What causes a broken axle?

[George_San_Jose1]

On a late night stroll a few years ago I saw a fairly new car loaded with teenagers hit a speed bump going a little too fast, with a little too much weight in the car, both front axles appeared to break at the outer cv joints. As you might expect the car came to a screeching halt, mid-road; meanwhile the cv joint ball bearings broke loose & went whirling down the road.