Emergency Stops with air?

[sparkplug188]

Is it really worth the posibility of flat spotting a bunch of tires to find out the answer to that question? If the brakes lock at 40 mph for 2 seconds, the tires will leave skidmarks over 100' long.  If for any reason the parking brakes can't be released, the skidmarks will be over 250' long before stopping.  The time and money spent balancing or replacing tires could surely be put to better use.

At 5 mph the worst that can happen is a skid mark 2' long.

[Oonrahnjay]

RJ, thanks... and that was a fantastic read.  Thanks for the link.

Still, there are some conflicting views on whether the wheels will lock at speed (dry pavement).

I suspected that they would not...as the parking brake has plenty of force to keep the vehicle from rolling, but not enough to stop the wheels at speed.

I would also imagine that the higher the speed, the less traumatic it would be... as the kinetic energy dissipates to the point where it will create an abrupt stop (at the end).

The test I might try at some point...in a safe place... might be to roll about 40 and pull the plug.... then try to push it back in before the microwave oven launch... if that makes sense?  

     Some people have tried it at 6 MPH, some have tried it (pulling the brake knob) at 20 MPH, some have had a sudden loss of air.  The consensus seems to be that many vehicles will lock their wheels at some speeds, under some conditions of road surface, with certain condition and setting of the brakes on the braked axle, some vehicle loads, and under some speed of air loss.  Some will lock their wheels all the time, some never will.  

     But if they're set up right, the spring brakes are there to apply a reasonably efficient emergency stopping force (again, this is where the adjustment and other factors that Niles quoted comes in) and their application should be strong enough to help stop the vehicle but not throw the vehicle out of control.  But you'll never know exactly how close you will come to locking the wheels on any given day and you're not going to get perfect stopping at any time and since your bus is certainly a special case with its multiple axles and heavy weight, it's probably not going to behave like the "average" vehicle.  
(How about those sentences, RJ?)

     And in real life, you have the issue of whether the air loss affects all the reservoir tanks and thus has an effect on all the axles as the compounding/anti-compounding function of your brakes cuts in.  (OK, RJ?)  Just pulling the knob at a certain speed won't tell you much -- in real life, you'll have the spring brake actuation and foot brake interaction (or not).  

     But locking the wheels on the spring brake axle (s) isn't really determinative of anything.  

HTH,   BH   NC USA

[06 Bill]

If I remember correctly years ago some truck tractors did not have front brakes period.  06 Bill

[skihor]

On the subject of "flat spots" I came around a corner during a bad rain storm, on a two lane highway, and the traffic was stopped. No shoulder and oncoming traffic. My only choice was a full on panic stop. On one hand I'm glad my brakes were adjusted, on the other hand the rears were a little too tight and locked up. I got stopped with at least 4 feet to spare,,, The Mini van in front of me must have filled his shorts. Anyway the resulting flat spot resulted in a thump, thump, thump vibration that eventually shook and sheared the bolts on the driveline flange, on the same trip. That was @40 MPH and on wet pavement.
Getting back to the subject, the emergency system is designed to apply the rear brakes HARD and INSTANTLY. Rather than "practicing" that scenerio I put my energy into keeping the entire brake system adjusted and in proper working order. Watching your guages can alert you of impending major loss and having a functional low air alert buzzer should give you enough time to get pulled over safely. I believe, if you're paying attention, in the event of a catastrofic air loss, you have enough time to react and get stopped safely before the emergency system engages.
Don & Sheila

[robertglines1]

one size diaphragm for parking different size for service brake?  A skidding wheel has no traction to steer ?  or help coach stay straight?  If you loose air your drive axle service brakes apply? On my coach tags also; plus I have a reserve emergency tank dedicated for this major failure to temporary supply( Emergency air supply tank)Green knob you must hold down; a short supply of air to allow you to move bus a very short distance.  Some food for thought .   Bob

[Oonrahnjay]

... . I believe, if you're paying attention, in the event of a catastrofic air loss, you have enough time to react and get stopped safely before the emergency system engages.     
Don & Sheila

    Yes, probably ... probably ...

[Oonrahnjay]

If I remember correctly years ago some truck tractors did not have front brakes period.  06 Bill   

     Yes, and then there was a stage where there was a two-way valve where the driver could cut the front brakes in or out depending on load, road surface, etc.  And there was also a stage with proportioning valves for the front axle.  Lots of different things have been tried.

