Hot Brakes

You need not agree with my, but you do need to agree with physics.

Which findings are you citing? Do you have actual data from a study to support what you're claiming so we know there isn't a difference between your physics and actual physics?

Ah, physics. Engineering too.

Metals and ceramics suffer damage more frequently when they are heated and cooled rapidly, as opposed to being operated at steady temperatures.

Snub braking will cause many rapid fluctuations in brake temperatures, while constant pressure leads to many fewer rapid fluctuations.

In a properly functioning brake system, constant 5-10 lb low pressure braking will lead to a equilibrium heat in the braking system that is not enough to damage the brakes or tires, or catch anything on fire. However, snub braking IS superior to high pressure constant braking of more than 10lb pressure, as I've said before.

This is what was taught in the past, (and in the present everywhere I was taught) and if your brakes are working properly, it's still true now. Physics doesn't change.

That said, based on the articles I researched, it seems as if the change to favor snub braking as the braking method of choice on slopes is because drivers are not properly maintaining their brake adjustments. Look at the articles' reasoning behind using snub braking. Practically every reason is based on some sort of adjustment or equipment failure.

The only exception to maladjustments or failure being responsible for substandard low pressure constant pressure is when tractor and trailer brake valves having significantly different system activation pressures. If that is the case, you won't get much braking from 10 lbs constant pressure, and may need to shift to snub braking, so the point is moot.

Ah, physics. Engineering too.

I asked if you were citing a scientific study or if you had any actual data to demonstrate your point. Typing the words "physics" or "engineering" does not mean your theories are scientific law. For instance:

Metals and ceramics suffer damage more frequently when they are heated and cooled rapidly, as opposed to being operated at steady temperatures

First of all, how quickly do you think 100 pounds of cast iron will heat up or cool when you apply and release the brakes? That is a massive chunk of iron. It can stand a ridiculous amount of heat and it's engineered for constant cycles of heating and cooling. Same with the pads of course. You're not doing any damage to anything with normal levels of heating and cooling in cycles.

So yes, as a general principle you want to pay attention to heating and cooling cycles of certain materials. But that doesn't mean you should change your braking methods based on that one theory alone. There's a lot of factors to consider beyond heating and cooling cycles that will ultimately determine the best braking method. That's why I asked if by chance you had any actual data that backs up your theories or if you're just making assumptions based on one or two high school physics principles.

There's a link to the report I think G was referencing. It does mention that proper brake adjustment is key under all braking procedures but it gets into why snub braking is the preferred method.

Brake report

Brake report

That's the abridged, summarized version...I will respond in a bit with the report link that influenced the downhill braking changes to CDL manuals.

Ah, physics. Engineering too.

I asked if you were citing a scientific study or if you had any actual data to demonstrate your point. Typing the words "physics" or "engineering" does not mean your theories are scientific law. For instance:

Metals and ceramics suffer damage more frequently when they are heated and cooled rapidly, as opposed to being operated at steady temperatures

First of all, how quickly do you think 100 pounds of cast iron will heat up or cool when you apply and release the brakes? That is a massive chunk of iron. It can stand a ridiculous amount of heat and it's engineered for constant cycles of heating and cooling. Same with the pads of course. You're not doing any damage to anything with normal levels of heating and cooling in cycles.

So yes, as a general principle you want to pay attention to heating and cooling cycles of certain materials. But that doesn't mean you should change your braking methods based on that one theory alone. There's a lot of factors to consider beyond heating and cooling cycles that will ultimately determine the best braking method. That's why I asked if by chance you had any actual data that backs up your theories or if you're just making assumptions based on one or two high school physics principles.

Constant low pressure braking would not have been the preferred method of braking in the recent past for no reason. Someone did the studies.

As I said, the reasoning for using snub braking as the preferred method today seems to be almost exclusively based on expectations of poorly maintained equipment. That just means that they want people to snub break because they don't trust them to maintain their equipment.

In terms of how much energy is transferred into the braking system, that's easy to explain as a rough, crude calculation.

First, you determine the loss of energy of the truck based on the change in speed and mass.

Then add that amount of energy as heat to the drums and pads for an approximation.

It takes 771kiloJoules to melt 1 kg of cast iron.

So, put some heat on 10x 100kg brake drums. They need 1000x771, or 771,000 kiloJoules of energy to melt them.

Now, cast iron can absorb a lot of heat after it is glowing before it melts. The heat of melting for cast iron is 126 kiloJoules per Kg. We have 1000 kg of brake drums, so 126,000 kiloJoules of energy is energy above what we need to get the cast iron heated up to as hot as it can get before melting. To get to that point, we need about 645,000 kilojoules of energy

Let's say we have a 66,000 lb truck, which is roughly 30,000 kilograms. It is traveling at 62 MPH which is 27.7 meters per second.

The total energy of that truck at 62MPH is 11,509,350 Joules, or 11,509 kiloJoules.

When we slow the truck with a snub brake to 52 MPH, it's new energy state is 8,073,600 Joules, or 8073 kiloJoules. That means that 3436 kiloJoules of heat transferred into the brakes in just a few seconds.

