How to increase linkDistance without affecting the node alignment,

example: http://mbostock.github.com/d3/talk/20110921/force.html

when I try to increase the circle radius and linkDistance the it collapse

<script type="text/javascript">

var w = 1280,
    h = 800,
    z = d3.scale.category20c();

var force = d3.layout.force()
    .size([w, h]);

var svg = d3.select("#chart").append("svg:svg")
    .attr("width", w)
    .attr("height", h);

svg.append("svg:rect")
    .attr("width", w)
    .attr("height", h);

d3.json("flare.json", function(root) {
  var nodes = flatten(root),
      links = d3.layout.tree().links(nodes);

  force
      .nodes(nodes)
      .links(links)
      .start();

  var link = svg.selectAll("line")
      .data(links)
    .enter().insert("svg:line")
      .style("stroke", "#999")
      .style("stroke-width", "1px");

  var node = svg.selectAll("circle.node")
      .data(nodes)
    .enter().append("svg:circle")
      .attr("r", 4.5)
      .style("fill", function(d) { return z(d.parent && d.parent.name); })
      .style("stroke", "#000")
      .call(force.drag);

  force.on("tick", function(e) {
    link.attr("x1", function(d) { return d.source.x; })
        .attr("y1", function(d) { return d.source.y; })
        .attr("x2", function(d) { return d.target.x; })
        .attr("y2", function(d) { return d.target.y; });

    node.attr("cx", function(d) { return d.x; })
        .attr("cy", function(d) { return d.y; });
  });
});

function flatten(root) {
  var nodes = [];
  function recurse(node, depth) {
    if (node.children) {
      node.children.forEach(function(child) {
        child.parent = node;
        recurse(child, depth + 1);
      });
    }
    node.depth = depth;
    nodes.push(node);
  }
  recurse(root, 1);
  return nodes;
}

    </script>