I've written a hull which in itself isn't dreadful, but which has revealed

fragility in the way OpenSCAD handles name collisions. This makes mixing
libraries highly problematic. Needs thought.

model/fourbar.scad

@@ -26,7 +26,7 @@ module fourbar_leg(length=1000, chord=100, long_skew=30, lat_skew=30) {
             cylinder(h=chord, r=12);
         
         // aerofoil section
-        skew([0, long_skew, 0, 0, 0, 0])
+        sskew([0, long_skew, 0, 0, 0, 0])
             rotate([lat_skew, 0, 180])
             color("black")
             airfoil(h=length, L=chord);
@@ -71,3 +71,4 @@ module fourbar(length=1000, chord=100, long_skew=30, lat_skew=30, shoulder=650)
 }
 
 
+// fourbar();

model/hull.scad

@@ -0,0 +1,65 @@
+// hull.scad
+
+// (c) Simon Brooke 2025; CC-BY-SA
+
+include <BOSL2/std.scad>
+include <BOSL2/skin.scad>
+
+// doesn't work, but it's promising
+module hull_i() {
+    color("darkgreen", 0.5)
+    difference() {
+        translate([1000, 0, 600])
+            rotate([0, 5, 0])
+            scale([3.49, .79, 1.19])
+            difference() {
+                sphere(r=500);
+                cube([1000, 1000, 1000]);
+            }
+
+        intersection() {
+            translate([1000, 0, 600])
+                rotate([0, 5, 0])
+                scale([3.5, .8, 1.2])
+                sphere(r=500);
+            
+            rotate([0, 95, 0])
+                translate([-500,0,0])
+                prismoid(size1=[1000, 1000], 
+                        size2=[500, 600], 
+                        height=3000);
+        }
+    }
+}
+
+module hull() {   
+    intersection() {
+        translate([2000,0,0])
+            rotate([180, 90, 0])
+            scale([20, 16, 20])
+            rotate_extrude()
+            //rotate([ 0, 90, 0])
+            import("library/hull-volume-of-rotation.svg");
+        
+        translate([-900,0, -110])
+        rotate([0, 90, 0])
+            prismoid(size1=[1100, 1000], 
+                    size2=[400, 600], 
+                    height=3000);   
+    }
+        
+    rotate([0,90,0])
+        translate([0,0,-2070])
+        difference() {
+            skin(
+                [ellipse($fn=64,r=[350,30],center=true),
+                 ellipse($fn=64,r=[430,340],center=true)], z=[0,1200],
+                slices=40,sampling="length",method="reindex");
+            skin(
+                [ellipse($fn=64,r=[344,24],center=true),
+                 ellipse($fn=64,r=[424,334],center=true)], z=[-1,1201],
+                slices=40,sampling="length",method="reindex");
+        }
+}
+
+hull();

model/library/skew.scad

@@ -8,7 +8,8 @@
  * z along x
  * z along y
  */
-module skew(dims) {
+/* renamed to sskew to resolve name conflict with BOSL2 */
+module sskew(dims) {
 matrix = [
 	[ 1, dims[0]/45, dims[1]/45, 0 ],
 	[ dims[2]/45, 1, dims[4]/45, 0 ],

model/tricycle.scad

@@ -4,10 +4,25 @@
 
 // Top level final rendering
 
+include <BentSim/Library/Velo_rider.scad>
+
 include <subframe.scad>
 include <fourbar.scad>
+// include <hull.scad>
 
 rotate([0, 0, 270])
-translate([0, 0, 500])
+translate([0, 0, 400])
 subframe();
-fourbar(700, 100, 30, 35, 650);
+fourbar(700, 100, 30, 35, 650);
+
+//hull();
+
+/* 
+prop=0.1;
+seat_ang=0;
+leg_ang=0;
+left_fold=0;
+head_ang=0;
+leg_prop=0.5;
+
+vRider(prop, seat_ang, leg_ang, left_fold, head_ang, leg_prop);*/