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Author Topic: Air Brakes - Application Pressure? Related to toad brakes  (Read 9532 times)
rv_safetyman
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Jim Shepherd


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« on: December 02, 2008, 01:06:58 PM »

I use a 1.5 inch diameter piston actuator for my toad system (plumbed to the bogie relay valve).  Today I unhooked the vacuum system and then drove around the yard and applied air to the cylinder.  It looks like at 60 PSI, I get some decent stopping.  I have been a bit concerned, since I now tow a service truck that weighs over 8K.

The question is, what does a typical hard stop air pressure go to?  As I recall some folks talked about light breaking resulting in pressures in the 20's.  I know that Jack Conrad played with his system at about 50 PSI.  If the pressures go over 60 PSI on a hard stop, I will feel comfortable with the cylinder I have.

I don't have an application gauge, so I hope some of you have an applied pressure gauge and looked at some of the pressures at various braking levels.

Jim
« Last Edit: December 06, 2008, 05:53:02 PM by rv_safetyman » Logged

Jim Shepherd
Evergreen, CO
85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
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Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
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blue_goose
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« Reply #1 on: December 02, 2008, 01:13:19 PM »

Jim, a max brake stop will put full air on your brakes.   Most of the time not over 60 lbs. for a highway stop, but in a bang bang stop look for over 90 lbs.
Jack
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Len Silva
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« Reply #2 on: December 02, 2008, 01:24:04 PM »

Jim,
I think you will be just fine.  You don't really need the toad brakes until you really need them. 
At 1.5" diameter, the area is 1.76"2,  so at full application of say, 90 psi you will be putting 158 lbs on the brake pedal, probably about what it would be from your foot in a panic stop.

I think the home built that Fred Hobe uses http://users.cwnet.com/~thall/fredhobe3.htm has a 1" x 6" cylinder.
« Last Edit: December 02, 2008, 01:28:39 PM by Len Silva » Logged


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rv_safetyman
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Jim Shepherd


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« Reply #3 on: December 02, 2008, 01:31:05 PM »

Thanks Jack and Len.  I had done the math as well and felt that it should be enough, but did not have a good handle on what a full application would give in the way of air pressure.  It makes sense that it would be full system pressure, but I wanted to verify that.

I used this cylinder for a lighter toad and never worried about it.  However, with this big hunk it became more of a concern.

Jim
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Jim Shepherd
Evergreen, CO
85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
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Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
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« Reply #4 on: December 02, 2008, 03:38:15 PM »

Hi:

Thanks for jogging my memory with this thread.

I have the 1/4" brake line plumbed now from the relay valve on the tag axle, through a 1/4" ball valve, then to a quick coupler at the rear bumper of my MC7.
My toad is a Geo Tracker 4 dr with power brakes, weighs in around 2200lbs I guess.

Question:

Fred's setup includes an 'air muffler' which I believe is threaded into the 2nd port on the air cylinder. His part number for the cylinder translates into a 1.5" cylinder with a 6" stroke. Our local supplier has these on the shelf. What does the air muffler do, and is it required?

The way I figured it, when the brake is released, the air must travel back to the relay valve to escape.

I am 'assuming' (Oh, I hate that word) that can just plug the 2nd port on the air cylinder.

Any suggestions would be appreciated before I buy/install the wrong thing.

Best regards.

Mark

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Mark Morgan    near Brandon, Manitoba, Canada
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« Reply #5 on: December 02, 2008, 10:29:27 PM »

When I was driving my truck with an application gauge, during normal braking, I made it a rule not to go over 30psi.   As consequence, my brakes have lasted over 300,000 miles.  So in my book, 60 psi would be too much.  Good luck, TomC
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JackConrad
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« Reply #6 on: December 03, 2008, 05:06:27 AM »

   We installed a brake application air pressure gauge when we installed the R&M dash in our MC-8. A "slow down" application is usally about 5-10 PSI. A "stop sign ahead" application is about 10-15 PSI. A "S%*T, the light just changed!" stop is 25-40 PSI.  When we do our pre-trip, while sitting still (after releasing the parking brake),  I make a HARD application to pressurize the break-away tank on the toad, (70-80 PSI).
   Our M&G installs between the brake booster and the brake master cylinder. It does require an air line connection to the bus air brake system.  We installed a Tee in the right rear tag axle brake line (before the tag axle brake regulator) and ran a 1/4" DOT tubing to a quick connect female fitting on the rear bumper.
    I did a test by inserting a stand quick connect (open) male fitting in the bumper female fitting and went for test drive.  Although I had a "major" (1/4") air leak when brakes were applied, I had no problem making a normal stop or a "panic stop". Only problem was when I kept the brake applied after coming to a stop. Setting the parking brake and releasing the service brake solved that problem.  Jack
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« Reply #7 on: December 03, 2008, 10:09:04 AM »

Question:

Fred's setup includes an 'air muffler' which I believe is threaded into the 2nd port on the air cylinder. His part number for the cylinder translates into a 1.5" cylinder with a 6" stroke. Our local supplier has these on the shelf. What does the air muffler do, and is it required?

The way I figured it, when the brake is released, the air must travel back to the relay valve to escape.

I am 'assuming' (Oh, I hate that word) that can just plug the 2nd port on the air cylinder.

Any suggestions would be appreciated before I buy/install the wrong thing.

