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Author Topic: Air Brakes - Application Pressure? Related to toad brakes  (Read 9788 times)
edroelle
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« Reply #15 on: December 06, 2008, 07:04:25 PM »

Tom,  I looked-up the  Air Force One and could not find out how they do what they claim.  A regulator can help to limit the maximum force, but the cylinder diameter sets-up the proportional braking.  All things considered, you would like your toad to brake at the same deceleration rate as your coach.

Jim,  Yes, the 150# is at the pedal.  One FMVSS test stipulated 150# max pedal effort, no power - stopping distance was verrry long.

"My problem was that the typical non-panic brake application was in the 30 psi range and that is almost no toad braking.  "   If it is a non-panic stop, is toad braking necessary for you?

Optimally, if you are stopping your coach alone at a medium decel and there is x psi air pressure to the rear bus brakes, your would like the x psi, operating the cylinder in your toad, to give you the same braking decel - proportional.    I think the 2" is too large.  When I went through the numbers a few years ago, I think the cylinder diameter was to be ABOUT  1 1/4"  to 1 1/2" - as I remember (my paperwork is at home and I am in Florida).  You will not be able to slide the toad tires on pavement under no power, below 150# pedal effort.

Jim, you are on the right track with some testing - light bus braking, ? decel, ? air pressure.  Repeat med bus braking, heavy bus braking.  Now go repeat in toad - light toad braking, ? pedal effort ...  Then it is just sizing of cylinder so a given bus braking/air pressure applies toad pedal force to give equal/proportional decel.

The difficulty in all this for us, is we just don't have decel meters and pedal force transducers readily available.

The difficulty for toad brake manufacturers is the multitude of combinations of coaches and toads.   So, the manufacturers have to design to assumptions.   When I asked them about their designs, they admitted that proportioning meant more toad braking when there was more coach braking - NOT equal proportioning.   

I support having toad braking.  Someone may have a true proportional toad brake system, I just have not seen an effective system available.

Ed Roelle
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Brian Diehl
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« Reply #16 on: December 06, 2008, 08:06:40 PM »


It sounds like it works well for Brian. 

Jim

I also tried a 1.5" cylinder and was not happy with it in the least.  2" provides braking that "feels" right to me.  I know, not much science there.  I wish I had access to the right tools to prove perfect levels of proportional braking, but don't.  So, seat of the pants testing like Jim descibes is the best I can do.  I do know I don't lock up the brakes and I also know there is a noticable difference in bus brake pedal travel/force required if I don't have the toad brake connected compared to when I do.
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rv_safetyman
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« Reply #17 on: December 06, 2008, 08:21:27 PM »

Ed, thanks for the reply.  BTW, I changed the subject to let folks know that the discussion involved toad brakes.

Obviously, the air pressure to the cylinder is the same as the air pressure applied to the bus brake chambers.  Thus the force to the master cylinder would be a direct function of air pressure to the air brake chambers on the bus.  Beyond that I don't know enough about the brake dynamics to say that they would be proportional.  I would think it would in the ball park of being proportional.

For the normal city driving, or to slow for a slower driver, the toad breaking is not a significant issue.  The one place that I am concerned about is really steep hills.  With the 10 speed and good Jakes, I don't have to use the brakes very often.  However, I have been on hills where I have to slow the engine from slightly over 2100 rpm to perhaps 1900 rpm, or to slow for a slower vehicle.  I try to use a moderate stab on the brakes for a perhaps  5 - 10 seconds.  However, I would bet that I never get over 40-50 psi and that does not apply any significant toad braking according to my testing.  In that case, I want as much toad braking as I can get, so that the slowing takes place quickly and does not heat the bus brakes any more than necessary.  I guess the other way of saying that is that I want the heavy truck to share the braking load under less than panic conditions (the truck is about 20% of the total GCVW).  Knowing that it will never be perfect, I want to defer to a bit of overloading the truck brakes.  With the 4W disc brakes on the truck, I don't think I will be doing any significant heat damage and the parts are fairly cheap.

