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The mechanics behind slowing down and stopping are not so simple anymore

By on March 2, 2015

Take a short walk around a truck and you will notice the evolution of the brake system. This has become a very sophisticated bit of engineering, often due to the need to meet six requirements of the Federal Motor Vehicle Safety Standards (FMVSS).

Separate air reservoirs

There must be separate air reservoirs for the front and rear brakes. Engineers and designers designate the rear brakes as the primary brake system, and the front brakes or steer axle brakes as the secondary system. Once this distinction is clarified, they install check valves in air lines and/or air tanks; route air lines directly from an air tank to the air system that it supports, and internally separate valves in a single housing to prevent mixing of systems. That’s why there are two air gauges or two air gauge needles on your dashboard. These are required by law so the operator can identify how much air pressure is in the primary and secondary brake systems.

Air volume

There must be 12 times as much air volume in the air reservoirs as in the combined volume of the brake chambers. That means that the number and size of air tanks will vary, typically based on the number of axles on the vehicle. If you run multiple axles you will notice multiple air tanks on your vehicle, which can be installed in various ways, but the air cannot mix between the primary and secondary air systems.

Brake release time

All tractors and trailers have a brake release time requirement, defined as the amount of time it takes to drop air pressure from 95 to 5 psi. Because the time requirement for your brakes to release once you take your foot off the brake pedal is so short, there is a very large hole in the bottom of your brake valves. Some truck manufacturers will install quick-release valves close to the brake chambers to aid in the speed of the air exiting the brake chambers. It is critical that these systems operate properly in order for you to maintain control of your vehicle.

Brake application time

The brake application time requirement for all tractors and trailers is defined as the amount of time it takes to raise chamber pressure from 0 to 60 psi, when reservoir is 100 psi. The engineering required to comply with brake application timing is a delicate science. The length of air lines; the angle of air fittings; the brake stroke adjustment and a very tricky design, called crack pressure, all contribute to how long it will take for each brake to apply. Crack pressure is the amount of air pressure needed to trigger the operation of a brake relay valve. Brake valves often look identical but may have different crack pressures. That is why it’s important to always have your brake system serviced by trained professionals.

Parking brakes

Parking brakes must apply within 3 seconds of application. While this requirement may seem less important, it could be very dangerous if your truck is not in compliance. In order for your parking brakes to apply, the air must escape from the brake chambers. Your parking brakes are applied by a very large spring inside the brake chamber and they are released by air pressure. That air pressure must quickly be routed out of the brake chamber as soon as you pull the parking brake knob. This quick-release system typically utilizes some of the same design principles as your foot brakes. A quick-release valve is mounted near the chambers or replaces the parking brake valve, and has a very large hole in the bottom to aid the escape of air.

Stopping distance

Different vehicles will be subject to certain stopping distance requirements. A recent revised application of this requirement has triggered many technology changes, such as the increase in disc brake systems on Class 8 trucks, larger brake chambers and huge brake shoes on the wheel end. A shorter stopping distance requirement has manufacturers scrambling to nail down the most competitive design in the market to control both build and assembly costs as well as maintenance costs for the end user. The good news is that the brake systems are more efficient and positively impact the safety of trucks on the highway. ABS, traction control and stability control are just some of the recent safety systems that directly interact with the brake system, and there are bound to be more. Brake systems keep getting more sophisticated, and I do not see the truck manufacturers slowing down the increase in technology.

Homer Hogg’s “Maintenance Matters” airs on the Dave Nemo Show (Road Dog Trucking, SiriusXM 146), 8 a.m. ET, the first and third Thursday of each month.

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