AIR BRAKES - GENERAL INFORMATION If    the    balance    ratio    is    below    0.75,    the combination   unit   is   out   of   balance   and   the tractor   is   providing    more    than    its    share    of braking efforts. If  the  balance  ratio  is  above  1.25,  the  trailer  is providing more than its share of braking efforts. DATA INTERPRETATION AND CORRECTIVE MEASURES After  the  test,  the  data  needs  to  be  interpreted.    Guide- lines  and  concepts  are  provided  below  but  it  must  be recognized that values which are acceptable  depend  on many factors. These factors include terrain, weather, operator and fleet preference,    maintenance    and    the    tractor    and    trailer foundation  brake  and  vehicle  characteristics  as  well  as the    interrelationship    of    pressure    balance,    pressure buildup rate and torque balance. Also   it   must   be   remembered   that   data   taken   on   one vehicle combination may or may not be representative of other units in a fleet. Pressure Balance Combination Units - A 28 kPa (4 psi) or less pressure differential  is  generally  acceptable  between  the  tractor rearmost axle and the trailer rearmost axle.  Some users prefer to have a slightly higher pressure at the trailer than at  the  tractor.    This  may  require  special  valves  on  the tractor. For   combinations   involving   multiple   trailers,   it   is   the preference of some operators to have lower pressure in the  dolly  axle  brake  chambers  than  at  other  axle  brake chambers. Individual  Valves--  If  the  pressure  differential  between the   tractor   and   trailer   brake   chambers   exceeds   the above   guidelines,   the   system   should   be   checked   for pressure drops across individual valves.  This is accom- plished   by   connecting   air   lines   from   the   duplex   test gauge to the inlet and outlet ports of the individual valves and checking the variation of input to output pressures at 34 kPa (5 psi) increments up to 207 kPa (30 psi). Combination Unit Overview The vast majority of brake applications are made at less than 207 kPa (30 psi).  Due to this fact, minimizing pres- sure differentials between the tractor and trailer(s) brake chambers  becomes  very  Important  if  each  axle  is  to  do its share of braking the combination.  For example, If on a  specific  brake  application  the  drive  axle  receives  103 kPa  (15  psi)  and  the  trailer  axle(s)  only  55  kPa  (8  psi), then the tractor brakes could be doing almost 50% more work  in  slowing  the  combination  than  the  trailer  brakes. Therefore,  minimizing  the  pressure  differentials  on  the trailer axle(s) is very important to optimize performance, wear and stability. It  is  extremely  difficult  to  obtain  exactly  the  same  pres- sure  on  all  axles  due  to  the  additional  valves  used  on each  of  the  trailer  axles.    However,  controlling  differen- tials   and   maintaining   them   within   the   28   kPa   (4   psi) noted earlier is important. Pressure Build-up Rate (Timing) Tractors   and   trailers   built   since   1975   are   required   to meet   FMVSS-121   application   and   release   timing   re- uirements.    However,  meeting  these  requirements  does not  insure  timing  compatibility.    Evaluating  the  time   it takes   to   build   up   pressure   in   the   various   axle   brake chambers  and  the  time  to  exhaust  this  pressure  from these chambers takes sophisticated equipment nor-mally not available to the user.  If such a check is desired, the vehicle manufacturer should be contacted. However, a general comparison of the pressure build-up rates in the tractor and trailer chambers can be checked by the procedure noted earlier.  This check will give you an   indication   if   the   pressure   build-up   differential   be- tween the tractor and trailer is reasonable.  Usually, there will be a difference (the trailer will usually be slower) due to  the  added  piping  and  valves  required  to  actuate  the trailer  brakes.    Minimizing  this  difference  is  important  in achieving  combination  compatibility.    As  noted  earlier, maintaining this lag to 69 kPa (10 psi) or less should be the objective. If the Pressure Build-up Rate Test indicates the lag is not within  the  69  kPa  (10  psi)  guideline,  the  system  should be   checked   for   restrictions   in   the   lines   and   hoses, damaged fittings, or incorrect lines or fitting sizes.  If no problem is noted, the individual valves should be check- ed for contamination and/or failed components. Torque Balance Normally,  acceptable  combination  unit  performance  can be   achieved   with   a   tractor-trailer   balance   ratio   of   be- tween .75 and 1.25. If  these  guidelines  are  exceeded,  the  modifications  to consider include: 1. If ratio is less than .75 Increase the power of the trailer brakes or Decrease the power of the tractor brakes 2. If ratio is greater than 1.25 Decrease the power of the trailer brakes or Increase the power of the tractor brakes NOTE   -   A   tractor-trailer   combination   (with   each   axle loaded to its GAWR) that meets the performance criteria for   pressure   balance   and   timing,   and   torque   balance should    also    reach    a    9-1/2    l1    ft./sec.2    deceleration (minimum) without any wheel lock from 64 kmh (40 mph) at 276 kPa (40 psi). CTS-4154Y - Page 34