Anti-Lock Brakes

ABS saved my butt when I was driving in the mountains and hit an icy curved bridged. As soon as I felt the car understeer, I knew I had hit a nice big patch of ice. I slammed on the brakes, and came up short of barrier in the oncoming lane.

 

It has worked well for me and thankfully the skids were not significant enough to create tire bald spots, which would have forced me to get new tires.

 

The brakes saved my hash coming in to work a few months ago. The car in front of me rear-ended another car and I had to slam on the brakes to avoid being next in the lineup. I went from 55 to 0 pretty quickly and stopped about a foot from the other car.

 

<div class='quotetop'>QUOTE(Locust43 @ Nov 15 2006, 02:32 AM) [snapback]349351[/snapback]</div>

The brake pedal on the Prius is attached to a spring loaded hydraulic system. The hydraulic system is there to make the brake feel like a normal hydraulic brake, but has no direct connection to the disk and drum brakes. There are two sensors on the brake pedal interface: the first senses the amount of pressure on the brake pedal through the hydraulic system; the second is attached directly to the pedal and is used to sense the rate of brake application, as in sensing the difference between a normal stop and a mash the brakes to the floor panic stop. The hydraulic pressure for the disks and drums is supplied by an electrical pump and accumulator. You will hear it cycle once in a while when you press the brakes. It makes a buzzing, rattling sort of noise for a few seconds. A bank of capacitors provide backup power for the brakes in case of an electrical failure.

As for your question about a wire breaking, that would be bad, but the chances are in the same order as a failure with normal brakes. If you have a complete brake failure, then you have to use the emergency (parking) brake.

Tom

 

On two or three occasions I have had to make a panic stop on ice. I was too flustered to remember now which dashboard icons might have appeared. But in every case the car stopped in time, and never swerved or skidded by any noticeable amount. The old '89 Civic (no ABS) would have skidded and slipped sideways.

The Prius with regular all-season tires does better on ice than the Civid did with studded winter tires.

And although you didn't specifically ask about VSC, that also has performed amazingly, preventing skids when taking an icy turn too fast. (On black ice, 5 mph can sometimes be too fast!) I strongly recommend getting a package that includes VSC. It is worth every penny.

 

Remember that the purpose of ABS is to retain steering control by not locking the wheels. ABS does not provide shorter stopping distances, because its brief releases of braking pressure allow slight rolling of the tires.

If you just slam the brakes, you might still ride right into the obstruction even with ABS. You have to actively steer around it, where possible, using your steering control to do so.

A car without ABS will simply skid all tires and cannot be steered. It may stop slightly shorter than an ABS-equipped vehicle, but it will be out of control during the skid.

 

<div class='quotetop'>QUOTE(Rick Auricchio @ Nov 15 2006, 03:35 PM) [snapback]349595[/snapback]</div>

It is my understanding that everything else being equal, the car with ABS will stop quicker than the car without ABS. I thought that was one of the points of ABS, the other of course as was mentioned that you can still steer the car even during a panic stop. The ABS allows the wheels to move ever so slightly during the stop to prevent the car from riding on liquified rubber, which is what leaves those neat black marks in the road and gives much less traction during the stop.

My experience in a panic stop was driving a '72 Pontiac Catalina and coming up on a school bus that stopped. I locked the brakes on and the car was sliding closer and closer, I was even looking at the kids in the back. The car stopped within inches of the bus, and I was extremely thankful. There was no ABS on that car.

 

<div class='quotetop'>QUOTE(Rick Auricchio @ Nov 15 2006, 11:35 AM) [snapback]349595[/snapback]</div>

Just a minor point here, but once a tire starts skidding the friction coefficient is lower than what it can generate when not skidding. In plain English that means that a car can stop in a shorter distance by not skidding than by skidding. A well designed ABS system will reduce the stopping distance for the "average" driver because the average driver is not skilled at threshold braking, (using the highest possible brake force without inducing a skid). The shortest possible stopping distance would be accomplished by threshold braking without ABS.

 

ABS will allow a vehicle to stop in a shorter distance:
1. On dry pavement.
2. On wet pavement.
3. On ice.
ABS will NOT allow shorter stops:
1. On gravel.
2. In snow.
The reason is the medium (gravel or snow) plowing up in front of a skidding tire will cause higher friction, so ABS is at a slight disadvantage in these two conditions.

