diff --git a/test/center-test.js b/test/center-test.js
new file mode 100644
index 0000000..ef0c14a
--- /dev/null
+++ b/test/center-test.js
@@ -0,0 +1,27 @@
+var tape = require("tape"),
+    force = require("../");
+
+require("./nodeEqual.js");
+
+tape("forceCenter repositions nodes", function(test) {
+  const center = force.forceCenter(0, 0);
+  const f = force.forceSimulation().force("center", center).stop();
+  const a = { x: 100, y: 0 }, b = { x: 200, y: 0 }, c = { x: 300, y: 0 };
+  f.nodes([a, b, c]);
+  f.tick();
+  test.nodeEqual(a, { index: 0, x: -100, y: 0, vy: 0, vx: 0 });
+  test.nodeEqual(b, { index: 1, x: 0, y: 0, vy: 0, vx: 0 });
+  test.nodeEqual(c, { index: 2, x: 100, y: 0, vy: 0, vx: 0 });
+  test.end();
+});
+
+
+tape("forceCenter respects fixed positions", function(test) {
+  const center = force.forceCenter();
+  const f = force.forceSimulation().force("center", center).stop();
+  const a = { fx: 0, fy:0 }, b = {}, c = {};
+  f.nodes([a, b, c]);
+  f.tick();
+  test.nodeEqual(a, { fx: 0, fy: 0, index: 0, x: 0, y: 0, vy: 0, vx: 0 });
+  test.end();
+});
diff --git a/test/collide-test.js b/test/collide-test.js
new file mode 100644
index 0000000..f9ed20f
--- /dev/null
+++ b/test/collide-test.js
@@ -0,0 +1,48 @@
+var tape = require("tape"),
+    force = require("../");
+
+require("./nodeEqual.js");
+
+tape("forceCollide collides nodes", function(test) {
+  const collide = force.forceCollide(1);
+  const f = force.forceSimulation().force("collide", collide).stop();
+  const a = {}, b = {}, c = {};
+  f.nodes([a, b, c]);
+  f.tick(10);
+  test.nodeEqual(a, { index: 0, x: 7.0710678118654755, y: 0, vy: 0, vx: 0 });
+  test.nodeEqual(b, { index: 1, x: -9.03088751750192, y: 8.27303273571596, vy: 0, vx: 0 });
+  test.nodeEqual(c, { index: 2, x: 1.3823220809823638, y: -15.750847141167634, vy: 0, vx: 0 });
+  collide.radius(100);
+  f.tick(10);
+  test.nodeEqual(a,  { index: 0, x: 174.08616723117228, y: 66.51743051995625, vy: 0.26976816231064354, vx: 0.677346615710878 });
+  test.nodeEqual(b, { index: 1, x: -139.73606544743998, y: 95.69860503079263, vy: 0.3545632444404687, vx: -0.5300880593105067 });
+  test.nodeEqual(c, { index: 2, x: -34.9275994083864, y: -169.69384995620052, vy: -0.6243314067511122, vx: -0.1472585564003713 });
+  test.end();
+});
+
+
+tape("forceCollide respects fixed positions", function(test) {
+  const collide = force.forceCollide(1);
+  const f = force.forceSimulation().force("collide", collide).stop();
+  const a = { fx: 0, fy:0 }, b = {}, c = {};
+  f.nodes([a, b, c]);
+  f.tick(10);
+  test.nodeEqual(a, { fx: 0, fy: 0, index: 0, x: 0, y: 0, vy: 0, vx: 0 });
+  collide.radius(100);
+  f.tick(10);
+  test.nodeEqual(a, { fx: 0, fy: 0, index: 0, x: 0, y: 0, vy: 0, vx: 0 });
+  test.end();
+});
+
+tape("forceCollide jiggles equal positions", function(test) {
+  const collide = force.forceCollide(1);
+  const f = force.forceSimulation().force("collide", collide).stop();
+  const a = { x: 0, y:0 }, b = { x:0, y: 0 };
+  f.nodes([a, b]);
+  f.tick();
+  test.assert(a.x !== b.x);
+  test.assert(a.y !== b.y);
+  test.equal(a.vx, -b.vx);
+  test.equal(a.vy, -b.vy);
+  test.end();
+});
diff --git a/test/find-test.js b/test/find-test.js
new file mode 100644
index 0000000..e7c0dd2
--- /dev/null
+++ b/test/find-test.js
@@ -0,0 +1,24 @@
+var tape = require("tape"),
+    force = require("../");
+
+require("./nodeEqual.js");
+
+tape("simulation.find finds a node", function(test) {
+  const f = force.forceSimulation().stop();
+  const a = { x: 5, y: 0 }, b = { x: 10, y: 16 }, c = { x: -10, y: -4};
+  f.nodes([a, b, c]);
+  test.equal(f.find(0, 0), a);
+  test.equal(f.find(0, 20), b);
+  test.end();
+});
+
+tape("simulation.find(x, y, radius) finds a node within radius", function(test) {
+  const f = force.forceSimulation().stop();
+  const a = { x: 5, y: 0 }, b = { x: 10, y: 16 }, c = { x: -10, y: -4};
+  f.nodes([a, b, c]);
+  test.equal(f.find(0, 0), a);
+  test.equal(f.find(0, 0, 1), undefined);
+  test.equal(f.find(0, 20), b);
+  test.end();
+});
+
diff --git a/test/nodeEqual.js b/test/nodeEqual.js
new file mode 100644
index 0000000..788f02e
--- /dev/null
+++ b/test/nodeEqual.js
@@ -0,0 +1,23 @@
+var tape = require("tape");
+
+tape.Test.prototype.nodeEqual = nodeEqual;
+
+function nodeEqual(actual, expected, delta) {
+  delta = delta || 1e-6;