[Oonrahnjay]

...  I have a reserve emergency tank dedicated for this major failure to temporary supply( Emergency air supply tank)Green knob you must hold down; a short supply of air to allow you to move bus a very short distance.  Some food for thought .   Bob   

     That sounds like a good idea, Bob.  Of course, if there's a really major air leak somewhere, the air from your emergency tank may just go out the leak with no effect on the spring brakes but that will offer you a good chance, especially if you have strategically placed one-way valves in your system. 
     And as Niles mentioned above, there's nothing like preventative maintenance.  If you can keep your system in good shape, you're much less like to have problems on the road.  And keep an inspection on the mechanical parts, too.  If something mechanical breaks, your brakes are going to fail on you even if you have good air and you're going to have a bad situation.  (How's that, RJ??)

[06 Bill]

I think the original configuration on my 1964 4106 had the front limiting valve arrangement. I re plumbed.
it to work as rear only parking brake. Big problem is if air pressure leaks down bus rolls away. Much not
good, always have to chock the wheels. Parking brake on drive shaft also works quite well.   06 Bill

[digesterman]

Everything you always didn't need to know about parking brakes,
www.nhtsa.gov/.../PDFs/VRTC/ca/capubs/HT_ParkingBrakeRpt.pdf

Then if you want to take a knowledge test,
www.spartanburg2.k12.sc.us/trans/CDL%20test/AIRBRAKES%20tests.pdf

Don't know how many truckers would agree with the answer to question 8 on the 2nd test tho, the ones that practice this are usually the ones driving the trucks that have the brakes smoking all the way down the hill. I think, not sure, that the state of Oregon changed the answer to that question. Maybe because all the 'experts' were nothing more than desk jockeys.

[Scott Crosby]

When I replaced my rear end gear I also cleaned and adjusted my Johnson bar drive shaft brake.  I tested it in my gravel lot at about 15 mph and was shocked that it locked up my rear wheels with only 1/5 of a pull (one click).  Before the procedure it took three clicks before the brake engaged and 4 clicks to be full on.  I was not expecting that kind of sensitivity or result from it.  

[sparkplug188]

8 On long downhill grades, experts recommend using a low gear and light, steady pedal pressure instead of on-again, off-again
braking. Why is that?

a. Because air usage is less when you have light steady pressure.
b. Because brake linings do not heat up as much with light steady pressure.
c. Because you can keep vehicle speed constant in a low gear with light, steady pressure.
d. All the above.

Answer Sheet: D

Every few years they change their mind.  When I got my CDL in 2011, the correct answer was: Use a low gear and firm application of brakes to reduce the vehicle speed, allowing the brakes to cool by completely releasing the brakes between applications.  On the 2007 CDL test the correct answer was steady, light brake pressure. 

[digesterman]

Sparkplug,
I know that you can always spot the guy using the light constant pressure on the brakes system, first their brake lites are on all the way down the hill, second, smoke coming from their brakes.

It just doesn't take a lot of thought to release that constant friction between any two surfaces, at least enough pressure to make a difference in slowing you down is going to create a lot of heat.

We had a driver years ago that drove with that mindset, went through more brakes and drums on his equipment than anyone else's. Jakes and a lower gear work  just fine with occasional braking.  Now if we didn't have Jakes,,,,,,,well that changes the entire picture.

[Jon]

Lightly loaded axles with spring brakes may lock up, but according to Prevost if the spring brakes are applied such as a loss of air pressure or intentional application they will bring you to a complete stop but gradually. They did not define "gradual".

From experience I can say if your brake diaphragm fails catastrophically and your spring brakes apply you can use engine power to overcome the spring brakes. But only for a very short distance because the brakes are going to get very hot.

[eagle19952]

When I worked on the Trans Alaska pipeline, Alyeska the maintenance contractor owned everything, when they moved equipment using sub contracted drivers/owner operators, we supplied the trailers under the loads....
That didn't last to long, because all we did was replace trailer brakes, LOTS of trailer brakes 
after awhile, no trailer...no load.
The 100T lowboys....we flipped them over to do almost anything, including brake work.