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Old 09-10-2014, 11:15 PM   #1
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How come there is no calibration for SMI brake system

I am in the process of installing a Air Force One system from SMI on my jeep Trailhawk. I have posed this question to SMI customer support, but I am having a hard time believing it...probably because there is something that I still don't understand.

How come there is no calibration needed so that you match the correct amount of braking force in the TOAD with the same force that comes from the MH.

Do all airbrake systems put out the same air pressure for the different amounts of braking effort?
Here is what troubles me. If the MH has 1/2 brake pedal applied,
this will cause a certain amount of air to go to the AF1 cylinder attached to the brake pedal arm. And the cylinder will extend a relative amount based on how much leverage it has. The leverage can vary if I mount the cyl. near the pedal pad (more leverage) or up high near the hinge point (less leverage). So what keeps the TOAD from braking too much or too little compared to what the MH is doing

What am I missing on how this works?

thanks,
Dan
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Old 09-11-2014, 09:56 AM   #2
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I'm not sure exactly how they've worked that one out, but their system works. We have had the same AF1 on three different coaches of different sizes, and brake capabilities and have never had a problem with it. (the exact same unit, not just the same model). When I apply the brakes, be it a panic stop from high speed or just a normal stop at a traffic signal, I never get the feeling that the toad is breaking too much, or too little. I can definitely tell when the toad brake is not working, but never an over or under feel. I can get a bit of a toad overbrake when already stopped and then I start just creeping forward, as in a traffic jam. At that point I feel the toad brake come on ahead of the Bus and it feels like the toad is slowing the Bus. This only happens at speeds LESS THAN a normal walking speed.

The one time I broke a toad cylinder activation cable on the way down from Alaska, it was immediately obvious that the toad was trying to overrun the Bus. I made a temp fix on the road with a nut, bolt and 2 washers to splice the cable, and sent the cylinder to SMI for repair. They had it back in my hands in less than 10 days at a cost of less than $30, which included shipping.

We've used the exact same system on a 2005 38' diesel Bounder, a 2008 40' Allegro Bus, and a 2013 43' tag axle Allegreo Bus. Our toads have been a 2006 Chevy Colorado and a 2011 Chevy Silverado. When we go to Alaska we're running the bus over 43,000 lbs and the toad around 8000 lbs. About 185,000 total miles on the AF1 system and if this one ever dies I'd have a new one in the mail in a heartbeat.
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Old 09-11-2014, 11:49 PM   #3
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Thanks Pigman (I would feel better using your first name...:>) )
That makes me feel better. I was concerned that I might not be able to tell if the TOAD was braking too much/little. But knowing that I will be able to feel when things are not right, puts me at ease.
And thanks for the info about a broken cable. I will stick a small split bolt in my tool kit incase this ever happens to me.

Regards,
Dan
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Old 09-12-2014, 08:13 AM   #4
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If you decide to go with the AF1, take a close look at the lug they use to fasten the cable to the firewall. In my unit it was a large electrical lug. The type you'd use to fasten a heavy wire to a screw terminal. Mine appeared to be made of titanium, and the in the area where the brake cable went the edge was a very acute right angle. Since the lug was so hard the right angle edge wore the cable until it broke. When I got the repaired cylinder back I took my Dremel tool and eased the sharp edge to a bit of a radius. No further problems with wear in that area.

Good luck with it if you decide to go that route. I feel quite sure you'll be pleased with the AF1.
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Old 09-13-2014, 08:45 AM   #5
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Yes, I have already started the install of the AF1. I will smooth out the edges where the cable exits as you suggest. Excellent tip. Installing the tow bar base plates today. I am just a little nervous about hooking into the air lines on my Phaeton. Air brakes are new to me...want to be sure I get the right connections.

Dan
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Old 09-14-2014, 07:27 PM   #6
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AF1 uses air pressure from coach to generate a vacuum on the car, just as the car creates vacuum when running. I believe they use an accelerometer in their control box that automatically adjusts the vacuum applied to the air cylinder. The more the force on the vehicle, the more braking it applies. Have used it for three years, 20,000 miles and feel its the best choice.
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Old 09-14-2014, 08:42 PM   #7
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No electronics in Air Force One

Quote:
Originally Posted by bertman View Post
AF1 uses air pressure from coach to generate a vacuum on the car, just as the car creates vacuum when running. I believe they use an accelerometer in their control box that automatically adjusts the vacuum applied to the air cylinder. The more the force on the vehicle, the more braking it applies. Have used it for three years, 20,000 miles and feel its the best choice.
I opened the control box. No electronics inside. Only a electrically actuated valve for the break away function. When you brake the motor home, the metered air goes in to the box, and then to the cylinder attached to the pedal. There is also a venturi device which creates a vacuum for the car booster.
Now, depending on where you mounted the cylinder the leverage is different.
that is t he part I don't understand.

