Turning the collar above an RC shock spring can raise the car’s resting position. It is tempting to describe that as “making the spring stiffer.” For a linear spring, however, preload and spring rate are different properties.
Spring rate describes the additional force needed for an additional amount of compression. Preload compresses the spring before the suspension moves through its working travel. Changing the starting compression does not change the linear spring’s rate, although it changes the forces and positions involved in the assembled suspension.
Start with the model’s intended adjustment
The Traxxas TRX-6 owner’s manual, as one model-specific example, treats ride-height adjustment and spring selection within its suspension guidance. Use the manual for your own vehicle; mounting geometry, travel limits and progressive components can make the whole system behave differently from a loose spring on a bench.
Place the model on a level surface with its normal running equipment installed. Check its resting height using the manufacturer’s method. Record the original adjuster positions before making a small, matched change where the instructions call for symmetry.
Check travel as well as the parked stance
A higher resting body is not automatically a better setup. The suspension still needs suitable travel in both directions, and excessive adjustment can create other limitations. Do not wind an adjuster to its limit merely to achieve a visual stance.
For a useful test, keep the tyres, battery position and surface consistent. Record the ride height and how the car behaves over the same modest obstacle or section of track. Change one relevant setting at a time so the result remains interpretable.
If the required resting position cannot be achieved within the recommended adjustment range, consult the manufacturer’s spring and setup guidance. Do not assume that additional preload can substitute indefinitely for an appropriate spring.
The distinction gives your notes more precision: “I increased preload and the resting height changed” describes an observation. “The spring rate increased” makes a different claim that the collar alone does not support.