AIR BRAKES - GENERAL INFORMATION TRACTOR-TRAILER BRAKE ANALYSIS PROCEDURE The   air   brake   system   on   a   tractor-trailer   combination must provide balanced braking at all wheels and axles of the combination for optimum brake performance and life. This   means   that   the   braking   effort   produced   at   each wheel  of  the  combination  must  be  capable  of  doing  its share  of  the  work  in  controlling  the  speed  of  the  two units. There    are    several    factors    which    can    affect    brake balance.  These are: 1. Loads    must    be    properly    distributed    between tractor  and  trailer.    A  balanced  condition  should not  be  expected  with  the  combination  vehicle  if the  load  is  not  properly  distributed.  The  brakes on   each   axle   are   designed   to   meet   specific performance  criteria  based  on  the  axle’s  Gross Axle   Weight   Rating   (GAWR).      For   example, 15400 kg (34,000 lb.) GAWR tandem axles used on both the tractor and trailer in combination are deigned to meet similar performance standards. Overloading or under loading either the tractor or the  trailer  tandem  axle  can  cause  unbalanced braking   in   the   combination   vehicle.      A   typical symptom would be early lockup of the brakes on an axle loaded to less than the rated capacity. 2. Tractor  and  trailer  brakes  must  be  compatible. The  foundation  brakes  used  on  the  tractor  and trailer  should  be  of  the  same  type.    Operating characteristics, such as fade and speed sensitivity, are different for wedge, cam and disc- type brakes. Consequently, under some operating   conditions,   the   intermixing   of   brake types   can   cause   or   exaggerate   an   imbalance condition between tractor and trailer(s).  Extreme care must be used when operating a combination   equipped   with   brakes   of   different types. 3. Tractor and trailer brakes must be maintained in proper  adjustment.    The  force  output  of  an  air brake  chamber  push  rod  Increases  linearly  for approximately     75%     of     its     maximum     travel. Beyond 75%, the force is reduced.  Therefore, if the   brake   adjustment   is   not   maintained   within specification  and  the  push  rod  travel  is  allowed to increase beyond 75% of maximum travel, the brake torque output at the wheel will be reduced. For example, when air pressure of 689 kPa (100 psi)  is  applied  to  a  Type  30  service  air  brake chamber,  approximately  3,000  lbs.    of  force  is produced with a push rod travel of 38mm (1-1/2 in.)    or    less.        When    the    push    rod    travel    is increased  to  57mm  (2-1/4  in.)  at  the  same  air pressure,    the    force    produced    decreases    to approximately  11,121N  (2,500  lbs.),  a  reduction of  about  17%.    A  reduction  in  force  at  the  push rod   causes   a   similar   reduction   in   the   brake torque  output  at  the  wheel.    A  reduction  in  the brake   torque   output   capability   at   any   one   or several  wheels  of  a  combination  vehicle  places added work on the brakes at other wheels.  The result  can  be  uneven  lining  and  drum  or  rotor wear (between brake assemblies), trailer surging and increased brake fade. If  automatic  adjusters  are  used,  they  should  be used    on    both    tractor    and    trailer    to    insure compatibility    between    the    two    units.        While automatic  adjusters  do  maintain  the  brakes  in reasonable  adjustment,  it  is  important  that  the adjusters   be   inspected   periodically   to   be   sure that they are functioning properly. 4. Tractor and trailer brake assemblies and air sys- tems must be Inspected and properly maintained.        Corroded    anchor    pins,    frozen camshafts,   weak   or   broken   return   springs,   or deformed  shoes,  etc.,  can  reduce  the  efficiency of   the   brake   assembly   at   any   wheel(s).      It   is important  to  inspect  the  air  system  -  reservoirs, valves, fittings and lines -periodically for damage.    This  includes  draining  the  reservoirs as required. Whenever    complaints    of    touchy    brakes,    inadequate braking,   short   service   life   of   brake   linings,   drums   or rotors on either the tractor or the trailer (but not both) are encountered,  there  may  be  an  imbalance  between  the tractor and trailer air brake systems. It is suggested that a brake analysis be performed.  This analysis   includes   testing   for   1)   pressure   balance,   2) pressure  build-up  rate  (timing),  and  3)  torque  balance. However,    before    conducting    the    tests,    the    following questions should be considered: 1. Are all or a  majority  of  the  drivers  operating  the combination  units  obtaining  the  same  results  or registering the same complaints? 2. Are    the    same    results    or    complaints    being registered on similar combination units? 3. Are    the    same    results    or    complaints    being registered under similar operating conditions? 4. Were complaints always received on the combination unit or did complaints originate after some  brake  service  was  performed,  or  vehicle operating conditions changed? The  answers  to  these  questions  may  help  in  identifying the cause(s).  Such problems could be the result of incorrect or malfunctioning brake components, or     simply     the     conditions     under     which     the combination vehicle is operated. If the response to these questions seems to indicate that a  brake  Imbalance  condition  may  be  present  in the air brake system, the preliminary checks and test   pro-cedures   in   the   following   text   can   be used to isolate the cause. CTS-4154Y - Page 26