Moved the lower knuckle out into its own file, and substantially improved it.

I'm still having problems with fudge factors which I need to resolve;
I need to learn more about filletting, rounding, radiusing and fairing.

model/fourbar.scad

@@ -2,15 +2,25 @@
 
 // (c) Simon Brooke 2025; CC-BY-SA
 
-
+include <BOSL2/std.scad>
 include <NACAAirfoils/files/Naca4.scad>
 include <library/skew.scad>
+include <lowerknuckle.scad>
 include <wheel.scad>
 
-module fourbar_leg(length=1000, chord=100, long_skew=30, lat_skew=30) {
+$fn=64;
+
+// Wherever you see unexplained numbers in this file, they are
+// fudge because I don't yet properly understand the coordinate
+// transforms and can't remember enough of the trigonometry I
+// learned 60 years ago. There REALLY should not be any unexplained
+// numbers.
+
+module fourbar_leg(length=1000, chord=100, lat_skew=35, long_skew=30) {
     h = length * cos(lat_skew) * cos( long_skew);
     w = length * sin( lat_skew);
     d = length * sin( long_skew);
+    pivot_radius = 12;
     
     translate([0, 0-w, 0]) {
         // upper pivot needs a complex trig transformation!
@@ -23,7 +33,12 @@ module fourbar_leg(length=1000, chord=100, long_skew=30, lat_skew=30) {
         translate([0 - chord, 0, (chord/2 * sin(long_skew))])
             rotate([0, 90+long_skew, 0])
             color("black")
+            difference() {
                 cylinder(h=chord, r=12);
+                        // through hole for pivot axle
+                translate([0, -1, 0])
+                    cylinder(h=chord+2, r=pivot_radius/3); 
+            }
         
         // aerofoil section
         sskew([0, long_skew, 0, 0, 0, 0])
@@ -34,72 +49,55 @@ module fourbar_leg(length=1000, chord=100, long_skew=30, lat_skew=30) {
 }
 
 
-module lower_knuckle(chord=100, pivot_radius=12) {
+module fourbar_axle_half( length=1000, chord=100, lat_skew=35, long_skew=30, shoulder=650) {
-    difference() {
-        cube([chord * 1.5, pivot_radius * 3, chord*.66]);
-        translate([ chord*0.25, -1, pivot_radius * 2])
-            cube([chord, pivot_radius * 4, pivot_radius * 4]);
-    }
-}
-
-module fourbar_axle_half( length, chord, long_skew) {
-    shoulder=650;
-    lat_skew=30;
     rise=8;
     
     // I've got something wrong in computing the half-lower-bar
-    // length, and I don't know what.
+    // length, and I don't know what. It scales with lat_skew, but
+    // (unsurprisingly) not liniarly. Fudge of 23 works for lat_skew of 30; 34 works for lat_skew of 35
     fudge=0;
     
-    w = ((length * sin(lat_skew) ) + ( shoulder / 2));
+    w = ((length * cos(lat_skew) ));
     l_hlb= w * (1/cos(rise)) + fudge;
-    v_offset=l_hlb * sin(rise);
+    v_offset=(l_hlb * sin(rise)) + 38;
     
-    echo("long_skew", long_skew);
+    translate([-15, 0, 0 - v_offset])
-    
-    translate([0, 0, 0 - v_offset])
         rotate([90-rise, 0, 180])
             color("black")
                 airfoil(h=l_hlb, L=chord);
     
     // lower knuckle
-    translate([(0 - (chord*1.25)), l_hlb * cos(rise), 30])
     rotate([0, long_skew, 0])
-        color("black")
+        translate([(0 - (chord*1.25)), (l_hlb * cos(rise) + 72), -62])
-        lower_knuckle(chord, 12);
+            lower_bar_end_knuckle(chord, 12, rise, lat_skew);
     
-    // wheel
+    rotate([0, long_skew, 0])
-    translate([-50, l_hlb + 72, -40])
+        translate([(0 - (chord*1.25)), (l_hlb * cos(rise) + 155), -62])
-        rotate([270, 0, 0])
+        wheel();
-        wheel(iso=451); 
+    rotate([0, long_skew, 0])
-        
+        translate([(0 - (chord*1.25)), (l_hlb * cos(rise) + 115), -62])
-    // brake disk
-    translate([-50, l_hlb + 35, -40])
-        color("silver")
             rotate([90, 0, 0])
-        cylinder(h=1.5, r=75);
+                color("silver")
-    // brake caliper
+                    cylinder( h=1.5, r=75);
+
 }
 
