Lockring Wheel casting Removable flange Removable flange Snap ring Bearing rollers Bearing rollers Grease retainer Fairing disc Flat base Drop center Fairing retaining screw Drop center wheel Flat base wheel 13-34 Figure 13-57.  Removable flange wheels found on older aircraft are either drop center or flat base types.  Figure 13-58.  Two-piece split-wheel aircraft wheels found on  modern light aircraft.  Figure 13-56.  A non-hydraulic shimmy damper uses a rubber piston  with lubricant that dampens via motion against the inner diameter  of the unit housing.  Aircraft Wheels Non-Hydraulic Shimmy Damper Aircraft wheels are an important component of a landing  gear system. With tires mounted upon them, they support the  entire weight of the aircraft during taxi, takeoff, and landing.  The typical aircraft wheel is lightweight, strong, and made  from aluminum alloy. Some magnesium alloy wheels also  exist. Early aircraft wheels were of single piece construction,  much the same as the modern automobile wheel. As aircraft  tires were improved for the purpose they serve, they were  made stiffer to better absorb the forces of landing without  blowing out or separating from the rim. Stretching such  a tire over a single piece wheel rim was not possible. A  two-piece wheel was developed. Early two-piece aircraft  wheels were essentially one-piece wheels with a removable  rim to allow mounting access for the tire. These are still  found on older aircraft.  [Figure 13-57]  Later, wheels with  two nearly symmetrical halves were developed. Nearly all  modern aircraft wheels are of this two-piece construction.  [Figures 13-58  and  13-59] Non-hydraulic shimmy dampers are currently certified  for many aircraft. They look and fit similar to piston-type  shimmy dampers but contain no fluid inside. In place of the  metal piston, a rubber piston presses out against the inner  diameter of the damper housing when the shimmy motion  is received through the shaft. The rubber piston rides on a  very thin film of grease and the rubbing action between the  piston and the housing provides the damping. and kept serviced. A fluid level indicator protrudes from the  reservoir end of the unit.  This is known as surface-effect damping. The materials  used to construct this type of shimmy damper provide a  long service life without the need to ever add fluid to the  unit.  [Figure 13-56]