To elaborate on Bill Cobb (excellent) post this is simple test you can do on your own FS / FSAE car.
Turn the steering wheel 45 degrees and measure the steering rack movement. Make a note of it. Try 45 degrees of steering angle more and look at the steering rack displacement again. Are the numbers the same? If not is it on purpose? If not where does the discrepancy come from? Is it from kinematics design, wrong steering column parts assembly or is there compliance?
Try yo make smaller steering angle and bigger number of increments by measuring the angle accurately. The digital inclinometer won't be good enough unless you mathematically take into account the steering wheel inclination angle. I have seen some guys "stealing" the disk that engine builders usually put on their crankshaft while they assemble their engine: that can be useful for accurate measurement of the steering wheel angle
Of course the steering wheel angle (deg) / steering rack movement (mm) ratio is not the steering ratio. The toe link angle Vs the chassis longitudinal axis (and the variation of this angle in steering), the Ackermann, caster angle and king pin angles and trails etc.. play a role too in the steering ratio. But starting with the steering wheel angle (deg) / steering rack movement (mm) ratio is a good beginning.
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Originally Posted by BillCobb
