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Author Topic: Intelligent Dumb Question:  (Read 16390 times)
gumpy
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« Reply #60 on: August 14, 2006, 10:04:52 AM »

Well, I certainly learned something because of this thread and the work Brian and Buswarrior did.  I, like many, thought my bus would come to a screeching halt if the button were popped at 40 mph. I, like many, had tried it at low speeds (i.e. < 10 mph), and it had stopped rather abruptly, so without really thinking about it, I assumed it would stop that way from 40 mph or higher. After viewing Brian's video, I got real curious, and brave, and so on the way to dump my tanks after my recent trip, I found myself on a section of road where there were no other cars and bravely popped the valve up while at 40 mph. Guess what. My MC9 reacted somewhere in between Brian's and Buswarrior's video clips. Very soft, yet steady braking to a stop. Nothing abrupt about it. Didn't break the fridge free from it's mount and throw it through the windshield or anything like that.

I didn't compare it to a full brake application, as I know how that one works from experience driving in MN.

I'm really glad this thread came up. This is something I'll do at least once a year.

Thanks for the great information. It's opened up my mind more to think about the actual workings behind this and now it's more clear in my mind what's happening and why. I don't think the parking/emergency brake would stop the coach on a steep downhill, but it might keep you from losing control till you got to the bottom should you suffer a catastrophic brake failure of the service brakes.

craig
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Craig Shepard
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buswarrior
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« Reply #61 on: August 15, 2006, 07:14:04 PM »

Hello Tom.

Long Stroke Brake Chambers are a great thing!

If you can get them to fit in your application. The bodies are bigger, in that they have greater depth in the same axis as the pushrod travel. In order to make them longer stroke, the body of the chamber has to be deeper so the plate that the diaphragm acts on has more depth to move through before hitting the end of the chamber.

These long stroke chambers exhibit a flatter power curve over their length of stroke, a fancy way of saying they don't weaken as much as they get out to the end of their stroke. Graphed, the long stroke is superior to a standard stroke as it gets out to maximum stroke. They are safe and legal out to 2 and a half inches for a 30-30, which gives a busnut a little more time to not worry about going underneath to re-adjust.

Even if you can't fit long strokes in the rears, you should be able to fit them on the steers and tags, when the time comes for replacements, longer stroke means more room for mistakes, misadventure and neglect, not that any of us would ignore our brakes....

Many? Most? Some? truck fleets are spec'ing long stroke chambers exclusively, and have been for quite some time.

Long Stroke chambers work especially well with automatic slack adjusters. More room for the adjuster to do its job while staying a legal stroke length.

On new equipment, I would not order anything else!

happy coaching!
buswarrior
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Barn Owl
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« Reply #62 on: August 15, 2006, 10:10:56 PM »

Do I qualify for a PhD. after reading this post? With this type of an education I feel like Iíve earned one. Great job everyone! This was one I thoroughly enjoyed.
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« Reply #63 on: August 16, 2006, 03:30:02 AM »

To all that contributed, this has been very interesting.  And as mentioned, there is a big difference between buses and trucks when comparing the two since axle weights between the two are different.  Here are some facts on brake area between the two:
My transit has 14" x 6" front and 14" X 10" in back.  That works out to be 4,926 sq in of brake area.  Parking brakes 3,079 sq in.  Based on a 36,000lb rated GVW that works out to be 7.3 pound per sq in on the service brakes and 11.69 pound per sq in on the parking brakes.
A typical 5 axle truck will have 15" x 4" front and 16.5" x 7" on all other brakes for a brake area of 13,388 sq in.  Parking brakes 11,974 sq in.  That works out to be (based on a fully loaded 80,000lb rig) 5.98 pound per sq in on the service brakes and 6.68 pound per sq in on the parking brake.
To put that into an easy comparison, for a truck to duplicate the transit performance, the truck should weigh 97,659lb.
On the other hand for the transit to duplicate the trucks performance, the bus should weigh 29,490lb.
But once again, we're talking momentum, and the ability to stop from highway speeds.  Because of the higher weight continuously on the buses axles, they will stop shorter.  The only time a truck will stop best, is when loaded-otherwise wheel lockup can occur.

And to clear up a point in my thread a few back, I said I had never been in a crash where I lost control of the vehicle, compared to someone else hitting you when they are out of control.  Good Luck, TomC
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Clarke Echols
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« Reply #64 on: August 16, 2006, 10:13:41 AM »

I'll pick a few nits, but Russ's treatise is well worth the read.

Momentum and kinetic energy are related, but not the same.  Momentum is mass times velocity.
Kinetic energy is mass times velocity squared, which is the same as momentum times velocity.

