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HTML

              
                <div id="wrap">
  <svg xmlns:svg="http://www.w3.org/2000/svg" xmlns="http://www.w3.org/2000/svg" width="800" height="500" id="svg3906" version="1.1" viewBox="0 0 800 500">
  <defs id="defs3908">
    <filter color-interpolation-filters="sRGB" id="filter4001" x="-0.08" width="1.15" y="-0.39" height="1.77">
      <feGaussianBlur stdDeviation="18.06" id="feGaussianBlur4003"/>
    </filter>
    <mask maskUnits="userSpaceOnUse" id="mask12847">
      <rect id="rect12849" width="562.5" height="112.5" x="145.27" y="425.5" style="fill:#fffcfc;filter:url(#filter4001)"/>
    </mask>
  </defs>
  <metadata id="metadata3911"/>
  <g id="layer4">
    <g id="meterParent" mask="url(#mask12847)" transform="translate(196.23395,-28)">
      <g id="meter">
        <rect y="475.5" x="-566.64" height="12.5" width="2" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="-503.809" height="12.5" width="2" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="-440.97" height="12.5" width="2" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="-378.15" height="12.5" width="2" style="fill-opacity:0.87;fill:#000"/>
        <rect id="rect3019" width="2" height="37.5" x="-315.32" y="450.5" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="-252.49" height="12.5" width="2" id="use3023" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="-189.66" height="12.5" width="2" id="use3025" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="-126.82" height="12.5" width="2" id="use3027" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="-63.99" height="12.5" width="2" id="use3029" style="fill-opacity:0.87;fill:#000"/>
        <rect y="463" x="-1.16" height="25" width="2" id="use3031" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="61.67" height="12.5" width="2" id="use3033" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="124.5" height="12.5" width="2" id="use3035" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="187.34" height="12.5" width="2" id="use3037" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="250.17" height="12.5" width="2" id="use3039" style="fill-opacity:0.87;fill:#000"/>
        <rect y="450.5" x="313" height="37.5" width="2" id="use3041" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="375.83" height="12.5" width="2" id="use3043" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="438.66" height="12.5" width="2" id="use3045" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="501.5" height="12.5" width="2" id="use3047" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="564.33" height="12.5" width="2" id="use3049" style="fill-opacity:0.87;fill:#000"/>
        <rect y="463" x="627.16" height="25" width="2" id="use3051" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="689.99" height="12.5" width="2" id="use3053" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="752.82" height="12.5" width="2" id="use3055" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="815.66" height="12.5" width="2" id="use3057" style="fill-opacity:0.87;fill:#000"/>
        <rect y="475.5" x="878.49" height="12.5" width="2" id="use3059" style="fill-opacity:0.87;fill:#000"/>
      </g>
    </g>
  </g>
  <g id="layer3">
    <g id="tWheelset" transform="translate(510,220)">
      <g id="wheelset">
        <g id="wheel" transform="translate(-150,-300)">
          <path id="path5301" d="m150 100 0 400" style="fill:none;stroke-width:4;stroke:#000"/>
          <path d="M350 300C350 410.46 260.46 500 150 500 39.54 500-50 410.46-50 300-50 189.54 39.54 100 150 100c110.46 0 200 89.54 200 200z" id="path3778" style="fill-opacity:0.85;fill:#dbdbdb;stroke-opacity:0.3;stroke:#040404"/>
          <path id="path3858" d="M262.5 300C262.5 237.87 212.13 187.5 150 187.5 87.87 187.5 37.5 237.87 37.5 300l225 0z" style="fill-opacity:0.85;fill:#f60;stroke:#000"/>
          <path id="path3849" d="m262.5 300-225 0c0 62.13 50.37 112.5 112.5 112.5 62.13 0 112.5-50.37 112.5-112.5z" style="fill-opacity:0.87;fill:#fffcfc;stroke:#000"/>
        </g>
        <circle d="M 40,0 C 40,22.09139 22.09139,40 0,40 -22.09139,40 -40,22.09139 -40,0 c 0,-22.09139 17.90861,-40 40,-40 22.09139,0 40,17.90861 40,40 z" r="40" cy="0" cx="0" id="sprocket" style="fill:none;opacity:0.5;stroke-width:15;stroke:#ddd"/>
      </g>
    </g>
  </g>
  <g id="layer5">
    <path d="m-120 10 40 0c24.93 0 45 20.07 45 45 0 24.93-20.07 45-45 45l-40 0c-24.93 0-45-20.07-45-45 0-24.93 20.07-45 45-45z" id="chain" style="fill:none;stroke-dasharray:15;stroke-linecap:round;stroke-linejoin:bevel;stroke-width:6;stroke:#000"/>
  </g>
  <g id="layer2">
    <g id="tCrankset" transform="translate(210,220)">
      <g id="crankset">
        <circle id="chainring" cx="0" cy="0" r="70" style="fill:none;opacity:0.5;stroke-width:15;stroke:#ddd"/>
        <g id="crankarm" stroke-width="5">
          <path d="M10 0A10 10 0 1 1-10 0 10 10 0 1 1 10 0z" id="path12042" style="fill-opacity:0.55;fill:#f9f9f9;opacity:0.98;stroke-width:5;stroke:#000"/>
          <path d="m-110 0a10 10 0 1 1-20 0 10 10 0 1 1 20 0z" id="path12046" style="fill-opacity:0.55;fill:#f9f9f9;opacity:0.98;stroke-width:5;stroke:#000"/>
          <path id="path12048" d="M-110 0-10 0" style="fill:none;stroke-width:5;stroke:#000"/>
        </g>
      </g>
    </g>
  </g>
  <g id="layer6">
    <path id="shiftFront" d="m75.2 379.86 0-18.99 16.44 9.49z" transform="translate(144.79695,119.13139)"/>
    <path transform="matrix(-1,0,0,1,275.20305,119.13139)" d="m75.2 379.86 0-18.99 16.44 9.49z" id="shiftFrontDown"/>
    <path transform="translate(444.79695,119.13139)" d="m75.2 379.86 0-18.99 16.44 9.49z" id="shiftBackUp"/>
    <path id="shiftBackDown" d="m75.2 379.86 0-18.99 16.44 9.49z" transform="matrix(-1,0,0,1,575.20305,119.13139)"/>
  </g>
</svg>
</div>
              
