Emergency Stops with air?

[TomC]

Spring brakes should not lock up the tires if you have sufficient weight on the axles-near the capacity of the axle.

When I was driving truck, I lost an air line that quickly exhausted my air pressure. I was also grossing about 65,000lbs. When the air pressure got down around 65psi, the yellow button (on my truck, I had three air brake buttons. Yellow was the master, red just the trailer, blue just the tractor. The yellow button had to be pushed in for either the red or blue to work) popped out and luckily there was a nice shoulder to pull over onto. I was doing about 65mph and it brought the truck and trailer smoothly to a rather quick stop in about 5 seconds. This is with 4 axles of spring brakes.

I strongly suggest you try your parking brake first from 20mph. Then if it doesn't lock up the tires, try faster speeds. You really need to know exactly how the bus operates under all circumstances-and suddenly loosing air pressure is one of them. Good Luck, TomC

[joel_newton]

When I used to run double bottom dumps one quarry we hauled from was up in the "hills."  One route out to Napa was the Oakville Grade.  A mile or so of apx 8% grade.  80,000 and no jake.  Used the technique from CA CDL Manual - "... apply brakes just hard enough to feel definite slow down.  reduce speed apx 5 mph below "safe" speed..."  Technique works well.  Ho hint of overheating brakes.  Air usage not an issue an your are running engine near max rpms for most efficient braking effect.  Works very well in my experience. 

[eagle19952]

I'll say it again I highly suggest you exercise your first attempt at
4-6mph Then if you still want to....try 20 or 40 or 60 or 100mph.
I sure wouldn't want  to experience this at 60mph in MY equipment.
That's not to say I haven't had the experience, cuz I have, in somebody else's....
You'll only do it once and hope like H E double toothpicks you never have to do it in a real world panic...
Re-Read the posts about flat spotted tires.

[bevans6]

With respect to adjusting brakes - the typical air brake design we have with slack adjusters and S-cams is designed to provide virtually identical performance throughout the range of legal adjustment.  A performance drop (of significant amount) happens when the cannister reaches the limit of it's travel - it locks at maximum and that's all the brake you got.  If the shoes or drums are too worn the S-cam can cam-over and you loose the brakes on that wheel.  So in reality, adjusting the brakes is important, but it doesn't actually change they way they work much if at all, if the brakes are well maintained.

With respect to spring brakes - the spring is the equivalent of a 60 psi brake application, as noted it takes about 65psi to back the spring off to release the brakes.  Given that a 60 psi application is about the equivalent of a half maximum application if your tank is at 120 psi, it's not all that much.  It will indeed lock the brakes on an unloaded van or flatbed, but it won't do much that is dramatic to a bus.  Doing that to all the wheels on a tractor trailer might be a different kettle of fish...

With respect to DD-3 brakes - the DD3 system is designed to give the driver three to four controlled brake applications from the emergency tank.  These applications go to the parking/emergency diaphragm on the drive axle only, and are pressure limited to 80 psi to a smaller diaphragm than the normal 30 square inches of the service diaphragm.  When the pressure in the emergency brake tank drops below the setting of the push pull valve spring (around 40 psi on my bus) the push pull valve will pop and apply the parking brake with whatever air pressure is left in the tank.  So, if you apply the parking brake at speed on a DD-3 equipped bus you will get the equivalent of around a 60 psi normal service brake application, on to the drive axle only, based on the smaller diaphragm being activated by 80 psi of air pressure.  Again not really a recipe for drama or launching microwaves.  

The spring brake stuff is taught in DOT air brake courses and you need to know it to get an air brake license.  The DD3 stuff you have to figure out on your own, or get someone to teach it to you.  Bottom line is there should really be no questions about this stuff from experienced bus drivers, it is part of the deal you should be signing up for before you get behind the wheel for the first time.


Cheers, Brian

[Oonrahnjay]

With respect to adjusting brakes - the typical air brake design we have with slack adjusters and S-cams is designed to provide virtually identical performance throughout the range of legal adjustment.  A performance drop (of significant amount) happens when the cannister reaches the limit of it's travel - it locks at maximum and that's all the brake you got.  If the shoes or drums are too worn the S-cam can cam-over and you loose the brakes on that wheel.  So in reality, adjusting the brakes is important, but it doesn't actually change they way they work much if at all, if the brakes are well maintained.

