Multiple timeouts not working?

Well, one guess is that (like in a browser) setTimeout(); doesn't set the this variable to what you'd expect, so when stop is called from setTimeout(this.stop, stopt); and it tries to do this.vel = 0, it's actually setting vel on the global variable, not on your wheel.

To fix it, try:

Wheel.prototype.spin = function(stopt) // stopt = stoptime
{
  this.vel = (2 + Math.random() * 5) / 40; // give the wheel a velocity
  var wheel = this;
  setTimeout(function() {
    wheel.stop();
  }, stopt); // tell it when to stop
};

There are other ways to do it, but that one is probably the most readable.