This video from Gazzetta Dello Sport shows the basic workings of the Brawn GP's rear suspension. It it though that the string from the center shock is what broke off in Barrichello's car. Some reports give the weight of the spring at around 800grams, Massa's speed at around 260km/h. You do the math.
Without translating the whole video, you can see that is the only coil spring and is used in conjunction with torsion bars embedded in the gearbox.