Momentum is measured in foot-pounds-mass whereas kinetic energy is measured in foot-pounds-force.
Mass is a measure of resistance to acceleration (from Newton's first law: bodies in motion tend to
stay in motion, bodies at rest tend to stay at rest unless acted upon by an external force).

To get mass into force, you multiply pounds mass times the acceleration of gravity (32 ft/sec/sec).
1 pound of force is the amount of force required to accelerate one pound of mass so that
for each second its velocity changes by 32 ft/sec.; hence, 32 ft/sec/sec or 32 feet per
second squared.

A bus that weighs 32,000 pounds, traveling at 50 ft/sec (a little over 40 mph), the momentum
is 1,600,000 foot-pounds (mass).  The kinetic energy is 80,000,000 foot-pounds (force).

The amount of work required to stop a bus is equal to the kinetic energy which must be
converted to heat through friction (including air resistance which can be significant at higher
speeds).  Thus 80 million foot-pounds of work must be done.  That is equivalent to the
amount of work required to lift the bus 80,000,000 divided by 32,000 feet into the air,
which is 2500 feet (almost a half-mile).

On the other hand, the  amount of work to stop the same bus at 10 ft/sec (about 6.5 mph)
is only 3,200,000 foot-pounds, equal to lifting the bus 100 feet.  Thus a 5-to-1 increase in
speed increases the energy required to stop (kinetic energy in the bus that must be
dissipated) by 25 times.  Kick the speed up to 70 mph, and the energy to stop goes up
100-to-1.

That's why they say "speed kills".  It's the kinetic energy that must be converted into something
else that can wreak huge damage in an accident where the vehicle comes to a dead stop in five
feet or less instead of 500 feet or more.  That allows you to bend a lot of metal.

A 35,000-pound bus at 60 mph hits a 3000 pound car, head-on, coming the other way at the
same time.  The bus slows down about 5 mph.  The driver of the car has what I call "an abrupt
experience".

A jack-knife occurs when the centerline of a tractor and the centerline of the trailer are not
parallel or coincident with each other.  If a trucker tries an abrupt stop on ice, for example, and
turns the steering ever so slightly, the two halves are no longer going in *exactly* the same
direction.  The slight angle causes the force from the kinetic energy in the trailer to have
a lateral component (velocity is a vector, having both magnitude (amount) and direction) along
with the longitudinal component that is still parallel to the axis of the tractor.  This longitudinal
force tries to push the fifth-wheel sideways.  If the available friction between the driver tires
on the tractor and the pavement is less than the lateral force, the tires slide sideways (or
creep by scouring thanks to the pliability of the tire tread), increasing the angle until it eventually
takes over and most of the trailer force is pushing the drivers sideways.  That's bad -- very bad.

Clarke
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DrivingMissLazy
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« Reply #65 on: August 16, 2006, 12:16:42 PM »

Hmmmmmmm! Would you mind saying that again please Clarke? I think I misssed something. LOL
Richard
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« Reply #66 on: August 16, 2006, 04:34:35 PM »

I'm going to jump into the skillet, so to speak, with this question and a comment.......

My MC5A has plenty of brake shoe area, more than enough drum area but for whatever reason (probably passenger comfort..totally disregarding driver panic and blood pressure...) MCI had a treadle valve design all their own..in fact the Service Manual suggests the methods for rebuilding that valve or suggests that you send it back to MCI for a rebuild.

I find that it's not aggressive enough for my emotional comfort. Yes, I can live with the DD-3 actuators, that was discussed at great length in another thread.  What options do we MCI owners have for a treadle valve replacement which might be more agressive?
And...what is the downside of changing it out to a Bendix truck type?

I'm sure I'm going to hear all the typical flack about re-designing the bus and the associated liabilities an owner would assume in doing so but what I'm really looking for here is some factual data...the difference in braking distances to the "Maximum point of Retardation" not necessarilly the ability to 'lock 'em up and put flat spots in the tires' at a given weight and speed comparison.

While I doubt that anyone has done these tests under a 'lab comparison' situation....I surely would appreciate some input.

Bob

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buswarrior
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« Reply #67 on: August 17, 2006, 03:16:57 PM »

Hello Bob.

Two things which you can do, and feel good about spending the money, and call it preventive maintenance, if nothing else...

Send the treadle valve in for re-build, and change out the relay valve down the back. Check to see if MCI recommends any upgrades to what your coach has currently.