Dan
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Old 09-14-2014, 09:09 PM   #8
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I installed a LED light on my coach dash that indicates when the AF1 has activated the towed vehicle's brake. It takes a fair amount of pressure on the coach service brake to activate the AF1 in the towed. More often that not I can almost bring the coach to a complete stop before the system activates.

I do a lot of mountain driving and I rarely have had the AF1 system activate when I am just snub braking.
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Old 09-14-2014, 09:38 PM   #9
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BobGed,
You might check every air and electrical connection confirming that everything is correct. Most importantly, be sure that the cable from the firewall is straight to the tube mounted to the brake pedal. Be sure to leave only about 1/4" side to side slack. That is the only adjustment, as I remember, that will affect how quickly the AF1 system begins to brake the toad. No slack at all in the cable would cause the toad to brake more aggressively than necessary; perhaps even unsafely. The adjustment is easy...a quarter inch...and it should work flawlessly. Good Luck JJ
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Old 09-14-2014, 09:52 PM   #10
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Quote:
Originally Posted by Jonnyboy 316 View Post
BobGed,
You might check every air and electrical connection confirming that everything is correct. Most importantly, be sure that the cable from the firewall is straight to the tube mounted to the brake pedal. Be sure to leave only about 1/4" side to side slack. That is the only adjustment, as I remember, that will affect how quickly the AF1 system begins to brake the toad. No slack at all in the cable would cause the toad to brake more aggressively than necessary; perhaps even unsafely. The adjustment is easy...a quarter inch...and it should work flawlessly. Good Luck JJ
Thanks for the suggestions, but mine is operating exactly the way I want it to. I do not want my towed brakes being activated every time I apply my coach service brakes. I only want/need them to activate when I really need them as in a panic stop. Other than that the most important thing to me is the breakaway function that would stop the towed vehicle should the tow bar and the safety cables fail.
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Old 09-14-2014, 10:14 PM   #11
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It's my understanding that vehicles with hyd brakes have to be able braked by a human being with no boosted or vacuum being supplied to the brake system. So if you loose the vacuum or boosted brakes you will still be able to brake the vehicle with just muscle force on the brake pedal. Not going to be easy to stop but doable. So if the car- truck manufactors have to build them that way all the manufactors would have to build them so that 90lbs or 120lbs of force on the brake pedal will stop you.( I'm not sure of the exact number). The supplemental brake manfactor just has to size the diameter of the air cylinder to put the correct force on the pedal that a person could put the pedal. That's why you are suppose to pump your brakes on the tow vehicle before you hook up the supplemental if you don't the first couple times you apply you supplemental brake it will lock up the tow vehicles wheels until the vacuum is depleted. Because the supplemental brake system is calculated to supply the correct force manually
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Old 09-15-2014, 07:27 AM   #12
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Quote:
Originally Posted by BobGed View Post
I installed a LED light on my coach dash that indicates when the AF1 has activated the towed vehicle's brake. It takes a fair amount of pressure on the coach service brake to activate the AF1 in the towed. More often that not I can almost bring the coach to a complete stop before the system activates.

I do a lot of mountain driving and I rarely have had the AF1 system activate when I am just snub braking.
Hi BobGed,
I look at it a little differently...maybe because I am a tightwad
I bought my Air Force 1 because it was 100% proportional. The reason I really want this is to save the brakes on the coach just because a brake job on the car is much cheaper (and I can do it) than the coach.

My thought is that anytime I touch the brake, I am trying to slow 40K lbs of mass. If the Toad brakes don't come on, the coach is doing all of the work and therefore wearing out the brakes a little bit faster.

The other poster said to check the cable slack. I am not a expert on physics, but I don't think that will make much difference. I think it has more to do with the mount point of the air cylinder. You might have it further away from the peddle pad...so that means it takes more force to apply the brakes in the TOAD (less leverage).

The important thing is that it is working the way you want...and getting you stopped when needed.

Cheers,

Dan
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Old 09-15-2014, 07:35 AM   #13
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My SMI Stay N Play Duo for gas MHs had an adjustment. Excuse me for not reading all the posts here but has anyone suggested calling SMI. They have a great CS dept.
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Old 09-15-2014, 08:35 AM   #14
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Quote:
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My SMI Stay N Play Duo for gas MHs had an adjustment. Excuse me for not reading all the posts here but has anyone suggested calling SMI. They have a great CS dept.
Yes, I did call them...and they confirmed there is no calibration.
Everyone seems to be happy with how the system works.
I guess I am just being a little anal in wanting to understand the details on how this is possible...I am never too old to learn something

Dan
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