With respect to spring brakes - the spring is the equivalent of a 60 psi brake application, as noted it takes about 65psi to back the spring off to release the brakes.  Given that a 60 psi application is about the equivalent of a half maximum application if your tank is at 120 psi, it's not all that much.  It will indeed lock the brakes on an unloaded van or flatbed, but it won't do much that is dramatic to a bus.  Doing that to all the wheels on a tractor trailer might be a different kettle of fish...

With respect to DD-3 brakes - the DD3 system is designed to give the driver three to four controlled brake applications from the emergency tank.  These applications go to the parking/emergency diaphragm on the drive axle only, and are pressure limited to 80 psi to a smaller diaphragm than the normal 30 square inches of the service diaphragm.  When the pressure in the emergency brake tank drops below the setting of the push pull valve spring (around 40 psi on my bus) the push pull valve will pop and apply the parking brake with whatever air pressure is left in the tank.  So, if you apply the parking brake at speed on a DD-3 equipped bus you will get the equivalent of around a 60 psi normal service brake application, on to the drive axle only, based on the smaller diaphragm being activated by 80 psi of air pressure.  Again not really a recipe for drama or launching microwaves.  

The spring brake stuff is taught in DOT air brake courses and you need to know it to get an air brake license.  The DD3 stuff you have to figure out on your own, or get someone to teach it to you.  Bottom line is there should really be no questions about this stuff from experienced bus drivers, it is part of the deal you should be signing up for before you get behind the wheel for the first time.


Cheers, Brian   

     Good info, Brian.  Well and logically thought out, and well presented.  Thank you. 

[Audiomaker]

<snip> Again not really a recipe for drama or launching microwaves. <snip> 


Cheers, Brian

Well yeah, that's what I was thinking, but as the speed decreases in this scenario, the spring pressure remains constant as the kinetic energy decreases to the point where it is no longer past the shear strength of the brake material in contact...resulting in a sudden lock at the end.

That's why the 4-5mph test might be a shocker.  As soon as that point is reached, it could launch a microwave.
The parking brake isn't letting off as the truck slows like we naturally do with our foot.
I would suspect even in a full lockup @ 50,  that you wouldn't get the violent deceleration of the 5mph test.  It's just a matter of the surface tension.
A brake shoe is just like a tire.... less traction when it's spinning (or when the drum is spinning).  As the tire slows, it will eventually grab...as does the brake shoe.

I don't think anyone is really concerned about what happens if you have an air emergency at 5mph, but as to my original question, I'm getting the impression that with the weight distribution of this type of vehicle, that the spring pressure is unlikely to lock up the tires at speed (on dry).

Thanks!

[Iceni John]

Some while ago I tested my spring brakes at about three or four MPH on clean dry level pavement.   One moment I was moving forward at walking pace, the next moment I was STOPPED, just like I hit something solid.   Wow!   Mind you, that's with 30/30 rear brake cans working on big 16.5 x 10" drums (Crowns have big brakes).   Most of my routine non-panic stops require only about 30 PSI for comfortable braking in traffic, according to my brake application force dual gauge  -  if I did a 65 PSI brake application at traffic speed, even allowing for the front brakes assisting, I would be through the windshield!

John

[eagle19952]

 if it doesn't lock the tires, something is wrong.
if you can power through the brakes something is wrong.
Try to do a converter stall test without properly functioning brakes or a wall.
at my shop I had 12 ft. long drill pipe pile driven into frozen ground for the sole purpose of testing brakes and performing convertor stall tests.
yes it will lock the tires, no it will not stop the bus, until the tires have flat spots on the tire contact area.
no it may not seem as abrupt as the 5 mph test and that's that.
if
you were to try that with DD3's how would you release them without enough having enough air or reflexes to perform the steps to release the brakes.

re-reference the trailer tire, driveshaft vibration, oh crap post.
anxiously awaiting the U-tube video...

[eagle19952]

if it doesn't lock the tires, something is wrong.
if you can power through the brakes something is wrong.
Try to do a converter stall test without properly functioning brakes or a wall.
at my shop I had 12 ft. long drill pipe pile driven into frozen ground for the sole purpose of testing brakes and performing convertor stall tests.
yes it will lock the tires, no it will not stop the bus, until the tires have flat spots on the tire contact area.
no it may not seem as abrupt as the 5 mph test and that's that.
PS sounds like John (post above) has done his maintenance..
if
you were to try that with DD3's how would you release them without enough having enough air or reflexes to perform the steps to release the brakes.

re-reference the trailer tire, driveshaft vibration, oh crap post.
anxiously awaiting the U-tube video...

