a cross bar smooth rod coupling that can be fitted without taking apart

the whole frame

Parametric_cross_bar_coupling/cross_bar_coupling.scad

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+
+//------------------------------------------------------------
+// Parametric pendulum rod coupling to increase frame stiffness
+//
+// http://www.thingiverse.com/wardwouts
+//
+//------------------------------------------------------------
+
+bar1_diameter = 6.5;
+bar2_diameter = 6.5;
+
+M3 = 3.2;
+
+thickness = 3;
+spacing = 1;
+
+
+FN=80*1; // 80 for nice prints; 16 for speed
+
+bar1r = bar1_diameter/2;
+bar2r = bar2_diameter/2;
+
+height=bar1_diameter+2*thickness;
+
+// I really need nut or rivetsize to make bar1mount the right size
+module bar1mount(){
+	translate([-(bar1_diameter+3*thickness),-thickness/2,0]){
+		difference(){
+			cube([bar1_diameter+3*thickness+1,thickness,height]);
+			union(){
+				translate([thickness + bar1r,-thickness,thickness+bar1r]){
+					rotate([-90,0,0]){
+						cylinder(h=(thickness*4), r=bar1r, $fn=FN);
+					}
+					translate([-bar1r,0,0]){
+						rotate([0,10,0]){
+							cube([2*bar1r,thickness*4,30]);
+						}
+					}
+				}
+			}
+		}
+	}
+}
+
+module bar2mount(){
+	translate([bar2r+thickness,0,0]){
+		difference(){
+			cylinder(h=height, r=bar2r+thickness, $fn=FN);
+			translate([0,0,-height]){
+				cylinder(h=height*3, r=bar2r, $fn=FN);
+			}
+		}
+	}
+}
+
+module screwmount(){
+	translate([bar2_diameter+2*thickness-1,-(thickness+spacing/2),0]){
+		difference(){
+			cube([thickness*2+M3, thickness*2+spacing, height]);
+			translate([thickness+M3/2, -thickness, height/2]){
+				rotate([-90,0,0]){
+					cylinder(h=thickness*5+spacing, r=M3/2, $fn=FN);
+				}
+			}
+		}
+	}
+}
+
+module cutout(){
+	translate([bar2r+thickness,-spacing/2,-height]){
+		cube([thickness*5+M3*2, spacing, height*3]);
+	}
+}
+
+module crossbar_coupling(){
+	difference(){
+		union(){
+			bar1mount();
+			bar2mount();
+			screwmount();
+		}
+		cutout();
+	}
+}
+
+crossbar_coupling();