The New Hotness

The bent-steel brake levers on our Drive Blue Streak wheelchair present themselves edge-on to the rider:

There are good mechanical reasons for shaping and orienting the steel like that, but the handle concentrates the considerable force required to push the brake tab into the rubberoid tire on your (well, my) palms. After a couple of weeks, I decided I didn’t need two more sore spots and conjured a palm-filling knob from the vasty digital deep:

Wheelchair Brake Knob - installed — Wheelchair Brake Knob – installed

Bonus part: the little octagon near the wheel prevents the leg rest (seen in the first picture) from smashing into the end of the brake tab and chipping the lovely blue powder coat. The brown fuzzy felt foot seemed like a good idea at the time, but isn’t strictly necessary.

A cylindrical handle on Thingiverse apparently fits on the bare steel underneath the rubberish “cushion”, but cutting a perfectly good, albeit uncomfortable, cushion off seemed like a step in the wrong direction. My knob thus descends from a doodle of the OEM dimensions:

Drive Wheelchair Brake Handle - dimensions — Drive Wheelchair Brake Handle – dimensions

The knob builds in two halves adjoining the bonus octagon, which stands on edge to eliminate support inside its slot:

Wheelchair Brake Mods - solid model - build layout — Wheelchair Brake Mods – solid model – build layout

You (probably) need two of all those shapes, a job your slicer is ready to perform. At three hours for each knob, I just printed the same G-Code twice.

You can customize the knob width to fit your palm, with the other two dimensions fitting themselves around the cushion. Mary and I settled on a knob size that fits both our hands reasonably well, so it’s probably not critical.

I tried building the knob halves without support for the first prototype, but the sloped upper surface produced awful bridging:

Wheelchair Brake Knob - unsupported interior — Wheelchair Brake Knob – unsupported interior

It’s easy enough to design a customized support structure:

Wheelchair Brake Mods - cross section — Wheelchair Brake Mods – cross section

I oriented the knob to put the split on the narrow sides of the brake handle in order to not have a seam facing my palm:

Wheelchair Brake Knob - rear half installed — Wheelchair Brake Knob – rear half installed

The quartet of M3×20 mm socket-head cap screws thread into brass inserts epoxied into the rear half. I recessed their heads deeply into the front half and avoided thinking too hard about plugs matching the surface curvature:

Wheelchair Brake Knob - front view — Wheelchair Brake Knob – front view

The low-vertex-count polygonal shape is a stylin’ thing and produces a nice feel during a firm shove, at least to my paws. Although I’d rather not need a wheelchair at all, setting the brakes now seems authoritative instead of annoying.

The OpenSCAD source code as a GitHub gist:

	// Pride wheelchair brake lever mods
	// Ed Nisley KE4ZNU 2020-11

	/* [Layout options] */

	Layout = "Build"; // [Build, Show, Fit, TabCap, Handle, Knob, Support]

	// Hold up the knob's inside
	Support = true;

	/* [Extrusion parameters] */

	/* [Hidden] */

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	HoleWindage = 0.2;

	function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
	function IntegerLessMultiple(Size,Unit) = Unit * floor(Size / Unit);

	Protrusion = 0.1; // make holes end cleanly

	inch = 25.4;

	ID = 0;
	OD = 1;
	LENGTH = 2;

	//———————-
	// Useful routines

	module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes

	Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);

	FixDia = Dia / cos(180/Sides);

	cylinder(r=(FixDia + HoleWindage)/2,h=Height,$fn=Sides);
	}



	//* [Basic dimensions] */

	WallThick = 4.0; // min wall thickness

	Screw = [3.0,5.5,20.0]; // thread, head, length under head
	Insert = [3.0,4.1,8.0]; // thread, knurl, length

	//———————-
	// Brake tab cap

	BrakeTab = [15,21,3.1]; // length to wheel, width, thickness
	BrakeTabSagitta = 8.0; // height of curved endcap

	CapOAL = [BrakeTab.y + 2WallThick,BrakeTab.y + 2WallThick,BrakeTab.z + 2*WallThick];

	module TabCap() {

	difference() {
	rotate(180/8)
	cylinder(d=CapOAL.y,h=CapOAL.z,center=true,$fn=8);
	translate([BrakeTab.x/2,0,0])
	cube(BrakeTab,center=true);
	rotate(180/8)
	cylinder(d=BrakeTab.y/cos(180/8),h=BrakeTab.z,center=true,$fn=8);
	}
	}

	//———————-
	// Brake lever handle
	// Soft covering with rounded sides that we square off for simplicity

