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Crash Forensics has the capability of performing complete testing of air brakes on site. The testing we perform can identify both catastrophic failures and brake deficiencies. Where other inspectors only check brake adjustment, we do full air system testing by checking the operation of all the valves, gages, warning devices, and pressure supply system. We are sensitive to concerns that the vehicle condition could be altered during testing. Therefore, the testing we perform is structured in a way that allows us to obtain complete results without removal of any critical parts. In most cases, complete testing can be performed by removing only one air hose and the brake dust covers. The following discussion outlines some of the more common problems with air brakes. These problems as well as others are identifiable through our testing. Link to our brake expert.

Air Brake Failure Analysis Explained
Truck brakes are often blamed for causing crashes. Most commonly, this claim comes from the truck driver, who is trying to transfer blame from himself to a failure of the truck. When this claim is made, interested parties often assume that a catastrophic failure caused a complete loss of braking force. Stated differently, the assumption is that a defective component of the brake system spontaneously failed causing the brakes to no longer function. In reality, brake systems are designed so that a complete catastrophic failure is an extremely rare event. Therefore, alleged brake failures usually are not failures at all but performance problems stemming from deficient maintenance.

Truck braking systems can usually still provide low levels of braking force even with maintenance deficiencies. This low-level braking force will allow the truck driver to adequately stop the truck for normal operations such as slowing for a stop sign. However, when a high level of braking force is demanded in an emergency, these deficiencies will show themselves. Even though the driver is applying the brakes very hard, he will not get the expected result, which is a high level of deceleration. In this case, the brakes are slowing the truck, but not as quickly as the driver expects them to. Most likely the driver will perceive that the brakes are not working at all. In reality, the brakes are working, but not at the level of performance expected for an emergency application.

Brake Imbalance
Another brake performance problem that results from poor maintenance is brake imbalance, which can be caused by deficiencies that affect some of the brakes in the system but not others. Brake imbalance can also happen by having mismatched brake system components that cause some brakes to work harder than others. Brake imbalances can lead to instability during braking, brake fade, and brake fires.

Good brake balance is a result of having properly matched, maintained, and adjusted brake system components, as well as a properly loaded trailer. There are two main types of brake balance; torque balance and pneumatic balance. Proper torque balance is created by having matched mechanical components, which are working properly and adjusted correctly. If a truck has a torque imbalance, some of the brakes will work harder and lock up easier than others. When a truck has a torque imbalance problem, brakes are usually affected individually. Proper pneumatic balance is created by having equal air pressure at all wheel ends. When a truck has a pneumatic imbalance, some of the brakes will also work harder and lock up easier than others. However, pneumatic imbalance will be manifested at one axle or one set of tandem axles, such as the tractor drive axles or the trailer axles.

Brake imbalance is one of the more common causes of loss-of-control crashes for air-braked trucks. If a truck does not have good brake balance, it will have a propensity toward either jackknifing or trailer swingout. Jackknifing occurs when the tractor's drive axles achieve a higher level of braking force than the trailer axles. When this happens in an emergency or low traction situation, the tractor drive axles will likely lock up while the trailer axles are still rolling. With the tractor's drive axles locked, they will lose directional stability and the unbraked trailer load will push the tractor into a rotation around the king pin. Trailer swingout is similar to a jackknife, but occurs when the trailer axles achieve a higher level of braking force than the tractor. With the trailer brakes locked and directional stability lost, the tractor will drag the trailer, which will then begin to swing out.

As you can see, if the brake system is not balanced for any one of a number of reasons, the result will be that some of the brakes will have to work harder than others. This imbalance can cause the hardest working brakes to become too hot, resulting in brake fade or fire. This usually happens when the truck with a brake imbalance is descending a long steep grade. As a truck is being braked, the properly working brakes will be doing more than their share of the work. This will cause the overworked brakes to become much hotter than they should be and may eventually lead to brake fade. When the good brakes fade, the only brakes available to stop the truck will be the deficient ones. Additionally, as the brakes get hotter, they can catch on fire and/or catch the corresponding tires on fire. A brake imbalance will also affect the stopping distance. Because the deficient brakes will not be working at their peak efficiency during hard brake applications, the truck will take longer to stop.

Brake Inspection
A brake imbalance can be discovered by inspecting the brake linings. If an imbalance exists, the linings at some of the wheel ends will wear faster than others. By law, a truck driver is required to check his vehicle on a daily basis and note any problems, such as improper brake balance, on his daily log. Then, the noted problems must be fixed before operating the vehicle (Title 49, Code of Federal Regulations, Part 392.6).

Since improper brake balance could have either caused or contributed to cause a crash, it is always a good idea to have the brakes tested after a truck collision. This kind of inspection requires specialized equipment that can supply and regulate air pressure to a vehicle that may have a damaged air-supply system. Information gathered during this inspection can be used to calculate brake force in order to determine the efficiency of each brake. Then, this information can be used in the reconstruction of the crash to determine not only what the pre-braking speed of the truck was but also if the brake condition was a causative factor in the crash.

This discussion shows the importance of proper brake maintenance. However, it must be understood that maintenance is one thing and proper maintenance is yet another. Truck brakes can be maintained or serviced in a way that can actually create brake imbalance. This is a very widespread problem that happens when a truck needs maintenance or service on just one brake, but not on others. When maintenance or repairs are done to that one brake, the same maintenance or repairs should at least be done to the brake on the opposite side of the axle, if not all the brakes. However, this kind of balanced maintenance is rare since truck owners are usually unwilling to work on brakes that have been functioning properly. As a result, the truck is put back in service with one brake that will be working harder than the rest. Applying this practice over time, each of the brakes on the truck will be serviced at a different time, creating a brake system that may be able to generate a high level of brake force, but one that has a severe imbalance.

As you can see, proper brake maintenance is extremely important in the prevention of crashes. Even though proper brake maintenance is so important, it is far from standard practice in the trucking industry. A large number of trucks are not regularly maintained, but the ones that are regularly maintained are most often not maintained properly. This fact is very evident to the crash investigator who rarely ever sees 18 skid marks from an 18-wheel truck that locked its brakes at an crash scene.

Crash : John C. Glennon, Chartered Contact Us Links Home