The Ramp Travel Index is for measuring how flexible the suspension is. It consists simply of a 20°-ramp on which you drive up to the point where one wheel leaves contact with the ground. You divide the distance traveled up by the wheelbase, so you get a percentage. Multiply this by 10 to get a more "handy" score. So if you have a Range Rover with a 100" wheelbase and you can travel up 40" you get a score of 400.
Anything over 500 is, on unmodified cars, a very good result. The stock NAS 90 scores 600. A few years ago 1000 was believed to be the maximum achievable.
On the left is the highly modified 90 from Chris Hinkle, on the right is a standard 1997 Defender with sway bars intact ramping about a 600 on the 20 degree ramp. The truck belongs to Kirk and Leefa of Solaros. The ramp is courtesy Southern Offroad Specialists, Kevin Barrett presiding
The softer a suspension the longer the wheels can stay on the ground thus providing traction. A few of the drawbacks of the softer suspension is the greater instability and reduced on-road usability.