top 20 has seven slits analogous to slit 170 171 through 177 in registered alignment with the slits in the hub bottom within hub bottom 10 are openings 180 through 187 these openings are recessed and are in registered alignment with openings 210 to 217 in hub top 20 each opening 180 through 187 and 210 to 217 receives a means for securing bottom 10 and hub top 20 preferably the securing means is a fastener 280 through 287 not shown in this view openings 180 through 187 in the hub bottom 10 and 210 to 217 on the top 20 are preferably recessed so the heads of the fasteners and the nuts not shown lie flush with the respective surfaces of the hub the nuts and fasteners referred to are tightened to secure hub top 20 to hub bottom 10 the openings in hub top 20 also are provided with recesses 220 to 227 recesses 220 to 227 are configured to both capture nuts 230 to 237 and enable them lie flush with the respective surfaces of hub top 20 the fasteners and nuts 230 to 237 are threaded so that they may be threadedly engaged to improved structural performance and minimize weight hub top 20.