On a recent trip to an FMCA International Comvention in Albuquerque, I returned home with a brand new tow car brake system (in the box) and a near new towbar.  This section discusses the installation of the barking system, and a modification of the towbar to clear my engine compartment door.

SMI Air Force One Braking System

After doing research on the available tow car braking systems, I narrowed my search to two units.  

The M&G system is very popular, and does much of what I want in a system.  They have a custom machined cylinder which attaches between the brake master cylinder and the power brake activator in the tow car.  The cylinder attaches to an air line from the coach air brake system and provides sufficient force to actuate the master cylinder directly.  It is totally non-invasive as far as driving the car is concerned.  The standard system does not include a breakaway system, but one is available as an option.  Two disadvantages of this system are:

1) The air cylinder is custom designed for each car, and would require the purchase of a new one when changing cars.  Most, but not all cars can be fitted with the cylinder.  

2) The air system on the coach is not protected against a failure of the car system, or a broken air line in case of a car breakaway.  If one of these events occur, the coach is left with brakes on only 3 wheels.

The SMI Air Force One uses a clamp-on air cylinder mounted on the brake pedal.  The mounting is high enough on the pedal that it does not interfere with normal car driving.  Air from the coach creates a vacuum which activates the standard car power brake booster.  This means the cylinder does not need to provide a very high force on the pedal.  

The coach is fitted with a small air tank which is surrounded with protective valves such that any break in the car system lines will only empty the small storage tank, and not interfere in any way with the motorhome braking.  This is the only system on the market which meets DOT requirements for this coach system protection.  The system comes standard with a breakaway system.

This is the system I chose.

The control unit of the Air Force One braking system is mounted with Velcro on the air cleaner.  There are lines from the incoming coach air, and to the brake pedal cylinder and the power brake vacuum. This box contains a small air storage tank and the needed valves to activate the brakes and hold them in case of a car breakaway.

I had to mount the air fitting (left) and the breakaway switch (center) under the bumper. As the bumper plastic is not too stable, I mounted them on reinforcing plates. The electrical plug is the original one with one added connection.

The air cylinder is designed to pull on a small aircraft cable when activated.  The cylinder mounts on the brake pedal arm and the cable is anchored to the floor.  When the cylinder activates, the cable pulls the pedal down, assisted by the normal power brake booster.

This side view shows the cylinder a little better.

The air brake isolating equipment is mounted under the rear of the coach. The air tank stores a supply of air from the main supply line.  It is protected by a special valve.  The output goes to a normal air brake relay valve.  The coach pedal air controls the amount of tank air that is sent to the car.

This shows the fittings that are used to tap into the main air supply (bottom fitting) and the modulated air from the brake pedal (top fitting). All the fittings are DOT air brake certified fittings.

The brake kit came with a small LED assembly which was to be placed on the car or towbar somewhere where you could see it through the backup camera.  This was attached to the car brake light circuiit and would illuminate when the car brakes were applied.  As I did not want to mount and unmount, and plug and unplug this light each time I used the car, I sent that signal through a spare pin in the existing electrical connector.  This runs forward to an LED on the dashboard which I had mounted long ago when I was having problems with the Air Conditioning.  I no longer need it for the AC, so am now using it as an annunciator for the braking system.

The output from the air relay goes to a small fitting to the right of the hitch. The power connector on the left had dragged on the ground sometime in the past, and the bracket was bent outward at about 45 degrees.  I cut it off, straightened the pieces and overlapped them about an inch.  This makes it an inch less likely to hit bottom again. I added a wire from the car brake light switch which runs to an LED on the dashboard.  This indicates when the car brakes are applied.

This is now what the car looks like when it is connected.  From left to right we have a safety cable, the airline for the brakes, the breakaway lanyard, the electrical cable, and the other safety cable. Disconnecting the air line will not affect the coach braking system, and pulling the breakaway lanyard will set the car brakes and hold them on.

I have tested the braking system and it all works fine.  I am about to take the rig out on the road, and will verify that it is all donig what it is supposed to do.


Towbar Modification

I am very well pleased with my new towbar.  It is actually two model revisions newer than my old one, and certainly in better condition.

The one problem which I encountered on my old towbar, and then corrected was that when the towbar is stowed, the engine compartment door hits the towbar as you open and close it.  On the old design of the towbar, I simply made a steel piece with 3 holes, bolted it to the towbar, and pinned to the added hole when storing it.  This allowed the towbar to sit at an outward angle to the motorhome and provide the needed clearance.

I want to do the same thing with my new towbar, but the design is totally different.  This bar has a spring loaded latch which engages the head of one of the pivot bolts and holds the bar in place.  It is much more convenient to use, but is much more difficult to modify.

I made a very detailed CAD drawing of that part of the towbar.  I then rotated the latch mounting assembly 25 degrees around the main pivot bolt to define what I needed to end up with.  I then designed a welded assembly that would mount on the old latch holder and provide the rotated latch holder.  

This is the assembly I made to re-position the latch to hold the towbar 25 degrees out from the coach. It mounts on the original latch holder, bolts in place, and then provides a mount for the latch in the new position.  It was quite tricky to build!

Here the new latch holder is mounted in place.  The latch plate originally pivoted where the stainless steel bolt is and was spring loaded against the vertical gray member.  You can just see the right hand stop just above the bolted bracket piece. The latch plate now pivots around the empty hole and is spring loaded against the left member.  The spring rests on the  1/8 plate protruding at an angle. The whole latch mounting pattern is duplicated 25 degrees CCW around the large bolt at the bottom. As with most of my modifications, the change is totally reversible if I should decide to sell the towbar or change coaches.

Here is the finished towbar modification.  The bar rests at a 25 degree angle away from the rear of the motorhome.  This is just sufficient to clear the large white door on the left as it opens and closes.

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