| noel |
| Had to make a very fast stop yesterday after a car in the next lane cut in front of me. To my surprise, the rear wheels locked up and tires let out a chirp as they do when this happens. This 2007 MDX is supposed to have ant-lock brakes so the lock up really surprised me. Has anyone else had this experience? |
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| G. COLTON |
The anti-lock feature on my 2004 worked perfectly just 3 days ago. Same situation as you. The car in the right lane pulled into the center lane and that car pulled into me. Was glad that my reaction time and brakes were both good.
G |
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| sigp228 |
They locked up? What makes you believe they locked up? Please I am not trying to be sarcastic or anything, i am just curious as there is not a alot of details about the incident you describe. Maybe this will help you.
ABS simply applies and releases the brakes many times per second (much faster than a human ever could). It bases this on the rotational speed of the wheels. It compares the speeds of all wheels relative to each other and when it senses lockup based on the sudden deceleration of the wheel, it releases pressure to that wheel. If it is a 4 channel system such as ours it can do each wheel independently and if it is a three channel system then it does the fronts independently and the rear as a pair (much less common these days). Threshold braking, if you are not familiar with it, is the technique to maximize your braking. You simply are applying the maximum amount of pressure to the brake pedal that will allow the wheels to rotate but not lock (ie right at the "threshold" line of locking up, hence the name). If a wheel locks, you simply release some pressure and when the wheel begins to rotate again, you apply the pressure again. Problem is that there is one brake pedal but 4 wheels so if one wheel locks, the others might not be near threshold but you have to release because of that one wheel. Plus. applying and releasing takes time (remember when you learned to "pump" the brakes"). ABS simply does the work for you. It can apply threshold braking independently at each wheel, release and apply pressure many many times per second, and constantly monitor the speeds of the wheels. Also, at threshold braking, the tires will usually screech as if they were locked because of the deformation and friction among many other variables being generated by the braking (shift in weight, surface traction, tread compound, etc). Also, because ABS pumps the brakes 15, 20 or even higher times per second, it can sometimes visually appear and/or sound like the wheels are locking. This is because your eyes and brain can't keep up with the speed of the apply and release the system is doing.
Now, another reason that I think that you may be mistaken is that, although you did not mention how fast you were going, at any sort of appreciable speed, the locking of the rear wheels only often tends to induce an oversteer condition (the rear of the car will swing around to the front ie spin out). If the front wheels lock up only a car tends simply continue on the same path it was going (known as understeer). Also, if there was a malfunction, I would think that your ABS light would be illuminated as well as the MID would indicate a system failure.
Since you did not mention how fast you were going, also be aware that most ABS systems actually do not operate at slow speeds, usually anything below 15 mph. The reason is that at that speed, the effect ABS would have is negligible and stopping distances are not really greater locked or unlocked. The car will simply stop. In fact, it might even be undesirable to have ABS working then. If you were on snow or dirt, locking the wheels can actually help you build a wedge of dirt under the tires to stop you whereas allowing the wheels to continue to rotate would increase your stopping distance. This is true on ice if you are going slow enough you might be able to stop if you are going less than 15mph and the wheels lock because they "dig' in and create a small wedge of ice under the wheels. Of course this is not going to happen at higher speeds or where inertia and gravity is too great (such as trying to stop on a downgrade) or you are driving on ice with summer tires or bald tires. If this does not make sense to you or does not seem to apply and you still believe that the ABS did not function as it should, then I suggest you bring it to the dealer ASAP. Again, I would think that there would be a warning light if the system was not functioning. Even if the ABS fuse blows out, I think there is redundancy built in to alert to a problem with the system. |
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| cvista |
Same happened to me yesterday.
I was trying to beat the yellow light, changed my mind in the last moment and decided to Brake.
Looked like the wheels did lock up. Didn't see ABS taking over at all. Atleast in my old car when ABS kicked in, I could feel it when the Break pedal started vibrating.
I believe I was doing 40 MPH and came to a dead stop in 10 - 15 meters meters.
Is there anything I'm missing ?? |
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| shootist |
A tire that chirps, rather than screeches, duing a panic stop is an indication that the ABS did kick in. ABS (or, in MDX terms "C-VSA") simply opens and closes one of the four brake fluid valves very quickly when it detects that one wheel is rotating a lot slower than the others during a hard stop. If ABS did not kick in, the tire or tires would have screeched long and hard as the wheel locked up.
MDX also has a very neat, and somewhat unusual feature called something like Emergency Braking Assist. If it detects that you stomped the brakes, or "intended" to stomp the brakes, it applies the brakes full force immediately and lets the ABS control the tires. THis was done because car makers learned that people still pump ABS brakes, which destroys the effectiveness of ABS. |
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| sigp228 |
quote:
Originally posted by cvista
Same happened to me yesterday.
I was trying to beat the yellow light, changed my mind in the last moment and decided to Brake.
Looked like the wheels did lock up. Didn't see ABS taking over at all. Atleast in my old car when ABS kicked in, I could feel it when the Break pedal started vibrating.
I believe I was doing 40 MPH and came to a dead stop in 10 - 15 meters meters.
Is there anything I'm missing ??
Nope. Most ABS systems these days do not pulse the pedal like they used to. You might feel it if you were concentrating on it but I suspect you were concentrating on whether you would make the light. |
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