wheel-bearing  construction  for  both  rear-wheel  and front-wheel  drives. TIRE  CONSTRUCTION Most  modern  passenger  vehicles  and  light  trucks use  tubeless  tires  that  do  NOT  have  a  separate  inner tube.  The  tire  and  wheel  form  an  airtight  unit.  Many commercial and construction vehicles use inner tubes which  are  a  soft.  thin.  leakproof  rubber  liner  that  fit inside the tire and wheel assemblies. However. in the last  few  years  tubeless  tires  have  been  introduced  to commercial  and  construction  vehicles.  reducing  the need  for  tube  type  tires.  Tires  perform  the  following two basic functions: They must act as a soft CUSHION between the road and the metal wheel. They   must   provide   adequate   TRACTION (friction) with the road surface. Tires   must   transmit   driving.   braking.   and cornering forces to the road in all types of weather. At the  same  time,  they  should  resist  puncture  and  wear. Although  there  are  several  tire  designs.  the  six  major parts of a tire are as follows: TIRE BEADS (two steel rings encased in rubber that  holds  the  tire  sidewalls  against  the  wheel rim). BODY  PLIES  (rubberized  fabric  and  cords wrapped  around  beads.  forming  the  carcass  or body of the tire). TREAD (outer surface of the tire that contacts the road surface). SIDEWALL (outer surface of the tire extending bead to tread; it contains tire information). BELTS (used to stiffen the tread and strengthen the plies; they lie between the tread and the inner plies). LINER  (a  thin  layer  of  rubber  bonded  to  the inside  of  the  plies:  it  provides  a  leakproof membrane for tubeless tires). There are many construction and design variations in tires. A different number of plies may be used and ran at different angles. Also, many different materials may  be  used.  The  three  types  of  tires,  found  on  late model vehicles, are bias-ply, belted bias. and radial. Bias-Ply Tire A bias-ply tire is one of the oldest designs, and it does  NOT  use  belts.  The  position  of  the  cords  in  a bias-ply tire allows the body of the tire to flex easily. This  design  improves  the  cushioning  action,  which provides a smooth ride on rough roads. A bias-ply tire (fig. 8-29) has the plies running at an  angle  from  bead  to  bead.  The  cord  angle  is  also reversed from ply to ply, forming a crisscross pattern. The tread is bonded directly to the top ply. A major disadvantage of a bias-ply tire is that the weakness of the plies and tread reduce traction at high speeds and increase rolling resistance. Belted Bias Tire A  belted  bias  tire  provides  a  smooth  ride,  good traction,  and  offers  some  reduction  in  rolling resistance over a bias-ply tire. The belted bias tire is a bias-ply  tire  with  stabilizer  belts  added  to  increase tread  stiffness.  The  belts  and  plies  run  at  different angles. The belts do NOT run around to the sidewalls but lie only under the tread area. Two stabilizer belts and  two  or  more  plies  are  used  to  increase  tire performance. Radial Ply Tire The  radial  ply  tire  (fig.  8-30)  has  very  flexible sidewall, but a stiff tread. This design provides for a very   stable   footprint   (shape   and   amount   of   tread touching  the  road  surface)  which  improves  safety, cornering,  braking,  and  wear.  The  radial  ply  tire  has plies  running  straight  across  from  bead  to  bead  with stabilizer  belts  directly  beneath  the  tread.  The  belts  can be made of steel, flexten, fiber glass, or other materials. Figures 8-29.—Bias-ply tire construction. 8-30