing and total loss of control. It's a very simple concept. Have you ever driven on icy roads?

 

[boston_kevin]

I think you guys are saying the same thing...

Iowa - Thai is simply stating that if the tires can't get a grip in the snow/ice, all of the stability systems in the world can't stop or control the vehicle. That being said, all systems are designed with the intent that there will be at least mininmal friction between the tires and the surface you are driving on (ice skating rinks, loose sand on pavement, oil slicks, all have near zero friction). To that end, I have seen a $70k land rover in Colorado hit a patch of snow covered black ice (about 3 years ago), and end up about 500 yards down the mountain... So much for VSC and TRAC on that car.

Thai - Iowa is restating what you are saying. I believe he is already assuming that there must be minimal friction, and the systems do in fact, work beautifully.

As far as systems reacting before any wheel slip occurs... Acura claims this with their MDX, but if you read the test results very carefully, wheel slip did occur and the system engaged within 1 millisecond, therefore the driver felt no slip whatsoever (and even with this system in Vail, many of the test drivers still ended up in the snow bank because of the laws of physics).

ATRAC and other related systems are phenomenol compared to what was offered before (lockers and previous gen's of TRAC), but it is not a perfect system. Don't think that will ever exist until we have flying cars, then who needs 4WD???

Thanks for the info Thai!! You are a wealth of knowledge.

 

[Iowa4Runner]

Re: I think you guys are saying the same thing...

Iowa - Thai is simply stating that if the tires can't get a grip in the snow/ice, all of the stability systems in the world can't stop or control the vehicle.

I know, and that's the problem. I'm not talking about stopping or cornering. I'm talking about the wheelspin that results from applying too much throttle in slippery conditions. Wheelspin doesn't mean you're out of control. It simply means the wheel RPM is too high for the speed you're traveling.

Over the last century, MILLIONS of slow reacting humans have kept their cars under control by simply letting off the gas when wheelspin occured. A modern vehicle control system could easily accomplish this.