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5.4.3 – Emergency Stops
If somebody suddenly pulls out in front of you, your natural response is to hit the brakes. This is a good response if there is enough distance to stop and you use the brakes correctly.
You should brake in a way that will keep your vehicle in a straight line and allow you to turn if it becomes necessary. You can use the “controlled braking” method or the “stab braking” method.
Controlled Braking. With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make a larger steering adjustment or if the wheels lock, release the brakes. Re-apply the brakes as soon as you can.
Stab Braking. Apply your brakes all the way. Release brakes when wheels lock up. As soon as the wheels start rolling, apply the brakes fully again. (It can take up to one second for the wheels to start rolling after you release the brakes. If you re-apply the brakes before the wheels start rolling, the vehicle will not straighten out.)
5.4.4 – Stopping Distance
Stopping distance was described in "Section 2: Speed and Stopping Distance". With air brakes there is an added delay called ”Brake Lag.” This is the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light/medium trucks), the brakes work instantly. However, with air brakes, it takes a little time (one-half second or more) for the air to flow through the lines to the brakes. Thus, the total stopping distance for vehicles with air brake systems is made up of four different factors.
Perception Distance + Reaction Distance + Brake Lag Distance + Braking Distance = Total Stopping Distance
The air brake lag distance at 55 mph on dry pavement adds about 32 feet. So at 55 mph for an average driver under good traction and brake conditions, the total stopping distance is over 450 feet.
Multiple-Choice Questions:
With air brakes there is an added delay called "brake lag." Define brake lag:
- This is the initial "surge" you feel when you're braking and the tires lock up
- This is the time it takes for the brake lining to make contact with the S-cam
- Is is the lack of braking pressure you experience when the air compressor turns on
- This is the time required for the brakes to work after the brake pedal is pushed.
What is the equation for total stopping distance with an air brake system?
- Sliding Distance + Resuming Distance + Rolling Distance + Initial Stopping Distance = Total Stopping Distance
- Pressure Distance + Reaching Distance + Brake Grip Distance + Braking Distance = Total Stopping Distance
- None of these are correct
- Perception Distance + Reaction Distance + Brake Lag Distance + Braking Distance = Total Stopping Distance
If somebody suddenly pulls out in front of you, your natural response is to hit the brakes. You should brake in a way that will:
- Lock up the tires to create maximum grip on the roadway
- Keep your vehicle in a straight line and allow you to turn if it becomes necessary.
- Allow people alongside of you to veer out of the way quickly
- Lock up the trailer tires, but allow the drive tires and steer tires to continue to roll
Describe controlled braking:
- Apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make a larger steering adjustment or if the wheels lock, release the brakes. Re-apply the brakes as soon as you can.
- None of these are correct
- Turn the wheel while applying the brakes hard enough to lock the drive tires. The drive tires locking will produce more grip on the roadway, giving you more braking force and more control.
- Apply the brakes as hard as you can so that the wheels lock. Keep the wheels locked and the steering wheel movements very small while doing this. If you need to make a larger steering adjustment, release the brakes. Re-apply the brakes as soon as you can.
Controlled Braking. With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make a larger steering adjustment or if the wheels lock, release the brakes. Re-apply the brakes as soon as you can.
The air brake lag distance at 55 mph on dry pavement adds about 32 feet. So at 55 mph for an average driver under good traction and brake conditions, the total stopping distance is:
- Over 800 ft
- Under 300 ft
- Over 450 ft
- About 50 ft
Describe stab braking:
- Apply your brakes all the way. Release brakes when wheels lock up. As soon as the wheels start rolling, apply the brakes fully again.
- Apply the brakes as hard as you can so that the wheels lock. Keep the wheels locked and the steering wheel movements very small while doing this. If you need to make a larger steering adjustment, release the brakes. Re-apply the brakes as soon as you can.
- Turn the wheel while applying the brakes hard enough to lock the drive tires. The drive tires locking will produce more grip on the roadway, giving you more braking force and more control.
- None of these are correct
Stab Braking. Apply your brakes all the way. Release brakes when wheels lock up. As soon as the wheels start rolling, apply the brakes fully again. (It can take up to one second for the wheels to start rolling after you release the brakes. If you re-apply the brakes before the wheels start rolling, the vehicle will not straighten out.)
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