Or, if you feel adventurous, plumb a good air gauge into the lines downstream from both of them and be sure they are delivering the proper pressures. A fellow I know went to Europe for his airbrake technician training and uses these techniques to find all manner of bad valves, sometimes right out of the manufacturer's boxes!!! The Europeans are way ahead of us on diagnosing brake problems. They plug gauges in all over to confirm proper pneumatic functioning, cut the new shoes for a brake re-line right on the vehicle to ensure they are truly round, centre the wheel-ends on install, put the vehicle on a dynometer to test the brake torque... Very few places in North America know, or bother, with these methods. Brake re-lines last longer if they are cut on the vehicle, ensuring they are round, in orientation to the axle.

Also, the MCI's, with that rod arrangment back to the bulkhead mounted treadle valve, also leaves us open for the ravages of time bending or otherwise weakening the mounts that all that linkage is attached to. If there isn't proper travel, we won't get full braking. And the mounts can flex when you stand on it. Get an assistant and watch it carefully.

happy coaching!
buswarrior

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NCbob
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« Reply #68 on: August 17, 2006, 04:37:37 PM »

Some great suggestions, BW...I believe I'll have time for the treadle rebuild (or perhaps I'll get lucky and get an exchange from MCI).

I'm in the process of replacing the rear actuators and if time permits...the fronts as well before we leave for FL for the winter.  I might even be able to set up a dial indicator on the brake treadle linkage and check for mounting, etc.  Again, thanks for the places to look.

I'll report back on any problems I find.

Bob
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Stan
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« Reply #69 on: August 17, 2006, 05:28:37 PM »

NCBob: You can get a lot more agressive brakes if you change to a softer brake lining.  The trade-off is shorter brake lining life.
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JackConrad
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« Reply #70 on: August 18, 2006, 04:50:25 AM »

   When we purchased our MC-8, the brakes were "adequate". Slack adjusters were checked for proper adjustment. Brakes were still "adequate".  We drove the bus like that for several years.  When I installed the Australian Cypress flooring in the driver's area, I had to remove the brake/throttle assembly from the floor. This required disconnecting the rod from the brake pedal to the service application valve (E1) that is mounted on the bulkhead below the driver's area. After installing the new floor and re-installing the brake/throttle assembly, the rod was now the wrong length due to the increased floor thickness. I re-adjusted the rod allowing only about 1/16" of play in the brake pedal. The first drive after this, I almost put my wife through the windshield! It seems that this rod had been out of adjustment, with too much free play, not allowing a full application to the brake service application valve.
   I also installed a brake application pressure air gauge in the new dash. A normal light application to start slowing down is about 10-15 PSI. A normal full stop application is about 25-30 PSI. A hard (maximun) application is about 85 PSI.  Jack
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NCbob
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« Reply #71 on: August 18, 2006, 05:17:08 AM »

Very interesting, Jack....and a good place to start my search.  My guess is that we take too much for granted with these systems.

I sure wish I could find the time to attend the Bendix Air Brake school in ATL that Jim Shepherd recommends so highly..but right now we're running in full throttle trying to finish everything we can in order to fulfill the already loaded schedule.

Thank you and the others for taking the time to post your reflections.

Bob
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« Reply #72 on: April 01, 2012, 09:54:24 AM »

I'm Bumping this because after several years of being on this board i just found this article... I think with all the new people that have joined this is a MUST READ for everyone... and perhaps a refresher for others.. as more and more of us go for older buses as i'm looking into now, this is something anyone looking at a bus needs to understand!
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buswarrior
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« Reply #73 on: April 01, 2012, 10:19:01 AM »

A word of caution from later posts.

The video is resident elsewhere, perhaps someone else remembers where?

After the video shoot, from the large difference in results, Brian's brakes were found to be contaminated by way of leaking wheel end seals, thus, its stopping distance was far longer than it should be.

The evidence, as recorded in the video, could be re-packaged to serve as a warning as to what happens to your ability to stop if the wheel end seals are leaking into the brake drums.

He tore down the wheel ends shortly after this thread, discovered this, and fixed it all up.

No matter spring or DD3 systems, when everything is up to snuff, they all stop much the same, with a transit coach having some advantage due to the larger swept area of the brakes installed for the constant stop and go.

So, take your coach out, ensure all is secure, which you should be doing every time it moves anyway, and give it a whirl!

happy coaching!
buswarrior

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RJ
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« Reply #74 on: June 11, 2014, 04:21:12 PM »

All -

Brian's great video has been found.  It's now on YouTube, here's the link:

Bus Parking Brake vs. Service Brake application


FWIW & HTH. . .

 Wink
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RJ Long
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S13406 Now
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