Shock absorbers dampen the compression and rebound of the suspension by controlling the speed at which a suspension cycles. Without them your vehicle would continue to bounce up and down. By forcing a piston through oil, shocks develop the hydraulic friction necessary to oppose the unwanted bouncing in your suspension. Most of the shock oil is forced through tiny holes in the piston as it travels. Shock absorbers are velocity-sensitive, which means the faster a suspension cycles, the resistance increases exponentially. If you look at a graph comparing a small hole piston vs large hole piston, the small hole piston has a steeper curve. A common term used to describe this is the small hole piston has more ‘pack’ than a large hole piston.
Imagine two identical trucks sitting next to each other on a work bench. One has small hole pistons with a light shock oil, the other with large hole pistons with a heavy shock oil. The dampening of the two feel the same when pushed up and down by hand, but when dropped from a couple feet above the table, the one with the large holes bottoms out and slaps the table harder than the one with the small holes. The small hole piston has more ‘Pack’ and is more velocity-sensitive.
So, what does all this mean. When running on tracks with larger jumps you may want to increase the pack in the shocks to prevent the vehicle from bottoming out hard by changing to a smaller hole pistion and compensating with a lighter shock oil. When running on a rough and choppy track, you will want to decrease the pack to allow the suspension to cycle faster so that the vehicle rides smoother by changing to a larger diameter hole and compensating with heavier shock oil.
When setting up your suspension for a new track, work on where you are the slowest first. Dialing in a race vehicle is all about compromises and sometimes you have to give up a little on the big jump to gain a lot in the small mogul section. Always keep in mind that shocks not only slow the compression and rebound of your suspension, but also reduce bounce, roll, sway, brake dive and acceleration squat. Do your testing with a stopwatch and go with what is fastest.