Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Anti-lock brakes

  1. Jun 19, 2008 #1
    When mine kick in it's like i'm sliding on ice. I'm pulling the fuse. Is there something wrong with them or do they just work badly by design? 04 Dakota.
     
  2. jcsd
  3. Jun 19, 2008 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    That's a pickup truck right?
    And you never have any cargo in it.
    So you put on the brakes but because there is no weight on the rear wheels they immediately lockup as the back off the truck bounces into the air, and so the ABS instantly releases them.
    Basically there is no point in having rear brakes on an empty pickup truck - of course there is no point in driving an empty pickup truck but that's a different arguement.
     
  4. Jun 19, 2008 #3
    Rethink and come back again....
     
  5. Jun 19, 2008 #4
    You're taking too long. This is the deal. The truck is always full, not empty. The back wheels don't leave the ground, I don't think. Back brakes do nothing anyway so why even consider them? Thank you for the response. I'm serious, don't jump up with your hair on fire like that. By the way, for clarification sake, it's when the ABS is engaged that it feels like ice. Not when it turns off.........
     
    Last edited: Jun 19, 2008
  6. Jun 19, 2008 #5

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    Do you realise that this forum is not a public service, nobody is obliged to give you an answer, and rude comments such as those won't get you help any more quickly?
     
  7. Jun 19, 2008 #6
    You're right. I apologize.
     
  8. Jun 19, 2008 #7

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    ABS just stops the wheel locking up - it monitors the wheel rotation and releases the brakes slightly if the wheel stops moving. A sliding tire doesn't do as much to stop you as one which is gripping the road and still turning.
    It only works if there is enough weight on the wheel to keep it turning and there is some grip between the tire and the road. Even my 4wd subaru with ABS slipped backward on a hill covered in deep snow this winter.

    The problem with pickup trucks is that they are designed to have a heavy load in them to keep all four wheels weighted down on the road. They have very simple chassis and suspension compared to a car which lets wheels lift up. And they have twice as much mass.
    As soon as you brake the momentum pushes the front of the truck down and the back up, so the rear brakes are lifted off the road ( or at least unweighted) and the ABS can't apply much braking force without them locking up.
    So all the braking is being done by the front wheels, but now they have all the weight of the vehicle on them. If you try and steer at all, the wheel is now slightly sideways to to the direction it is being pushed and stops rotating as much and so the ABS releases the brakes.
    Without ABS in an unloaded pickup, all that happens is that you lock up the wheels and it bounces along the road.
     
  9. Jun 19, 2008 #8
    Thank you. Should I pull the fuse now or wait for an accident?
     
  10. Jun 19, 2008 #9

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    I wouldn't pull any fuses - you don't know what else they are disabling!

    I would recognise that if you are driving a large pickup truck you have to drive more like a professional truck driver. Leave a bigger gap in front of you, like twice the stopping distance of a car, check traffic and lights further ahead.
    Basicaly just think that driving 3tons of heavy iron gives you more capability than a minivan but it also gives you more responsibility to drive better.

    If you are often driving it empty you might have to keep some extra weight (bags of sand) over the back axle - especially in winter.

    Or you can buy a Subaru and rely on it's amazing brakes and accelaration to get you out of trouble!
     
  11. Jun 19, 2008 #10
    Try an experiment: Find a large, empty parking lot and perform some brake tests. Accelerate to, say, a steady 30 mph (50 kph) then nail the brakes at a marked spot. Measure your stopping distance with and without ABS enabled.
    Note: For ABS to work properly, you need to keep the brake pedal firmly depressed until the vehicle is stopped. If you try modulating/pumping the pedal, you defeat the ABS system.
     
  12. Jun 19, 2008 #11

    NoTime

    User Avatar
    Science Advisor
    Homework Helper

    A) Did the ABS always work as described?
    B) Just what kind of road conditions are you on when this occurs.
    C) How hard are you trying to brake in those conditions.
     