Best regards.

Mark



The muffler allows the air on that side of the piston to escape. It also keeps dirt out.

If you plug the port, you won't like the results . . .
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Dallas
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« Reply #8 on: December 03, 2008, 10:34:40 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/dissconnect 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
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« Reply #9 on: December 03, 2008, 10:41:06 AM »

Can't speak for others, but in my case, I already had the quick connect fittings and one of those 10' coiled air hoses.  Didn't even have to drive to town (25 mile round trip) for any parts.  Jack
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rv_safetyman
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« Reply #10 on: December 06, 2008, 08:34:08 AM »

Even though folks think I might be OK (at least for the panic stop), I went ahead and ordered a 2 inch cylinder.  That will give me about 80% more force.   

Hopefully, it will come before we leave (just after Christmas - have to be in Indio by Jan. 1 -- wish we were heading towards the other coast).  That way I can do some testing at various pressures.

That cylinder will give me much better braking at less that full pressure (touching the brakes going down big hills when the Jakes just can't quite keep up).  My guess is that I will have to put a pressure regulator on it to keep from getting too much brake in a panic stop.

I want to make sure I keep the bus/truck in good control, since I am at about 45K total. 

The truck (3/4 ton)has all disk brakes, so a little extra use will not hurt anything.

Jim
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Jim Shepherd
Evergreen, CO
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Somewhere between a tin tent and a finished product
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« Reply #11 on: December 06, 2008, 09:03:14 AM »

Jim, I use a 2" cylinder myself.  I can DEFINITELY tell that the toad brakes are working when it is hooked up.  Like you, my toad has all wheel disk brakes and I've never had any issues with the toad brakes locking up even on "hard" stops.  The peace of mind with the braking system hooked up is immense especially knowing I'll be okay if I have to make a hard stop while going down a steep grade.  I consider a toad brake essential equipment.
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edroelle
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« Reply #12 on: December 06, 2008, 01:34:58 PM »

Jim,

I have stayed out of these discussions but you disrupted my comfort level.

If you use a 2" cylinder, I am afraid your toad may be trying to stop your bus.   You will have a tendancy to wear-out your toad brakes also.   If you have vacuum in your brake booster, you have the potential of bending the brake pedal bracket.   Dependent on brake system design, the brake pedal ratio is somewhere between 3.3 and 5:1

Years ago, I designed brake pedals, brackets, and systems.  I would be OK with 150# brake pedal effort, under no vacuum, on an OCCACIONAL basis.   We designed for 600# max pedal effort.    Some of you may say that was over design, but we had seen brackets that showed that level of panic braking.

I do not like any of the commercial toad braking systems.  How can a company, supply a brake toad system, that provides effective braking for motorhomes ranging from 12-53,000 pounds, and  toads 3-8,000 pounds?  They can't, or haven't yet. 

Jim, if you have access to a brake decelerometer, and pressure gauge connected to the rear brakes, we can design a relatively effective system for your vehicles.   Aren't you coming to Bussin?

(That said, of the systems out there, I like the M&G the best.)

Ed Roelle
 
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« Reply #13 on: December 06, 2008, 02:26:35 PM »

Jim,


I do not like any of the commercial toad braking systems.  How can a company, supply a brake toad system, that provides effective braking for motorhomes ranging from 12-53,000 pounds, and  toads 3-8,000 pounds?  They can't, or haven't yet. 

(That said, of the systems out there, I like the M&G the best.)

Ed Roelle
 

We are planning on coming to Bussin.  we just ordered an air force one from smi.  it looks the same???  but they specified a pressure regulator which they are going to set before shipping becuase of our power brakes.

any thoughts?
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« Reply #14 on: December 06, 2008, 05:52:14 PM »

Hi Ed.  I wish we were heading to Florida, but we have shows to do in CA the first week of Jan.

I need to do a design check with you.  When you are talking about 150#, I am assuming you are talking about the force at the pedal as opposed to the master cylinder rod.  That would make sense.  When I first started playing with my system I grabbed a bathroom scale and tried some pressures.  Seems like I got to at least 100 pounds.

I want to clarify that I do not have a vacuum pump for the toad brakes, I am careful to pump the brakes a few times to empty the chamber, so we are talking about un-assisted brakes.

The 1.5 inch piston has an area of 1.77 sq. inches while the 2 inch has 3.14 sq inches.  At 60 psi that gives forces of 106 pounds (1.5 cylinder) and 188 pounds (2.5 inch cylinder).  My problem was that the typical non-panic brake application was in the 30 psi range and that is almost no toad braking. 

As I mentioned, I did some testing (seat of the pants) and at 60 psi with the 1.5 inch cylinder, it slowed down reasonably well, but was far short of strong stop.  I was on dry grass, and I was not even close to sliding the tires. 

My plan is to use the 2 inch cylinder and add a pressure regulator.  I will probably set it to 50 psi (157 pounds force) or perhaps 60 psi.  If I understand your numbers, that is right in the design range of the pedal. I will do that testing on dry grass as well (hopefully there won't be snow on the ground Angry).  If I do slide the tires, I will probably try it on dry pavement.  One of the problems is that I do not have the truck fully loaded right now - probably at least 1000 pounds short of the typical load of product and show equipment.

Ed, give me a cross check and see if I messed up in my thought process.

It sounds like it works well for Brian. 

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/
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