Anti-Lock Brakes

<div class='quotetop'>QUOTE(bsoft @ Nov 22 2006, 03:23 AM) [snapback]352928[/snapback]</div>

Agreed, ABS gives you better control. But what if you're in a situation, like the one above, where you need the shortest possible stopping distance (ie: where there's no time and/or room to steer around the obstruction). I was pretty sure the stopping distance of my car was lengthened by ABS. This means you're more likely to run into the back of another car that stops abruptly on dry pavement.

Obviously there's a trade off. The thing that bothers me, is that when I stomp down on the brake pedal, I want the car to cooperate and lock up the wheels. If I don't want wheel lock up (to maintain control in a skid or turning situation), then I don't stomp on the brakes.

Overall, I guess I'm probably better off with ABS. But if I run into someone because the wheels didn't lock up, I'm not going to be too happy.

 

I remember reading the report saying that ABS lengthens stopping distance because drivers do not activate ABS aggressively enough to reduce stopping distance. Study also shows that if you stomp on the brake pedal hard enough and don't let go, ABS does provide shorter stopping distance (as physics predicts) in addition to retaining steering control. However, most drivers, due to lack of practice, suffer from longer stopping distance. Practice is called for. I do it sometimes on rainny days when no vehicles are around me to trigger ABS just to get a feel of it.

 

<div class='quotetop'>QUOTE(ceric @ Nov 22 2006, 02:47 PM) [snapback]353112[/snapback]</div>

Around these parts, we do this in snow. Much easier to get ABS to engage. Like you, I also "practice" just to feel it working. As mentioned, ABS requires constant and increasing pressure as it activates. I guarantee one can stop better in snow with ABS than without.

 

<div class='quotetop'>QUOTE(Fatfenders @ Nov 22 2006, 11:11 AM) [snapback]353126[/snapback]</div>

Not all ABS systems are created equal. I've driven a few cars (mostly SUVs) in the snow (hard pack conditions with an icy surface beneath the snow) where ABS definitely lengthened stopping distances. The way I could tell was to threshold brake, and then nail the brake pedal and activate the ABS. In some vehicles, there was a distinct decrease in deceleration (felt like I was letting off the brake) when the ABS activated, compared to the deceleration when I continued to threshold brake. I attribute this to the ABS system constantly locking and releasing the brake (due to the minimal traction, and the system's inability to apply the brakes lightly) rather than applying light steady pressure and avoiding wheel lockup completely.

I've not driven my Prius in the snow so I can't comment on how it's ABS performs. I'd expect most modern systems to outperform even the most skilled drivers with the possible exception of those cases where the driver is highly skilled and prepared, and the brake balance and individual tire's traction is such that there is no premature lockup. I've been pretty happy with how all ABS systems I've tried have performed in the dry - they stopped as well or better than I could, and do that regardless of driver preparation, road conditions, etc.

I agree that it's a good idea to find a safe spot where you can practice activating the ABS and get a feel for how the system performs in panic situations. That's a lot better than learning while you're trying to avoid an accident.

 

Back to the original question, I used ABS (and probably VSC) only once, but it saved my butt, earlier this month.

Due to idiots cutting in front of the car in front of me, coupled with morning sun in my eyes, piled upon the usual morning brainfarting, I was faced with a full panic stop to not rear-end the guy in front of me on the freeway.

At speeds of about 55mph I slammed the pedal, and soon realized that despite this I would NOT stop in time. I jerked the wheel left and right to change lanes, expecting the usual swerving and overcorrection problems.

Instead, I changed lanes with no more dramatics than if I was off the brake. I could hear the beginnings of skid noises, but very quietly, as it went to the brink of skidding but not quite past it.

I am sure that without ABS I would have lost steering control and without VSC I would have overcorrected, and perhaps still caused an accident. But with them, I merely drove on without a scratch.

I'm a believer!

 

I believe that is why many new cars (Toyota included) now has Brake Assist. The system in effect helps you push the brake pedal harder and faster when it senses an emergency stop.Thus if you hit the pedal quick and hard, it pushes it all the way to maximize the ABS.

I have experienced it twice now with the Prius and a number of times with the 4Runner. Actually works really well.

 

<div class='quotetop'>QUOTE(narf @ Nov 15 2006, 03:52 PM) [snapback]349726[/snapback]</div>

This is not what I've read in the major magazines when writing about ABS and tire traction. Unless something has changed recently, ABS will not shorten the stopping distance, it lengthens it. But, only slightly, and it allows you the ability to steer while panic-stopping.