	HandleRibs = [15,34,14]; // ignoring slight taper from end
	HandleCore = [50.0,24.0,12.0]; // straight section of lever to top of ribs
	HandleTipWidth = 30.0; // ignoring actual sector height

	module Handle() {

	union() {
	hull() {
	rotate(180/8)
	cylinder(d=HandleTipWidth/cos(180/8),h=HandleCore.z,center=true,$fn=8);
	translate([-HandleCore.x/2,0,0])
	cube(HandleCore,center=true);
	}
	translate([-(3*HandleCore.x/2 – Protrusion),0,0]) // extend base for ball trimming
	cube(HandleCore,center=true);
	translate([-HandleRibs.x/2,0,0])
	cube(HandleRibs,center=true);
	}

	}

	//———————-
	// Support structure for handle cavity inside knob
	// Totally ad-hoc tweakage
	// Remember it's lying on its side to match the handle

	NumRibs = 2 + 1; // must be odd
	RibSpace = floor(HandleCore.z/(NumRibs + 1));

	module KnobSupport() {

	color("Yellow") { // support overlaps in the middle
	render(convexity=3)
	intersection() {
	for (k=[-1,1])
	translate([0,k*ThreadThick,0]) // shrink inward to break adhesion
	Handle();

	translate([(HandleCore.x – HandleRibs.x)/2 – HandleCore.x – Protrusion,0,0])
	cube([HandleCore.x – HandleRibs.x,HandleRibs.y,HandleCore.z],center=true);

	union()
	for (k=[-floor(NumRibs/2):floor(NumRibs/2)])
	translate([0,0,k* RibSpace])
	cube([2HandleCore.x,HandleRibs.y,2ThreadWidth],center=true);
	}

	translate([(HandleCore.x – HandleRibs.x)/2 – HandleCore.x,0,0])
	cube([HandleCore.x – HandleRibs.x,4ThreadWidth,NumRibsRibSpace],center=true);
	}
	}

	//———————-
	// Brake handle knob
	// Largely built with magic numbers
	// Includes support because it's not really optional

	KnobOD = 55.0;
	KnobOffset = HandleRibs.x/1;

	KnobSides = 243;

	module Knob() {

	difference() {
	hull() {
	resize([0,HandleRibs.y + 4*WallThick,HandleCore.x + HandleTipWidth/2 + WallThick])
	sphere(d=KnobOD,$fn=KnobSides);
	}

	translate([0,0,KnobOffset])
	rotate([0,-90,0])
	Handle();

	for (i=[-1,1],k=[-1,1])
	translate([iKnobOD/4,0,kKnobOD/4]) {
	rotate([90,0,0])
	PolyCyl(Insert[OD],1.5*Insert[LENGTH],6);
	translate([0,-Screw[LENGTH]/2,0])
	rotate([-90,0,0])
	PolyCyl(Screw[ID],KnobOD,6);
	translate([0,Screw[LENGTH] – Insert[LENGTH],0])
	rotate([-90,0,0])
	PolyCyl(Screw[OD],KnobOD,6);
	}
	}

	if (Support)
	translate([0,0,KnobOffset])
	rotate([0,-90,0])
	KnobSupport();

	}

	//———————-
	// Lash it together

	if (Layout == "TabCap") {
	TabCap();
	}

	if (Layout == "Handle") {
	Handle();
	}

	if (Layout == "Support") {
	KnobSupport();
	}

	if (Layout == "Knob") {
	Knob();
	}

	if (Layout == "Show") {
	translate([60,0,0])
	TabCap();
	Knob();
	}

	if (Layout == "Fit") {
	translate([60,0,0])
	difference() {
	TabCap();
	translate([0,0,CapOAL.z/2])
	cube(CapOAL,center=true);
	}
	difference() {
	Knob();
	translate([KnobOD + KnobOD/4,0*KnobOD,0])
	cube(2*KnobOD,center=true);
	translate([-KnobOD,-KnobOD,0])
	cube(2*KnobOD,center=true);
	}
	}

	if (Layout == "Build") {
	translate([KnobOD/2,0,(CapOAL.y*cos(180/8))/2])
	rotate([0,-90,90])
	TabCap();

	for (j=[-1,1])
	translate([0,-j0.75HandleCore.x,0])
	difference() {
	rotate([j*90,0,0])
	Knob();
	translate([0,0,-KnobOD])
	cube(2*KnobOD,center=true);
	}

	}

view raw Wheelchair Brake Mods.scad hosted with ❤ by GitHub

A doodle with dimensions of other parts:

Drive Wheelchair - brake footrest tab dimensions — Drive Wheelchair – brake footrest tab dimensions

The angled tab on the middle left is for the leg rest release latch, but I decided not to silk-purse-ize the thing.

The New Hotness

Drive Wheelchair Brake Knob