BY CHRIS DALY
There are some policies and procedures around the firehouse that I commonly refer to as “Old Guy Rules.” Old Guy Rules are informal procedures that have been passed down throughout the years from an old guy to a new kid.
While most of these procedures are justified and useful, some are not. What is common among many of them is the fact that no one knows where they came from. The only background or justification for such a rule is the fact that “an old guy told me to.”
TURN OFF THE JAKE BRAKE
One of the most well-known Old Guy Rules is the fact that an apparatus driver is supposed to “turn off the Jake Brake in the rain.” While many of us grew up with an actual Jake Brake on our apparatus, the term has come to generically define any auxiliary braking device. An auxiliary braking device is defined in National Fire Protection Association 1901, Standard for Automotive Fire Apparatus, as “A braking system in addition to the service brakes, such as an engine retarder, transmission retarder, driveline retarder, or exhaust retarder.” For an excellent discussion regarding the different types of auxiliary braking systems and how they operate, read Bill Peters’s article, “Secondary Braking Systems: A Second Chance,” in Fire Engineering (September1995).
- Apparatus Rollovers, Part 9: Wet Road Friction
- Apparatus Rollovers, Part 8: More on the Friction Circle
- Apparatus Rollovers, Part 7: the Friction Circle
While many fire apparatus operators have been told to turn off the auxiliary brake on a slick road, few of those operators can tell you why. If you have been reading the recent series of articles on the friction circle and g-force, I am hoping that you may have an idea of why this practice is so important. Either way, let’s put some science behind an Old Guy Rule.
AUXILIARY BRAKING AND THE FRICTION CIRCLE
To better understand the danger of using an auxiliary braking device on a slick road, let’s first examine the use of an auxiliary braking system on a dry road. We will once again examine our trusty 320-foot curve that has served us so well in past articles. We will assume that the fire apparatus operator is rounding this curve at a speed of 37 miles per hour (mph), which equates to a lateral g-force of 0.28 g (see Table 1). You may remember from past articles that drivers will often begin to feel uncomfortable when the g-force rises above 0.20 to 0.30 g. Therefore, as the apparatus operator rounds the curve at 0.28 g, we will assume that he begins to feel uncomfortable.
Realizing that he may be driving a bit too fast, the driver takes his foot off the accelerator pedal to slow down. When the driver takes his foot off the accelerator pedal, the auxiliary braking device kicks in. When the auxiliary braking device engages, it decelerates the vehicle at 0.15 g in the longitudinal direction (front-to-back).
When we combine the lateral g-force that the vehicle was experiencing as it rounded the curve with the longitudinal g-force that the vehicle experienced when the auxiliary brake engaged, we find that the combined g-force is 0.31 g. When we plot this g-force on the friction circle of a dry road, which has a drag factor of 0.75 g, we see that the combined g-force is well within the friction circle (see Figure 1). Because the combined g-force remains inside the friction circle, the tires hold their traction, and the vehicle does not lose its grip on the road.
Figure 1: As the driver rounds this 320-foot curve at 37 mph, he creates 0.28 g in the lateral direction. When the driver takes his foot off the accelerator pedal, the auxiliary brake engages and decelerates the vehicle 0.15 g in the longitudinal direction. The combined g-force equates to 0.31 g, which is well within the friction circle of a dry road.
Now let’s assume that inclement weather has arrived, and the drag factor of our roadway has dropped from 0.75 to 0.30. We will assume that the driver is rounding the curve at the same speed of 37 mph and lifts his foot off the accelerator pedal, causing the auxiliary brake to engage with the same intensity. When we plot this scenario on the friction circle of the slick road, we see that the combined g-force now falls outside the friction circle (see Figure 2). As a result, the drive wheels on the apparatus break traction and the vehicle loses its grip on the road.
As we can see, the issue of using an auxiliary braking device on a slick road is straightforward. If the vehicle is rounding a curve or making a turn at a relatively high speed for the existing road conditions and the auxiliary brake engages, the combined lateral and longitudinal g-force may exceed the grip of the tires. If the driver creates a g-force that exceeds the grip of the tires, the vehicle will break traction with the road and most likely crash. This is why the old guys have always told us to “turn off the Jake Brake in the rain.”
NEW OLD GUY RULE
So, now let’s touch on a new Old Guy Rule that I have been hearing lately. This rule states that “you don’t have to turn the Jake Brake off in the rain if you are driving a new vehicle.” I thought this was interesting, so I began to investigate this rule a bit further.
When I pondered the question, I couldn’t figure out how an auxiliary braking device would know when the road is slick and change its behavior accordingly. Unless there is some sort of weather or moisture sensor on a new apparatus, an auxiliary brake would still create a braking force that could combine with a lateral g-force and exceed the grip of the tires. So, I began to pester salespeople at conferences and eventually called a manufacturer directly.
As a result of this investigation, I determined that if the tires on the apparatus begin to skid and the electronic stability control system (ESC) or antilock brake system (ABS) is triggered, it deactivates the auxiliary braking device in the process. This appears to be the source of the urban legend that the auxiliary braking device would “automatically shut off in the rain.” When I pressed this question with one of the manufacturers, I confirmed that the auxiliary brake would still create a longitudinal braking force in wet weather; however, it would shut itself off when the ABS or ESC engaged.
Figure 2: Now we will assume that the fire apparatus operator rounds the same curve on a slick day. As a result of the slick road, the drag factor has been reduced from 0.75 to 0.30. When we redraw the friction circle, we see there is much less grip to work with. When the auxiliary braking device engages, it creates a longitudinal g-force that falls outside the friction circle.
So, I ask you: Should we drive in a manner that causes the ABS or ESC system to have to activate to prevent a loss of control—especially since the activation of the auxiliary braking device could have triggered the ABS or ESC event in the first place? In my opinion, it turns into an evil cycle. I will be glad to reevaluate my position if an auxiliary braking device manufacturer can provide more information, but for now, my opinion stands: If an auxiliary braking device engages, it could create a combined g-force that could exceed the available friction of a slick road. Relying on the ABS or ESC to engage and automatically turn off the auxiliary braking device is not a good idea.
Moral of the story: Turn off the auxiliary braking device on a slick road. Just remember that when you do this, the stopping distance of the vehicle could increase dramatically. The driver will not be able to rely on the added braking force from the auxiliary braking system. Furthermore, turning off the auxiliary brake may cause the brake temperatures to rise, which can lead to a brake fade situation. Therefore, drivers must remember to adjust their driving habits accordingly and remember to slow down and account for the significant change in stopping distance that they may encounter when the auxiliary braking device is turned off.
Also, remember that driver training programs should include drive time with the auxiliary brake deactivated. This will allow the fire apparatus operator to get a good feel for how the vehicle will decelerate without the assistance of the auxiliary braking system. Most driver training programs do not provide a driver trainee with the opportunity to drive with the auxiliary braking device turned off. If a driver has never experienced this scenario, it may come as quite the shock when he is driving for the first time to a real emergency without the auxiliary braking system. Train accordingly to prevent such a surprise.
Chris Daly will present “Drive to Survive” at FDIC International in April 2020. The concepts discussed in this series will be reviewed in great detail. Go to www.fdic.com to register.
CHRIS DALYis a 22-year police veteran, serving as a patrol supervisor in West Chester, Pennsylvania. He has served 29 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 22,000 firefighters and police officers at more than 500 emergency service agencies across the United States.