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ABS Question (different, please look)

  1. May 10, 2005 #1


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    This is an almost typical question about Anti-Lock Brakes. I know that through college we've all heard that ABS works because static friction is greater than kinetic friction. It also helped us conceptualize the theory of rolling (as the wheel "pivoting" about the point touching the ground for the differential amount of time dt, or as a combination of rotation and translation).

    Anyways, I just have a hard time believing that is the case that makes ABS work. I have a different view on it, and I would like other's opinions. We all know that basically your car stops by turning mechanical energy into heat. We will take a car with disk brakes as an example. As you apply the brakes, the calipers lock the pads onto the rotor. The mechanical energy is turned into heat, and the car slows.

    I just fail to believe that a rolling wheel causes more friction than a wheel sliding across the pavement. Here are my two situations. First, non-ABS, and the wheels are locked up. You basically have only one source of friction, between the tire and the ground. It is sliding friction, with a rather high coefficient of friction. Because the wheel is locked up, there is no friction between the brake pad and the rotor.

    Situation two, a car equipped with ABS. Now you have a situation where friction exists in two cases. Not only do you have the rolling friction of the car on the pavement, but you now also have friction between the pad and the rotor. I believe what ABS does is not maximize the friction between the wheel and the ground, but maximize the friction between the brake pad and the rotor.

    The reason I believe people focus on the tires is because the friction between tires and the ground is higher than brake pads and rotor. Because of this, the tires will lock up sooner than the brake pad would on the rotor. However, again my point being, the big difference when you lock your tires up is not a loss of friction from the tire switching from static friction to kinetic, but rather a large loss of friction from the brake pad on the rotor.

    p.s. Sorry about the length and disarray of this post. The thought just came to me, and I just started typing without any reasonable flow.

    edit: I forgot the question. I guess my question is, what are you thoughts on this. I'm tired of hearing about the Static Friction > Kinetic Friction argument because I refuse to believe it unless someone can quantify it.
  2. jcsd
  3. May 10, 2005 #2


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    I'm not quite sure whether this is all relevant, but my thoughts are as follows:

    Yes there is. It's the friction between the brake pad and the rotor which has stopped the wheel from rotating in the first place.

    I don't see why you think that in the case without ABS there is no friction between the brake pads and the disc. This friction is present in both cases, the only difference with ABS is that the wheels are prevented (electronically, or hydraulically I believe) from locking up, so that the wheels can carry on rotating.

    I don't understand this bit. The moment your wheels lock is the moment when the frictional forces generated by the braking system exceed those generated between the tyres and the road. This imbalance can be caused by an excessive braking force (the driver slamming on the anchors) or by a change in road surface under braking (the car going from dry tarmac to black ice, for instance).

    In any case it should be noted that ABS is not designed purely to minimise stopping distances, but to maintain directional control and stability during an emergency stop. Most occasions when the wheels would otherwise lock occur on wet roads, and in these situations it is much more preferable to have the wheels rotating and displacing water (to maintain static friction between the tyre and road surface), rather than aquaplaning with locked wheels.
  4. May 10, 2005 #3


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    Forget ABS brakes for a moment. Have you ever tried to stop with normal brakes on a slippery surface, and had the car suddenly slide freely? It begins braking, but the moment the wheels lock, you lose all deceleration and just slide forward from your momentum.

    It is quite physically apparent in the seat of your pants that you are decelerating better when your wheels are engaged, but are hardly decelerating at all when your wheels lock.
  5. May 10, 2005 #4

    "The reason I believe people focus on the tires is because the friction between tires and the ground is higher than brake pads and rotor."

    No, the reason for focusing on the tires is because it's the friction between the tires and the road that stops the car. The friction between the pads and rotor doesn't have any direct effect on the cars speed. It only acts indirectly to control the friction between the tire and the road. Stopping power is maximized when the breaks slow the tire rotation as fast as possible without allowing any slipping between the tire and the road.
  6. May 10, 2005 #5


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    Like what has already been said...The ABS monitors the speed of each wheel at all times. You are correct in saying that the ABS maximizes the friction at the pads to the disc, but the end result is to eliminate lock up and thus maximize the friction between the tire (or tyre for Brewnog) and the road. All the ABS does is look at deceleration rates.

    I think if you think about a situation on wet or icy surfaces vs. a dry surface the argument for rolling/sliding friction might make more sense.
  7. May 10, 2005 #6


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    Two more things: while the coefficient of friction between the tires and ground may be higher, that is irrelevant - the friction force is quite obviously higher in the brakes, otherwise, the brakes wouldn't lock up. More simply, the dynamic friction force in the brakes is higher than static friction between the tires and the ground, causing the brakes to lock up.

    Also, while it may be possible that the theoretical friction coefficient of the tires is higher than that of the brakes, in reality when you approach that level of friction, the tires start to disintegrate (skid marks).
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