Measure Roll/Steering Angle

Hi guys, I'm a student from Italy pretty new in the ES world!
I'm doing a project with a longboard and ones of my purpose is measure the steering angle. Maybe is better measure the roll angle of the board and from the well know mechanic relationship of the truck gets the steer angle.
The tilt sensors or IMU sensors are not accurate because during motions there are other forces summing to g vector!
I would like a sensor that doesn't suffer the effect of other forces except the gravitational one, so moving the longboard I'm interested on only the roll angle.

Thank to all!

I would do it like this:

Attach small potentiometer to deck and make an arm that connects to truck in a way when you are turning the potentiometer changes value.
I have done some tests with inertial navigation based on IMU sensors (accel, gyro) and for longboard with that much vibrations, damn, that would be a pain...

Thanks a lot Njakts for your suggestion!
Using a potentiometer is as I thought, but the problem is that truck respect deck has a relative motion so we need a spherical joint between the two arms; or maybe is better a string-pot or other solution?