Hot Brakes

What in the hell are you doing driving a Truck Farmerbob. With all of your vast knowledge, you could be designing Nuclear Weapons for the Iranians! You make Sheldon Cooper look like a DUNCE!!!

Ok so instead of just typing the words "engineering" and "physics" as proof of your theories you've instead decided to use fictitious and theoretical numbers to demonstrate your point?

Anyhow, this is an interesting topic and today we're going to be looking into that 100 page report G-Town cited. I'm thinking that "Mountain Braking" might make for a great new module for our High Road Training Program.

Ok so instead of just typing the words "engineering" and "physics" as proof of your theories you've instead decided to use fictitious and theoretical numbers to demonstrate your point?

Anyhow, this is an interesting topic and today we're going to be looking into that 100 page report G-Town cited. I'm thinking that "Mountain Braking" might make for a great new module for our High Road Training Program.

Brett, YOU asked if I knew what it took to heat a 100kg brake drum. I told you what you would need to come just short of melting all ten drums on a truck weighing 66,000 lb. There were no fictitious numbers. The numbers I used were accurate. I clearly indicated that I wasn't going to spend a month trying to generate a fully accurate solutions.

Feel free to create a new module, and describe it however you want.

However, here is a quote from the article you guys have been citing, where they describe how they would suggest wording the CDL manual:

"To control speed going down a mountain, some people favor using a light, steady pressure to drag the brakes while others favor a series of snubs, each sufficient to slow the vehicle by approximately 6 mph in about 3 sec. The snubbing strategy uses pressures over 20 psi for heavy trucks while the light drag may involve pressures under 10 psi. Tests have shown that either method will result in approximately the same average brake temperature at the bottom of the mountain as long as the same average speed is maintained. However, the snubbing method, due to the higher pressure involved, will aid in making each brake do its fair share of the work. Hence, the snubbing method will result in more uniform temperatures from brake to brake and thereby aid in preventing brakes from overheating. Furthermore, light, steady pressure at highway speeds on short grades of roughly one mi in length can lead to problems with "hot spotting" and drum cracking and fragmenting if the brake linings are new."

What the bolded sections MEAN is that if your brakes are improperly maintained or installed, snub braking will allow them to work better, and if you have brand new brake shoes, snub braking is a good idea until you have worn the shoes down a little bit so they match the drums properly.

If you are going to cite a study, and use it in an argument, understand it.

I do understand it. The reason why snub braking is preferred by the DoT is not because it works better, it's because it compensates for the failures of professional drivers better.

Since I don't care to be insulted by people who can't be bothered to actually understand what they are talking about, I'm done in this thread unless someone generates a meaningful argument, rather than citing a study that doesn't say what they think it says.

The melting point of a brake drum isn't relevant because excessive heat would cause the brake drum to expand so much that the shoes would no longer put much pressure on the drum. That's why hot brakes fail, right? So it wouldn't be possible to melt the drums. The brakes would fail long before that happened. But I do have a cast iron wood stove in my family room and now I'll know how to figure out how much heat it will take to melt it.

I'll tell you one huge assumption you're making in all of this. You're assuming that all of your brakes are almost perfectly adjusted and all of the components throughout the system are functioning perfectly at all times. But how do you know that? You don't have thermometers or pressure gauges for each of the individual brake drums. You're adjusting the brakes by pressing on the pedal but that doesn't guarantee that every brake has now adjusted itself correctly and is functioning identically to the others, especially when you're using a variety of different trailers in different states of repair. So just the fact that there could be an undetected variance in your braking system means it would be safer to use snub braking:

the snubbing method, due to the higher pressure involved, will aid in making each brake do its fair share of the work

You think all of your brakes are doing equal work and you're not heating up any one drum more than the others. But you don't know that. Especially with all of the different trailers you'll use. You could have easily heated up one or two drums on a trailer quite a bit more than the others because of some undetected problem with that trailer and you wouldn't have known it.

Tests have shown that either method will result in approximately the same average brake temperature at the bottom of the mountain as long as the same average speed is maintained.

So clearly you're not putting so much heat into the brakes, even at maximum temperature for snub braking, to make any discernable difference to the components of the system. The variance between maximum and minimum temps when snub braking is negligible. But it has been demonstrated that with snub braking you can expect each of the brakes to do an equal share of the work. That may not be the case with light steady pressure. So you believe you're using a method that will give you even temperatures and wear but you won't know that unless you take measurements of your shoes and drums.

