During the launch sequence, between the shuttle and the hold back bar, a huge amount of force is being applied to the nose gear. At the point of launch, there is enough force being applied, that it will accelerate the aircraft to flying speed, whether the engine is producing trust or now. Here is a decent video that shows how much loading is done. The nose gear is compressed about 6" just prior to release and squats another couple of inches just after release, as you can see with the nose bobbing up and down.
www.youtube.com/watch?v=tFGzgvgltMw
Tire pressures too low and you have blowouts on launch.