Willie,
Geometry regarding angles, rake, trail etc.
Design regarding length, strength, support, cockpit structure, distance from rear axle to swing arm pivot on a chassis with a swingarm/shock design.
Material thickness, what may be strong enough in a 72" chassis may not be as strong in a 96" chassis
What works at 50 mph may not work the same at double the speed, same thing with Boost or Nitrous fueling and timing.
Looks regarding wings for down force, engine/turbo placement that is a bad aero design.
Now I need to go get some PVC for the new bike..
John