to operatively receive and support the substantially circular disc member 222 thereon as described more fully hereinafter and an outer base element generally indicated as 226 attached to the wheel 212 by a plurality of attachments each indicated as 228 as best shown in fig1 the inner base element 224 comprises a substantially cylindrical base hub 230 having a centrally disposed aperture 232 formed therethrough to operatively receive and house a bearing 234 with a channel 236 therein and a concentrically disposed recess 238 formed in the outer face or surface 240 thereof to receive a portion of the substantially circular disc member 222 as described more fully hereinafter as best shown in fig1 the outer base element 226 comprises a substantially annular plate generally indicated as 242 extending between the substantially cylindrical hub 230 and a portion of the wheel 212 the substantially annular plate 242 comprises an inner member 244 inclined relative to the substantially cylindrical hub 230 and an outer member 246 substantially parallel to and engaging the outer surface of the wheel base 248 as best shown in fig1 each of the attachment means 228 comprises an elongated externally threaded member 250 extending through corresponding a channel 252 formed through the wheel base 248 and a corresponding aperture 254 formed in the substantially cylindrical hub 230.