Tractor Bob-Tail Problems

One of the engineering problems with tractor/trailer design, is that some times the tractor is operated by itself. While operating by itself, the rear tandem wheels are very lightly loaded and this causes two problems.

One of the problems is the rough ride because of the heavy duty rear springs with no load weight on them. This problem has been overcome with air-bag rear suspension. The air bags are filled by an air valve which senses the frame height above the rear axles. As the load increases, the air bag pressure increases to keep the same inflated air bag height. As the load decreases, the air bag deflates to keep the same frame height, and this results in a softer unloaded ride.

Another problem with bob-tailing is that the rear tandem tires slide easily due to the slight pounds per square inch of weight on these tires. Another problem is that the front brake power must be reduced while pulling a trailer to reduce the possibility of jackknife. Yet, when bob-tailing, the front brakes must effect more braking power because the front tires have more pounds per square inch of weight than the rear tires. These problems are solved with use of the bob-tail proportioning valve and the bob-tail ratio valve.

Bendix LQ-5 Bob-Tail Ratio Valve

The proportioning valve has one purpose. It must reduce the percentage of front wheel brake air pressure when a trailer is attached. When the trailer is disconnected, then 100% of the service brake pressure is applied to the front axle brakes. This results in more front axle braking power when the trailer is disconnected, and less front axle braking power when the trailer is connected (reduced chance of jackknife).

Although it is not mandatory to use the rear axle BP-R1 (described below) brake proportioning relay valve with the bob-tail ratio valve, the combined valves are highly recommended, and will reduce stopping distance and improve tractor control in bob-tail mode.

The LQ-5 has two spring loaded pistons, a blend back piston, and a proportioning piston. The stepped proportioning piston determines actual air pressure reduction pressure, and the blend back piston allows the valve's delivery air pressure to return to a 1:1 ratio during high pressure brake applications.

The stepped proportioning piston allows 100% brake pedal service pressure during the initial charge of the air chambers. This ensures quick contact of the brake shoes to the brake drums. After that initial pressure transfer, then the stepped piston provides delayed pressure rise to the front air chambers while the trailer is connected, and the brake pressures are light to medium. When the service air pressure becomes high due to heavy braking, then the blend back piston comes into play and allows 100% pedal pressure to be applied to the front axle air chambers.

When the trailer is disconnected, the front air chambers always receive 100% of the pedal service pressure to maximize braking on the front tires which have the heaviest loading (pounds per square inch) during tractor bob-tail.

Bendix BP-R1 Bob-Tail Proportioning Valve

The Bob-Tail Proportioning Valve is two valves combined into one. The lower body is a standard service brake relay valve which also includes quick release. The service relay valve speeds up brake application and release. The cover of this valve contains a brake proportioning valve, which reduces normal service brake application pressure when the tractor is not towing a trailer.

The relay valve description can be found at brake_valves. This valve employs a stepped proportioning piston which delays the service brake application pressure while the trailer supply lines are charged.

When the service brakes are first applied, the valve passes the pressure on to the brake chambers to ensure that the shoes rapidly contact the drums. Then the step comes into play, and commences limiting of the brake chamber pressures for low and medium service brake application.

When the service brake pressures become high, the valve blends back to 1:1 ratio, which applies full service brake pressure for high brake pressures. The graph to the right is courtesy of AlliedSignal Truck Systems, and shows how the service brake chamber pressure is delayed while a trailer is connected.

Notice how bob-tail line on the graph allows the full service brake chamber pressure to be passed on to the delivery port initially, and then the service brake chamber pressure is delayed to the delivery port until we reach the upper most part of the graph. At the higher service brake pressures, the bob-tail blended 1:1 ratio is achieved, and full service brake pressure is passed to the delivery port even while in bob-tail mode.

This graph also shows that when the trailer is connected, the full service brake pressure is always applied to the delivery port. With the trailer connected, the valve simply acts like a relay valve and a quick release valve.

This concludes the bob-tail valve descriptions. With both of these valves installed, the tractor is much safer to drive bob-tail.

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