Anti Skid Brakes

[MaxTorque]

I have a question regarding the operation of the anti skid brakes. I recently had the requirement to make a maximum effort quick stop. I have always been told to simply stomp on the brake pedal and the brake system will bring you to a stop in the shortest distance. It seemed to me that it took a longer distance to stop than I expected and also as I kept my foot on the brake the pedal eventually went to the floor board much as it would if there was air in the lines. Does anyone know if this is the correct opereation of the brakes? The truck did not skid but it certainly was alarming to feel the pedal go to the floor. I called a local Dodge tech. and he told me this was normal, however I am not convinced. Do we have a brake experet out there in TDR Land?

 

[RBender]

I think with ABS, there's a trade off with straight line braking distance and the ability to maintain directional control while braking.

Originally developed for aircraft, ABS basically works by limiting the pressure to any wheel which decelerates too rapidly. This allows maximum stopping force to be applied without brake lockup (skidding). If standard brakes are applied too hard, the wheels "lock" or skid, which prevents them from giving directional control. If directional control (steering) is lost, the vehicle skids in a straight line wherever it is going. ABS allows the driver to steer during hard braking, which allows you to control the car much better. In the old days, drivers had to know how to "pump" the brakes or sense the lockup and release foot pressure in order to prevent skidding. This meant that if only one wheel lost traction and started to skid, the driver would have to reduce braking force to prevent a skid. The advantage of ABS is that the brakes on the wheels with good traction can be used to the fullest possible amount, even if other wheels lose traction.



In operation, the wheelspeed sensors at each wheel send electronic pulse signals to the control unit. If wheel lockup (rapid deceleration) is detected during brake application, the computer signals the valve unit to limit the hydraulic pressure to the wheel cylinder. This is usually accomplished by diverting the fluid into a small reservoir. The fluid is later pumped out of the reservoir and returned to the main fluid reservoir when the brakes are not being applied.



The anti-lock brake system tests itself every time the vehicle is started and every time the brakes are applied. The system evaluates its own signals. If a defect is detected, the system then turns off, leaving normal braking unaffected.

I think that your experience sounds normal.



Robert

 

[MaxTorque]

Robert, thanks for your response. Does your answer as being "normal" include the pedal going soft and to the floorboard?

 

[rbattelle]

While I have found that the pedal WILL go "soft" when the ABS kicks in (this seems logical to me as a natural side-effect of the ABS brake pressure modulator), I've never paid attention to whether the pedal goes to the floor. However, if you think logically about it, it makes sense that the pedal could go to the floor with ABS engaged during a panic stop. The brakes are fully applied and ABS is in full modulation - in order to achieve maximum brake pressure, the pedal would probably be at the floor (or very close to it anyway). I think your brakes are fine. But if you think there could be a problem and it's keeping you up at night, take it in and ask them to bleed the brakes under warranty.



-Ryan

 

[mhstevens]

The most efficient and fastest way to stop is a technique called "threshold braking. " This is just prior to lockup, before ABS braking kicks in. It will stop you faster both in wet weather and dry, however it requires practice to use the technique effectively. The ABS will give you better control, but it will release your brake pads (that's the kick you get from the pedal when you stomp on the brakes) and therefore will take a little longer to stop.

 

[B.G. Smith]

Brakes

I have a question regarding the operation of the anti skid brakes. I recently had the requirement to make a maximum effort quick stop. I have always been told to simply stomp on the brake pedal and the brake system will bring you to a stop in the shortest distance. It seemed to me that it took a longer distance to stop than I expected and also as I kept my foot on the brake the pedal eventually went to the floor board much as it would if there was air in the lines. Does anyone know if this is the correct opereation of the brakes? The truck did not skid but it certainly was alarming to feel the pedal go to the floor. I called a local Dodge tech. and he told me this was normal, however I am not convinced. Do we have a brake experet out there in TDR Land?

IF THE PEDAL GOES TO THE FLOOR UNDER ANY BRAKING SITUATIONS, BY ALL MEANS HAVE THE BRAKES CHECKED. Sorry for the screaming but if the pedal goes to the floor, it is not normal. bg

 

[ROCKJEEP]

my truck did this also. Two days ago in the rain I had to do a panic stop and the truck wouldn't stop. The front brakes were pulsing with the abs but the rears felt like they never came on. I also wore out a set of front pads with only 30,000 miles on the truck so some thing is wrong. Its at the dealer right now and will let you know what they found.

Kory

 

[MaxTorque]

mhstevens

Didn't your mother teach you that it is impolite to introduce a subject and then not explain the subject? For the rest of us what is "threshold braking" and how is it applied and how does it work with the ABS? How will it shorten the stop distance?

 

[SilverFox]

On a recent 13000 mile RV trip I had 2 occasions to apply heavy breaking with trailer attached and on both those occasions I was getting good stopping power until the ABS took over and released the truck brakes. I managed to stop in one case and the other I had to take to the paved shoulder. I'm not happy with the way the brakes react in an emergency stop. I also believe the brakes are responding just like they are programed to respond. It's one of those things that could be better but NOT, just like the head lights. In neither case did my peddle go to the floor and I would be very concerned if it had.