As stated above, the advantage of ABS is that -while panic braking- you can still steer around things you -don't- want to hit. In fact, with ABS you don't think about braking, you think about steering. It's just hard for all us "old guys and gals" to adapt after having to think about both for so long.

It used to be ABS was designed to allow a slip of about 30% (wheels rotate about 70% of non-braking rate). This gave the highest friction or highest braking rate. Tires have changed since then (mid-80s), so the number may have changed slightly.

 

<div class='quotetop'>QUOTE(David Beale @ Nov 15 2006, 05:31 PM) [snapback]349765[/snapback]</div>

This is sometimes called Controlled Slip, and is used on train locomotives for maximum traction going uphill.

Classical physics shows us that the static coefficient of friction is higher than the dynamic coefficient, which is why locking the brakes makes for a longer stop. This seems to argue against a controlled slip, but that is because we are using an overly simple model. In the real world, there are very complex dynamics occurring with the surfaces, and the coefficient of friction actually increases as slip occurs, but only to a limiting point, and then the coefficient folds back to a lower level, which we call the dynamic coefficient of friction. Riding that maximum point will produce the best braking.

Tom

 

While a car with ABS will stop in a shorter distance than a car without ABS, it will still take longer to stop on ice than on dry pavement. While this may seem obvious to most of us, new drivers, especially in their first winter, need to understand that even with ABS, great caution is needed when driving on ice, or on roads that may have patches of ice.

 

<div class='quotetop'>QUOTE(daniel @ Nov 16 2006, 10:45 PM) [snapback]350619[/snapback]</div>

We see this with 4wd. People buy a 4wd SUV or truck and foolishly think it will stop better just because they have better traction for getting started. Actually it's not foolish; the 4wd fools you into thinking the roads are better than they are, and first tight corner or fast stop, and there you go...yeehaaa <"sleigh ride" playing in the background>

Tom

 

Thanks for the clarification on stopping distance; I always thought it was slightly longer, but it's good to know it's often slightly shorter.

The 4wd comment is so true! My first thought when I had the AWD Volvo SUV was for stopping, then I realized that AWD has nothing to do with stopping: it's only to get you moving!

 

I personally don't like the feel of the brakes. In sudden braking there seems to be a split second lag between brake application and actual braking. I don't know if this is because of the electronic braking system (some drive by wire systems have a similar throttle lag) or if its the brakes pushing through the regenerative brakes into the service brakes, but it is VERY unnerving to me. Maybe if you drove the Prius all the time you'd get used to it, but going back and forth its really a heart stopper at times.

 

<div class='quotetop'>QUOTE(Rick Auricchio @ Nov 15 2006, 02:35 PM) [snapback]349595[/snapback]</div>

This seems correct to me -- that ABS lengthens, not shortens the stopping distance on dry pavement. In my crazy youth, I did many intentional skid stops, as well as many unintentional ones (due to crazy driving). I know very well how quickly a car stops when you lock up the brakes.

Several months ago, I was behind a car getting on a fast highway. The overly cautions driver stopped, then went. I looked, went, then realized the car before me had stopped! I slammed on the Prius brakes. The pavement was clean and dry. I could tell it took longer to stop than if I had been able to lock up the tires.

It made me trust the Prius brakes less. I've had to stop fast on icy/snowy/slippery pavement many times in cars without ABS. I know how to pump the brakes to minimize stopping distance. Maybe a computer can do this faster than me, but the computer can't identify road conditions. I suppose I'm better off with ABS. But I still feel I'd rather have full control of the tires, rather than relinquishing it to a computer.

 

<div class='quotetop'>QUOTE(nyprius @ Nov 21 2006, 11:54 PM) [snapback]352925[/snapback]</div>

The benefit of ABS is not stopping distance - it's control. You may be able to pump the brakes without ABS, but you probably would have a hell of a time trying to steer the vehicle at the same time. With ABS, you can brake and steer at the same time.

Moreover, most people can't pump the brakes anyway in a crisis. You may know how to do so, but unless you have practiced it extensively, spinning out is a definite possibility.

ABS and VSC take control away from the driver. That's not necesarily a bad thing, considering that the driver is the most dangerous and unpredictable component of any vehicle.

 

After 97k miles (2001 Prius) I have twice used the brake pedal with enough gusto to engage ABS. What surprised me about it was the high rate at which the system was cycling. It felt a bit like driving over narrowly spaced grooves in the pavement, or actually 'gritty' like gravel.