I remember either reading or hearing somewhere once that they made the tilt too good, the body's "internal gyroscope" felt one thing, your eyes said something different, the brain gets confused and that's what made people feel sick
100% correct. If the carbody is tilted (whether using active/passive tilt or just the superelevation of the track) to the ideal banking angle for the train's speed on that curve's radius, there will be no lateral force experienced by the passengers, which does indeed cause dizziness and motion sickness for people who are not used to it due to the mixed messages.
The ideal banking angle for a curve on a road or railway can be worked out using the equation
tan(A) = v[sup]2[/sup]/g.r where
A is the banking angle,
v is the vehicle's speed (we're doing some physics here, so this is in metres per second),
g is the strength of the earth's gravitational field (9.81 m/s[sup]2[/sup]) and
r is the curve's radius in metres.
The ideal banking angle is not actually the most practical banking angle though! If you have vehicles using the curve at different speeds - as is the case for almost all railways and roads, there has to be a compromise found. A train travelling faster than the ideal speed for the curve's banking angle will need to find extra lateral force from the taper of the wheels' profile or from the flange contacting the rail. This is perfectly safe up to a point, but simply not too comfortable for passengers if the lateral forces get too great.
Where tilt-body trains come in handy is when you want to go at a higher speed which is still safe, but not comfortable. By tilting the carbody into the curve, the passengers on board experience a greater banking angle than on a non-tilt train, which in turn reduces the lateral forces they feel and allows the train to travel faster without compromising the comfort of the passengers.
The 'internal gyro' for the human body consists of fluid moving around a number of channels in the brain. If you want to have a play with this, try spinning on the spot a few times (maybe use an office chair) while holding your head straight forward, and then stopping - you'll feel dizzy for a few seconds as the fluids keep on moving. Next, try spinning a few times while using a trick practiced by ballerinas - 'snapping' your head from one angle to the next (maybe 12 o'clock, 2 o'clock, 4 o'clock etc) as your body spins instead of staying face forward - you won't feel dizzy when you stop because the 'snapping' motion doesn't get those fluids moving as much as a constant spinning motion does.
