Parts Back-left and Back-right
I changed the area where the wheel sits. Now this part can be printed without support. Also, the connection part (wh_connect) is now joined to back-right/back-left part so you don't need to print it separately.

back left
back left

 This part is not necessary anymore

not needed anymore
not needed anymore

 

Parts Front_right / Front_left
The parts are printable without support. Also the holes for mounting the part to back_right/back_left are modified so a washer and nuts for M3 scews fits.

front left
front left