Anyone had to use a runaway truck ramp?

That's fine. In a perfect world, most rigs would be in proper condition to always achieve equal braking under light applications, but that's not the case, sadly.

You're misunderstanding how pressure works and brake systems are set up. Trailers have valves with slightly lower crack pressures so that they actuate slightly before the truck. Air pressure increases propagate from the source, and move at the speed of sound down the line. So the lower crack pressure of the trailer valve allows the trailer brakes to get pressure a fraction of second sooner than the tractor, even though the brake signal takes longer to get to the trailer. I've not been around heavy haul trailers to know exactly how the extra axles are piped up, but on a tandem, both axle's brakes get the air at the same time, as they are all plumbed into the same control valve circuit. And once the pressure has stabilized in the control circuit, every brake in the system is then getting the same service pressure to the chamber. The problem with light applications isn't the pressures, it's the forces from the chambers to the wheel ends. Testing has shown that variances to stiction and spring forces can lead to substantial differences in applied torque at the wheel ends.

I never advocated using constant 20-30 psi applications, because of the issues you've pointed out. I was merely pointing out the hazards of very light brake applications. Although if you were to use the higher pressure applications, you would see the brake fade by the increased pressures required, just like you do at lower pressures.

What it comes down to is preference. I'd rather hang on the jake at a slightly lower speed with completely cool brakes, as to go down faster with the brakes substantially warmed up. The downgrade already will increase your braking distance if an emergency stop is needed, and the already warm brakes will start to fade sooner into the stop than cool brakes, further increasing your stopping distance. Especially on any heavily trafficked grade, I want the maximum braking power available to me at all times, because the time will come when you wish you had that distance lost to brake fade still in front of you.

 

Can you explain?

 

Years ago we were doing a road job cutting down a hill west of Quemado on U.S. 60. They brought an old 64 white cabover daycab single axle out to pull the water tanker with. I could hold on to the steer tire and the rear tires, thatis how short this thing was. You actually sit kind of in front of the steer tire and the cab did not even have a jack, you just shoved it over ny hand. It had a big six gas engine and straight 5 spd. we had the trailer brakes unhooked so we could use that valve for the water valve.
I loaded it and started up the mountain, motor screaming in first gear ( trying to blow this baby up) I had to dump half the water to pull the mountain and when I went through the first section we had a crew working on, the guys would walk up and climb on the truck and get sodas out of my cooler, while I was running WIDE open. lol
I got to the top, which was 3 or 4 miles and broke over, shifting up to fifth gear, still wide open and started down hill at probably a whopping 45 mph, maybe. lol Anyway after I went maybe a half a mile or a little more the grade would start getting steeper, when I let off to slow down the tranny jumped out of gear with an ear splitting boom and we were immediately in a serious race, no way possible are we going to get this pos back in gear, so just a wild several mile ride off that mountain and 3 miles once I hit the flats. I thought I was going to get to the Arizona line before I got it back in gear. lol
I do not have a clue how fast we got, but faster than I ever wanted to ever see again, especially in a 64 white. lol
Coming off that mountain I had dumped the rest of the water, so when I did get turned around and got back to the crew working on the down hill side, I didn't have anything for them and had to go get another load. lol

 

These new guys would mess their pants if they had to drive some of ratty old iron we had to when we were young!

 

My first truck was about a 58 or 59 white. Two axles and a short hood. Not very much to it with its' 17 foot single axle trailer. Docking was learned in a hour.

Instructors said great. Get up there to the big tractor trailer and dock that.

Now that was harder.

 

Were do you get the info about the actual brake force difference at each drum. I see your using the Snubbing method. That were the 20-30 psi come from and the values not opening and applying the same pressure to all brakes using the light steady pressure method. I never used the Snubbing method, what do you do when the road is snow covered or icy. I could not imagine applying 20-30 psi. Because of causing a possible start of jacknife. Plus using the engine brake on icy roads is not a good idea.

 

There is one I forget exactly where that used to ban trucks less than 400 horse. A trainer had me on it, we had a 450 that day and I was pretty tired when he woke me up sitting near the sign to get a eyeful of the hill above us. That's why the memory did not stick other than that sign.

We ran it ok. But trainer was teaching me that if she did break, lose grip, spin drives on the pavement and so on and begins to go backwards, we put her into the ditch next to us quickly while the speeds were very low. Because once gravity gets a hold, your other option is to leave the cab through the door. And anyone behind you probably will be hurt bad or killed.

I at least remember that.

 

I rarely use my brakes, only when coming off a hwy, city driving or if I have to stop faster than my engine brakes can do the job.


Is that wrong?

 

What you call snubbing I know as stab.

That's a form of braking which is applied hard enough for JUST LONG ENOUGH to dip the RPMS below 1000 and then take the next gear at the bottom and reapply jake. It should settle and then cool.

There has been situations in old iron pre ABS days where one car did stop short on US 15 in PA on me at a light right where I was going to stop while empty in the light rain that just got started. That car forced me to apply a little more braking. She did not have that traction left to accomodate it. So the tractor broke loose on me and viciously tried to fold and slam into the side of my own trailer.

I sawed the wheel while stabbing the brakes very fast. But not fast enough. Ended up in the intersection under the red light all crooked as the car fled after he got a eyeful of what was coming. Stupid idiot. (Then again I should have guessed he was going to do that... which is something I have never forgot again ever.)

I spend the next 30 miles going through a pack of smokes and considering going home to try something else in life that did not involve trucking. It was pretty scary the way that bucking cabover tried to do what it wanted to do.

But that's over and done with has been for 30 years now. But days like that you learn and do better.

 

You drop a gear. That also slows you down. Eventually the engine and gear ratio applied against the total vehicle weight on a slow enough speed will be held by the most #### jake in the world. You might drop two gears until you find that balance with that jake.

Nothing is wrong as long you maintain control. And no one gets hurt. That gives you a small range of what is possible in any situation depending on Sir Issac Newton's School of Physics and Laws applied to same.