Intelligent Dumb Question:

[gus]

Don't get pressure and force mixed up. 50 psi on a 1 square inch piston is 50 lbs of force. 

The same 50 psi on 10 square inches is 500 lbs of force, same air pressure.

So, there is more to braking force than psi of air.

If, as someone said in the string, the spring brake cans are smaller than the service brake cans then the 50 psi required to release the rear brakes does not mean that the springs apply the same force as the service brakes at 50 psi.

My god, I'm beginning to sound like the professor!!

[JackConrad]

In an average, "I am going to start applying a little brake to start slowing down" application, I use about 10-15 PSI of air pressure, a firm application uses about 25 PSI and a PANIC stop uses about 85 PSI. (these pressure are with an E-1 service brake application valve) Multiply that pressure times the can size (which is total square inches) and you have the brake application force. On our MC-8, the front brakes are 24, the drive axle is 30 and the tag axle is 12. So a specific amount of air pressue applies a different amount of braking force on different axles.  A 50 PSI application to a 30 brake can applies 1500 PSI to the S cam. I have seen 30/24 spring brake cans as well as 30/30 cans. I do not think you can get a can that has a parking chamber larger than the service chamber.  The return spring forces also vary based on the size of the brake can. On our bus this ranges from 39 to 12-1/4, with a stonger return spring on the larger cans.  Jack

[RJ]

OK, everybody. . .

For your reading pleasure, choking over your morning coffee, or fuel for your flames, here's my take on this whole thing:

http://r4106.blogspot.com/2006/08/intelligent-dumb-question-answered_10.html

Enjoy!   

(URL to blog edited 8-10-2006 @ 2231)

[JackConrad]

Russ,
   Thanks for a great article. This shoulkd be required reading for avery busnut.  Jack

[belfert]

The busnut I had talked about with Sceniccruiser on the first page of the thread actually had a seperate emergency brake I think called an ICC brake.  That brake was the one that stopped him right now at 15 MPH.

Brian Elfert

[DrivingMissLazy]

Excellent writeup Russ, as always. You are a credit to the Bus Nuts community and to the busing industry as a whole.

I agree with you regarding the bus portion of the article. I am not experienced in the trucking industry so I can not comment on that portion.

I agree because your statements exactly match the tests I performed on my Eagle several years ago and with results as I posted earlier in this thread:

I did it on my Eagle which probably had spring brakes. I started off slow at 20 mph and did it several times till I got to 50 mph. Never skidded a tire as far as I could tell. A lot difference in just applying brakes to one axle as opposed to applying them to all axles. [/b] Richard

Thanks again for all your excellent posts.
Richard

OK, everybody. . .

For your reading pleasure, choking over your morning coffee, or fuel for your flames, here's my take on this whole thing:

http://r4106.blogspot.com/2006/08/intelligent-dumb-question-answered_10.html

Enjoy!   

(URL to blog edited 8-10-2006 @ 2231)

[Chris 85 RTS]

While I have read and understand and agree with your article, I was wondering if the parking brake setup like that on my RTS could be an exception.  It is single drum mounted on the pinion of the differential.  It provides great holding power when stopped, I can not move the bus even under full throttle, but it seems to me that putting the parking brake in that location could be a weak spot in the driveline.  I'd hate to think that the pinion could be snapped by the sudden application of the parking brake at speed.  Now, I suppose that the designers were smart enough to recognize this and designed the system so that this would not happen, but I am not sure I am ready to test this theory as it would be very costly to replace the differential should something bad happen.  I've done the try to accelerate test with the parking brake so I know it works.  Seems like the discussion here was mainly about emergency braking systems that apply braking using the normal service brakes in a different manner of application.  Any comment about this slight varient? 

[FloridaCliff]

Russ,

Excellent article.

I had mine trip at low speed and it was an almost instant stop.

It gave me the impression that it would always stop that way, but your information and Brians test

have shown me my expectation from the earlier emergency stop was wrong.

Very informative.

