Understanding Braking Capacity

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.

 

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.

 

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.