I'm willing to be corrected here with some links to credible sources.

GeronimoPFudgemuffin

 

<div class='quotetop'>QUOTE(Tideland Prius @ Nov 22 2006, 01:15 PM) [snapback]353207[/snapback]</div>

Huh? What am I missing? If you slow down faster, your stopping distance is shortened.

Dave M.

 

<div class='quotetop'>QUOTE(dmckinstry @ Nov 23 2006, 09:57 AM) [snapback]353520[/snapback]</div>

He's saying that in loose snow, if you do not have ABS, the brakes will lock and the tires will skid. But the car then becomes a snow plow, pushing the snow rather than rolling over it, and that snowplow effect would slow the car faster and shorten the distance.

However, it would do this at the cost of complete loss of control. I'll stick with ABS, thank you very much.

 

<div class='quotetop'>QUOTE(ceric @ Nov 22 2006, 02:47 PM) [snapback]353112[/snapback]</div>

I've been weightlifting for over 30 years. I can dead lift over 400 lbs and leg press near 1000 lbs. If I stomped on the pedal any harder, I'd have put my foot through the floor.

Regarding physics, it seems to me that the maximum amount of friction against dry pavement will result in the shortest stopping distance. This would be produced by locking the wheels up. As others have said here, there's a trade off with ABS, as there are with many things. For a slightly longer stopping distance (due to cycling the brakes when cycling is not needed on dry pavement), you get more control.

I've done many hard controlled stops in my life. At a speed of about 20 mph on dry pavement, I could tell that the Prius took longer to stop when I stomped on the brakes, than if the tires had simply locked up.

 

No, in studies during development of ABS they found a 30% slip rate gave max braking (wheel turning 70% as fast as when brakes not applied). The reason is the tire heats up and the hotter rubber has less stick to the tire so comes off easier if the wheel is locked. When turning you constantly get a "cooled portion" of the tread to work with. Sorry, it's been so long I have no idea where the studies would be posted. Maybe check Bosch. They make or licence most of the ABS systems used these days.

There are two instances where ABS will result in a longer stopping distance - and both for the same reason.
1. Loose snow (packed snow can be considered ice). With ABS the snow will get under the wheel and act as "ball bearings" as the wheel will be turning over it. Without ABS the wheel grinds down through the snow to the hard surface, and, as stated above you also get added resistance from the "plow effect".
2. Loose gravel. Same effect as snow - gravel becomes "ball bearings" under the tire with ABS. Without ABS you grind down to the hard surface and also get the "plow effect" added resistance.

All studies I've seen indicate any other situation with ABS will result in equal or shorter stopping distances. It was found "professional drivers" including talented race car drivers can equal ABS stopping distances, but in doing so they loose some ability to control the vehicle. Notice that many race cars are now including ABS so the driver can concentrate on passing the thing blocking him, rather than concentrating on not locking up the wheels!

Also as stated above by other posters with ABS you concentrate on missing the object causing you to panic brake not the braking, and you can steer around it most of the time.

As for whether the Prius system is 3 or 4 channel, look at the ABS module. Does it have three or four lines coming out to the wheels? Other than checking the literature (manuals and brochures) that is the easiest way to tell. IF the Prius has an ABS module - the Prius brake system is different than any others I've seen, at least in the master cylinder. My guess is it's four channel to mate with VSC.

Back from looking at vehicle crash ratings - nhtsa rates brakes as "4 wheel ABS". Does this mean 4 channel ABS?

 

<div class='quotetop'>QUOTE(nyprius @ Nov 24 2006, 02:35 AM) [snapback]353657[/snapback]</div>

Sorry, but this is wrong. The highest coefficient of friction (which will slow you down the fastest) is when the tire is not sliding (actually just a tiny amount of slip may increase the grip, but we are talking about maybe 10 percent at most for most street tires. There's no way the average driver could maintain 10 percent slip in a panic situation, so in the real world on pavement "threshold braking", or holding the braking force just below the point of skidding will stop you far faster than skidding.

