Air Brakes

It occurred to me as I was trying unsuccessfully to stop on rain slicked I285 in Atlanta last Thursday that brakes are a pretty important component of trucking.

Heavy and most medium duty trucks all use air brakes. Why you ask? Most cars and trucks use "hydraulic" brakes, with a majority being vacuum assisted. What does this mean? When you push on the brake pedal, it pushes on a piston that tries to compress brake fluid, which is a very light oil. The fluid's properties include very little compression, so the force is transmitted to brake cylinders, where the pressure is converted back into mechanical motion to drum brakes, or a piston directly pushes on a brake pad - disk brakes. Engine vacuum is stored in a chamber at the end of the master brake cylinder to add a multiplying effect to the force of the brake pedal - if you've ever driven a set 'o wheels without power brakes, you will know and appreciate the vacuum assist.

However, this system has it's limitations. The braking force has it's limits, and the fluid can heat in a hurry. So, for heavy vehicles, vacuum assist hydraulic braking isn't the best option. So, what is?

The answer is air brakes. Air is the force conductive substance. However, the system has considerable differences compared to hydraulic brakes. Rather than using vacuum generated by the engine sucking for air, an actual air compressor is required, and storage tanks to hold the compressed air. The brake pedal (foot valve or treadle, as it is termed) does not directly pressure the air going to the brakes - it merely meters more air as the pedal is depressed. The more travel, the more air pressure will be applied. The treadles have some resistance, so "feathering" the brakes by feel can be simulated. Make no mistake, there is no feedback from the pedal like the hydraulic systems have - just changes in the environment that the driver has to observe. Mostly through the butt planted in the seat. The more you stomp, the more you slow down, and possibly lock up a wheel or eighteen. It just takes practice.

And the air from the treadle doesn't actually go directly to the brakes - it's merely a signal to several valves to release a correspondingly larger amount of pressure. Trailers have air storage tanks, and this "signal" goes to the valves using the air from those tanks to brake.

This is a diagram of s-cam drum brakes, probably the most common hardware trucks use. The brake pod transfers air pressure into rotational motion through the slack adjuster and turns the s-cam. As the cam turns, it spreads the brake pads against the drum, causing friction and therefore (hopefully) slowing the truck. The pods are spring loaded, so when the brakes are released, the pod retracts it's rod, and the cams return to rest. As the linings wear, the geometry changes, and the brakes become "out of adjustment."



This is the standard manually set slack adjuster. It fits on the splined shaft that has the s-cam on it's end. The brake pod rod attaches to one of the holes in the arm. This adjuster is made for three different applications - thus the three holes. A 9/16" box end wrench is used to push a locking ring back to expose a hex head for adjustment - it turns the splined insert to tighten or loosen the tolerance of the linings.

This is an self adjusting slack adjuster. It's set up to adjust to a certain torque - not enough torque means the linings are off the drum too far. So, as the brakes are applied, the adjusting mechanism constantly measures itself to keep the brakes tight.


This is a single chamber brake pod. As the air pressure pushes on an enclosed diaphragm (slang - pancake), it in turn pushes the threaded rod out - then to the slack adjuster and so on.



This is a double chambered brake pod. As a safety measure, air brakes are required to have these. If you run out of air for whatever reason, you still have the need to stop. The emergency circuit is designed to cut in at sixty pounds or less air pressure in the pressure tanks. It's all mechanical - there is a very strong spring in the pod that the separate air circuit has to compress before the truck can move. This retracts the rod, loosening the shoes from the parking position. Then when air is applied to brake, it must overcome the spring and the "backup" air to move the brake rod out.

So, if your truck is parked overnight and loses all it's air, it ain't gonna go nowhere without "building" air pressure to a bit over sixty pounds of indicated air pressure. There are actually two separate storage systems for safety and redundancy, and most trucks have two air pressure guages to show the levels in each. If, for whatever reason, the truck loses air while driving down the road, it will stop. Maybe not under complete control, but you can count on it coming to a halt in a hurry. Most systems recommend limiting the onboard air pressure not to exceed 120lbs.

I'm sure you've all seen the blue, red and black lines running from the tractor to the trailer. The electrical umbilical cord is generally black - but that may vary. It may or may not be coiled. The brake lines are sometimes black or gray without being coiled, but usually they are color coded. The air lines have "glad hands" on the ends.


They are designed to "break away" if you forget to unhook them when dropping a trailer, for instance. Sometimes they are color coded - you can see one is just anodized and not painted. The red line is the "emergency" line. It charges the emergency system on the trailer, and fills the trailer's air tanks. The blue is the "service" line. It carries the air that actuates the brake system.

