Acton  which  Determine  Power  Reqnired. — ^When  a  motor  car  is
in  operation  there  are  various  forces  tending  to  resist  its  motion.    On


68  The  Modem  Gasoline  Automobile

a  level  road  these  are  the  rolling  resistance  at  the  point  of  contact  of
the  traction  wheels  and  the  roadway;  friction  in  the  power  trans-
mission mechanism  and  atmospheric  resistance.     If  the  vehicle  is


Fig.  SL—Apflicttlioa  of  For»-Door,  nve-Pasaengw  Touring  Body  to  Gasoline


moving  on  a  gradient  there  is  still  another  factor  to  be  considered,
and  this  is  the  amount  of  power  needed  to  raise  tlie  weight  of  the
car  against  the  force  of  gravity.  Obviously  when  the  car  is  descend-
ing a  hill  the  force  of  gravity  assists  the  power  plant  instead  of  work-
ing against  it.
tig.  SS.— The  ConpC,  a  Popokr  Form  of  Closed  Body  Favored  by  PiofMnoiul


The  Modem  Gasoline  Automobile  69

The  resiBtance  of  the  road  depends  yery  largely  upon  the  nature
of  the  Burface,  and  it  is  .also  dependent  upon  the  size  of  the  wheels
and  type  of  tire  with  which  they  are  fitted.  The  second  item,  that
of  friction  of  rear  axle,  drive  shaft,  bearings  and  change  speed  gears,
is  comparatively  low,  and  it  is  generally  included  with  that  of  road
resistance.  It  will  be  evident  that  other  factors  must  also  be  con-
sidered in  determining  power  required  to  propel  a  vehicle.  One  must
figure  on  the  total  weight  of  the  car,  the  capacity  of  the  body,  and


Fig.  S3.— The  Rockwell  Taziub,  or  Publk  Service  Vehicle,  with  Convertible
Type  Bodj',  which  Hay  be  Used  as  Shown  and  which  Becomes  a  Closed
Car  when  the  Top  is  Raised.
