a pair of bearing assemblies 240 around a fixed nonrotating stub axle 220 the bearing assemblies 240 like their similarly numbered counterparts in the drive hub assembly 300 are inserted into opposite sides of the hub unit 230 after a pair of retaining rings 242 which engage internal slots 232 the bearings 240 are then sealed with bearing seals 244 fig7 is an exploded perspective view which illustrates the preferred interconnection between the front roller 100 f the back roller 100 b and the right side plate 400 r the interconnection at the left side plate 400 l is a mirror image of that shown in fig7 except for the width of the peg groove 405 l as illustrated in fig8 a and 8b as shown the preferred side plate 400 r includes a front axle guide plate slot 401 and a vertical slot 402 which leads to a back axle slider plate opening 403 the front axle guide plate slot 401 is sized to receive a front axle guide plate 410 which has an aperture 412 through which the front roller 39 s drive stub axle 320 f extends the drive stub axle 320 f is thereafter secured in an axle clamp 430 the back axle slider plate opening 403 is similarly sized to receive a rear axle slider plate 420 the rear axle slider plate 420 also includes an aperture 422 through which the back roller 39 s drive stub axle 320 b extends.