I've hauled hay in dry vans, and on flats, and on steps. Yes, I have been pushed ahead by the loader. Not 30 feet, but maybe about 3 feet. Not the same as not setting your trailer brake at a loading dock. Long sets of forks on an end-loader that's sitting on the ground is a bit different than being backed into a dock.

 

Here's an example of what can happen when the trailer brakes are not set as you try to unload a D8 Cat dozer:

http://break.com/index/how-not-to-tow-a-bulldozer.html

The weight of the dozer on the ass end of the trailer took enough weight off the tractor's drive wheels that the tractor drives just skidded along as the entire unit rolled away out of control.

Note that the tractor drive wheels are not turning as it rolls away.

Kranky; Always sets tractor & trailer brakes because it's easier than explaining to the boss why something is wrecked.

 

Stan, I made no comment whatsoever regarding the ABS system...I understand very well how it works, and how they have NOTHING to do with the spring brake system. The only relevence that has to this discussion is that the ABS will NOT keep your wheels from locking from the spring brakes. That means that if you suddenly lose pressure in your trailer during slippery weather, yes...you're gonna likely lock up all 4. I understand that. That's as FAR as ABS goes...it was NEVER meant to protect the brake system from iteself...it was meant for vehicle control and stability.

Now this, I did not know. I've never see the spec for the amount of energy stored in those springs when they are compressed. But it makes sense...Stan, does your truck have a brake application gauge? Mine does. 30 PSI is around 1/4 of the pressure that can go in a brake chamber using the foot pedal. 30 PSI is a moderate stop to be sure, but hardly an emergency panic stop. I use 30 PSI of brake application every day when I'm heavy, and it's well within what the brake systems are designed to apply to the drums.

No, I've never had the experience you had with damaged brakes from panic stops, and I do believe you when you say they happened. But, I do know that 30 PSI is nowhere near a panic stop.

Now, earlier you mentioned something about how much energy a moving vehicle has. KE = 1/2(MV^2)...which means it goes up exponentially as speed increases linearly. Meaning that a vehicle going 60 MPH has well over twice the energy of a vehicle going 30 MPH, even though the speed is "only" double.

Because of this, if you were to "pop" your tractor brakes at a higher speed, you would be using ONLY two brake systems to stop the vehicle form a higher rate of speed, when the system was designed to use 10 sets of brakes. You would CERTAINLY overheat the brakes on those two axles. Seems like common sense to me. Now, I do not know how much, if any, damage can be done by overheating the brakes. I've never done it. Unlike some on here, I don't like taking chances, so I don't plan on ever doing it! I do, however, know that if a modern air brake system could NOT handle 30 PSI of pressure without damage, we would all be in very serious trouble.

BJ...I currently drive for Conway Truckload, and have ever since I left Roehl. I do, honestly hope you bring your attitude with you to orientation.

On edit: Kranky, great video!

 

To see what the spring brakes have for stored energy, turn off the engine. Pump the brake peddle (just like DOT has you do for an inspection), and when you're near zero, stop pumping and start the engine. As your pressure goes up, hold the button in and try to move the truck. Until you get enough pressure, you will find that the truck does not want to move. The point where it will move is the amount of energy the springs have. Application is a different chamber.

A note about ABS braking... On very slick roads, and you're moving about 10 to 15 mph and a car cuts you off... You jam the brakes... And slide. The ABS computer senses wheel rotation, not ground movement. At slow speeds, if all the wheels lock up, the computer says: "VEHICLE IS STOPPED", even if you aren't. You may find yourself sliding at about a 45 degree angle toward the curb... If there is one. with the rig staying completely parallel to your direction of travel. If you have a curb, like I did, it will stop you. If not, you could slide completely off the road.

 

Windwalker, great post...now I wonder why I didn't think of that....gonna hafta try that!

 

You be mighty careful just how you try that. No parked cars on the lower side of the road from you or you'll cream all of them. And, with no curb, you could find a bit more adventure than you're ready to handle.

I got very lucky. I had no cars alongside me, and I had a 5 inch curb to stop me. Without that, there's a good chance I would have laid it over in the ditch. Not something to play a game with. Even when I let go the brake, the inertia kept me going to the side.

 

This is what I meant...definitely NOT messing with ABS on a slick public road

 

Usually about 60 PSI.

 

Originally Posted by YankeeTURBO
As your pressure goes up, hold the button in and try to move the truck. Until you get enough pressure, you will find that the truck does not want to move. The point where it will move is the amount of energy the springs have.

Could be wrong, but if you hold the button in, about 30 to 35 it will move. It takes about 50 or 60 to keep the button in. But the brakes will release before that.

 

Those springs are notorious for breaking as well - especially on trucks that are handled rough, either by the driver, or by loading and unloading.

I remember when I was pulling hopper bottoms, I would have a broken brake chamber spring at least once per year. Sometimes, I'd have several broken ones per year. Since they aren't an item that is subject to regular inspection (and can be dangerous to mess around with), they tend to go unnoticed. I got my first taste of what a broken spring can do when I was parked on an on ramp one night. I set my tractor brakes, and climbed in the sleeper. Suddenly I felt the truck shaking. I ran up into the driver's seat to find myself slowly beginning to creep down a very steep embankment. Luckily, I caught it in time, and after some gentle coaxing, got the tractor back onto the road.

From what I've been told by mechanics, something as simple as accidentally driving the truck for any period of time with the brakes applied can be enough to snap a spring. Whether or not this is true, I don't know.