Keep up the dialog, as I want to make sure I am doing the right thing.

Jim
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Jim Shepherd
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Len Silva
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« Reply #18 on: December 07, 2008, 08:20:14 AM »

Your case is different from most of us with the heavy truck and four wheel disk brakes.  For me, towing a little CRV, I don't want any toad brakes until I REALLY need them.
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« Reply #19 on: December 07, 2008, 10:04:48 AM »

This may be an off the wall question, except to those that aren't old truck drivers, but...

Why do you use a shop air quick connect/disconnect on the air line from the bus to the toad instead of a glad hand connection?

It's always been my experience that the brass or steel connections will eventually start leaking when exposed to the elements, but, while a gladhand connection may start leaking, it only costs about $1.00 to replace the glad hand rubber.

Another plus of the glad hand is, if you mess up and forget to unhook, the gladhand will come apart, leaving you to straighten the lock plate back with a pair of pliers. If you do the same with a quick connect, you'll be replacing either an air line or a couple of connectors.

I'm not saying that your are wrong to use those fittings, I'm just looking for the rationale behind it.

Thanks,

Dallas

Dallas,
You make a valid point! How ever as a heavy duty wrecker operator I can add one more reason for the use of the shop air quick disconnects. On a system such as our buses not having a trailer valve on the dash would mean that that air line is always "charged". Which is exactly as Jack described on his test run of a "1/4" air leak while holding the pedal down." Unless you used glad hand plugs you'd have the same problem & glad hand plugs are #1) not readily accessible to all & #2) in my opinion are more prone to some kid or clown tampering with than just a female quick connect that is empty.
JMHO FWIW Grin  BK  Grin
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« Reply #20 on: December 07, 2008, 10:18:36 AM »

Ed,

No expert here. However, I too leaned toward an M&G toad brake system. Only problem was they do not make one for my toad (2003 Dodge Ram 2500 4x4). The factory says there is not enough room in the engine area for their unit.

I found the SMI Air Force One on the web and made arrangements to have one installed at a rally last spring. This year we have logged over 7,000 miles and knowing the toad had its own brakes was a comfort to both of us. We even went over the Applachians and Rockies twice without problems. I really learned how to use the Jake also.

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Will Garner, Jr
Southern Pines, NC
1991 Prevost Conversion by Country Coach
makemineatwostroke
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« Reply #21 on: December 07, 2008, 10:55:00 AM »

I changed over to the M&G system from Brake Buddy for my Jeep in Oct after the unit failed what I don't like about Brake Buddy or the Roadmaster unit they will take a toll on the firewall as the vehicle wasn't design for that type force on the pedal or the seat without power assist so I would me careful in the design of a unit that puts out a great amount of pressure on the pedal, firewall on the floor board.I saw one I don't know who the manufacture was but it ripped the floor board and another at PPL in Houston that stripped the gears in a power seat on a Jeep    have a great day
« Last Edit: December 07, 2008, 11:07:52 AM by makemineatwostroke » Logged
luvrbus
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« Reply #22 on: December 07, 2008, 11:27:34 AM »

Jim, I have a Blue Dot vacuum pump for the brake booster new still in the box I bought to go on the Lexus if you want it I will bring it to Quartzsite in Jan I know we can trade for something.But I would be concerned about putting anything on a power assist braking system pedal without one, didn't Ben have a problem with his brake pedal using a air system with no vacuum to the booster.   

good luck
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Sean
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« Reply #23 on: December 07, 2008, 02:13:04 PM »

Jim,

Just additional data for you -- I have dual application gauges (one for each half of the redundant system) and a normal "service" application rarely runs above 20psi, a "hard stop" runs about 30psi.  If I put 60 psi to the brakes, I'd probably lock 'em up.