 
 
-module fourbar(length=1000, chord=100, long_skew=30, lat_skew=30, shoulder=650) {
+module fourbar(length=1000, chord=100, lat_skew=30, long_skew=30, shoulder=650) {
-    w = (length * sin( lat_skew)) + ( shoulder / 2);
-
-    
     translate([0, 0-(shoulder/2), 0])
-        fourbar_leg( length, chord, long_skew, lat_skew);
+        fourbar_leg( length, chord, lat_skew, long_skew);
     
     translate([0, (shoulder/2), 0])
         mirror([0, 1, 0])
-        fourbar_leg(length, chord, long_skew, lat_skew);
+            fourbar_leg(length, chord, lat_skew, long_skew);
     
-    fourbar_axle_half(w, chord, long_skew);
+    fourbar_axle_half(length, chord, lat_skew, long_skew, shoulder);
     
     mirror([0, 1, 0])
-        fourbar_axle_half(w, chord, long_skew);
+        fourbar_axle_half(length, chord, lat_skew, long_skew, shoulder);
 }
 
-// lower_knuckle();
+
-// fourbar();
+// fourbar(length=1000, chord=100, long_skew=30, lat_skew=35, shoulder=650);
-// fourbar_axle_half(1000,100, 30);
+// fourbar_axle_half();

model/lowerknuckle.scad

@@ -0,0 +1,144 @@
+// lowerknuckle.scad
+
+// (c) Simon Brooke 2025; CC-BY-SA
+
+include <BOSL2/std.scad>
+include <NACAAirfoils/files/Naca4.scad>
+include <library/skew.scad>
+include <wheel.scad>
+
+$fn=64;
+
+
+module lower_bar_end_knuckle_old(chord=100, pivot_radius=12, rise=8,lat_skew=30) {
+    l_body=chord * 1.5;
+    w_body=pivot_radius * 2.5;
+    h_axis=pivot_radius * 2.75;
+    
+    difference() {
+        union () {
+            translate([l_body/2, w_body/2, h_axis])
+                rotate([0, 90, 0])
+                    cyl(h=l_body, r=w_body/2, rounding=5);
+            
+            // 'except=TOP' in phrase introduces the spurious hulls
+            translate([l_body/2, w_body/2, h_axis*.75])
+                cuboid([l_body, w_body, h_axis * .75], rounding=5);
+            // bulge for wheel axle; the fork insertion end of
+            // a lefty axle is 25mm diameter
+            translate([chord * 0.25, w_body/2, 0])
+               rotate([90, 0, 0])
+                    cyl(h=w_body, r=25, rounding=5);
+            // fairing for this
+            translate([chord * 0.25, w_body/2, -3])
+                rotate([80, 0, 90])
+                    prismoid([w_body, 45], [w_body, 10], rounding=5, h=l_body * .75);
+
+        }
+        translate([ chord*0.25, -1, pivot_radius * 3])
+            cube([chord, pivot_radius * 4, pivot_radius * 4]);
+        // scoop for leg-bottom knuckle to rotate within
+        translate([ chord*0.25, w_body/2, h_axis])
+            rotate([0, 90, 0])
+                cylinder(h=chord, r=pivot_radius * 1.3);
+        // through hole for pivot axle
+        translate([-1, w_body/2, h_axis])
+            rotate([0, 90, 0])
+                cylinder(h=l_body+2, r=pivot_radius/3); 
+        // through hole for wheel axle  
+        translate([ chord * 0.25, w_body +1, 0])
+            rotate([90, 0, 0])
+                cylinder(h=w_body-5, r=12.5); 
+    } 
+    
+    // airfoil stub to be epoxied into the lower bar of 
+    // the linkage
+    translate([chord*1.25, 1, 30])
+        rotate([90 + rise, 360 - lat_skew, 0])
+            airfoil(h=chord, L=2 - chord);
+    
+//    // brake disk
+//    translate([(0 - (chord*1.25)), (l_hlb * cos(rise)), 1])
+//        rotate([90, 0, 0])
+//            color("silver")
+//                cylinder( h=1.5, r=75);
+//    // wheel (obviously)            
+//    translate([(0 - (chord*1.25)), (l_hlb * cos(rise)), -10])
+//        wheel();
+
+}
+
+module lower_bar_end_knuckle(chord=100, pivot_radius=12, rise=8,lat_skew=30, colour="blue") {
+    l_body=chord * 2;
+    w_body=pivot_radius * 2.5;
+    h_axis=pivot_radius * 2.75;
+    
+    difference() {
+        union () {
+            // (more streamlined) body
+            translate([160, w_body/2, -22])
+                rotate([0,0,180])
+                    scale([1, 2, 1])
+                        color( colour)
+                            airfoil(h=70, L=l_body);
+            // boss for wheel axle
+            translate([ chord * 0.25, w_body * 0.9, 0])
+                rotate([90, 0, 0])
+                    color( colour)
+                    cyl(h=w_body * 1.25, r=20, rounding=5); 
+
+        }
+        // scoop for leg-bottom knuckle to rotate within
+        translate([ chord*0.25, w_body/2, h_axis])
+            rotate([0, 90, 0])
+                cylinder(h=chord, r=pivot_radius * 1.3);
+        // space for the airfoil leg to emerge
+        translate([ chord*0.25, w_body*.75, pivot_radius * 2.25])
+            rotate([lat_skew + 45, 0, 0])
+                cube([chord, pivot_radius * 4, pivot_radius * 4]);
+        // through hole for pivot axle
+        translate([-1, w_body/2, h_axis])
+            rotate([0, 90, 0])
+                cylinder(h=250, r=pivot_radius/3); 
+        // through hole for wheel axle  
+        translate([ chord * 0.25, w_body *1.5, 0])
+            rotate([90, 0, 0])
+                cylinder(h=w_body * 1.4, r=12.5); 
+        // slice off the front bottom
+        translate([chord * 1.25, w_body/2, 0 - w_body])
+            rotate([270-lat_skew, 0, 90])
+                color( colour)
+                    cuboid([2* w_body, 2*w_body, l_body *.75]);
+        // slice off the upper back
+        translate([0, w_body/2, w_body * 2.5])
+            rotate([270-lat_skew, 0, 90])
+                color( colour)
+                    cuboid([2* w_body, 2*w_body, l_body *.75]);
+    } 
+
+    // airfoil stub to be epoxied into the lower bar of 
+    // the linkage
+    translate([chord*1.25, 1, 30])
+        rotate([90 + rise, 360 - lat_skew, 0])
+            color( colour)
+                airfoil(h=chord, L=2 - chord);
+    
+//    // brake disk
+//    translate([(0 - (chord*1.25)), (l_hlb * cos(rise)), 1])
+//        rotate([90, 0, 0])
+//            color("silver")
+//                cylinder( h=1.5, r=75);
+//    // wheel (obviously)            
+//    translate([(0 - (chord*1.25)), (l_hlb * cos(rise)), -10])
+//        wheel();
+
+}
+
+//translate([-125, 0, 0])
+//    lower_bar_end_knuckle();
+//translate([-125, 0, 125])
+//    color("blue")
+//    lower_bar_end_knuckle();
+//translate([-100, 74, 0])
+//    wheel(); 
+

