AIR BRAKES-GENERAL INFORMATION c. Initial Test Speed - If other than the recom- mended 64 kmh (40 mph).  If speeds of 64 kmh  are  not  permitted  in  the  area  chosen for  the  test,  the  test  speed  for  the  snubs can   be   reduced   to   30   mph.      Whatever speed  is  used,  it  should  be  the  same  for each of the checks noted below. 4. Check the brake torque output of the tractor and trailer in combination and individually as follows: NOTE - An observer should be used to record all data for these    tatas.        Only    the    SUSTAINED    decelerometer reading a should be recorded and not the peak readings at the beginning and end of each step. a. Tractor-Traller  Combination  Torque  Output The initial step in evaluating the performance of a tractor-trailer Is to determine     whether     the     combination is capable of operating at its designed performance level. While operating the tractor-trailer combination  in  a  suitable  test  area,  make three  (3)  stops  from  64  kmh  (40  mph)  at 276 kPa (40 psi) brake application pressure. Record the sustained deceleration    reading    (to    the    nearest    .5 ft./sec.2)  for  each  stop  in  the  appropriate space  on  the  Data  Sheet.  Determine  the average for the three readings. b. Tractor Torque Output With the service brakes released, close the shut-off     cock     in     the     double     gladhand connection between the tractor and trailer. WARNING - SINCE CLOSING THE SHUT- OFF COCK MAKES THE TRAILER SERVICE BRAKES INOPERATIVE, ALLOW MORE DISTANCE FOR STOPPING THE COMBINATION VEHICLE.      IF   POSSIBLE,   USE   A   NON- PUBLIC    AREA  FOR  CONDUCTING  THE TEST. Again, operate the tractor-trailer combination,    this    time    using    the    tractor brakes only to make three (3) stops from 64 kmh  (40  mph)  at  276  kPa  (40  psi)  brake application pressure. Record the decelerometer readings on the Data Sheet; determine and record the average reading. c. Trailer Torque Output With the service brakes released, open the shut-off     cock     In     the     double     gladhand assembly. Again, operate the tractor-trailer combination and, using only the trailer hand control  valve  (Figure  34)  make  three  stops from 64 kmh (40 mph) at 276 kPa (40 psi) and record the readings and average on the Data Sheet. NOTE - It may be helpful to preload the traller air lines    for    smooth    brake    application    and    more accurate  test  results.    This  Is  accomplished  by maintaining   vehicle   speed   while   lightly   applying the hand control valve just prior to the test stop. Figure 34 - Trailer Hand Control Valve 5. Add   the   average   deceleration   reading   for   the tractor  to  the  average  deceleration  reading  for the trailer. The  sum  should  be  within  +.5  ft./sec.2  of  the  average deceleration reading obtained from the combination test. If not, the individual tractor and trailer torque output tests must be rerun. Evaluating the Performance of Tractor and Trailer 6. From   the   test   results,   compute   the   Balance Ratio using the following formula: NOTE - Typical Weights In the formula are those weights  normally  being  carried  by  the  vehicle.  If a    realistic    estimate    cannot    be    made,    it    Is recommended  that  the  tractor  and  trailer  (with typical load) be weighed. 7. Record the Balance Ratio on the Data Sheet. a If the    balance    ratio    is    1.00,    the    tractor-trailer combination is in ideal torque balance. CTS-4154Y - Page 33