How to separate different "gravitational fields" in D3? - d3.js

Question

I have a force enabled SVG visualisation where I want smaller circles to be attracted to bigger circles. This attraction works by calculating the elements' centre point and change it in iterations for every "tick" in the visualisation, to keep the items from going over the centre of the nodes I use a function to change the charge of the items depending on their size.
I used Mike's code here as a basis: http://mbostock.github.io/d3/talk/20110921/#14
My problem comes here - it seems like the bigger circles are affecting each others "gravitational fields" - is there a way I can separate them from eachother?
Force layout setup:
var w = 1280,
h = 800,
color = d3.scale.category10();
var force = d3.layout.force()
.gravity(0.0)
.charge(function(d){
return -10 * d.r;
})
.size([w, h]);
Element drawing:
var g = svg.selectAll("g.node")
.data(nodes)
.enter().append("svg:g")
.attr("transform", function(d) {
return "translate(" + d.x + "," + d.y + ")";
})
;
g.append("svg:circle")
.attr("r", 40)
.attr("transform", function(d) { return "translate(" + 0 + ","+ 0 + ")"; })
.style("fill", fill)
.call(force.drag);
g.append("svg:text")
.attr("x", 0)
.attr("dy", ".31em")
.attr("text-anchor", "middle")
.text(function(d) {
return d.label;
});
Animation loop:
force.on("tick", function(e) {
var k = e.alpha * 0.5;
nodes.forEach(function(node) {
var center = nodes[node.type];
dx = center.x - node.x;
dy = center.y - node.y;
node.x += dx * k;
node.y += dy * k;
});
svg.selectAll(".circle")
.attr("cx", function(d) {
return d.x;
})
.attr("cy", function(d) {
return d.y;
});
});
Adding smaller circles:
svg.on("mousemove", function() {
var p1 = d3.svg.mouse(this),
node = {
type: Math.random() * 3 | 0,
x: p1[0],
y: p1[1],
r: 1.5,
px: (p0 || (p0 = p1))[0],
py: p0[1]
};
p0 = p1;
svg.append("svg:circle")
.attr("class", "circle")
.data([node])
.attr("cx", function(d) {
return d.x;
})
.attr("cy", function(d) {
return d.y;
})
.attr("r", 4.5)
.style("fill", fill);
nodes.push(node);
force.start();
});