645000 / 3436= a bit less than 188, so simply slowing that truck by 10 miles per hour on level ground will add roughly 1/188th of the heat required to turn all ten drums into barely-solid brightly-glowing pieces of metal. (The drums will fail catastrophically long before they absorb that much energy, but I'm damn well not going into that much detail.)

Moderate brake pressure can slow a 62 MPH truck from 62 to 52 mph in about 3 seconds. (Stab braking will stop a truck in @ 8 seconds at 65 MPH)

So, in a very simple calculation, all ten brake drums will be glowing as bright as they possibly can before melting if you brake moderately for 564 seconds straight. (3 seconds x 188 applications) They will fail catastrophically FAR sooner than this, due to losing rigidity, but I do not know at what temperature cast iron begins to lose rigidity, and I'm not going to hunt for that data, even though it would dramatically reduce the amount of time under constant moderate braking before failure.

It doesn't happen that fast because heat bleeds out of the drums into the rest of the braking system, wheels, axles and tires, as well as the air, and through any evaporating moisture from the road, and even radiant heat emissions. The hotter the drums are, the faster the heat leaks into the rest of the system or the environment. At the same time, the hotter the environment is (from absorbing brake heat) the slower the heat will be absorbed (diminishing returns.) I'm not going there because it'd take me a month to get all the information I need to do the calculations. It's been 25 years since engineering school.

Yes, I went to engineering school. I did not get a degree, but I finished 3/4 of the curriculum. That's not to say I'm always right, but I'm not just pulling facts and numbers out of my ssa.

I am so impressed!

You guys are letting Farmer Bob drag you down to his level and now he is beating you with experience!

I studied mechanical engineering also, and I still have never used this constant brake pressure method. I don't think Farmer Bob realizes that Brett could run circles around him when it comes to calculations.

If he would just cite the study that Brett requested he could put this all to rest. But now he chose to give us some impressive calculations as if that would divert our attention from what we're asking.

The whole reason for this discussion is critical because proposing the use of an outdated method that has been proven in studies to be less effective is not a smart move in a forum like this.

I am so impressed!

You guys are letting Farmer Bob drag you down to his level and now he is beating you with experience!

I studied mechanical engineering also, and I still have never used this constant brake pressure method. I don't think Farmer Bob realizes that Brett could run circles around him when it comes to calculations.

If he would just cite the study that Brett requested he could put this all to rest. But now he chose to give us some impressive calculations as if that would divert our attention from what we're asking.

The whole reason for this discussion is critical because proposing the use of an outdated method that has been proven in studies to be less effective is not a smart move in a forum like this.

Have you read the studies that have been linked? I have, which is why I have mentioned that the reasoning for moving to snub braking over constant low pressure braking is very clearly indicated as being based on an expectation of improper maintenance. If your equipment works properly, constant low pressure braking works just fine, and probably better, as it will cause less damage to the braking systems.

There are quite a few people in this world that can out-calculate me. However, if I do not make any errors, there is nobody in the world that can get a more correct answer.

As I said above, I do not need to cite another study. The study that they are citing, if you pay attention to the wording in the study, clearly explains WHY the change was made, and it was based on expectation of truck maintenance failures, not due to any improved performance over low pressure braking.

What was proven in the studies is that the snub method is better for poorly-maintained equipment. If I were driving a poorly maintained piece of junk truck with a trailer old enough to vote, I'd use snub braking too.

Welp, gonna hit the sack so I an be on schedule to start driving to final after my 34 ends. Will check in tomorrow and see what sort of responses have popped up.

Old School wrote:

You guys are letting Farmer Bob drag you down to his level and now he is beating you with experience!

Not exactly...I do this in my spare time and am at the point where I realize arguing "Matt" is becoming futile, a waste of time because I have seen him do this very thing when defending other opinions offered as fact (like "crabbing"). But overall I agree with your entire reply OS. All of it!

This is the link that contains the pdf report on research and subsequent findings that changed the wording of the CDL manuals and core teaching of downhill braking: The Influence of Braking Strategy on Brake Temperatures on Mountain Descents

Nothing personal Matt, but you are abdicating a technique that has not been widely taught or generally accepted for over a decade. You seem to be completely ignoring that fact. You can defend 5/10lb constant braking all you want, but until your research and math challenges current wisdom and guidance specifying snub-braking for safe and efficient downhill control, I will stick to what I know works, has worked for 4+ years, and what my safety and engineering departments require from me to control my 45k load descending a long grade.

One final note, has it occurred to you, the reason for the lack of brake wear on your truck is primarily because you are intelligently applying your engine brake as your primary source for controlled braking? I would strongly suggest that is the case because the findings of your mechanic are not all that unusual for an experienced driver.

FarmerBob wrote:

There are quite a few people in this world that can out-calculate me. However, if I do not make any errors, there is nobody in the world that can get a more correct answer

How many exactly can "out calculate you"? OMG...this (above) is all I need to know about you Matt (FarmerBob), I know how to drive a truck, my performance and safety record speaks for itself. However that does not make me superior to anyone else in this forum.