I know this is not the ultimate resource, but please read the How Stuff Works article on brakes. It's at :
How Stuff Works: Brakes

Note the box near the bottom of the page marked "Coefficients"

 

<div class='quotetop'>QUOTE(narf @ Nov 24 2006, 09:37 AM) [snapback]353717[/snapback]</div>

This is easily demonstrated on a vehicle where you can brake the front and rear wheel(s) independently. Part of the MSF (Motorcycle Safety Foundation) beginning riders course is to have students brake with only the rear wheel, first locking it, and then by threshold braking. The stopping distances with a locked tire were always much longer than those with threshold braking. Students would slide through the end of the braking lane, knocking down cones with locked brakes, but stop short of the end of the lane with threshold braking.

The fact that a sliding tire has a lower coefficient of friction is also the reason why a car which locks its rear wheels before the fronts will tend to swap ends under hard braking. Since the front (unlocked) tires have more grip than the (locked) rears, the rears will try to pass the fronts if they can. In the absence of ABS, a car which locks its front brakes before the rears is more stable and therefore safer.

I've got to wonder if any people who think locked brakes stop quicker actually have any experience with panic stops, because they're just completely wrong.

 

The misunderstanding of friction during braking is not hard to understand. On a NASCAR race the commentators (who should know better) agreed that when you locked the wheels on wet grass the car accellerated! Funny.

 

<div class='quotetop'>QUOTE(narf @ Nov 24 2006, 01:37 PM) [snapback]353717[/snapback]</div>

As usual, few things are black and white. It appears this and other posts are saying ABS stops the car faster on dry pavement at all speeds. This may be true at high speeds, but I don't think it's true at lower speeds, say 10-20 mph. I've done many hard stops in non-ABS cars. I am very familiar with how quickly a car will stop at slow speeds on dry pavement if you lock up the tires. A few months ago, I had to slam on the Prius brakes while going about 20 mph to aviod hitting someone that had stopped quickly in front of me. The car felt like it was not responding. I could tell the stopping distance was lengthened by not locking up the wheels. I could feel the wheels still turning when I wanted them to stop. At slower speeds, there will be little skidding if you lock up the tires. The car will simply stop. In emergency situations when you want the quickest stop a slow speeds, there isn't time for the ABS to be fooling around with cycling the brakes. There's no need to cycle at slow speeds.

I suppose I'm better off overall with ABS. However, I suspect it would be more effective if ABS was automatically de-activated at slower speeds.

 

<div class='quotetop'>QUOTE(nyprius @ Nov 26 2006, 09:20 PM) [snapback]354368[/snapback]</div>

In this case they are. You're confusing physics (static vs. sliding friction) with the performance of a particular implementation of ABS.

When you wrote:

<div class='quotetop'>QUOTE(nyprius @ Nov 24 2006, 02:35 AM)</div>

that's physics, and you're wrong, period.

How the Prius performs in a panic stop at low speeds, and whether the ABS produces a shorter stopping distance than locked brakes has much more to do with the performance of a particular implementation of ABS than it does with static and sliding coefficients of friction of tires on pavement. Also, as others have pointed out, maintaining steering control under hard braking, as you can with ABS, is probably more important than obtaining the shortest possible stopping distances.

 

<div class='quotetop'>QUOTE(David Beale @ Nov 15 2006, 04:31 PM) [snapback]349765[/snapback]</div>

But the average driver needs to understand that it is all relative. ABS will stop you faster on ice compared to locking your wheels on ice. Same with dry or wet pavement. ABS is not really intended to shorten the stopping distance, but rather, as you mention, to help you maintain control during hard braking/panic stops. With or without ABS, the stopping distance of the car on any given surface is the pretty much the same.

I was in an accident last summer I in a car that did not have antilock brakes. ABS probably would have reduced the impact a little, but not prevented the collision. I didn't have enough room/time to stop. Though I was decelerating very fast, the last fraction of a second was a "Hail Mary." I am an experienced enough driver to know that for the couple of car lengths, I was just going along for the ride - and trying to restore traction would have been a waste of time.

The accident was my fault (Ever notice how people in forums like this will always start their stories by describing how careful they were being, and it was always some other "jerk" that caused whatever happened). Though I was maintaining an adequate following distance, I got distracted by something another driver was doing behind me (passing me). I paid a fraction of a second too long keeping an eye on something in the rear view mirror and my following distance closed when traffic came to an abrubt stop in front of me. I was boxed in on both side by cars slowing down. The prefect storm for a rear ender, I guess.

The only thing that might have saved my Prius (had I been driving it at the time) is that it is lighter. Combined with the ABS and good tires, I might have stopped 20 feet sooner. MIGHT HAVE. Hard to say.