model/subframe.scad

@@ -11,7 +11,7 @@ include <epicyclic.scad>
 module subframe_cycleparts(rim=451, chainring_teeth=82, crank=140) {
         // wheel
     translate([0, 382, 93])
-        rotate([0, 90, 0])
+        rotate([0, 0, 90])
         wheel(iso=rim);
         
     translate([-42, 382, 93])

model/tricycle.scad

@@ -11,8 +11,8 @@ include <hull.scad>
 rotate([0, 0, 270])
 translate([0, 1300, 180])
 subframe(structure="carbon");
-translate([0, 0, 320])
+translate([0, 0, 250])
-fourbar(700, 100, 30, 35, 650);
+fourbar(length=1000, chord=100, long_skew=30, lat_skew=35, shoulder=650);
 
 translate([800, 0, 120])
 rotate([0, 4, 0])

model/wheel.scad

@@ -20,6 +20,7 @@
 module wheel (iso=451) {
     radius=(iso/2)+7;
     
+    rotate([270, 0, 0]) {
         // rim
         color("grey")
         difference() {
@@ -52,10 +53,10 @@ module wheel (iso=451) {
         color("silver") {
             translate([0, 0, -45])
                 cylinder(h=70, r1=12.5, r2=7.5);
-        translate([0, 0, -85])
+            translate([0, 0, -60])
-            cylinder(h=40, r=12.5);
+                cylinder(h=25, r=12.5);
+        }
     }
-    
 }
 
 // wheel();