I've noticed on my NC24 theres a good 14cm from the top of the wheel to the top of the wheel well. I'm wondering how much of that space is actually needed... I must get 2 people to sit on my bike and measure it, and then consider a big bump. any idea how much the rear suspension travels on bumps? Am I correct in thinking that preload doesn't affect where the suspension bottoms out?
is there any limit to how far the rear suspension can travel? or a way to tell where it would bottom out?

From memory the NC30 has 120mm of travel, measured on the wheel, I can only imagine that 24 is the same since 90 procent of roadbikes got this measurement.
The 120mm is from fully topped out to fully compressed (bottomed out), so you can subtract your sag from this, rider sag should be around 20-30mm or the rear on a roadbike. This leaves you with 90-100mm for bumps, where roughly the last 5mm will be on the bumpstop (rubber on the rear, hydralic on the forks). The 20-30mm sag is the travel you will use when hitting potholes and more importently when you brake (weighttransfer will take up some of the sag). Too little sag on the rear will have your rear wheel hovering in the air under heavy braking and you will loose traction on bumpy roads. I haven't touched upon weight distribution from playing with sag, but that will be affected aswell.

Ronni