It is big, it is clever, and it is a rite of passage. However it isn't so that you can find the bikes limits. If you were close to doing that, you'd already be racing. On the track, your knee will go down without trying as the tarmac is grippier and you can take the right lines as no oncoming traffic. On the road, you have to overcompensate for the fact your bike isn't being pushed at all. Ideally, you'll bein 3rd gear, at about 30-35mph.
Rossi is banked over, practically lying it down at well over 120mph.... If you can take highworth roundabout that fast, you won't need to ask how to get your knee down! ;D
