THE BOBTAIL PROPORTIONING RELAY VALVE
WE'VE GOT TO SEPARATE VALVES
TO SHOW THE OPERATION OF IT
BASICALLY THE LOWER HALF HERE
IS A NORMAL RELAY VALVE
THE NORMAL RELAY VALVE HAS A SUPPLY CIRCUIT
OR SERVICE CIRCUIT
OR CONTROL CIRCUIT THAT COMES FROM AN APPLICATION VALVE
IN THIS CASE IT WOULD BE THE TREADLE VALVE
THE TREADLE VALVE WILL SEND AIR TO THE RELAY PISTON
THE RELAY PISTON WILL MOVE DOWN
SEALING THE EXHAUST PORT IN THE RELAY VALVE
OPENING THE SUPPLY PORT, RIGHT HERE
NOW THAT ALLOWS AIR TO COME FROM THE PRIMARY AIR TANK
OUT TO THE SERVICE BRAKE CHAMBERS
ALLOWS IT TO COME TO THE SERVICE BRAKE CHAMBER
ON OUR PARKING BRAKE CHAMBER
AND IT WILL PUSH ON THE DIAPHRAGM
AND PUSH THE PUSH ROD OUT
WHICH REACTS ON THE SLACK ADJUSTER'S
S-CAM AND THEN APPLIES THE FOUNDATION BRAKES
SO AS THE AIR COMES THROUGH HERE
IT OPENS UP AND ALLOWS AIR TO COME IN ON THE SUPPLY
AS SOON AS WE GET THE EQUIVALANT ON THE BOTTOM OF THE PISTON
AS WHAT IS BEING APPLIED ON THE TOP
THE PISTON IS SAID TO GO INTO A STATE OF BALANCE
SO THE INTAKE VALVE DOWN HERE CLOSES
AND WE GO INTO A STATE OF HOLD
IF THE DRIVER WERE TO STEP ON THE TREADLE VALVE
AND GIVE IT 20 PSI
THE RELAY PISTON WOULD MOVE DOWN
NOW BECAUSE WE HAVE 20 PSI ON TOP AND ONLY 10 ON THE BOTTOM
IT WILL OPEN UP THE SUPPLY PORT
AND AS IT OPENS UP THE SUPPLY PORT
ONCE WE GET 20 PSI ACTING ON THE BOTTOM HERE
IT CLOSES THE SUPPLY PORT
THE SIGNAL IS TAKEN AWAY
SO THE DRIVER LIFTS HIS FOOT OFF THE TREADLE VALVE
THEN AIR WILL BE EXHAUSTED FROM THE TOP SIDE OF THE RELAY PISTON
NOW THE 20 PSI ON THE BOTTOM OF THE PISTON
WILL CAUSE IT TO MOVE UP
OPENING THE EXHAUST PORT
AND WE EXHAUST ANY AIR IN THE CIRCUIT
FROM THE SERVICE BRAKES
BACK TO THE RELAY VALVE
THROUGH THE EXHAUST PORT ON THE RELAY VALVE
NOW WITH THE BOBTAIL PROPORTIONING RELAY VALVE
WHAT THEY'VE DONE, IS THEY'VE TAKEN THE LIMITING VALVE
AND THEY'VE PUT THE LIMITING VALVE
ON THE TOP HALF OF THE RELAY VALVE
AND WHAT THEY DO, IS THEY
SEND AIR FROM YOUR TREADLE VALVE
TO THE INLET OF THE RATIO VALVE
AND THEN AS THE AIR COMES THROUGH
BELOW 60 PSI AND WITHOUT A TRAILER ON
THE AIR DELIVERED
TO THE RELAY VALVE AND ON TO YOUR SERVICE BRAKES
WOIULD BE AT HALF THE RATE
OF WHAT IS BEING APPLIED TO IT
SO IF WE PUT 10 POUNDS OF AIR INTO THIS VALVE
BECAUSE THE SURFACE AREA ON THE BOTTOM
BEING TWICE AS MUCH AS ON THE TOP
AS THE VALVE IS OPENED UP
TO ALLOW AIR TO GO BY
THE BOTTOM HALF OF THE RELAY VALVE
AS SOON AS WE GET 5 PSI ON THE BOTTOM
IT WILL CLOSE IT
SO IT GOES UNBALANCED
WHEN WE GET THE EQUIVALENT AIR PRESSURE
BY HALF ON THE BOTTOM SIDE OF THIS PISTON
BECAUSE OF THE COUBLE SURFACE AREA
IT CLOSES OR, GOES INTO A STATE OF BALANCE
SO THIS VALVE
WAS DESIGNED TO HELP PREVENT REAR WHEEL LOCK UP
WHEN YOU WERE LIGHTLY LOADED
AS IN THE CASE OF BOB-TAILING WITH A TRACTOR.
