Fine tuning an RC Car can be a very challenging process of you do not know what you are doing beforehand. The tuning process includes a multitude of things, especially for crawlers, this includes motors and drag brakes, suspension and ride height. Some things are easier than others, but it is good to know what each adjustment does before you start messing with it.
Motors in crawlers are the biggest factor of tuning in crawlers, they are usually a brushed motor. Most crawlers have a 27 to 35 turn motor, but they range from 20 to 55 turn. The difference is that a smaller turn number goes faster and that a higher turn number has much more torque. For crawlers the higher turn is better as speed is not really needed for rocky terrain and steep slopes. Another function you can tune on crawlers is the drag brake on the motor.
Drag brake is the amount of brake that is applied when the throttle is released, it is measured as a percentage from zero to a hundred percent. Drag brake is important for steep uphill or downhill climbs so the car will stop automatically without the car rolling downhill. The reason you tune it is so that if you release the throttle and the car comes to an immediate stop odds are that it will flip and tumble down. So instead the drag brake can be set where it is just enough to stop the car without flipping.
Suspension is another item that is tuned very frequently, there are a few different types of suspension, including links, leaf and droop. The most popular type of suspension system is link, there are also two different types of link suspension, this includes three link and four link suspension. Four link suspension is better than three link suspension since their are more links to allow more travel and flexibility. Axial crawlers come stock with three link suspension that are made out of plastic. Upgrades can be made by purchasing metal links that slide off of rocks easier and that come in four link systems, this upgrade can be considered a tune that gives a lot more mobility and a huge upgrade. Leaf suspension consists of four leafs and four springs installed on the axle and on the chassis.
Four link suspension is the superior suspension system as it allows the most travel over all sorts of terrain. You would want less travel if trying to achieve high speeds to allow less movement and momentum moving the car side to side or off course.
One last thing that is highly tuned for both crawlers and short course cars is the height of a ride, this means how far the chassis sits above the ground. For a crawler you would want a higher chassis to clear taller objects. For a short course on the other hand the lower the car the lower the center of gravity, this also means less handling but for speed runs it is an advantage. Then if you put the height to high the center of gravity will also be to high and therefor it will flip easy.