Why do ABS systems work so much better than traction control systems?
February 8, 2013 4:15 PM   Subscribe

Why do anti-lock brake systems [seem to] work so much better than traction control systems on modern sedans?

For many years I drove a car without what you might call electronic safety systems, by which I mean anti-lock brakes, traction control, and electronic stability control. Since 2010, I have been driving a 2010 Ford Fusion with all three of these systems. It seems to me that the anti-lock braking system works much better than the traction control system does, and I am wondering why this might be.

To clarify, by the "traction control system," I mean the system which tries to keep you from losing traction (acceleration) if you depress the accelerator pedal too far while driving in a straight line on a slippery surface.

When it is safe to do so, such as on an empty road or in an empty parking lot under appropriate conditions, I like to "experiment" with my car's performance in slippery conditions. I do this because I am curious about the performance and limits of the modern brake/traction/stability systems, and because I think it is important to know how one's car handles at the edges (or outside) of control in case I encounter that type of situation by mistake or in an emergency while driving.

By "experiment," I mean doing such things as:

a) Increasing brake pedal pressure on a slippery surface to see at what point the ABS kicks in and how quickly the car can stop once it does.

b) Increasing accelerator pedal pressure to see when the wheels start to slip and the traction control kicks in and how quickly the car can continue accelerating once it does.

c) Other things involving steering in conjunction with braking and/or accelerating which would normally send the car out of control, in order to see what type of order the ESC system is able to maintain.

I have been very impressed with the car's ABS system whenever I experiment with it. In every case I have tried, the ABS has been able to stop the car more quickly than I would be able to if I back off (and then modulate pressure on) the brake pedal once the ABS kicks in.

On the other hand, it seems like I can often make the car accelerate faster (in slippery conditions) if I back off and then finesse the accelerator pedal than if I simply hold the accelerator pedal at a position past where traction control kicks in and let the traction control system do its job.

[I have been *extremely* impressed with what the stability control system is able to do, even under the most extreme/unwise sets of control inputs, but that's not germane to this question.]

So, my questions are this:

1) If you drive a front-wheel-drive sedan with ABS and traction control and have experimented with or used the systems, do you agree that the ABS works substantially better than the traction control?

2) Why do you think this is? I can come up with some ideas off the top of my head (e.g., braking is frequently more safety-critical than acceleration, so they put more work/money/etc into it), but I would be very interested to hear actual engineering, mechanical, scientific, or otherwise fact-based explanations of why this might be.

Thanks in advance and, as always, I look forward to reading your input.
posted by Juffo-Wup to Technology (15 answers total)
 
ABS isn't about stopping quickly- it's about stopping while maintaining the ability to steer. In fact, if you didn't have ABS, you'd stop in a shorter distance almost every single time.

Similarly, traction control isn't (just) about acceleration, it's about maintaining grip on the road. The whole point of traction control is that it stops your wheels from over-spinning, and thus it is probably reducing your acceleration. If you were to do these same tests while also maneuvering through a slalom course, you'd see the difference in ABS and traction control.
posted by thewumpusisdead at 4:29 PM on February 8, 2013


thewumpusisdead:

Those are good points as to the purpose of the systems. Since I don't have the ability to (easily) disable the ABS and traction control systems on my car, I have only been able to compare their relative abilities when accelerating or decelerating in a straight line.

And, in that case, it seems like I can "beat" the performance of the traction control almost every time (by backing off on the accelerator and using it carefully), while it seems like I can pretty much never "beat" the performance of the ABS (by backing off on the brake and using it carefully), and that is what intrigues me.

[End of threadsitting, I promise!]
posted by Juffo-Wup at 4:45 PM on February 8, 2013


Most traction control systems use the same electro-hydraulic brake actuators as ABS systems. If you're accelerating and encounter a traction problem, your car will brake the wheels spinning faster than others, and may even reduce engine power by cutting spark to your engine or reducing fuel supply. This will in effect slow you down so you do not lose traction. When trying to accelerate as fast as possible, you want to be on the limit of traction, without going over. Going over and letting traction control systems take over keeps you going, but going over and losing traction may cause a crash.
posted by dobi at 4:53 PM on February 8, 2013


In fact, if you didn't have ABS, you'd stop in a shorter distance almost every single time.