  13. Jun 19, 2008 #12

    Mech_Engineer

    User Avatar
    Science Advisor
    Gold Member

    Seeing as how it's only an '04, I would suspect the ABS is working fine. If by "it feels I'm sliding on ice" you mean the brake pedal seems to go somewhat soft, that is a normal symptom of ABS engagement. You have to make sure to keep solid pressure on the pedal even if ABS engages to ensure maximum stopping power. You might also check to see if your rear brake proportioning valve is working properly, and that your brake pads/rotors/drums are all in proper working condition.

    The idea of checking if the system is working properly by testing your brakes with and without the ABS fuse seems like a good one, as long as you are careful (you should stop faster with ABS and continuous pedal pressure). Alternatively, you can have the system inspected by the dealership. I personally would not be comfortable with just pulling the fuse and calling it a day.
     
  14. Jun 20, 2008 #13
    Oh you bought an american car. PROBLEM FOUND!
     
  15. Jun 20, 2008 #14
    Excellent questions. I think the system always worked as described. The second and third questions I'll explain. On dry cement, when the system kicks in I get that "no brakes" feeling if I have to make a quick stop. I always have to modulate the brakes to get a decent stopping distance. On wet surfaces I can apply the brakes hard enough to cause the front wheels to lock up. If I have to modulate my foot pressure and the wheels lock up on hard stops, what the heck is the system accomplishing. I really don't think ABS has evolved very much and it needs a complete rethink. I find the whole subject really fascinating.
     
  16. Jun 20, 2008 #15
    I have another brake related question. I've noticed that the calipers on some cars are in different places. To my thinking the front caliper should be behind the wheel. The back of the rotor is going up. I think this would keep the front from diving on hard braking. Also the back caliper should be in front of the wheel. To keep the back of the car from lifting. I don't see this consistently on all cars. I just saw a Buick SUV with the calipers in exactly the opposite locations.
     
    Last edited: Jun 20, 2008
  17. Jun 20, 2008 #16
    Dodge Dakota large truck? LMAO It's the largest of the small pickup trucks. It hardly weighs 6000 Lbs. I would give an estimated guess of around 3500. I am not a fan of ABS to start with so I'd disable them if it were me. I've never noticed an unloaded pickup truck slide it's back wheels when stopping quickly and I've driven a few of them. In virtually any vehicle excluding semi-trucks a large majority of the braking is in the front end. Up until the mid-70s semi tractors didn't even have brakes on the front (steering) wheels.
     
  18. Jun 20, 2008 #17
    If the calipers were attached to the road instead of the spindle assembly then you would have something. Otherwise it will make no difference to what you are referring to.
     
  19. Jun 20, 2008 #18
    Okay, now I understand -- your truck has rear-wheel anti-lock. That was/is offered on trucks only. (I think my friend's Chevy Blazer had it, too.)

    This article describes some of the problems you are experiencing:
    http://www.aa1car.com/library/abs_kelseyhayes_rwal.htm
     
  20. Jun 20, 2008 #19
    Pantaz. thanks for the link and the response. I read what they said and it's not quite what i've got going. #1 No pedal collapse....pedal acts fine. #2 I know what they said but I think this truck has ABS in the front. I'll tell you why. The back brakes on any vehicle are pretty week. They do almost nothing. I usually change the backs every other time I do the fronts. I also drove a motorcycle for a while and I know from that, that the only way to stop is to hit the front brake. The back brake only made the back end swirve out if you hit that one by itself. Now, having said that, why would any influence of rear wheel ABS have such a large effect on the braking as a whole. I contend that you could go and cut your rear brakes right out of the loop and nothing would happen. If you don't agree, try stopping your car with the hand brake next time.
    Anyway it's an interesting problem.
     
  21. Jun 20, 2008 #20
    I just dug up the window sticker to see if you were right. you were right. I'm not getting the pedal collapse but something is still wrong. I think you're on the something. This explains why the front wheels can be locked up. But I wouldn't have thought that the back brakes were doing so much work. I think it's like this. When you apply the brakes there's a balance between how fast you want to stop and how hard you push on the brake. If something changes while you're braking, its influence is felt quite a bit. Is rear ABS a useful thing of is it just hype? thanks
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Anti-lock brakes
  1. Engine Braking (Replies: 42)

  2. Brake regeneration? (Replies: 3)

  3. Lock in amplifier (Replies: 1)

Loading...