            
!

CSS

              
                body {
  background: #f7f4ee;
  display: flex;
  flex-direction: row;
  justify-content: center;
  align-items: center;

  height: 100vh;
}

#wrap {
  width: 100%;
  max-width: 700px;
  margin: 0 auto;
}

svg {
  width: 100%;
  height: 100%;
}

#layer6 path {
  cursor: pointer;
  fill: black;
  stroke-width: 0;
}

#layer6 path:hover {
  fill: #f60;
}

              
            
!

JS

              
                window.onload = function () {
	
  (function(){ 

  	// Representation of the drivetrain modules
		var crankset = Crankset(); 
		var chain = Chain ();
		var wheel = Wheel();

		crankset.init(1);
		chain.init( crankset.getChainspeed() );
		wheel.init( crankset.getChainspeed() );

		// Update/control functions. Perhaps this is just a overcomplicated way to
		// define event handlers 
		var shiftFrontSprocket = function ( d ) {
			var r = crankset.chainring.getRadius ( ) + d;
			if ( r <= 0 || r > 200 ) return;
			crankset.chainring.setRadius ( r );

			chain.updateFrontSprocket( r );
			wheel.update ( crankset.getChainspeed() );
			meter.update ( Math.PI / wheel.getAngVel() );
		};

		var shiftBackSprocket = function ( d ) {
			var r = wheel.sprocket.getRadius ( ) + d;
			if ( r <= 0 || r > 200 ) return;
			wheel.sprocket.setRadius ( r );

			chain.updateBackSprocket( r );
			wheel.update ( crankset.getChainspeed() );
			meter.update ( Math.PI / wheel.getAngVel() );
		};

		Shifter ( 'shiftFront', shiftFrontSprocket, 15 );
		Shifter ( 'shiftFrontDown', shiftFrontSprocket, -15 );
		Shifter ( 'shiftBackUp', shiftBackSprocket, 10 );
		Shifter ( 'shiftBackDown', shiftBackSprocket, -10 );

		// The terrain
		var meter = Meter();
		// How long should the pan take? Just as long as it takes for the wheel to do a half loop,
		// also PI/ang_vel
		meter.init( Math.PI / wheel.getAngVel() );
	})();
}

var Meter = function() {
	var sprite = document.getElementById( 'meter' );
	var tween = null;
	var orig_dur;
	
	return {
		init: function ( dur ) {
			orig_dur = dur;

			// Start pan:
			tween = new TweenMax( sprite, dur, {
				x: Math.PI*200, 
				ease: 'Linear.easeNone', 
				repeat: -1
			});	
		},
		update: function ( dur ) {
			tween.timeScale ( orig_dur / dur );
		}
	};
};

var Crankset = function(  ){
	var sprite = document.getElementById( 'crankset' );
	var tween = null;
	var ang_vel; // Constant after initialization
	var chainspeed;

	// As a reminder, to show what's the difference with the modular pattern
	// here a normal literal object: observe the use of 'this'
	var chainring = {
		sprite: document.getElementById( 'chainring' ),
		setRadius: function ( r ) {
			this.sprite.setAttribute('r', r);
			chainspeed = r * ang_vel;
		},
		getRadius: function ( ) {
			return parseInt(this.sprite.getAttribute('r'));
		}
	}; 

	return {
		chainring: chainring,
		init: function ( w ) {
			ang_vel = w;
			chainspeed = ang_vel * chainring.getRadius();
			tween = new TweenMax(sprite, 2*Math.PI/w, {
				rotation: -360, 
				ease: 'Linear.easeNone', 
				repeat: -1
			});			
		},
		getChainspeed: function () {
			return chainspeed;
		}
	}
};

var Chain = function ( ) {
	var sprite = document.getElementById( 'chain' );