 

[mhstevens]

I apologize for making a statement without giving the details. Threshold braking involves applying the brakes firmly, with enough pressure to get the brakes to provide maximum stopping power without engaging ABS (on the threshold of lockup). In pre-ABS vehicles, most people jumped on the brakes and got into 4-wheel skid--all 4 wheels locked and sliding. In that case, you actually built up a layer of molten rubber (from the tires generating heat on the pavement) and slid on the rubber. Once the wheels locked, you lost all directional control, and therefore could not avoid road hazards and usually impacted another object in order to stop. ABS was designed to let the wheels roll freely to regain directional control. While very effective at allowing you to avoid hazards, it is not the most efficient way to stop a load in motion.



The threshold braking technique, if practiced, is very good at providing both. If you take your vehicle to an empty lot where you can avoid hazards and no one is around, get up some speed (usually around 40) and apply the brakes as hard as you can. Get a sense of where the ABS will kick in, then try it again, but back off a little and stop without kicking in the ABS. I think you'll see that the stopping distance is shorter, and you can make directional changes. Most people don't steer during a panic stop, or if they do its fairly slight (going to the right or left to avoid a vehicle in front). In wet or icy roads, ABS will allow better control without locking the wheels, but will increase3 your stopping distance. I even taught the wife to do threshold on full icy roads and she was able to learn it fairly rapidly (and the final kicker was that she was impressed with my ability to teach the technique, an unexpected bonus). I've taught law enforcement driving for several years and have repeatedly gotten into discussions over ABS vs threshold, and have usually won the stopping distance argument.



In the trucks, another dynamic that takes over and can be more pronounced is unloading of the rear wheels. In a panic stop, the nose dips and weight is transferred to the front end. That unloads the rear, which is usually light to begin with, and the rear wheels can slide a lot easier. That can kick in the ABS earlier than if the vehicle is loaded. Practice before you do it in a real-world situation and I think you'll be able to use it when necessary.



Class dismissed, time for the practical test. And no, I am not personally liable for your misjudgements that end up with the nose of your trucks against a solid object.

 

[MaxTorque]

Thanks for the explanation. I am going to find a place to give it a try.

 

[SilverFox]

I wonder how many of us could apply less than maximum force to the brake peddle when we are approaching a solid object and it looks like you aren't going to stop... And if you don't apply max force, how many are sure that they are applying enough force to stop in the shortest possible distance... Is the force the same at 7500 lbs. and 23000 lbs. ? Another problem I have with the less than max force method is my brake controller depends on the brake peddle position to send the correct signal to the trailer brakes. The harder you push, the more current the trailer brakes receive. I can see lots of problems with threshold braking, especially when towing. Probably be ok with an almost empty truck, but then, that's when you seldom need maximum braking.

 

[Boonieman]

Doesn't the trailer brake controller work by adjusting output by looking at the pressure in the brake system? At least I thought mine did, not peddle position. If that's true, wouldn't the highest amount of pressure available in the brake lines be just prior to ABS kicking in? Also, isn't the sensor in the top of the rear pumpkin the one that detects lockup and also causes the ABS to kick in? So, seems to me different road conditions (ice, snow,etc) and loads in the truck would affect this pre-ABS threshold.



By the way, I just had to do a "bat stop" for my neighbors dog the other day. Peddle was definitely not on the floor.

 

[SilverFox]

I'm sure some controllers work off hydraulic pressure and some work on inertia. Others work on duration. Mine works on peddle position. I have a Jordan Brake Controller. I have no idea where the sensor is located for the ABS on the Dodge.

 

[CAgnoli]

IF THE PEDAL GOES TO THE FLOOR UNDER ANY BRAKING SITUATIONS, BY ALL MEANS HAVE THE BRAKES CHECKED. Sorry for the screaming but if the pedal goes to the floor, it is not normal. bg

ABS Actually does operate this way. I've owned probably ten vehicles manufactured by Dodge, Ford, Chevy, Toyota, and Nissan, and ALL of them with ABS when you stomp on the pedal, the pedal drops to the floor, and the vehicle stops straight.



NOTE: I said the vehicle will stop straight, not stop faster. You will LOSE braking distance if you rely on the ABS, but I'll never, NEVER, own another vehicle without them! They are a lifesaver, you just need to relearn your braking.



I took my wife out on a snowy night, and didn't let her drive home until she was comfortable with the pedal "going soft".

 

[B.G. Smith]

ABS Actually does operate this way. I've owned probably ten vehicles manufactured by Dodge, Ford, Chevy, Toyota, and Nissan, and ALL of them with ABS when you stomp on the pedal, the pedal drops to the floor, and the vehicle stops straight.



NOTE: I said the vehicle will stop straight, not stop faster. You will LOSE braking distance if you rely on the ABS, but I'll never, NEVER, own another vehicle without them! They are a lifesaver, you just need to relearn your braking.



I took my wife out on a snowy night, and didn't let her drive home until she was comfortable with the pedal "going soft".

My truck has 4 wheel ABS as does our car and I like it, it has saved me a couple of times. I have not experienced the pedal to the floor though. bg

 

[jsimpson]

Thank God!

My 94 with rear-only ABS behaved as you describe, but my 03 with full ABS does not. I had the opportunity to experience a full pedal stop today on the way to church when a little old lady ran a stop sign in front of me. The ABS worked perfectly - no soft pedal - and God gave this old man a quick enough reaction time today to stop an inch or less from the right door of her car. I thought I had hit her - it was that close. There was no tire noise and no breaking loose of the tires.

 

[jsimpson]

Threshhold braking was an indispensable technique with my 94. Once the ABS kicked in it was like I was hit from the rear and the truck would lurch forward before regaining braking control. I disconnected the ABS and the brakes worked fine from then on. My 03 ABS works flawlessly, loaded or unloaded.