By Chris Daly
Last month’s article addressed the issue of fire apparatus braking efficiency. Braking efficiency is the amount of available roadway friction a vehicle can put to use to skid to a stop. A vehicle with reduced braking efficiency will have a longer stopping distance.
In September 2016, a large-scale training exercise was conducted to evaluate the braking efficiency of fire apparatus. The results of this testing confirmed that fire apparatus have a reduced braking efficiency when compared to a passenger car. While most hydraulic-braked passenger vehicles have near 100 percent braking efficiency, the braking efficiency of a large fire apparatus averaged around 68 percent.
One of the interesting side effects of this testing exercise was a realization that many fire apparatus have improperly adjusted brakes. Of the 10 fire apparatus that arrived to participate, two were put out of service by law enforcement truck inspectors because of bad brakes. If a vehicle does not have properly adjusted air brakes, it will not be able to generate an effective braking force. As a result, the vehicle’s stopping distance could increase dramatically.
|A comparison of stopping distances on a dry road based on skid tests conducted in September 2016. At each speed, the top vehicle represents the stopping distance of a passenger car. The middle vehicle represents the stopping distance of a fire apparatus with properly adjusted brakes. The bottom vehicle represents the stopping distance of a fire apparatus with brakes out of adjustment.|
These facts were made evident during the skid tests. Of the two apparatus that were found to have brakes out of adjustment, one also had a leaking airline. This truck was immediately placed out of service and was not available for testing. However, we were able to skid the other apparatus before and after the brakes were adjusted. This provided an invaluable opportunity to examine the difference in stopping distance between properly adjusted brakes and those out of adjustment. Needless to say, the results were startling.
On average, a fire apparatus with properly adjusted brakes had a 45 percent increase in stopping distance when compared to a passenger car. However, the apparatus with brakes out of adjustment had a 95 percent increase in stopping distance compared with a passenger car. The stopping distance of the fire apparatus with brakes out of adjustment was nearly twice that of a passenger vehicle. It is difficult enough to bring a moving fire apparatus to a safe stop. If brakes out of adjustment are added to the equation, they will significantly compound the problem.
How Air Brake Systems Work
To understand the importance of brake adjustment, it is important to discuss how an air brake system works. In summary:
- To apply the brakes, the driver presses down on the “foot valve,” otherwise known as the “brake pedal.”
- Air travels from the air tank reservoir through the air lines and into a brake chamber.
- Inside the brake chamber, the compressed air presses against a rubber diaphragm. The air causes the diaphragm to expand and push on a pushrod.
- The pushrod pushes on a slack adjuster, which turns a camshaft.
- When the camshaft turns, it will twist an S-Cam, which will expand the brake shoes and press the brake pads against the drum.1
The braking force available at each wheel is directly related to how far the pushrod travels. The distance the pushrod travels is called “stroke.” As the stroke increases, the braking force at the wheel decreases. If the pushrod travels too far, it will “bottom out,” and there will be no braking force at the wheel.
Under normal circumstances, there is a very small gap between the brake pad and the brake drum. When the driver presses the brake pedal and charges the system with air, the brake shoes only need to travel a short distance to press the brake pads against the drum. However, as time passes, the brake pads wear down. As the brake pads wear down, the pushrod will have to travel a longer distance to press the pads against the drum. As the pushrod has to travel farther, the braking force at the wheel will decrease. If the pushrod has to travel too far, it will bottom out, and the braking force at the wheel will be nonexistent.
|1 Prior to skid testing the fire apparatus, each rig was weighed and inspected by law enforcement commercial motor vehicle enforcement officers. The officers measured the pushrod stroke of each apparatus. Two of the apparatus were placed out of service because the brakes were out of adjustment. (Photos by author.)|
A similar situation can occur if the brake drum heats up from overuse. If the vehicle is descending a long hill or the brakes are being used forcefully in heavy traffic, the brake drum will begin to overheat and expand. As the drum expands, the pushrod will have to travel a longer distance to press the brake pads against the drum. As the pushrod stroke increases, the braking force decreases. This is known as brake fade.
