After posting i had a thought, what if you used a large high speed actuator instead of a servo? since a quad's bars only move forward or backward about 9-12 inches from the riders position then an actuator should work and be cheaper. I would use a robotics actuator if it were me. As long as its not a giant grizzly quad then the steering shouldn't need a crazy amount of torque to work and it would make setting up much less complicated. Just some food for thought.