A few special designators may also be found for aircraft tires.  When a B appears before the identifier, the tire has a wheel  rim to section width ratio of 60 to 70 percent with a bead taper  of 15 degrees. When an H appears before the identifier, the  tire has a 60 to 70 percent wheel rim to section width ratio  but a bead taper of only 5 degrees. Tire plies are reinforcing layers of fabric encased in rubber  that are laid into the tire to provide strength. In early tires,  the number of plies used was directly related to the load the  tire could carry. Nowadays, refinements to tire construction  techniques and the use of modern materials to build up aircraft  tires makes the exact number of plies somewhat irrelevant when determining the strength of a tire. However, a ply rating  is used to convey the relative strength of an aircraft tire. A  tire with a high ply rating is a tire with high strength able to  carry heavy loads regardless of the actual number of plies  used in its construction.  Another means of classifying an aircraft tire is by the  direction of the plies used in construction of the tire, either  bias or radial. Traditional aircraft tires are bias ply tires. The  plies are wrapped to form the tire and give it strength. The  angle of the plies in relation to the direction of rotation of the  tire varies between 30° and 60°. In this manner, the plies have  the bias of the fabric from which they are constructed facing  the direction of rotation and across the tire. Hence, they are  called bias tires. The result is flexibility as the sidewall can  flex with the fabric plies laid on the bias.  [Figure 13-128] Some modern aircraft tires are radial tires. The plies in radial  tires are laid at a 90° angle to the direction of rotation of the  tire. This configuration puts the non-stretchable fiber of the  plies perpendicular to the sidewall and direction of rotation.  This creates strength in the tire allowing it to carry high loads  with less deformation.  [Figure 13-129] It is useful to the understanding of tire construction to  identify the various components of a tire and the functions  contributed to the overall characteristics of a tire. Refer to  Figure 13-130  for tire nomenclature used in this discussion. An aircraft tire is constructed for the purpose it serves. Unlike  an automobile or truck tire, it does not have to carry a load for  a long period of continuous operation. However, an aircraft  tire must absorb the high impact loads of landing and be able  to operate at high speeds even if only for a short time. The  deflection built into an aircraft tire is more than twice that of  an automobile tire. This enables it to handle the forces during  landings without being damaged. Only tires designed for  an aircraft as specified by the manufacturer should be used. As stated, aircraft tires can be tube-type or tubeless. This  is often used as a means of tire classification. Tires that  are made to be used without a tube inserted inside have an  inner liner specifically designed to hold air. Tube-type tires  do not contain this inner liner since the tube holds the air  from leaking out of the tire. Tires that are meant to be used  without a tube have the word tubeless on the sidewall. If  this designation is absent, the tire requires a tube. Consult  the aircraft manufacturer’s maintenance information for any  allowable tire damage and the use of a tube in a tubeless tire.  Tube-Type or Tubeless Tire Construction Bias Ply or Radial Ply Rating Figure 13-128.  A bias ply tire has the fabric bias oriented with  and across the direction of rotation and the sidewall. Since fabric  can stretch on the bias, the tire is flexible, and can absorb loads.  Strength is obtained by adding plies.  Figure 13-129.  A radial tire has the fiber strands of the ply fabric  oriented with and at 90° to the direction of rotation and the tire  sidewall. This restricts flexibility directionally and the flexibility  of the sidewall while it strengthens the tire to carry heavy loads. 13-77