AND NO TRAILER ON
NOW WHEN THE DRIVER HAD THE TRAILER ON
THIS VALVE BECAME ONE TO ONE
BECAUSE WHAT THEY WOULD DO IS THEY WOULD SEND A SIGNAL
FROM OUR DASH VALVE
THE RED DASH VALVE HERE
TRAILER AIR SUPPLY VALVE
WHEN THEY PUSH VALVE IN TO SUPPLY AIR TO THE TRAILER
THERE WAS A SIGNAL ALSO SENT
TO THIS VALVE AS WELL
WHEN THAT OCCURRED, IT CAUSED THIS OUTER PISTON HERE
TO COMPRESS
AND WHEN THAT OUT PISTON IS COMPRESSED
IT COMPRESSES THE SPRING IN HERE
IT HOLDS THE SMALLER PISTON ON THE BOTTOM HALF
AND IT GIVES IT ONE TO ONE
SO IF THE DRIVER GIVES IT 10 PSI HERE
10 PSI GOES TO THE SERVICE BRAKES
20 IN, 20 TO THE SERVICE BRAKES
WHEN THE DRIVER PULLS OUT THE RED DASH VALVE
AIR IS EXHAUSTED FROM THIS PORTION OF THE VALVE
THEREFORE WE NEED AT LEAST 60 POUNDS TO COMPRESS THE UPPER PISTON
SO ANY APPLICATION
BELOW 60 PSI WILL DELIVERED AT HALF THE RATE
AS THE DRIVER IS REQUESTING
UP TO 60 PSI
AT 60 PSI THE VALVE FULLY OPENS
AND YOU GET ONE TO ONE
THESE VALVES
BOBTAIL PROPORTIONING RELAY VALVES
ARE PRE ANTI-LOCK BRAKES
THEY ARE STILL AND OPTION AND STILL AVAILABLE
HOWEVER THE ANTI-LOCK BRAKES DO A BETTER JOB
PREVENTING WHEEL LOCK-UP
UNDER MOST ROAD CONDITIONS
THIS WAS A GOOD IDEA
BUT THE PROBLEM IS
WHEN YOU REDUCE THE AIR PRESSURE MY HALF
TO THE SERVICE BRAKES
IT CHANGES THE ROAD FEEL FOR THE DRIVER
SO IF THEY HAVE BEEN DRIVING FOR SOME TIME
WITH A TRAILER ON, SUDDENLY THEY BOBTAIL
THEY HAVE HALF THE SERVICE BRAKES AT THE REAR AXLE
SO THEY HAVE TO RE-LEARN HOW TO BRAKE
SO THAT CAN BE A LITTLE BIT OF A PANIC STRICKEN DRIVE
WHILE THEY ARE RE-ADJUSTING TO THE BRAKE FEEL.
WHEN THEY FIRST CAME OUT WITH BOBTAIL PROPORTIONING RELAY VALVES
WE HAD A LOT OF COMPLAINTS FROM DRIVERS
THAT THEY DIDN;T HAVE PROPER BRAKING
OR ADEQUATE BRAKING WHEN THEY WERE BOBTAILING
AND THAT'S SIMPLY BECAUSE
IN THIS SITUATION THEY REDUCED
THE AIR PRESSURE TO THE REAR BRAKES BY HALF
HELPING TO PREVENT WHEEL LOCK-UP
BUT WHEN YOUI DO THAT YOU CHANGE THE ROAD FEEL
THAT NEEDS A LITTLE BIT OR A LEARNING CURVE
FOR MOST PEOPLE TO RE-LEARN