Robbie said he was just a little fast when he rode off the roof, leading to the long landing. He said it would have been even longer, except he was able to force the rear tire down and save it. 
Had he landed in the flat area, it would have been a train wreck.
Also notice that on the take off, the g forces pull him dangerously back on the bike. He manages to pull himself forward as he decelerates in the air to save it, this causes a rotation to an unexpected front wheel landing.
He did receive a hand injury from the force of the landing.
I suspect the knobby's were needed because the landing ramp was very soft to help decelerate him. Notice how fast he is able to slow down. He is almost stopped by the time he comes off the ramp.
Here is a video of how he practiced for this: http://www.youtube.com/watch?v=xdA8xJKgSK4
Had he landed in the flat area, it would have been a train wreck.
Also notice that on the take off, the g forces pull him dangerously back on the bike. He manages to pull himself forward as he decelerates in the air to save it, this causes a rotation to an unexpected front wheel landing.
He did receive a hand injury from the force of the landing.
I suspect the knobby's were needed because the landing ramp was very soft to help decelerate him. Notice how fast he is able to slow down. He is almost stopped by the time he comes off the ramp.
Here is a video of how he practiced for this: http://www.youtube.com/watch?v=xdA8xJKgSK4
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