The bottom line here is that we're not going to recommend light steady pressure to anyone because it hasn't been scientifically demonstrated to be the safest method for the greatest number of people. If you want to use that method that's fine. But when it comes to teaching new drivers the safest way to do something we're gonna go along with the best science we have right now and that seems to mean snub braking is the way to go.

The reason why snub braking is preferred by the DoT is not because it works better, it's because it compensates for the failures of professional drivers better.

That sure sounds arrogant. Why would you consider an undetectable problem within a braking system to be the failure of a professional driver? A driver can only do a visual inspection of the system. There are all kinds of hidden problems that could be creeping around in the brake system including obstructions in the air lines, any number of brake chamber issues, metal fatigue, or problems with the numerous valves and connectors within the system. It's entirely possible that a driver has done a thorough inspection but still goes down the road with a slight problem that could cause uneven braking if using light steady pressure but even braking when using snub braking. That's not a failure on the driver's part. That's just the reality sometimes when using complex machinery in the real world.

One time I had a problem somewhere in the valving where the tractor brakes were working but the trailer brakes were not. There was nothing whatsoever visually wrong with the system and in fact the mechanic didn't believe me at first. He said, "There's no way you can push the pedal and send air to the tractor brakes and not the trailer brakes."

But I told him I had felt the brake drums at the bottom of the hill. The tractor was hot, the trailer was ice cold. You can't argue with that. They eventually figured out it was something to do with the valves somewhere on the tractor. Now that isn't a problem that would have been affected by snub vs steady pressure braking but it shows you can have an undetectable problem with the brakes that would not be considered a failure of the driver.

FarmerBob wrote:

What the bolded sections MEAN is that if your brakes are improperly maintained or installed, snub braking will allow them to work better, and if you have brand new brake shoes, snub braking is a good idea until you have worn the shoes down a little bit so they match the drums properly.

It doesn't say that Matt. Where does it say in the report; "this is what we mean?" That is your interpretation spinning the facts, supporting your opposition to what is obvious to the majority.

The CDL manuals, handbooks, training manuals, and current guidance reflect the facts of the report and not what you think it "means".

Like I have said I will continue practicing and advising the merit of snub braking to control downhill speed until a much higher authority than you, mandates otherwise.

Based on your final paragraph I don't expect a reply...since I am not an ME.

The reason why snub braking is preferred by the DoT is not because it works better, it's because it compensates for the failures of professional drivers better.

That sure sounds arrogant. Why would you consider an undetectable problem within a braking system to be the failure of a professional driver? A driver can only do a visual inspection of the system. There are all kinds of hidden problems that could be creeping around in the brake system including obstructions in the air lines, any number of brake chamber issues, metal fatigue, or problems with the numerous valves and connectors within the system. It's entirely possible that a driver has done a thorough inspection but still goes down the road with a slight problem that could cause uneven braking if using light steady pressure but even braking when using snub braking. That's not a failure on the driver's part. That's just the reality sometimes when using complex machinery in the real world.

One time I had a problem somewhere in the valving where the tractor brakes were working but the trailer brakes were not. There was nothing whatsoever visually wrong with the system and in fact the mechanic didn't believe me at first. He said, "There's no way you can push the pedal and send air to the tractor brakes and not the trailer brakes."

But I told him I had felt the brake drums at the bottom of the hill. The tractor was hot, the trailer was ice cold. You can't argue with that. They eventually figured out it was something to do with the valves somewhere on the tractor. Now that isn't a problem that would have been affected by snub vs steady pressure braking but it shows you can have an undetectable problem with the brakes that would not be considered a failure of the driver.

So, now I am being arrogant, simply for stating facts?

I went down Monteagle yesterday, going Westbound, in the rain. It's not a bad downgrade, but it's significant. Obviously, I am not going to be using my jakes on wet, curvy downgrade roads, because THAT would be stupid. I did it in 8th gear (out of 10), at 1500 RPM, using 5-10 lb pressure most of the way down, coming completely off the brakes a couple times on flatter points.

Now, if my trailer brakes were not working properly, then the tractor brakes alone probably wouldn't have been enough to keep the truck going a safe speed in a couple places, and I would have started snub braking. But, it WAS enough to keep the truck going at a safe speed, and it could not have damaged the braking system, because 5-10 lbs pressure can't generate enough heat to damage braking systems. Even if only some of the brakes are working.

