It’s only my experience, misguided or not, but we have a short but steep hill to go down from our house with a stop sign at the bottom. The cross street is fairly well traveled so you want to make sure you stop. The hill gets pretty icy and I have found I have more control either stopping or evading without ABS than with it. Of course I don’t have anything without it anymore but it is not a good feeling to simply sit back with the ABS going off hoping the car will stop in time. Even creeping down the hill doesn’t help when the last 20 feet are glare ice. Otherwise it is a rare event when the ABS goes off, but I really would not prefer to have it on that hill.
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You’ll get no argument from me on that issue. I know I will get the shortest stopping distance if I don’t lock the wheels.
The ability to steer around what you are braking for in the first place. With wheels locked up you will not be able to steer properly, When I was a kid they would preach “pumping” the brakes to stop on snow and ice. ABS does that for you much better and faster than you could ever do it.
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ABS saved my but more then once on icy roads. Especially black ice.
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@mustangman, I can’t understand your graph. There is no scale on the Y-axis. I don’t which way is up. And the X-axis only shows the extreme limits, and no intermediate vales. Ye Gods, man, it could be logarithmic, and I would have no idea. Geeks need fully labeled graphs, my friend. Please see to it that in the future, you Feed the Geek.
The only time abs has kicked off for me was on snow/ic stops, did I like it, not really, could I have stopped sooner maybe, but I leave enough room for whatever, just not enough experience with it on a lock up the wheels freeway stop, but can understand your stopping distance was not what you were expexting due to refinements in technology. Sorry to hear the van is so badly mangled.
The Y axis labled coefficitn of friction, the linear plot of a non-dimensionalized value that is defined as tractive force (tire normal force / tire braking force), the X axis is linear in percent slip. Percent slip is the actual travel of the car / the linear travel of the tires. I.e. the tire is not rotating as much as it should be to cover the distance the car actually traveled (murky, huh?).
The absolute numbers matter little, it is the shape that tells the story. Skidding tires make less brake force than still-rolling tires. Maximum braking comes at 6, 8, 10, 12, or 14% slip depending on the tire and the coefficient of friction of the surface. That is the key take-away. A rolling tire can brake harder than a skidding tire. That is how ABS can stop faster than a driver. Each tire can be braked at the peak force point no matter what the conditions.
Sorry, so used to seeing this plot, I forget it can be murky to others. Even when you understand the axis, it is still heard to grasp the concept.
I beg to disagree. By altering the Y axis, I can make a four degree F temperature variation over the course of a day look like an 80 degree variation. It’s a common trick with statisticians trying to make data look like something it isn’t. It’s also a common trick with politicians.
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Huh? There is no temperature component in the graph, only coefficient of friction in the Y axis. That value cannot be zero and the graph shows that. If you alter the scale, the shape stays the same although the relative peak-to-sliding appears closer together, the conclusion is the same - max braking effort is with a rolling tire, not a sliding one.
And. . . with the inception of ABS on dang near everything. . . crash scenes no longer have any tell-tale signs called . . SKID marks. Cars don’t skid ? no skid marks !
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What I have seen are tiretracks I the snow that went around the accident instead of plowing in.
Yes, they used to use the skid marks as a factor in determining speed prior to impact. I asked a cop that used to do accident reconstruction about it and at the time he had no idea…
I think the prevalence of dashboard cameras in people’s cars is making measuring skid marks obsolete. Most of the major intersections where I live have cameras.
How do they derive speed from a camera with reasonable accuracy?
Unfortunately, not all accidents occur at monitored intersections in cities…and nobody I know has a dashboard camera (yet).
If the distance between two objects is known, then the time it to go from one object to the next will give you speed.
Some dashcams also record speed readouts based on GPS.
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I was just having fun, but @“the same Mountainbike” has a point. I assume that the coefficient of friction is a base ten number. It could be massaged by using a log plot to reduce the effect. It depends on what the user is trying to show. I believe you were trying to illustrate the effect of tire slip on the coefficient of friction without any hidden motive.
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No skid marks on the road, anyway.
Pretty easy, actually. The camera has a frame rate. Say 24 frames per second. It doesn’t vary so each single picture frame is a defined moment in time. The car its taking the pictures of has length that doesn’t change so that provides a distance reference. Time and distance give you velocity. Done.
I had the same thought for accident reconstruction but then I remembered the little black boxes everyone has in their cars. No need to measure the skid marks anymore. All the speed and braking information is stored right in the box for big brother to look at. So if you screw up, don’t lie. Just be silent and let your solicitor do the talking.
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“prevalence of dashboard cameras in people’s cars . . .”
I know that in Russia, that statement would be accurate
But not in my neck of the woods . . . I live in Los Angeles and see plenty of new, or nearly new, cars and trucks
Makes me wonder where you live, and why your neighbors think such dashboard cameras are necessary
I know in Russia, they are common, because many people get into accidents and drive away without stopping. So the cameras are used as a kind of “witness” to the accident