I seriously doubt this, even for a highly skilled driver at the top of his game. Fact is, for my racecar I have chosen to run the ABS rather than rip it out. Theoretically it is possible, but in the real world, with bumps and other imperfections on the road surface, a good four-channel ABS will out-perform a good human 99.5% of the time. The only exception that I am aware of is on dirt surfaces where you use the locked wheel(s) to dig in in order to brake effectively.

I don't know the answer to your question, but I think that part of the answer may be in the difference between the instantaneous modulation of brake line pressure of the ABS, and the more indirect techniques of the TC.

On preview, it is normally possible to disable these systems by pulling the right fuses, though some cars have switches to disable them.
posted by GeeEmm at 4:55 PM on February 8, 2013


I'm just guessing here, but there is obviously a difference in application time between letting the brakes off when the wheel locks up and slides, and cutting off the gas when the wheel loses traction to let the engine slow down. The first is much more instantaneous than the other. Then re-applying the brake when the wheel starts to roll again is again a quicker procedure than giving the engine gas again when the wheel starts to grip. It takes more time for the gas to feed the engine and work that energy through the transmission to the wheels. So ABS gains on both sides of the procedure compared to traction control. Both work on the principle that starting friction is greater than sliding friction, i.e., it takes more force to start pushing your desk across the floor than it does to keep it moving it once it is sliding. So both ABS and traction control work to keep the wheels just below the threshold of sliding.
posted by weapons-grade pandemonium at 5:22 PM on February 8, 2013


In fact, if you didn't have ABS, you'd stop in a shorter distance almost every single time.

No. You may get this impression from poor/shoddy ABS systems (they're out there - every time you see a dotted skid mark on the freeway), but there is no way a human driver using one pedal can match, let alone outperform a proper ABS that is maximizing the braking force in all four wheels individually.

As regards Traction Control, In addition to the reasons mentioned, on my car the traction control is slow by design - in the previous model of the car, drivers complained that that the traction control was too effective and they hated that. People like to be able to use their engines, give it some gas and "let the back out to play" from time to time. You can't do that unless the TC lets you. So the TC is designed to let you.
This wouldn't make sense in a minivan, but it can be a design factor in cars the more they are aimed to appeal as a pleasurable driving experience.
posted by anonymisc at 5:29 PM on February 8, 2013 [1 favorite]


(Also, I don't think ABS was originally about maintaining the ability to steer while stopping - though this is a fantastic feature. It was about stopping much faster. It came about as a way to save untrained drivers (ie almost everyone) in emergency stops, because the normal human panic reaction is to hit the brakes as hard as possible, which locks the wheels, which breaks your traction, and then you can't stop in time. ABS means that a panicked driver stomping the brake pedal will still come to a rapid stop despite their error.)
posted by anonymisc at 5:46 PM on February 8, 2013 [1 favorite]


anonymisc, given that antilock brakes first debuted on aircraft, you are probably right that they were more about stopping distance. However, given the amount of engineering required to ship them on a mass-market automobile, I find it impossible to imagine that the ability to steer while stopping wasn't part and parcel of the earliest automotive ABS offerings.
posted by Good Brain at 7:10 PM on February 8, 2013


In fact, if you didn't have ABS, you'd stop in a shorter distance almost every single time.

I seriously doubt this, even for a highly skilled driver at the top of his game. Fact is, for my racecar I have chosen to run the ABS rather than rip it out. Theoretically it is possible, but in the real world, with bumps and other imperfections on the road surface, a good four-channel ABS will out-perform a good human 99.5% of the time.


Yes, exactly. Perhaps some of the older systems were heavy handed and slow, but modern systems are way faster and smarter than people. ABS is about maintaining braking power on a slippery surface. If the wheels lock, you lose both stopping distance and steering ability. I don't think one is more important than the other, they are just effects of a system that makes braking safer. It's just physics- skidding wheels have less friction than wheels that are maintaining contact with the road. And despite the kvetching of "expert drivers", ABS only kicks in when wheel spin is detected. So if they are as skilled as they claim, ABS will never be an issue for them.

