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Tow Car Brakes and Towbar

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) from a convention vendor, and a near new towbar I spotted on the local Craigslist.  This section discusses the installation of the braking 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 of my Subaru.  There really was no other practical space that I could find to mount this unit.  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 doing what it is supposed to do.

Mods as of 3/9/2011

A couple of weeks ago I traded in my Subaru Forrester for a new Jeep Liberty.  Of course before turning in the car, I removed the Air Force One braking system to transfer over to the Jeep.

I ordered a baseplate for the Jeep to connect to my Roadmaster All Terrain tow bar, and installed it when it arrived.  I ran wire in split loom sleeving and finally managed to feed it all the way back back through the frame.  I used this to wire the tail and stop lights for towing.  I originally was going to mount separate bulbs speciffically for towing, but after looking carefully at the light assemblies I decided not to, as the new bulbs would be far from the focal center of the reflectors, and the light output would be much less.  I then opted for the normal diode isolating method which uses the original bulbs both for normal car driving, and for towing.

I installed the Air Force One system in the Jeep.  It fit much better than it did in the Subaru.  There was an open place on the inside of the left front fender where the unit fit nicely, in easy reach of the various wires and hoses it needed to connect to.

The final step was to mount the air fitting, break-away switch, and the light connector on the front of the Jeep.  The flat horizontal part of the plastic bumper assembly leading back to the top of the lower grill was fairly flexible and would be a problem if these items were simply bolted to it.  I decided to back up the mounting area with a metal plate.  I cut a piece of 3/16 hard aluminum to about 2 inches by 15 inches.  I drilled matching mounting holes in this plate and in the bumper, threading the ones in the plate..  I was able to fish the plate into the proper area through the lower grill and maneuver it into place using screw drivers and an ice pick.  Once aligned, I threaded the mounting bolts through the item to be mounted, through the bumper, and threaded them into the backing plate.  Loctite on the threads assures continuing tightness.  I was very pleased with the results.  All three items are very solidly mounted and show very little flexure as I attach the mating fittings.


The brake actuating air cylinder mounts on the Jeep brake just like it did on the Subaru.  The main difference is I had no worries about it interferring with the clutch pedal, as the Jeep has an automatic transmission.
The Air force One braking system fit in very nicely on the left front fender.  All the wires and hoses were easy to run.
This view shows the tow sockets for the base plate inserts, the safety cable loops, the air fitting on the left, the break-away switch in the center, and the light connector on the right.  The break-away switch swings 90 degrees out for towing and stows nicely sideways for normal driving.

The aluminum plate on the top of the bumper horizontal section at the top of the lower grill provides a very stable mounting for the various fittings.

Here the Jeep is connected and ready to tow.  Shortly after taking the towbar picture with the Subaru, I replaced the coil cord electrical and the coiled air line with straight ones which pass through the wiring channels of the towbar.  This makes the connection much neater and controls these items better when the towbar is stowed.
A side view shows the towbar is pretty level without the need for a dropped or raised receiver on the motor home.  By actual measurement, it is aabout 2.5 inches higher on the car than on the motorhome.  This is within the tolerances called for by Roadmaster, the maker of my towbar and baseplate.  Their specs are for the car to be between 3 inches higher and 4 inches lower than the motor home.  Other manufacturers recommend level to 4 inches lower.  I would like to lower the mounting on the car to more nearly approach level.
To accomplish this lowering on the car, I modified how the brackets mount to the baseplate inserts.

The bars that insert into the baseplate have flat plates on the front end with two slotted holes.  The piece pictured here bolts to one of these plates.  The crossbar that the tow bar attaches to slips into this pair of plates.  The original mounting was by the two slotted holes in this plate.

To lower the towbar mounting on the car, I drilled an additional set of holes near the top of this bracket, so that when bolted to the inserts, the mounting would be lower.
This shows the original mounting of the bracket to the insert.  Note how high the bracket extends above the mounting plate.
This is the bracket mounted using the new holes.  The net result is that the mounting  for the towbar on the car is about 1 3/4 inches lower than it was originally, resulting in a towbar that is less than 1 inch higher on the car than it is on the motorhome.


Addition of 3/30/12
Tow Hook

When I first received the Jeep it had a couple of heavy duty tow hooks under the front bumper.  I guess this is part of the "Trail Rated" equipment.  I was pleased, as if I am ever in need of being pulled out of a ditch, mud, snow, or whatever, this would be a very convenient place to attach a tow line.  I was disappointed when I had to remove these hooks to install my towbar baseplate. 

I have been thinking of ways I could restore the function of these hooks if I were to need to.  I considered (1) fastening the hook to a heavy steel bar machined to duplicate one of the removable towbar inserts, (2) purchasing a duplicate towbar attachment insert and attaching the hook directly to it, or (3) making a bracket that would allow me to attach the hook to the inserts I already have.  I chose the third alternatitive.

I designed a bracket which would drop over the top pin of the existing inserts with a pin that drops through the bottom hole, just like my existing towbar crossbar attaches.  The folowing photos show what I came up with.




I machined a 3/8 piece of steel, making a large hole which would drop over the top pin of the towbar insert, and two smaller holes to mount the hook.  I bent a couple of 1/4 in steel pieces to taper down from the 2 inch top plate to the 3/4 inch pin, and welded it all together.
The hook bolts to the top plate with its two original bolts.  This shows the assembled bracket and hook, as I will carry it with me. 

Here I have inserted one of the towbar inserts into the Jeep baseplate and dropped the hook assembly over it.  It is ready to use.  I can use the hook on either side insert, and can put a lock through the hole in the top pin to secure it as I do for my towbar, but for just a quick pull, even that would not be required. 

I thought of mounting the other hook on a 2 inch square hitch bar so I could put it in my trailer hitch receiver, but decided that the normal insert with a ball would work just as well If I need to tow someone else, or if I need a backward tow..




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 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.



This is what the folded towbar looked like before any modifications.
And this is what it is like now.  The 25 degree angle provides enough clearance that the engine door will now clear it.


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 right as it opens and closes.



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Dick Mason, Prescott, AZ  4/16/10