In essence, if low pressure constant braking is sufficient to slow the truck, then it can't damage the brakes. If low pressure constant braking ISN'T enough to slow the truck, then you snub brake.

Further, it doesn't matter why the low pressure braking doesn't work. If it doesn't work, for whatever reason, you snub brake.

This is what I was taught by both schools I attended (TDI and Stevens) and both trainers that I drove with. It is backed up by physics and engineering. Snub braking also works, but there is one thing about snub braking that should worry every driver:

The harder you brake on a curve, the more likely you are to lose control on a curve. If you find yourself in a situation where you have allowed your speed to build back to snubbing speed, and you come around a corner and see that you are about to enter into a wet curve, then you might be in trouble and the state might be buying new guardrails. If you start to snub on the curve, you might break free because of your brakes. If you don't snub, you might break free because you are going too fast into a turn. If I am feather braking into the same curve, I just put a tiny bit more pressure on my brakes before I get to the curve, then release my brakes for a second until I'm done with the curve, and I'm never in any danger of losing control. There is never any point where I don't have full control when I am feather braking.

Now, you might say that a good driver will not allow themselves to go fast enough to get caught entering a wet mountain curve at snubbing speed. I'll say that signs do get run over or go missing, and sometimes signs are obstructed by other trucks as you pass them, or they pass you. That doesn't change what might happen if you are forced to snub in a curve.

I have a strong objection to people advocating braking methods that might lead to other drivers doing Rocky the Flying Squirrel imitations on curvy downgrades, even if those people doing the advocating are with the DoT, and basing their recommendations on studies that don't consider all the elements of safety on downgrades.

If anyone wants to get into braking mechanics on curves, lets start a different thread. I'm pretty sure I'm done in this thread.

The thing you have to keep in mind is that every mechanical system has resistance in it. You have to overcome that resistance to make the system operate and fulfill its mission. In these braking systems there is resistance pretty much anywhere that two parts are connected. If you use light steady pressure you may not fully overcome the resistance throughout the system in order get the expected pressure going to each brake drum equally.

In other words, the system may be bound up in places. So even though you have light pressure coming from the foot pedal, not all of that pressure is being transmitted fully and equally to all of the brake drums. If you press harder, like with snub braking, you'll overcome the resistance in the system, the system will no longer be bound up, and you'll get equal and full pressure to each brake as expected:

the snubbing method, due to the higher pressure involved, will aid in making each brake do its fair share of the work

That's what I mean when I say you have to consider more factors than just "don't apply heat in cycles" - there's more to it than that.

The thing you have to keep in mind is that every mechanical system has resistance in it. You have to overcome that resistance to make the system operate and fulfill its mission. In these braking systems there is resistance pretty much anywhere that two parts are connected. If you use light steady pressure you may not fully overcome the resistance throughout the system in order get the expected pressure going to each brake drum equally.

In other words, the system may be bound up in places. So even though you have light pressure coming from the foot pedal, not all of that pressure is being transmitted fully and equally to all of the brake drums. If you press harder, like with snub braking, you'll overcome the resistance in the system, the system will no longer be bound up, and you'll get equal and full pressure to each brake as expected:

the snubbing method, due to the higher pressure involved, will aid in making each brake do its fair share of the work

That's what I mean when I say you have to consider more factors than just "don't apply heat in cycles" - there's more to it than that.

Brett, don't you see that it doesn't matter if every brake is engaged when using light continuous pressure? If light continuous pressure works, then it works, and the brakes will not overheat. If it doesn't work, for any reason, then you snub brake.

If you know that light continuous pressure SHOULD have worked on X mountain, from experience, and you have to snub brake anyway, then you know there is a braking problem, and need to try to figure out what is wrong.

You are factually incorrect when you say that you will "get equal and full pressure to each brake as expected" by snub braking.

If there is a braking pressure problem/discrepancy where some brakes activate before others because they are operating at different pressures or experiencing different mechanical resistances, they may operate at different pressures when snub braking as well, meaning that you might smoke some of your brakes halfway down the mountain while snubbing, but you can't smoke brakes when feather braking.

Totally glad I stayed out of this one.

Anyone know if having to use an "escape ramp" is still a firing offense at most larger companies?

Rick