Understanding Braking Capacity

Seeing as it appears (based on other threads) that far too many don't understand the basics of how much braking is available on their truck, and how heat effects total stopping power, I thought I'd throw this out there.

Picture a 5 gallon bucket, that's the total heat capacity of your brakes before fade. In the side of the bucket is a line of holes from top to bottom, those represent the ability of your brakes to shed heat. Above the bucket is a large water hose with a valve that opens further the more the brakes are applied, that's the heat you're putting into the braking system.

Let's start with a normal hard stop, say 25psi. That valve opens and water starts to fill the bucket. That's the heat you're putting into the brakes as you slow. The water level will rise up the bucket, quickly at first but then slower as more holes are used to drain the bucket. By the end of a normal stop, the water is still well below the top of the bucket.

But what if we have to make another normal stop before the water all drains from the bucket (the brakes have fully cooled)? The level will be higher than the previous stop, because we put the same amount of water in, but there was still water in the bucket when we started. Hopefully you can see that if we make too many normal stops to close together, the bucket will overflow. That overflow is brake fade, we have exceeded the heat capacity of our brakes, and performance drops off.

Now let's look at a fully loaded hard stop. The valve goes wide open or nearly so, and the bucket will fill rapidly. Especially from high speed or while going downhill, the bucket fills up before the end of the stop and you'll feel braking performance start to degrade before you're fully stopped. You'll likely smell the brakes, but they did what they needed to do.

But what if that hard stop is needed at the bottom of a big hill? Let's suppose you came down the grade with a steady 10psi application helping the jakes, allowing a faster descent. You're bucket will probably be around 3/4 full, as at that point the water going in and out are roughly equal. Now throw the valve wide open for a sudden stop. The bucket will overflow within seconds, and not only have you got brake fade, but you'll likely have smoking brakes as all that spilled water is heat that's pouring into your brakes.

Hopefully you can see that just because your brakes aren't overheating, doesn't mean they're not in a state with significantly diminished braking ability. Use the engine brake, and drop gears so that you're only using the brakes infrequently. The old rule of thumb of 6 seconds of cooling for every 1 second of braking will keep you from overheating the brakes, but it doesn't necessarily preserve your braking capacity. And trust me, there is no worse feeling than mashing harder on the pedal with less and less effect.

 

Here's a calcutator for kinetic energy, the energy of motion.

Kinetic Energy Calculator

Change the units to pounds and mi/hour

You can see the difference in letting your speed increase just 5 mph (mass of the truck will not appreciably change coming down a hill). Little bitty increases of speed put LOTS more energy/heat into the brakes. The huge additional heat you put into the brakes from slowing down just the small speed increase will be far more than the amount of cooling the brakes will receive when you let off the brakes and begin letting speed build again. The heat energy the brakes will need to get rid of increases at the square of the increase. So if your speed goes up 1% you put a lot more than 1% more heat into the brakes. If your speed increased 5% you almost increase heat by 5 times 5 or 25%. Letting speed build is like borrowing money from a loan shark that doubles the interest rate every time you ask for more money. You can never borrow enough money from him to pay him what you owe.

 

Add in the likelyhood that all brakes are not adjusted exactly the same, so some will be doing a lot more work than others.

 

New drums will brake harder then warped drums. We all know most trucks don't get all new drums at the same time.

Adjusted shoes will brake harder then out of adjustment.

Only way I can see 10 psi slowing anything down is new drums and properly adjusted brakes.

Otherwise. It's 15. Maybe 20.

 

Then you add in some misadjusted brakes on top of that. Doesn't take much expansion on a brake drum to take up all your brake chamber stroke. One chamber maxes out its stroke and that's 10% of your braking power gone on a 5 axle unit.

 

That's exactly why they changed the originally recommended light, steady pressure to snub braking, way back in the early days of the CDL. Manual adjusters were still on most equipment back then, that was a few years before ASA's were required. The firmer snub application made sure that even out of adjustment brakes were doing some of the work. Under light steady, those brakes often barely touched the drum.

 

I know I probably shouldn’t say anything since most don’t seems to like or believe it possible. You can ride the brake all day with 10psi or less. Yes you can still stop if needed even going downhill. Most people don’t figure the drums can dissipate heat. At 10PSI you are generating the same amount of heat the drums can dissipate. That’s why they don’t overheat and fade out. If you need to stop you push brake pedal and stop.

It’s better way of braking in my opinion, not just because it was how I was trained. You can go down any hill with any amount of weight and it easy to maintain the brakes. You know at top of hill if you are at 15 psi you need to slow down because you are generating more heat then the drums can dissipate. If going downhill and you notice 10psi won’t hold the truck and trailer then you need to slow down. Yes the brakes will be hot. I believe Virginia Tech did a study comparing light steady pressure vs the snubbing way I believe. The temperature difference on the drums on a long hill/mountain was almost the exactly the same.

 

What you describe is my experience as well. Between my school and my first trainer I was taught both ways. I tried both ways and saw that when I did what you describe, I didn't smoke my brakes. I was lucky my truck, with no jake brake, had a pyrometer to monitor exhaust temps when climbing hills and a brake application gauge to monitor braking PSI from the foot pedal. "Light, steady pressure" worked every time. I think the alternative system is snub braking, where I would start downhill at the same slow speed and once speed increased 5-10 mph I would slow with moderate brake pressure, and repeat that cycle until at the bottom of the hill. That last method even using proper manual gear selection often produced smoking brakes. I'd urge every driver to experiment with both techniques when they have identical loads on one hill they routinely descend if they have a brake application gauge.

 

Y'all are totally missing the point. Yes, you can drag 10 psi all day and the drums won't overheat. But if think that is not affecting your total braking capacity, you're not understanding the difference between temperature and heat capacity.

Also, there is vast difference between that 10 psi at low speed without jakes and using that 10 psi with jakes to get down faster.
In the first instance, the fact the brakes are somewhat warm isn't a factor, as the energy required to stop isn't that high, and won't "fill the bucket"
In the second, there will substantially more heat in the brakes, the energy needed for a hard stop at the higher speed will quite likely take you into brake fade. A heavy, hard stop, especially downhill, will generate fade with absolutely cool brakes at the speeds 10 psi and full jakes will allow. @Brandt, as you kept asking in the other thread, why take that risk?

 

I don’t think there is a big risk you can stop just push the pedal. At 10psi you making the same amount of heat the drums can dissipate. If you do that hard stop then you will be making more heat then the drums can handle. You need to stop so it doesn’t matter. That’s the point of 10psi you still have stopping power. The hard brake if you need to stop is not enough to make the brakes fade out so you can’t stop.

I did this all the time going south on I-15 into Southern California. The scale was basically at the bottom of the hill and if open you needed to stop. That’s was using the engine brake most time, but you would just be going slower is the engine brake was not working or if you just use the engine brake you will be going slower. That’s why I say I use the engine brake to go faster. It sounds kinda funny to say.

Drivers need to be able to get downhill if the engine brake is not working. You can use just the engine brake and save your brake if you want, nothing wrong with that when the roads are dry. If you don’t mind driving that slow you can never touch you brakes if you want.

I would use both on dry roads and that would let me go faster. I’m not saying crazy fast or something you can do 25-35 mph it just depends on how much weight you have. If you only use engine brake then you will be going slower. I use everything the truck has to help get me downhill on dry roads. If the engine brake failed for some reason I can still get downhill just going to be slower ride.