	// Deriving the chain from the svg is hard because getClientRect and getBBox
	// do not give the result in svg screen coordinates. (How about snap's getBBox()?)
	// (Or how about extracting the coordinates from the transform property?)
	// Just type in the numbers then

	var sprocket0 = { m: { x: 210, y: 220}, r: 70};
	var sprocket1 = { m: { x: 510, y: 220}, r: 40};

	// Define the shape of the chain by calculating the 2 lines tangent to both sprockets
	var tangentLines = [];

	calcTangents = function () {
		// Calculate angle between the line through the centers of the circles and 1 of the 2 lines tangent to both circles
		var dx = sprocket0.m.x-sprocket1.m.x;
		var dy = sprocket0.m.y-sprocket1.m.y;
		var d = Math.sqrt(dx*dx + dy*dy);
		var angle = Math.asin ( (sprocket0.r - sprocket1.r) / d );

		// Now do some math to get the actual tangent lines
		angle -= Math.PI/2;
		var ca = Math.cos( angle );
		var sa = Math.sin( angle );

		tangentLines[0] =[
			sprocket1.m.x + sprocket1.r * ca,
			sprocket1.m.y + sprocket1.r * sa,
			sprocket0.m.x + sprocket0.r * ca,
			sprocket0.m.y + sprocket0.r * sa,
		];
		tangentLines[1] = [
			sprocket0.m.x + sprocket0.r * ca,
			sprocket0.m.y - sprocket0.r * sa,
			sprocket1.m.x + sprocket1.r * ca,
			sprocket1.m.y - sprocket1.r * sa,
		];
 	};
	
	/*
	 * Draw the convex hull around the two circles
	 */
	getChainPathData = function ( ) {
		var sweep = sprocket1.r > sprocket0.r ? 1 : 0;

		var d = "M" + tangentLines[0][0] + " " + tangentLines[0][1] + " L" + tangentLines[0][2] + " " + tangentLines[0][3];
		// add arc sprocket1
		d += " A " + sprocket0.r + " " + sprocket0.r + ", 0, " + parseInt(1-sweep) + ", 0, " + tangentLines[1][0] + " " + tangentLines[1][1]
		d += " L " + tangentLines[1][2] + " " + tangentLines[1][3];
		// add arc voorblad
		d += " A " + sprocket1.r + " " + sprocket1.r + ", 0, " + sweep + ", 0, " + tangentLines[0][0] + " " + tangentLines[0][1];

		return d;
	};
	var update = function() {
		calcTangents();

		// draw new calculated chain shape
		sprite.setAttribute('d', getChainPathData() );	

		// timescale tween
		tween.timeScale(sprocket0.r/sprocket0.orig_r);
		
	};

	return {
		init: function( speed ) {
			// remember original radius, to alter tween afterwards
			sprocket0.orig_r = sprocket0.r; 

			tween = new TweenMax( sprite, 30/speed, {
				'strokeDashoffset': -30, 
				ease: 'Linear.easeNone', 
				repeat: -1
			});			
			update();
		},
		updateFrontSprocket: function ( r ) {
			sprocket0.r = r;
			update();
		},
		updateBackSprocket: function ( r ) {
			sprocket1.r = r;
			update();
		}
	}
}

var Wheel = function ( ) {
	var sprite = document.getElementById( 'wheelset' );
	var tween = null;
	var ang_vel,
			orig_ang_vel;

	var sprocket = {
		sprite: document.getElementById( 'sprocket' ),
		setRadius: function ( r ) {
			this.sprite.setAttribute('r', r);
		},
		getRadius: function ( ) {
			return parseInt(this.sprite.getAttribute('r'));
		}
	}; 

	return {
		sprocket: sprocket,
		init: function ( chainspeed ) {

 			ang_vel = chainspeed / sprocket.getRadius();
			tween = new TweenMax(sprite, 2*Math.PI/ang_vel, {
				rotation: -360, 
				ease: 'Linear.easeNone', 
				repeat: -1
			});			

			orig_ang_vel = ang_vel;
		},
		update: function( chainspeed ) {
			// timescale tween
 			ang_vel = chainspeed / sprocket.getRadius();
			tween.timeScale(ang_vel/orig_ang_vel);
		},
		getAngVel: function() {
			return ang_vel;
		}
	}
};	

var Shifter = function( spriteId, callback, d ) {
	var sprite = document.getElementById ( spriteId );

	sprite.onclick = function() {
		callback( d );
	};
};
              
            
!
999px

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