Also, make sure you use a "protection valve" ahead of your coupler (and I'm with Dallas on this: use a glad-hand).

Without such a valve, a tear-away of the toad (or road debris severing the air lines, whatever) will empty your service line on application, resulting in either 50% reduction of coach braking power, or full emergency application, depending on how your system is plumbed.  Neither is good, which is why federal law mandates the protection valve for truck tractors.  FWIW.

-Sean
http://OurOdyssey.BlogSpot.com
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rv_safetyman
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« Reply #24 on: December 08, 2008, 09:35:14 AM »

Clifford, I think I will pass on the vacuum pump.  Want to keep the system as simple as possible.

Sean, no sure what you mean by protection valve.  The valve that comes to mind is the one that shuts off at, say 60 psi, and is used on one of the supply tanks to feed the aux. tank.  If that is the case, I can't quite see how that would work.  If I would put in in the line that runs between the relay valve and the toad, it would never allow pressure to get to  the toad, except for the panic stop (pressure would be below the opening pressure).

I want to make sure that I am clear about how I plumbed this system.  I simply plumbed into a port on the bogie relay valve and when the brakes are applied, the toad air line is pressurized.  If a line brakes in this system, there is no air loss until the brakes are applied.  I use a 1/4 inch line, so the loss is minimized (but would be a problem with extended brake pedal application). 

It seems to me that a glad hand would not help if the air line between the glad hand and the toad broke (most likely scenario).

A good friend who read this board, but will not post, called to strongly remind folks that have this type of system to use a pedal return spring.  He had a friend who had a battery go down because the brake pedal did not return sufficiently to turn off the brake lights.  He also had a friend who lost a toad to a fire that could have been caused by the brakes not releasing fully (they are not sure).  I use a bungee cord and that seems to do a good job.  I check it at each stop and the brake pedal is retracted and the brake lights are not on (on most cars, the first key position will unlock the steering, and allow the brake lights to work, but nothing else).
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Jim Shepherd
Evergreen, CO
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« Reply #25 on: December 08, 2008, 03:51:49 PM »

Jim,

A protection valve allows air to flow only at a certain rate, above which the valve closes.  Note that this has to do with flow rate, not pressure.

The idea is that, under normal service brake application, the pressure builds smoothly and the flow rate in the lines, which terminate in slave cylinders, is actually very low, remaining below some threshold.  The valve is pre-set above this threshold.  If a line is open to atmosphere, air under pressure will try to leave the line at a much higher rate, and the protection valve will close, preventing further air loss.

The (possibly deadly) danger of not using such a valve is that, under normal but prolonged braking, an open line caused by the toad air line being severed would ultimately result in loss of service pressure, as the compressor could not keep up with the loss.  Now this is not likely to be a problem in normal stops on normal roads, but let's say you're coming down from Monarch Pass and you're on the brakes pretty steady, when you lose your toad (or the line comes apart, for whatever reason).  Now you're going to run out of air before you get to the bottom of the hill -- or your spring brakes (if equipped) will engage, completely outside of your control.  At that speed, on that hill, I would figure a spring brake application to be a sure way to catch the bearings on fire, and then you'll get to do a live test of your Cold Fire extinguishers Wink

The glad hand comment, BTW, was unrelated to the protection issue.

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #26 on: December 09, 2008, 09:35:59 AM »

Tom,  I looked-up the  Air Force One and could not find out how they do what they claim.  A regulator can help to limit the maximum force, but the cylinder diameter sets-up the proportional braking.  All things considered, you would like your toad to brake at the same deceleration rate as your coach.

Optimally, if you are stopping your coach alone at a medium decel and there is x psi air pressure to the rear bus brakes, your would like the x psi, operating the cylinder in your toad, to give you the same braking decel - proportional.    I think the 2" is too large.  When I went through the numbers a few years ago, I think the cylinder diameter was to be ABOUT  1 1/4"  to 1 1/2" - as I remember (my paperwork is at home and I am in Florida).  You will not be able to slide the toad tires on pavement under no power, below 150# pedal effort.

