tricycle-mechanical-design/microcar/suspension.scad

/**
 * suspension.scad
 *
 * First sketch of a microcar suspension subframe.
 *
 * The purpose of the subframe is sumply to carry the inner ends of the
 * wishbones, so it is simply the minimum size required to do that.
 *
 * TODO: this does not yet consider caster, camber, anti-dive geometry,
 * or other such niceties.
 */

include <BOSL2/std.scad>
include <shockabsorber.scad>
include <wheel.scad>
 
// assumes we're building out of square section aluminium tobe;
// the section width of that tube.
stock_section = 25;
 
lug_thickness = stock_section/6;
 
// radius of suspension pivots
pivot_radius = stock_section/3; //for now

// aft tilt of kingpins in degrees from vertical
caster_angle = 7;

// inward tilt of the kingpins in degrees from vertical
// TODO: should be calculated from hub width and wheel radius?
camber_angle = 10;

module kingpin( h = 300, r = 12.5, 
        cmb = camber_angle, cst = caster_angle, 
        al = 100) {
    kl = h * (1/cos(90 - cmb)) * (1/cos(90 - cst));
            
    // kingpin itself        
    rotate([cmb, cst, 0])
        cylinder( h=h, r=r, center=true);
    // stub axle
    // axle offset as a fraction of the lenth of the kingpin
    aof = 0.35 * h;
    camber_offset = aof/tan(90-cmb);
    caster_offset = 0 - aof/tan(90-cst);
        
            
    translate([caster_offset, al + camber_offset, 0-aof])
    rotate([ 90, 0, 0])
        cylinder( h=al, r=r);
            
    translate([caster_offset, al, 0-aof])
        wheel();

    
}

 
module lug() {
    difference() {
        union() {
            translate([0, stock_section/2, 0]){
                cube([ stock_section, stock_section, lug_thickness],
                    center=true);
            }
             
            cylinder( r=stock_section/2, h=stock_section/4, 
                center=true);      
         }
         cylinder( r=pivot_radius, h=stock_section, center=true);
    }
}

module wishbone(l, w, skew = 0, is_upper = true) {
    ssd2 = stock_section/2;
    ssm2 = stock_section * 2;
    ssm3 = stock_section * 3;
     
    // wishbone inboard pivot lugs
    translate([ (l - lug_thickness)/2, stock_section, ssd2])
        rotate([180,90,0])
            lug();
    translate([ 0 - (l - lug_thickness)/2, stock_section, ssd2])
        rotate([180,90,0])
            lug();
     
    // shock absorber end lugs
    if (is_upper) {
        translate([ ssd2, ssm2, ssd2])
            rotate([180,90,0])
                lug();
        translate([ -ssd2, ssm2, ssd2])
            rotate([180,90,0])
                lug();
    } else {
        translate([ ssd2, w-ssm3, ssd2])
            rotate([180,90,180])
                lug();
        translate([ -ssd2, w-ssm3, ssd2])
            rotate([180,90,180])
                lug();
    }
    
    color("silver")
    union() {
        intersection(){
            linear_extrude(stock_section)
                polygon( points=[[0-(l/2), ssd2], 
                        [l/2, ssd2],
                        [skew + (3*ssd2), w-(3*ssd2)], 
                        [skew -(3*ssd2), w-(3*ssd2)],
                        // hole in the middle
                        [(0 - (l/2 - 3*ssd2)), 3*ssd2],
                        [(l/2 - 3*ssd2), 3*ssd2],
                        [skew + ssd2, w-(5*ssd2)], 
                        [skew - ssd2, w-(5*ssd2)]], 
                    paths=[[0, 1, 2, 3], [4, 5, 6, 7]
            ]);
            translate([0 - (l - ssd2)/2, 0, 0 - stock_section])
                cube([l-ssd2, w, ssm3]);
        }
        translate([skew-3*ssd2, w-3*ssd2, 0])
            cube( [stock_section, 3*ssd2, stock_section]);
        translate([skew+ssd2, w-3*ssd2, 0])
            cube( [stock_section, 3*ssd2, stock_section]);
    };
    color("darkgray")
    translate([0, stock_section, ssd2])
        rotate([0, 90, 0])
            cylinder(h=l, r=pivot_radius, center=true);
    color("darkgray")    
    translate([skew, w - ssd2, ssd2])
        rotate([0, 90, 0])
            cylinder(h=ssm3, r=pivot_radius, center=true);    
}
 
 module subframe(l, w, h) {
     q = stock_section * 2;
     color( "silver")
     difference() {
         cube([l,w,h], center=true);
         cube([l+200, w-q, h-q], center=true);
         cube([l-q, w+200, h-q], center=true);
         cube([l-q, w-q, h+200], center=true);
     }
 }
 
module suspension( l = 300, w = 300, h = 300,
        track = 1400, hub_width = 135, wheel_diameter=600, 
        cmb=camber_angle, cst=caster_angle) {
    subframe(l,w,h);
            
    wishbone_length = 500; // TODO: calculate
            
    camber_offset = h/tan(90-cmb);
    caster_offset = h/tan(90-cst);
     
            
    // Upper wishbones
    translate([0, w/2, (h/2) - stock_section])
        wishbone( l, wishbone_length - camber_offset, caster_offset);
    mirror([0,1, 0]) {
        translate([0, w/2, (h/2) - stock_section])
            wishbone( l, wishbone_length - camber_offset, caster_offset);
    }

    shock_extent = wishbone_length - (5.5 * stock_section);
    shock_angle = atan(shock_extent/(h-stock_section));
    shock_length = shock_extent * tan(shock_angle);

    // lower wishbones
    translate([0, w/2, 0 -(h/2)]) {
        wishbone( l, wishbone_length, 0, false);
        translate([0, (stock_section*2), h-(stock_section/2)])
            rotate([shock_angle +180, 0, 0])
                shockabsorber(length=shock_length,eid=stock_section/2);
    }
    mirror([0,1, 0]) {
        translate([0, w/2, 0 -(h/2)]) {
            wishbone( l, wishbone_length, 0, false);
            translate([0, (stock_section*2), h-(stock_section/2)])
                rotate([shock_angle +180, 0, 0])
                    shockabsorber(length=shock_length,
                    eid=stock_section/2);
        }
    }
    
    translate([caster_offset/2, 
        wishbone_length + (stock_section * 4), 0])
        kingpin(cst=cst);
    mirror([0,1, 0]) 
        translate([caster_offset/2, 
            wishbone_length + (stock_section * 4), 0])
            kingpin(cst=cst);
}