|House Projects|| Machine
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.
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.|
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 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.
is now what the car looks like when it is connected. From
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.
|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.|
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
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.|
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.|
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
from the 2 inch top plate to the 3/4 inch pin, and welded it all
|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.|
have inserted one of the towbar inserts into the Jeep baseplate and
dropped the hook assembly over it. It is ready to
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..
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!
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.
what the folded towbar looked like before any modifications.
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.|