Jim, you are on the right track with some testing - light bus braking, ? decel, ? air pressure.  Repeat med bus braking, heavy bus braking.  Now go repeat in toad - light toad braking, ? pedal effort ...  Then it is just sizing of cylinder so a given bus braking/air pressure applies toad pedal force to give equal/proportional decel.

The difficulty in all this for us, is we just don't have decel meters and pedal force transducers readily available.

The difficulty for toad brake manufacturers is the multitude of combinations of coaches and toads.   So, the manufacturers have to design to assumptions.   When I asked them about their designs, they admitted that proportioning meant more toad braking when there was more coach braking - NOT equal proportioning.   

I support having toad braking.  Someone may have a true proportional toad brake system, I just have not seen an effective system available.

Ed Roelle

i'm trying to talk smi or a local dealer into making a presentation and/or exhibiting at the Bussin rally in Arcadia.  They explained to me tha their system is not really that complex.  air line from relay valve on bus, to air force one system on toad, with special disconnects at the bus bumper.  Their system manipulates the brake pedal while systems such as m&g manipulate the brake cylinder fluid.  The proportional part comes from the amount of air pressure being applied to the air brakes on the bus being applied to the pedal on the toad.

that's my extremely limited, quick explanation, or interpretation, of what they said.  like i said, i'm trying to get them to come to the rally.
sounds like what Jim is doing himself.
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Tom
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JackConrad
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« Reply #27 on: December 09, 2008, 03:04:05 PM »

Tom,
   I would love to have them do a seminar at our rally.

Jim,
   Not sure how they figgered their cylinder size, but the M&G unit uses air pressure from the tag axle brakes to move an air cylinder that pushes the plunger in the toad brake master cylinder. More air pressure in tag axle air line= stronger toad brake application.  Jack
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« Reply #28 on: December 10, 2008, 07:02:54 PM »

Hi Sean.  I went all through my Bendix Air Brake Handbook and looked at all the various components and none work based on flow limitation that I could find.  I did find the pressure protection valves (PR-2, PR-3, and PR-4) but they are pressure operated.

I really like the idea of the valve you suggest, but I can't find anything that works on flow.  I did a general google search and found Haldex and Midland pressure protection valves, but they also operated on pressure.

If you have a source, let me know.  It is a great idea and I would guess someone has them, but not the major suppliers in the air brake market that I can find.

Jim
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Jim Shepherd
Evergreen, CO
85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
Blog:  http://rvsafetyman.blogspot.com/
Sean
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« Reply #29 on: December 10, 2008, 09:55:49 PM »

Jim,

I don't think you'll find such a valve in an air brake catalog, for a simple reason: DOT air brake systems forbid single service-line connections to trailers.  In a commercial hauler such as a truck tractor, you will have a "supply line" to the trailer, also known as the trailer emergency line, as well as a "control line" also known as the trailer service line.

With this arrangement, full breakaway protection, as well as protection from failure of either line, can, indeed, be detected and prevented by means of pressure alone, and this is normally done with a pair of pressure-operated control valves such as a PP-7 and TP-3 (which ought to be well described in your Bendix catalog).

So, I think, to get the required protection in your non-DOT arrangement of only carrying the service line back to the trailer, you will need to get a valve from another application.  I don't have a specific part # for you, since I haven't had to solve this particular problem.  But in gas lines (in the strict sense of the word, e.g. air, helium, etc.), the type of valve is known as an "excess flow valve," and you should be able to find one that meets your needs by Googling around a bit.  Incidentally, in fluid lines, such a valve is known as a "hydraulic fuse."

BTW, any one out there who has an LP cylinder aboard already has an excess flow valve -- there is one built into the cylinder to protect against a rupture in the high-pressure line leading to the regulator.

HTH.

-Sean
http://OurOdyssey.BlogSpot.com
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