Hey all,
Since my truck is about the same size and configuration as a bus, I was wondering...
Has anyone had to pull the air brake knob while underway?
If so, what happens? Do any of the wheels lock up? Do you think they might in the wet..on a grade...etc?
How long to come to a stop? ..etc ..etc.
I am curious to the experiences of those who have engaged the parking brake while still moving.
Thanks!
Sean
Talk to Luke on that one!!!!!!!!!!!!
IF it does NOT lock the wheels you NEED a brake adjustment NOW!!!!!!!!!!!!!!!!
to pass some states inspection you must do 15 or 20mph and pull the e brake and it must stop within spec.
Not sure I would want to do that, slamming on the brakes can't be good on things if you are going at any speed. I am not sure but when hitting the parking brake aren't you allowing the spring brakes to engage? I have tested mine by just engaging them on a steep hill when fully stopped and they held when coach was in neutral. Mine adjust automatically so I have concluded that everything is in working order.
I know on a fully loaded Pete when you blow a main air line you will leave enough of your tires on the pavement to see them in the form of some very long black marks for a few months,,,,,,happened to me once years back in Sacramento, during traffic of course, not fun.
Quote from: digesterman on June 09, 2014, 06:45:06 PM
Not sure I would want to do that, slamming on the brakes can't be good on things if you are going at any speed. I am not sure but when hitting the parking brake aren't you allowing the spring brakes to engage? I have tested mine by just engaging them on a steep hill when fully stopped and they held when coach was in neutral. Mine adjust automatically so I have concluded that everything is in working order.
I know on a fully loaded Pete when you blow a main air line you will leave enough of your tires on the pavement to see them in the form of some very long black marks for a few months,,,,,,happened to me once years back in Sacramento, during traffic of course, not fun.
Yes, it would engage the spring brakes.
I'll have to go back and look, but the CDL manual makes mention of it "slowing" the vehicle. Because all vehicles are different, I wanted to see if anyone knew what a 40-45' 30-40,000lb tandem did under those circumstances.
I figured someone here might have been through it.
tires are 6-800$ apiece...you test that at 4-6 mph max. if it works it might be useful in an emergency....but they are not called emergency brakes, they are parking brakes... ??? ::) ???
PS the emergency brake is between your ears, the foot brake, and the transmission shift lever...then the yellow button... :o
Quote from: eagle19952 on June 09, 2014, 07:31:51 PM
PS the emergency brake is between your ears, the foot brake, and the transmission shift lever...then the yellow button... :o
Yes, of course, but the Oregon CDL manual does go into pulling that yellow button while underway. I don't have a CDL, but I am studying to get one, so I came across that. The manual describes spring brakes as slowing the vehicle. I would imagine in an empty or lightly loaded dry van, that this would lock up the wheels...at least on the trailer, but the weight distribution and whatnot is a little different on these buses and I kind of wanted to know if anyone had done it and knew the result (and the resulting behavior).
Here's one... is the parking brake only on the rears, or does it apply the steers too? It seems like it would be better only to apply the rears and the steering tires would apply as the truck lost it's air pressure while parked.
Thoughts?
Pulling the knob to apply the parking brakes, be it Maxis or DD3s, will not lock up the wheels on dry pavement. Everyone should practice this because if you have a catastrophic loss of air pressure one day, this is what will happen when you loose enough air: the emergency/parking brake will come on. On a bus or a truck, they are on the drive axle, not on the front steer. Of course on wet pavement or snow/ice, it could easily lock up the wheels.
JC
On a semi it locked up my tires as I previously said, there was no slowly slowing down, it was a very rude stop, slid over to the side.
Successful emergency brake engagement is a function of air pressure/volume, brake type/condition, square inches of braking surface of and condition of braking surface, brake diaphragm size/volume, and condition of brake shoes, calipers, etc... No individual past history of an emergency brake engagement is determinate of any future outcome. You just have to maintain your brake system as diligently as possible and hope you have done enough to survive a catastrophic failure - HTH and FWIW
Sean -
Looks like it's time to resurrect this thread I started awhile back:
http://www.busconversions.com/bbs/index.php?topic=1489.0 (http://www.busconversions.com/bbs/index.php?topic=1489.0)
Be sure to read the whole thing, it's quite entertaining!
