Crash Forensics provides crash analysis services for large truck mountain grade crashes. All aspects of these large truck crashes including the road, the driver, and the truck require special consideration. Mountain grade crashes commonly occur on steep mountain grades and passes such as:

1. Berthoud Pass (US-40 CO)
2. Cajon Pass (I-15 CA)
3. Donner Pass (US-40/ I-80)
4. The Eisenhower (I-70 CO)
5. Loveland Pass (US-6/ I-70 CO)
6. Monarch Pass (US-50 CO)
7. Monteagle Pass (I-24 TN)
8. Parley Summit (I-80 UT)
9. Red Mountain Pass (US-550 Co)
10. Sandstone Mountain (I-64 WV)
11. Tejon Pass, aka The Grape Vine (I-5 CA)
12. Vail Pass (I-70 CO)
13. Wolf Creek Pass (US-160 CO)

The causative factors of mountain grade crashes are explained below. Link to one of our experts.

Mountain grades are one of the most unforgiving truck driving environments. To safely traverse mountain grades requires: (1) an alert, well trained, and humble truck driver; (2) a properly loaded and maintained truck; and (3) a properly signed roadway that gives timely information to the truck driver. Any deviation from these requirements will increase the crash probability.

Many mountain passes like Loveland Pass, Berthoud Pass, Monarch Pass, and others are twenty to thirty miles long from base to base. These passes can take an hour or more to traverse in a truck. On these long ascents and descents, truck drivers often get too aggressive trying to stay on schedule and push the limits of their truck. Mountain grade crashes are not only caused by runaway trucks but more commonly by roadway curves, switchbacks, and other unexpected hazards for which truck drivers are unable to adjust their speed.

When truck drivers aggressively descend grades, their ability to adjust speed for unexpected hazards depends largely on the braking system and how they use it. However, most truck drivers have been taught to improperly descend grades based on an old trucker's tale. This trucker's tale was perpetuated in 1989 when the Commercial Drivers License (CDL) manual published it as a recommendation for downhill braking in its first edition. The recommendation was that a truck driver use a light and steady brake application (controlled braking) when descending steep grades. This recommendation was based on the misguided theory that assumes heavy brake applications generate more heat than light applications.

The following is the incorrect recommendation published in the 1989 CDL manual:

Some people believe that letting up on the brakes from time to time will allow them to cool enough so they don't become overheated. Tests have proven this is not true. Brake drums cool very slowly, so the amount of cooling between applications is not enough to prevent overheating. This type of braking requires heavier brake pressures than steady application does. Heavy pressure on the brakes from time to time builds up more heat than light continuous pressure does. Therefore, select the right gear, go slow enough, and maintain a lighter, steadier use of the brakes. Driving Safely

Anyone who understands conservation of energy and its relationship to automotive brake systems knows that this old truckers' tale and the first edition of the CDL manual are obviously wrong. Right after the first CDL manual was published, a scientific study proved that controlled braking was wrong. This study resulted in downhill braking recommendations that are effectively the exact opposite of what was first recommended by the CDL manual. As a result, changes were made to the CDL manual to mirror the recommendations of this study and published in the 1993, second edition. Yet, the controlled braking method that was taught to truck drivers for years is today still taught to and practiced by many stubborn truck drivers. Snub braking became the recommended method of downhill braking as a result of the study done by the National Highway Traffic Safety Administration (NHTSA) and University of Michigan Transportation Research Institute (UMTRI). The following is the correct recommendation published in the current CDL manual:

Apply the brakes just hard enough to feel a definite slowdown. When your speed has been reduced to approximately five mph below your "safe" speed, release the brakes. (This brake application should last for about three seconds.) When your speed has increased to your "safe" speed, repeat steps 1 and 2.

For example, if your "safe" speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade.

To help explain the snub braking method, the following is a basic discussion of the brake concepts of conservation of energy, brake fade, and pneumatic balance. Conservation of Energy tells us that energy is neither created nor destroyed and can only be converted from one form to another. Truck brakes are energy converters, converting kinetic energy (movement) of the truck into heat energy through the friction between the brake shoes and drums. The heat energy is then removed from the brake primarily by convection (air moving over the brake). Since Ke=½ m·V² (Kinetic energy equals one half mass times velocity squared) and the brakes are converting Ke, then the amount of heat energy produced depends only on the weight of the truck and the desired speed reduction. If the weight and the desired speed reduction remain constant, the way the brakes are applied, either hard for a short time or lightly for a long time, will not change the amount of heat energy the brakes output.

Brake fade is largely a result of saturation. When the brake is converting heat faster than it can be removed, at some point it will become "full" or saturated. A brake system's, resistance to saturation is a result of its ability to store and "drain" heat. The brake systems ability to store and "drain" heat is largely affected by the number of brakes in the system. As an example, a one-gallon bucket can only hold one gallon of water even if you try to put two gallons of water in it. The two gallons of water can be distributed to two one-gallon buckets, but no more water can be added since the system is full (saturated). Now if the two one-gallon buckets each had a drain, then more water could be poured into them as they drain as long as the water is poured slow enough that they don't become full again. Alternatively, if you pour the two gallons of water into four one-gallon buckets with drains, you now have a 2-gallon reserve and the water will drain faster because two more drains were added to the system. In the same way, the more brakes a braking system has working, the more capacity it has to hold heat and the more contact the system has with moving air to "drain" the heat.

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 work harder than others. Pneumatic balance is largely affected by the relay valves that 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. These relay valves are controlled by air pressure from the foot valve (brake pedal). This control side pressure opens the relay valves allowing the desired amount of supply side pressure from the air tanks to pass through the valves and supply air pressure to the brakes. Pneumatic imbalance is a result of these valves opening 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 may have a relay valve with a 3psi crack pressure. A truck set up this way will only apply the trailer brakes during a 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 tractor and trailer valves. This type of imbalance can also result from contaminants and alcohol in the air system that cause these valves to hang-up and have higher than normal crack pressures.

The UMTRI study found that trucks with properly balanced brake systems had basically the same average brake temperature when using either controlled or snub braking. 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 different trailers over relatively short time periods. Therefore, because it is virtually impossible to determine if a truck has good brake balance, snub braking is the recommended method.

Although snub braking does help prevent brake fade caused by imbalances in the brake's air system, there is a misconception that snub braking also helps prevent brake fade caused by torque imbalances such as uneven brake adjustment. 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 the 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 torque imbalance. If one brake wears faster than the rest, there is an imbalance 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, the cause of an identified torque imbalance need to 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 and overheat. Equally important is 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 side.

Snub braking is the method that every truck driver should be using. Although snubbing is a very good precautionary measure, it is not a 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 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.

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