[Graham]

Is there a reason why 'drop arms should have a reach from the bottom of the tyres to the track surface of minimum of 5cm' for R2, R3 &R4 vehicles?

This is because the Pro Car RTR models provide this gap for being able to maintain braid contact on some of the more challenging obstacles (eg SCX ramp and suspended bridge tracks)

          

The Category 1 vehicles will be the only ones not required to run over these obstacles.