If one of the brakes begins to fade, the other brakes will have to work harder to burn off the vehicle’s kinetic energy. Because the other brakes are working harder, they may begin to overheat and fade as well. This situation could lead to a “cascade failure” of the braking system as all of the brakes quickly fade one after the other.
Federal regulations set strict limits on how far a pushrod can travel before it is out of compliance. When roadside truck enforcement officers inspect a truck, one of the first things they will do is climb underneath the vehicle and measure the pushrod stroke. The allowable pushrod stroke will depend on the size of the brake chamber it is attached to. If the pushrod stroke exceeds the federal standard, the truck may be placed out of service. Furthermore, if the truck is involved in a crash and an investigator determines the brakes were not properly adjusted, there will be significant legal implications for both the fire department and fire apparatus operator.
Fire departments must recognize the importance of properly adjusted brakes. The department must find a certified emergency vehicle technician (EVT) or other qualified person to measure the brake adjustment on a regular basis. Many brake manufacturers provide training programs that will train fire apparatus operators on how to inspect and maintain the braking system in-house. Having drivers trained to perform these checks would be a huge benefit to any fire apparatus safety program (see http://bit.ly/2uw71j8 for a great article on in-house brake inspection procedures).
On September 5, 2004, a fire apparatus in Missouri was en route to a fire in an apartment complex. As the engine was traveling on a straight road, it encountered a civilian vehicle that appeared to be stopping in the left lane to yield the right of way. The fire apparatus began to pass the stopped vehicle on the left when the vehicle suddenly turned directly in front of the fire apparatus. The fire apparatus struck the civilian vehicle and then traveled into the oncoming lane. While in the oncoming lane, the fire apparatus struck a stopped vehicle, sheared through a utility pole, and came to final rest after crashing into a tree. The acting captain who was riding in the officer’s seat was fatally injured during the collision with the tree.
During the crash reconstruction, a police investigator inspected the air brake system. During this inspection, the police officer discovered that the rear air brakes were out of adjustment. While the allowed pushrod stroke was two inches, the rear brakes measured 2¼ and 21⁄8 inches. The police investigator determined that this excess pushrod stroke reduced the braking efficiency to a point where the fire apparatus was unable to stop before striking the tree. While the braking issues did not cause the initial crash, the police investigator concluded that if the brakes had been properly adjusted, the vehicle would have been able to stop before striking the tree.
|2 A police officer measuring the pushrod stroke of the fire apparatus.|
Further investigation revealed that the braking system on the apparatus had not been serviced for nearly 16 months prior to the crash. The city fire chief attributed this oversight to the fact that the city did not have enough reserve fire apparatus to use while front-line apparatus were being inspected and maintained. The city later issued a statement that read, “The city had no knowledge, or reason to believe, that any of the brakes on this fire truck were out of adjustment.” No reason to believe the brakes were out of adjustment after 16 months of hard use? Really?
Fire departments must ensure that the apparatus brakes are regularly inspected. In career departments, fire apparatus operators should be conducting brake inspections as part of daily truck checks. Brake inspections should look for obvious issues such as the brakes failing to hold and issues with the air system. In volunteer departments, this inspection should be done weekly.
A more detailed inspection of the air brake system should be conducted monthly. The more detailed inspection should include a trained and qualified inspector measuring the pushrod stroke at each brake chamber. If a brake is out of adjustment, place the vehicle out of service and turn it over to a qualified EVT to make repairs. Finding and repairing issues with the brake system will help reduce the risk of a crash and create a safer working environment.
1. Pennsylvania Department of Transportation, Commercial Driver’s License Manual, pages 5-1 to 5-11.
CHRIS DALY is a 20-year police veteran, currently serving as a patrol supervisor in West Chester, Pennsylvania. He has served 27 years as both a career and volunteer firefighter, holding numerous positions, including the rank of assistant chief. He is an accredited crash reconstructionist (ACTAR #1863) and a lead investigator for the Chester County (PA) Serious Crash Assistance Team. Daly is a member of the Fire Apparatus & Emergency Equipment editorial advisory board. Daly has also developed an emergency vehicle driver training program called “Drive to Survive,” which has been presented to more than 18,000 firefighters and police officers at more than 440 emergency service agencies across the United States.