Pre-load is a fairly straightforward setting for an RC car. It is simply used to set the ride height of the car.
Pre-load is the amount of pressure you add to the shock spring, usually by lowering the spring retainer at the top of the shock. The more you lower that retainer, the more force the spring generates, and the higher your car will sit above the ground.
Pre-load is usually measured as the distance between zero pre-load (where the spring is as long as possible while still being on the shock) and the current position of the spring retainer.
it is important to note that you should NOT use pre-load to change the actual damping behavior of the car - it won't. Changing pre-load is only effective at changing the ride height of the car.
Learn more about ride height tuning.
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