Maximum braking no matter the system is achieved using "threshold braking." This entails keeping the brakes right on the threshold of locking up. One thing I did learn in college was the difference between static friction and sliding friction. Static friction is a higher figure than sliding - so as far as brakes are concerned, if you keep the tires from sliding, you have better braking power, not to mention directional control. Threshold braking is far easier to achieve with hydraulic braking systems because of the non compressibility of brake fluid. Air brakes, not so much. Anti-lock brakes on a car can adjust the braking power several times a second, but the air brakes have a natural delay due to the air compressing - there is a delay when the brakes are first applied until the necessary air gets to the brake pods, and there is a delay if they lock - it takes a bit to bleed off the necessary air pressure. Trailer brakes always have a time delay under severe and sudden loads.

So, generally, when threshold braking a semi, ya stomp on the pedal until the wheels lock or nearly lock, release quickly to keep directional control - if your drivers and steer axle wheels lock up, you have no control of where the rig is headed. Then, ya stomp on them again. Too much of this will run the rig "out of air." Also, this technique virtually guarantees the trailer brakes will be locked up continuously because of the delays involved. While the tractor brakes are released and you are starting to stomp on them again, the trailer brakes are just beginning to release. Stomping on the brakes stops that, so the trailer brakes stay "locked." Which is what starts jackknifing. Antilock brakes are supposed to take care of this in a perfect world, but we don't live in one. Lightly loaded or empty trailers have far more braking power than traction, period. The ideal braking situation has a truck loaded - but not to capacity. This brings up the static braking force the tires have on the surface without the full momentum of a heavy load to overcome.

Plus, antilock brakes don't work very well under certain conditions. One is on a gravel road. The vehicle burns off more kinetic energy if the wheel locks up and digs into the gravel, creating a wedge in front of the tire as it slides. The vehicle has to climb over that continuously renewing wedge of road surface. Of course, the brakes have to be released a bit to keep directional control, but electronically managed threshold braking is far less effective.

And, I've noticed that truck antilock brakes leave something to be desired on slick surfaces as well. It seems that the brakes lock up just like the old systems did. How do I know this?

We were going north on I285 on the west side of Atlanta during intermittent showers. One of the guys noted that the little bit of rain didn't seem to be staying on the road - but that was on the porous looking pavement. We crossed over a concrete surfaced overpass, and I noted how slick it looked - it had a shiny look I didn't like at all.

Rain slick roads are the most dangerous when the rain first starts. The accumulated grime - dirt, oil dripped from thousands of vehicles, particulates from exhaust, you name it - gets wet and turns to slime. Continuous rain washes all of this off and traction is better. As the rain starts to end, the crud washed from the thousands of vehicles starts to accumulate again, and it gets slicker.

But the worst is intermittent rain. The crud never gets rinsed off, and more is added by traffic.

Anyways, we had seen a couple of accidents and traffic tie ups in the other lane already. Suddenly, I saw dozens of brake lights in front of me. I had been shadowing a semi, and his trailer was fishtailing across another lane. I was in the lead, and hollered back to my buds that we were coming to a halt. We all had plenty of time to get shut down. A small box truck with a small trailer spun out, seemingly without hitting anything or jackknifing. He got 'er pointed in the correct direction and off we went.

I'm not one of THOSE PEOPLE who tailgate and switch lanes willy nilly like I've just got to win the race to wherever. I keep a big distance between me and the traffic. I generally have a bunch of Andrette wannabes cut into that space (including trucks), so I just back off and let them have it, and I get my safe space back. This drives some of my compadres nuts, because I don't bull my way through traffic. I don't care.

So, I'm back behind the clot of traffic without much going on around me - just like I like it. Once again, the interstate started turning into a parking lot ahead of me. This time, it was slightly downhill on concrete. I was having trouble getting 'er whoaed down. I had time to holler back about the situation, plus my traveling companions got to hear my running commentary about whether or not I was gonna get 'er shut down. My trailer was fishtailing, I was stomping on the pedal and releasing, stomping again, lather, rinse, repeat. I heard a buzzer go off. My air pressure had hit seventy pounds, getting close to the sixty pound lockup. I was also looking to thread my way through the lanes to dodge the vehicle in front of me - I was closing far too quickly.

But, I got it shut down just in the nick of time. My air compressor pumped the system back up as the traffic started to inch forward. A semi had hit something on it's driver's side pretty hard and ended up partially in the ditch, with the trailer sticking out in traffic. We didn't see what he'd hit. The driver was out in the rain on his cell phone.

Oh, and my trailer? It was trying to pass me? One of my buds started laughing and told me: "Some four wheeler was honking at your trailer for holding him up!" Apparently he was upset because I was nearly jackknifing and inconveniencing him on his journey.

Frankly, I could give a shit less about his delay. I was more concerned about the seat fabric that had been sucked into my rectal region, if you know what I mean, and I think that you do. He's just lucky he wasn't beside me to be swatted by that mean old trailer.