Cliff

[pvcces]

Russ, excellent article. I especially liked your explanation of the kinetic energy involved.

Thanks.

Tom Caffrey
Suncatcher
Ketchikan, Alaska

[Happycampersrus]

Chris 85RTS,

I don't believe you would want to try this "experiment". The maintenance manual C-8123 for the RTS section 5C-1 says "The control valve is not spring-loaded and will not "pop-up" to automatically apply the brakes in the event of low system pressure" So that got me curious then I called 505-347-7500 and spoke to Bob Benton at Millennuim Transit Service (They build the RTS) and he does not recommend trying this on a bus with the pinion mounted PARKING brake. He did say that you can and should use it in an emergency situation, but no test have been conducted to see what the effects would be from repeated use in this way (every 3 days).

Dale

[MC7S50]

Hey Russ

Thanks for the informative lesson about the well-designed safety features of our braking systems.  I can tell you were a professional trainer!  I certainly learned some valuable info. 

Safety is always a big concern of mine because I see evidence that proper maintenance is not always carried out on bus conversions.  Also, understanding of how the systems work is not always what it should be. 

My congrats on furthering our understanding while having a good time doing it!

John

[Buffalo SpaceShip]

A most excellent, informative article, Russ, as always. Your dedication to safety and thoughtful posts are always appreciated. I've learned sooo much from you over the years, sir.

Buswarrior sent me some stills of his transit garage AND a video of pulling the spring brakes on one of their Fishbowl transits. I edited a new version of the video, and now must fully retract anything I've said about spring brakes taking longer to stop a coach than DD3s!

New video (same link)  is here: http://www.thefamilybus.net/videos/parkingbrake.wmv

Cheers,
Brian

[buswarrior]

Wow, Brian's editing skills are GREAT!!!

What a great video, and not because I helped!

happy coaching!
buswarrior

[buswarrior]

Hello all.

One of the little things I caught up in this long thread, and would like to help with is the concepts around what the brake chamber pushrod is doing when we brake.

First, the brake chamber's job is to squeeze the brake linings against the inside of the drum when we apply air pressure with the brake pedal, or spring pressure when we put on the parking brake control, or regulated air pressure if we apply the parking brake control with DD3.

So, no matter which parking brake system, or whether we choose service or parking, the brake chamber push rod extends the linkage (slack adjuster, camshaft, S cam, rollers, shoes to linings) and rubs the linings against the inside of the drum.

MYTH BUSTER:

The distance that the pushrod extends does NOT change according to the pressure applied. Once the linings are against the drum, that's it for pushrod travel. What changes is the amount of squeeze, once they are against the drums. The rods do not push out any further once the linings are in contact with the drum. (unless you want to measure the very small deflection as the brake spider flexes under the extremes of full air pressure, but I don't think that matters for our discussion here)

Now, as the drum heats up during a stopping event, it grows slightly, and as it grows, the push rod will extend to compensate for the linings moving outward to stay in contact .

This is why we need to keep our brakes well adjusted. They "grow" longer under heat.

For a given application pressure, the amount of squeeze will also slightly weaken, as the volume available to it slightly increases with the slight extension of the rod. Pressure drops slightly. This is normal, and you don't really knowe about it when you are sitting upstairs. This is one of the smaller parts of what is referred to as brake fade.

The reason for regulated push rod travel limits is the serious situation you would be in if the drums heated up enough that the push rod ran out of travel, due to plate the diaphragm pushes on, hitting the inside of the chamber.  It is commonly considered that pushrod stroke can lengthen over half an inch between cold and the heat of a hard stop from highway speeds.

Descending a mountain using the brakes wrong, will extend them further, and ruin the brake linings, another whole thread!

Great thread and explanation, Russ!

happy coaching!
buswarrior

[pvcces]

Thanks, Brian.

I really liked the work you did on this thread.

Buswarrior, what do you think of the newer extra long travel chambers? I don't think that we can use them because of insufficient room.

Tom Caffrey
Suncatcher
Ketchikan, Alaska