FWIW & HTH. . .
;)
PS: Brian's video link in the above thread is now dead, I believe he moved it to a private YT account. I'll contact him and see if I can bring it back.
Quote from: lostagain on June 09, 2014, 08:43:04 PM
Pulling the knob to apply the parking brakes, be it Maxis or DD3s, will not lock up the wheels on dry pavement. Everyone should practice this because if you have a catastrophic loss of air pressure one day, this is what will happen when you loose enough air: the emergency/parking brake will come on. On a bus or a truck, they are on the drive axle, not on the front steer. Of course on wet pavement or snow/ice, it could easily lock up the wheels.
JC
Lost again: do you happen to know if the steers simply don't have spring brakes, or is it that they just don't dump with the parking knob, but would apply once the air pressure dropped off over time (ie...when parked)?
I didn't (and still do not) know if the fronts are air against spring... I mean "something" must be handling the brake return pressure or?
Darn, left out the most important thing - BRAKE ADJUSTMENT - sorry = HTH
Quote from: Audiomaker on June 09, 2014, 09:53:28 PM
. . .do you happen to know if the steers simply don't have spring brakes. . .
Sean -The emergency/parking brake, be it DD3 or Spring, is only installed on the drive axle. In the case of your tandem rears, most likely the emergency cans are on the front, or engine side, axle.
Most commonly, the third axle and the steering axle have simple brake cans with no parking/emergency feature, altho your beast may have them on the third axle.
You'll never find parking/emergency brake cans on the steering axle.
FWIW & HTH. . .
;)
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?
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.
Quote from: Audiomaker on June 09, 2014, 10:24:42 PM
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
If I remember correctly years ago some truck tractors did not have front brakes period. 06 Bill
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
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
Quote from: skihor on June 10, 2014, 05:10:23 AM... . 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 ...
Quote from: 06 Bill on June 10, 2014, 05:01:02 AMIf 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.
Quote from: robertglines1 on June 10, 2014, 05:32:45 AM... 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??)
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
Everything you always didn't need to know about parking brakes,
www.nhtsa.gov/.../PDFs/VRTC/ca/capubs/HT_ParkingBrakeRpt.pdf (http://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 (http://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.
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.
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. ??? ::) ???
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.
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.
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 8)
after awhile, no trailer...no load.
The 100T lowboys....we flipped them over to do almost anything, including brake work.
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
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.
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... ;D
Re-Read the posts about flat spotted tires.
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
Quote from: bevans6 on June 10, 2014, 09:25:45 AM
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.
Quote from: bevans6 on June 10, 2014, 09:25:45 AM
<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!
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
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...
:)
Quote from: eagle19952 on June 10, 2014, 05:51:54 PM
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...
:)
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. . .
;)
Link to RJs blog: http://r4106.blogspot.com/2006/08/intelligent-dumb-question-answered_10.html (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 ;D
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.
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 ?
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.
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.
My 1650 ft.lbs. can overcome a set of spring breaks - FWIW
All -
Brian's video has been located. I posted a link at the end of the original discussion (Intelligent dumb question), but for those who can't wait:
Bus Parking Brake vs. Service Brake application (http://www.youtube.com/watch?v=EOx7J2_qetU&feature=youtu.be#)
Pay close attention, everyone should learn from this!
FWIW & HTH. . .
;)
Thanks for posting that, RJ.
Mike
Audiomaker:I think in this instance ,Ignorance might be bliss.Just pray that you never get into a situation where you would need to do this,because I'll promise you if you have a 40 or 45 thousand lb load and you pull that knob at 40 or 45 mph something is going to give in a big way.That is why you have alarms on your air system ,to give you enough time to get stopped so you won't have to pull that yellow knob while you are at highway speeds