ignoring air friction, 400m=0.5*9.8t^2 ===> t=9s which is faster than how fast they can travel on land.

if you want to take into account air friction, you have to use some fluid dynamics. However, there's no way to judge what the velocity profile of a motorcycle streamline is which makes it hard to say. Also, there's a good chance that the motorcycle will be at terminal velocity after 9 seconds. Ignoring friction, the bike will be approximately 90 m/s == 324 km/h.

So, looking at terminal velocity...

http://en.wikipedia.org/wiki/Terminal_velocity
shows that terminal velocity on a person is about 54m/s. On a bike, I'd imagine it'd be smaller. Let's assume 45m/s. Therefore, the bike would reach terminal velocity at 45m/s. So let's neglect air friction for the first 5 seconds and integrate the velocity over time to get distance.

400 = 0.5*9.8*5^2+45*t ===> t= 6.1667

total time = 5 + 6.1667 = 11.1667

Taking figures from

http://www.motorcycle-usa.com/Articl...ID=4514&Page=2
which has the cbr600RR 1/4 mile time clocked at 10.6, we can compare the two times.

IF we were to consider air friction for the first 5 seconds, the time for the free fall would be even higher.

Also, to make sure, using the same math for terminal velocity = 54m/s, we still yield 10.9 which is still larger than 10.6

in conclusion, on the ground = faster