As for the traction control, I think it is two fold. One, it is just not as important as ABS, so it likely isn't as finely tuned. Maybe in a $250,000 supercar the traction control will be like magic, but not in a normal car. Two, modern traction control simply takes over the throttle from you. It ignores what you are telling it and reduces RPMs until wheelspin stops. Then it gradually reapplies throttle. That's not nearly as fast of a process as ABS. (The older methods of pulling fuel or spark are no longer done now that throttle by wire is common.)

And yes, I hate traction control too. I love me some ABS and the electronic stability control in newer vehicles is amazing. But traction control just feels wrong. The first time I drove my new car in the snow, I was enraged at how badly it was driving. It wasn't listening to what I was telling it! Go, godammit! I know the wheels are spinning, there is 6 inches of snow on the ground! Let 'em spin or we aren't going to move! Then I turned off traction control and it started acting right and my blood pressure went back down.
posted by gjc at 8:07 PM on February 8, 2013


thewumpusisdead: " In fact, if you didn't have ABS, you'd stop in a shorter distance almost every single time.
"

Uh, no. I'm not aware of any surface/material combination for which the coefficient of static friction (what you're using if you're still rolling, i.e. with ABS to keep you from skidding) is less than the coefficient of sliding friction (skidding).
posted by notsnot at 9:19 PM on February 8, 2013


I would think marketing. Put it this way: you've got a car with at least 175hp. What are you going to think if you can't spin its wheels, even for a little bit? You might appreciate the improved actual acceleration, you might switch off the TC, or you might think to yourself "underpowered POS" and check out the competition. I would imagine that the fight between engineering and marketing must be interesting nowadays. On the one side you want something that's comfortable and refined, on the other you want your modern-day exoskeleton to allow you to hulk up, and shake, rattle and roar - and skid your wheels ... at the lights, that just turned green ... next to the guy in the Mazda :)
posted by labberdasher at 10:29 PM on February 8, 2013


I love me some ABS and the electronic stability control in newer vehicles is amazing. But traction control just feels wrong.

OK, so what's the difference there? Power delivery to individual wheels/axles is adjusted separately? Seems like that would require AWD/4WD, no? Or is "stability" control electronically dampened steering?
posted by snuffleupagus at 8:18 AM on February 9, 2013


ABS is certainly about, at least in part, maintaining steering while under heavy braking. When ABS was first making it's way into mainstream cars GM had a travelling demonstration with a dozen or so pairs of cars (one with ABS and the other with ABS disabled) for people to test drive. They'd set up a cone course where you'd accelerate towards a set of cones and then a light system would tell you to brake and which direction to steer around the cones. The course was set up on an asphalt parking lot which water sprayed down with water.

It was very informative. The ABS equipped cars could avoid the cones under heavy braking, the non ABS cars would plow through.
posted by Mitheral at 8:42 AM on February 9, 2013


I could be wrong but A lot of auto sites state traction control n most cares is not anything that a driver cannot do manually.

I have a 2004 corolla that has abs but not traction control. Since my car is a loley 4 cylinder compact I feel every bump in the road. I can feel when my wheels are losing traction before I skid . I think traction control can help in 2wd cars for people who do not pay attention to their cars but for people like me who feel every bump and know whats going on from even small differences I think its not a big thing.

The abs in my corolla is not heavy handed. Usually only kicks in if I slam on my brakes real hard or in the snow.
posted by majortom1981 at 1:17 PM on February 9, 2013


OK, so what's the difference there? Power delivery to individual wheels/axles is adjusted separately? Seems like that would require AWD/4WD, no? Or is "stability" control electronically dampened steering?

You could think of stability control as an additional steering mechanism. It's a little bit like the differential-drive that tanks use, except a tank pivots around one side (on which the caterpillar track is slower) by sending more power to the caterpillar track on the other side, and that speed difference turns the tank. While a car does it by using the same individual-per-wheel braking system as the ABS to increase drag on one side, and the car pivots around the wheel that is dragging. It's fantastic, like you're some kind of drift king (plus saving lives and stuff)

Since it's using the same infrastructure as the ABS, it's not surprising that it's similarly seamless.
posted by anonymisc at 4:58 PM on February 10, 2013 [1 favorite]


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