[RJ]

Left a message for Brian today about the video, hoping to hear from him tomorrow.

Pretty obvious to me from the posts after my "resurrection thread link" that many have commented without understanding - or perhaps never even reading - my explanation blog.

Some things never change. . .

Shall we discuss the proper oil for a two-stroke Detroit now?

FWIW & HTH. . .

[sparkplug188]

Link to RJs blog: http://r4106.blogspot.com/2006/08/intelligent-dumb-question-answered_10.html

Quite a lot of thought was put into that blog post-- defiantly worth reading.  I guess it is safe to say there is less risk of destroying tires than I initially thought.  However, I still don't intend to test it out anytime soon either.

Anyone notice Clifford hasn't posted on either thread?  He knows how to stay out of the fire 

[Jon]

With all due respect to Eagle19952 my spring brakes will not lock up as has been described by others but they are working properly. At low speeds if I apply them I will stop like I hit a wall. But at highway speeds they do not lock up the axle.

The spring brakes will hold my 53000 pound coach on my driveway which has a 29% grade. That is not a typo. But despite their effectiveness I can overpower them with the engine. I did it with my 8V92 coach and I can do it with my current Series 60 coach.

[eagle19952]

well ok then, at some point the brakes will lock up, exactly where/when I don't know, I will concede not instantly, but during this test scenario the vehicle will come to a stop, when it does try to move it without releasing the park knob....does your bus move ?

if it does something is not right.
I did learn just now that when you yellow knob stop, you lose the foot pedal...That would bother me.

And I still believe there has to be some deformation of the tire.

I am often confused in presenting my thought  to paper....
anyone have a link to the video ?

[Audiomaker]

It has to do with the shear strength of the brake shoe material.

It takes a certain amount of kinetic energy to "shear" the material off the brake liner.

This is happening continuously as you use the brakes, which is why they get thinner over time.

That energy is provided by a spinning drum.  The faster it spins, the more energy it has to shear (at a constant psi for this exercise).

As the wheel loses speed (and the brake drum), it becomes harder and harder for the drum to shear off that material (ie...you increase stopping power).
This is why we let up on the brakes as we come to a stop.   This can be demonstrated by the fact that if you apply normal brake pressure with your foot from say a 50mph stop, and keep the same pressure as you approach the stop sign (or rear end of someone's car), that very quickly you will start to be launched forward (having not lowered the braking pressure that was quite comfortable at 50mph).

The energy required to shear the brake material falls below what the spinning drum has and...whammo...it stops.

As I machinist, I see this demonstrated on a lathe frequently.   You have large part...say a 6" diameter piece of round stock aluminum in the chuck, and it's spinning. 
As it spins, you drive a cutter into to remove material...
If you cut power to the lathe motor with the cutter still in the material, the spinning will slow.....and then VERY quickly slow, and then violently stop.
This is a mistake in machine work...or an accident.  The cutter is always pulled away from the work prior to shutting down the spindle for just this reason.
The same cutter that was happily shearing off material at 500rpm can actually break as it comes to this halt.
To formulate in reverse, if you apply the cutter to the material prior to starting the spindle...at the same depth that you were already cutting at, you are likely to break the cutter or stall the machine. 
Simply there is not enough kinetic energy developed yet to shear the material and break it's surface tension.
This is why a parked vehicle has a tremendous amount of holding power from it's parking brakes.
The amount of energy required to stall your wheels at speed is likely enough to hold your buss near vertical when parked.

That doesn't mean it can't be done.  Enough PSI applied at any speed could overcome the liner's ability to shear and stop the wheel.
Also keep in mind that the tire is having the same day... to shear vs not shear.
When the brake liner's resistance to shearing is greater than the tires' resistance to shearing, a lockup occurs.

The tires' resistance to shearing is more or less constant as the PSI to the road is much more consistent than the ever changing brake material PSI to the drum.

Shear resistance is also affected by the two materials working against each other.  In the case of metal to brake liner, the metal is stronger.
In the case of asphalt to tire, the asphalt is generally stronger.  In the case of rubber to water, the rubber is usually stronger, so as the percentage of water molecule to rubber vs asphalt molecule to rubber increases, at some point you're shearing water molecules (which are very easy).  Yes...obviously... you lose traction in the wet.

[gus]

Sometimes mine releases without brake application and sometimes it doesn't. If not, a full brake application releases them.

Make sure you don't use full brake pressure to set them, it only takes a light pressure to set. Full set pressure will make them hard to release.