-module wheel_black(radius, width, shaft_radius, shaft_flat_width, wall_width){
+module tread(wheel_radius, tread_radius){
+ rotate_extrude(convexity = 10){
+ translate([wheel_radius, 0]){
+ circle(tread_radius);}}}
+
+module wheel_black(radius,
+ width,
+ shaft_radius,
+ shaft_flat_width,
+ wall_width,
+ tread_radius){