+  this._assert(nodeEqual(actual, expected, delta), {
+    message: "should be similar",
+    operator: "nodeEqual",
+    actual: actual,
+    expected: expected
+  });
+  
+  function nodeEqual(actual, expected, delta) {
+    return actual.index == expected.index
+        && Math.abs(actual.x - expected.x) < delta
+        && Math.abs(actual.vx - expected.vx) < delta
+        && Math.abs(actual.y - expected.y) < delta
+        && Math.abs(actual.vy - expected.vy) < delta
+        && !(Math.abs(actual.fx - expected.fx) > delta)
+        && !(Math.abs(actual.fy - expected.fy) > delta);
+  }
+}
\ No newline at end of file
diff --git a/test/simulation-test.js b/test/simulation-test.js
new file mode 100644
index 0000000..e4d09bd
--- /dev/null
+++ b/test/simulation-test.js
@@ -0,0 +1,21 @@
+var tape = require("tape"),
+    force = require("../");
+
+require("./nodeEqual.js");
+
+tape("forceSimulation() returns a simulation", function(test) {
+  const f = force.forceSimulation().stop();
+  test.deepEqual(Object.keys(f).sort(), [ 'alpha', 'alphaDecay', 'alphaMin', 'alphaTarget', 'find', 'force', 'nodes', 'on', 'restart', 'stop', 'tick', 'velocityDecay' ]);
+  test.end();
+});
+
+tape("simulation.nodes(nodes) initializes a simulation with indices & phyllotaxis positions, 0 speed", function(test) {
+  const f = force.forceSimulation().stop();
+  const a = {}, b = {}, c = {};
+  f.nodes([a, b, c]);
+  test.nodeEqual(a, { index: 0, x: 7.0710678118654755, y: 0, vy: 0, vx: 0 });
+  test.nodeEqual(b, { index: 1, x: -9.03088751750192, y: 8.27303273571596, vy: 0, vx: 0 });
+  test.nodeEqual(c, { index: 2, x: 1.3823220809823638, y: -15.750847141167634, vy: 0, vx: 0 });
+  test.end();
+});
+
diff --git a/test/x-test.js b/test/x-test.js
new file mode 100644
index 0000000..51a989f
--- /dev/null
+++ b/test/x-test.js
@@ -0,0 +1,84 @@
+var tape = require("tape"),
+    force = require("../");
+
+require("./nodeEqual.js");
+
+tape("forceX centers nodes", function(test) {
+  const x = force.forceX(200);
+  const f = force.forceSimulation().force("x", x).stop();
+  const a = { x: 100, y: 0 }, b = { x: 200, y: 0 }, c = { x: 300, y: 0 };
+  f.nodes([a, b, c]);
+  f.tick(30);
+  test.assert(a.x > 190);
+  test.assert(a.vx > 0);
+  test.equal(b.x, 200);
+  test.equal(b.vx, 0);
+  test.assert(c.x < 210);
+  test.assert(c.vx < 0);
+  test.end();
+});
+
+tape("forceY centers nodes", function(test) {
+  const y = force.forceY(200);
+  const f = force.forceSimulation().force("y", y).stop();
+  const a = { y: 100, x: 0 }, b = { y: 200, x: 0 }, c = { y: 300, x: 0 };
+  f.nodes([a, b, c]);
+  f.tick(30);
+  test.assert(a.y > 190);
+  test.assert(a.vy > 0);
+  test.equal(b.y, 200);
+  test.equal(b.vy, 0);
+  test.assert(c.y < 210);
+  test.assert(c.vy < 0);
+  test.end();
+});
+
+tape("forceX respects fixed positions", function(test) {
+  const x = force.forceX(200);
+  const f = force.forceSimulation().force("x", x).stop();
+  const a = { fx: 0, fy:0 }, b = {}, c = {};
+  f.nodes([a, b, c]);
+  f.tick();
+  test.nodeEqual(a, { fx: 0, fy: 0, index: 0, x: 0, y: 0, vy: 0, vx: 0 });
+  test.end();
+});
+
+tape("forceY respects fixed positions", function(test) {
+  const y = force.forceX(200);
+  const f = force.forceSimulation().force("y", y).stop();
+  const a = { fx: 0, fy:0 }, b = {}, c = {};
+  f.nodes([a, b, c]);
+  f.tick();
+  test.nodeEqual(a, { fx: 0, fy: 0, index: 0, x: 0, y: 0, vy: 0, vx: 0 });
+  test.end();
+});
+
+tape("forceX.x() accessor", function(test) {
+  const x = force.forceX().x(d => d.x0);
+  const f = force.forceSimulation().force("x", x).stop();
+  const a = { x: 100, y: 0, x0: 300 }, b = { x: 200, y: 0, x0: 200 }, c = { x: 300, y: 0, x0: 100 };
+  f.nodes([a, b, c]);
+  f.tick(30);
+  test.assert(a.x > 290);
+  test.assert(a.vx > 0);
+  test.equal(b.x, 200);
+  test.equal(b.vx, 0);
+  test.assert(c.x < 110);
+  test.assert(c.vx < 0);
+  test.end();
+});
+
+tape("forceY.y() accessor", function(test) {
+  const y = force.forceY().y(d => d.y0);
+  const f = force.forceSimulation().force("y", y).stop();
+  const a = { y: 100, x: 0, y0: 300 }, b = { y: 200, x: 0, y0: 200 }, c = { y: 300, x: 0, y0: 100 };
+  f.nodes([a, b, c]);
+  f.tick(30);
+  test.assert(a.y > 290);
+  test.assert(a.vy > 0);
+  test.equal(b.y, 200);
+  test.equal(b.vy, 0);
+  test.assert(c.y < 110);
+  test.assert(c.vy < 0);
+  test.end();
+});