Downhill Braking

John C. Glennon, Jr., BSAT

[ Reprinted from the Trucker's World Magazine, Volume 5, Issue 6, June 2001]

When the Commercial Drivers License (CDL) manual was first published, it recommended that a driver use a light and steady application of the brakes when descending steep grades. This recommendation was based on an old theory that heavy brake applications would generate more heat than light applications. This method (controlled braking) was commonly taught to drivers and, even after changes were made to the CDL manual because this theory was proven wrong, the method is still taught and practiced today.

Snub braking is now the recommended method of downhill braking. This method works by: first, choosing the correct gear for the hill; second, allowing the truck to speed up to the maximum safe speed as it descends the hill; third, applying the brakes hard to slow the truck down 5 mph; and then repeating this process to the bottom of the hill. To understand why this method is recommended takes some understanding of the basics about how brakes work. Slowing a truck with it.s brakes, creates friction between the brake shoes and brake drum to convert the kinetic (forward movement) energy of the truck into heat energy dissipated by the brakes. The amount of heat energy produced is dependent upon the weight of the truck and the amount of slowing desired. Assuming these two factors remain constant, the manner in which the brakes are applied, hard for a short time or lightly for a long time, will not change the amount of heat energy and heat produced by the brakes. This heat energy will be distributed among all the brakes that are working. Again, assuming all other factors constant, the more brakes the system has working the cooler each brake will be.

This explains why the old theory of light and steady braking is incorrect. However, to understand why snub braking is the recommended practice, you must also understand the basics of pneumatic balance. Trucks have relay valves to control the application and release of the air brakes. A standard truck-trailer usually has one relay valve for the tractor drive axles and one for the trailer axles. The relay valves are controlled by air pressure from the foot valve (brake pedal). This control pressure opens the relay valve allowing the desired amount of air pressure from the air tanks to pass through the valve and supply pressure to the brakes. Pneumatic balance is created by having equal air pressure at all wheel ends. Pneumatic imbalance is a result of these valves that open at different pressures. For example, a tractor may be setup with a relay valve that opens at 15psi (15psi crack pressure relay valve) and the trailer being towed may have a relay valve with a 3psi crack pressure. A vehicle setup this way would only apply the trailer brakes during controlled brake application, which typically has an application pressure of less than 10psi. However, a .snub. brake application of 20 to 30psi will open all valves and apply all brakes. This type of imbalance can also be a result of contaminants and alcohol in the air system that can cause these valves to hang-up and have higher than normal crack pressures.

Snub braking became the recommended method of downhill braking as a result of testing done by University of Michigan Transportation Research Institute. This research found that trucks with properly balanced brake systems had basically the same average brake temperature when using either the controlled or snub braking method. However, trucks with poor brake balance were found to have more uniform brake temperatures when the snub method was used. Unless pneumatic testing is performed on a truck to ensure that proper brake balance is maintained, there is no way to know if a truck has good brake balance. This type of testing is difficult to perform in most trucking operations since a tractor is usually hooked to several trailers over relatively short time periods. Therefore, for the purposes of deciding which braking method to use, it would be virtually impossible to determine if a truck has good brake balance. This is why snub braking is the recommended method.

Although snub braking does compensate for imbalances in the pneumatic system of the brakes, there is a misconception that snub braking also compensates for brakes that are not evenly adjusted. Brakes that are not evenly adjusted have a torque imbalance. Torque balance is created by having matched mechanical components that are working properly and adjusted correctly. Snub braking has limited ability to compensate for torque imbalance. A good example of this would be a truck with a six-inch slack adjuster on one side of the axle and a five-inch slack adjuster on the other side. This truck will always have an imbalance at any pressure because the brake with the six- inch slack adjuster has more leverage. The same imbalance can happen with uneven brake adjustment because the force output of a brake chamber is directly related to the brake adjustment (push rod stroke).

Since the snub braking method cannot compensate for torque imbalance, trucks should always be inspected and repaired with the following in mind. A truck.s brake system should have matched mechanical components such as the same size brake chambers and same length slack adjusters on both sides of an axle and, most of the time, on all brakes in a group of axles (i.e. tractor drive axles). When inspecting the condition of the brakes, any isolated premature wear found is an indication of a balance problem. If one brake wears faster than the rest, there is a torque balance problem and that brake is doing more work than the rest. If one brake wears much slower than the rest then that brake is not working as hard as the rest. When brakes are repaired, it is important that the cause of an identified torque imbalance be found before repairs are made. Repairs made without correcting the torque imbalance could amplify the problem causing the overworked brake to work even harder. It is equally important to ensure that the same repairs are done on both sides of an axle. If the brake hardware is replaced on the right side of an axle it should also be replaced on the left side. If the s-cam bushings are replaced on the right side they should be replaced on the left.

Snub braking is the method that every truck driver should be using. Although snubbing is a very good precautionary measure, it is still no substitute for a properly balanced brake system. Brake imbalances not only cause brakes to overheat when driving in the mountains, but also can cause instability both on slick driving surfaces and during hard brake applications. These stability problems (to be discussed in a future article) are the primary cause of jackknifes and trailer swingouts. Therefore, I recommend not only that trucks be tested, repaired, and maintained to ensure that they have good brake balance, but also that the snub braking method be used to compensate for any variances that result from interchanging tractors and trailers.

About the Author

John C. Glennon, Jr., is a forensic automotive technologist who performs crash reconstruction and detailed vehicle testing for trucking companies, insurance companies and lawyers involved in investigating and litigating motor vehicle collisions. He has a B.S.A.T. degree in Automotive Technology, he is a triple-certified Master Automotive Technician, and he holds a Class A CDL in the state of Kansas.


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