emergency/parking brake usage
05-27-07, 08:04 PM
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emergency/parking brake usage
I wrote this article when I was Tech Advisor to another club. You may find it interesting. RX7MAVEN
THE PROPER USE OF THE EMERGENCY BRAKE/PARKING BRAKE (PB),
and The PARK position on Automatic Transmissions
Most automobiles use their rear brake shoes or disc as a PB. There are some exceptions that use drive shaft PB’s, and some with PB drums in the center of their rear disc brake. The PB’s are normally actuated by a foot-peddle, a twist and pull leaver, or a center mounted or side mounted brake lever.
Virtually all cars use a cable between the actuator and a fulcrum leaver at the rear to press the brake shoe or pad against the rotating drum or disc.
Older cars use adjustable cables to take up cable stretch. Newer cars use automatic mechanical ratchet adjusters to compensate for cable stretch.
If you can picture all these pieces working together you get the same result.
That would be a friction material (shoe or pad) pressed hard enough against a drum or disc to keep the drum or disk from turning. The locked drum or disc preventing the rear wheels from turning.
With few exceptions the usefulness of a PB as an emergency brake is a joke.
Do not hesitate to use it in an emergency. It is still better than nothing
In an emergency situation, where there is a hydraulic brake failure, the emergency brake only works on the rear wheels. Also realize that applying pressure to a cable is actuating the brake. You are correct to think that I am essentially describing 1920 mechanical brakes. Those brakes were scary, but top speeds rarely exceeded 35 miles per hour. The point I am trying to make is that hydraulic brakes replaced mechanical brakes for a darn good reason. They can apply immense pressure to the stopping media.
Now that I have bored you with the history of the PB, lets get to the subject. After prolong use PB cables reach the end of their adjustments. The cables stretch. Think about why the stretching occurs. It is a stranded cable that tightens and thus becomes longer when pressure is applied at both ends to actuate the PB. To extend the life of the cable, and to almost eliminate the need for adjustment, follow these steps:
Apply the hydraulic foot brake firmly. This will push the friction material against the drum or disc at a greater pressure that could be obtained with actuating the cable.
The PB can now be applied while holding the hydraulic brakes. You are using the cable to take up the slack rather than to apply the required pressure. Apply the pressure gently until the cable tightens. Jerking the PB leaver or pedal will put unnecessary pressure on the cable.
Try it. It works.
While on the subject of parking devices it is worthwhile to investigate the PARK position on your automatic transmission. I have found that most people use PARK as their PB, and not using the PB. It you ever saw the PARK mechanism in an automatic transmission you would feel uncomfortable about using it as a PB. It is usually a pin dropped into a cog. The pin being no bigger in diameter than your small finger. Using the PARK as your only hold when the car is on a reasonable grade could cause the following:
1.) Sheer the cog.
2.) Sheer the pin.
3.) Jam the pin in the cog making it difficult or impossible to get the vehicle out of PARK. When you pull the car out of PARK there should be no resistance. Hearing a clunk when you do this indicates too much pressure has been applied to the pin/cog.
On many occasions our shop was called to get a vehicle parked on a grade out of park. The procedure was to gently push the vehicle in a direction opposite the slope. This would reliever the pressure on the pin/cog until the pin could be released.
There is also the remote possibility that, when parked, another car could tap you car, and cause too much instant pressure on the pin/cog.
Do it properly. Set the PB as previously described. Make sure the PB is holding. Then put the car in PARK. This will assure that no undue pressure is being applied to the PARK mechanism.
THE PROPER USE OF THE EMERGENCY BRAKE/PARKING BRAKE (PB),
and The PARK position on Automatic Transmissions
Most automobiles use their rear brake shoes or disc as a PB. There are some exceptions that use drive shaft PB’s, and some with PB drums in the center of their rear disc brake. The PB’s are normally actuated by a foot-peddle, a twist and pull leaver, or a center mounted or side mounted brake lever.
Virtually all cars use a cable between the actuator and a fulcrum leaver at the rear to press the brake shoe or pad against the rotating drum or disc.
Older cars use adjustable cables to take up cable stretch. Newer cars use automatic mechanical ratchet adjusters to compensate for cable stretch.
If you can picture all these pieces working together you get the same result.
That would be a friction material (shoe or pad) pressed hard enough against a drum or disc to keep the drum or disk from turning. The locked drum or disc preventing the rear wheels from turning.
With few exceptions the usefulness of a PB as an emergency brake is a joke.
Do not hesitate to use it in an emergency. It is still better than nothing
In an emergency situation, where there is a hydraulic brake failure, the emergency brake only works on the rear wheels. Also realize that applying pressure to a cable is actuating the brake. You are correct to think that I am essentially describing 1920 mechanical brakes. Those brakes were scary, but top speeds rarely exceeded 35 miles per hour. The point I am trying to make is that hydraulic brakes replaced mechanical brakes for a darn good reason. They can apply immense pressure to the stopping media.
Now that I have bored you with the history of the PB, lets get to the subject. After prolong use PB cables reach the end of their adjustments. The cables stretch. Think about why the stretching occurs. It is a stranded cable that tightens and thus becomes longer when pressure is applied at both ends to actuate the PB. To extend the life of the cable, and to almost eliminate the need for adjustment, follow these steps:
Apply the hydraulic foot brake firmly. This will push the friction material against the drum or disc at a greater pressure that could be obtained with actuating the cable.
The PB can now be applied while holding the hydraulic brakes. You are using the cable to take up the slack rather than to apply the required pressure. Apply the pressure gently until the cable tightens. Jerking the PB leaver or pedal will put unnecessary pressure on the cable.
Try it. It works.
While on the subject of parking devices it is worthwhile to investigate the PARK position on your automatic transmission. I have found that most people use PARK as their PB, and not using the PB. It you ever saw the PARK mechanism in an automatic transmission you would feel uncomfortable about using it as a PB. It is usually a pin dropped into a cog. The pin being no bigger in diameter than your small finger. Using the PARK as your only hold when the car is on a reasonable grade could cause the following:
1.) Sheer the cog.
2.) Sheer the pin.
3.) Jam the pin in the cog making it difficult or impossible to get the vehicle out of PARK. When you pull the car out of PARK there should be no resistance. Hearing a clunk when you do this indicates too much pressure has been applied to the pin/cog.
On many occasions our shop was called to get a vehicle parked on a grade out of park. The procedure was to gently push the vehicle in a direction opposite the slope. This would reliever the pressure on the pin/cog until the pin could be released.
There is also the remote possibility that, when parked, another car could tap you car, and cause too much instant pressure on the pin/cog.
Do it properly. Set the PB as previously described. Make sure the PB is holding. Then put the car in PARK. This will assure that no undue pressure is being applied to the PARK mechanism.
05-27-07, 10:39 PM
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To extend the life of the cable, and to almost eliminate the need for adjustment, follow these steps:
Apply the hydraulic foot brake firmly. This will push the friction material against the drum or disc at a greater pressure that could be obtained with actuating the cable.
The PB can now be applied while holding the hydraulic brakes. You are using the cable to take up the slack rather than to apply the required pressure. Apply the pressure gently until the cable tightens. Jerking the PB leaver or pedal will put unnecessary pressure on the cable.
Try it. It works.
Apply the hydraulic foot brake firmly. This will push the friction material against the drum or disc at a greater pressure that could be obtained with actuating the cable.
The PB can now be applied while holding the hydraulic brakes. You are using the cable to take up the slack rather than to apply the required pressure. Apply the pressure gently until the cable tightens. Jerking the PB leaver or pedal will put unnecessary pressure on the cable.
Try it. It works.
BTW... welcome back!
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