CRASH COURSE

PHOTOS JOEL NAVARRO

UNDERSTANDING SHOCK ABSORBERS

Whether you've got an on-road car or an off-road machine, its shock absorbers are integral to its performance. Without shocks, off-road RC would be virtually impossible, and on-road cars wouldn't be able to grip the track. They're important parts that work hard, but they're not hard to understand. And when you know how they work and what they do, adjusting your shocks' settings will allow you to tune your machine's ride to suit your terrain and driving style. Here's what you need to know.

SHOCK ANATOMY

RC shocks are a “coil over” design: a coil spring is placed over a damper.

WHAT SHOCK ABSORBERS DO

That's easy. Shock absorbers absorb shocks, right? Well, it's really the springs that “absorb” the energy generated by hitting a bump or landing off a jump. Springs are also very efficient at releasing that energy, as you've discovered if you've ever bounced on a trampoline or pogo stick. What we think of as the “shock absorber” is properly called a damper, and its job is to damp the oscillation of the springs that support the car. Without dampers, your car (full size or RC) would constantly bounce and jitter like a bobblehead figure. A shock absorber's damping action comes from the resistance of a piston moving through fluid. In the case of RC shocks, that fluid is silicone oil. When the damping force and the spring's rate are properly matched, the spring compresses to accommodate jolts, and the damper quickly settles the spring so the car doesn't rebound like a pogo stick.

PISTONS AND SHOCK OIL

Oil-filled shocks allow damping force to be adjusted in two ways: by altering the size of the holes in the pistons, and by changing the viscosity (or “thickness” ) of the shock oil. Both methods alter how much force is required to move the piston through the shock oil. The greater the force, the greater the damping effect.

PISTONS

The piston with two holes will require more force to move through the shock oil than the 3- or 4-hole pistons.

Nearly all kits and many RTRs include extra shock pistons for suspension tuning. If you look closely at the pistons, you'll notice that they have different numbers of holes, different sizes of holes, or both. The smaller or fewer the holes, the more force will be required to move the piston through the oil. Generally speaking, smaller or fewer holes are better for landing big jumps, because they will prevent the vehicle from bottoming out. Conversely, larger or more piston holes increase handling response and allow the vehicle to soak-up small bumps more easily. This assumes, however, that the springs are not too firm or too soft for the amount of damping the shocks provide—more on that later.

SHOCK OIL VISCOSITY

The higher the number, the “thicker” the oil.

The easiest way to alter damping is by changing the shock fluid. The higher the “weight” number of the shock oil, the “thicker” it is and the more damping it will provide. Most cars and trucks arrive with 30-weight oil in the shocks, and you can typically go as high as 40-weight or as low as 20- weight and still be within the damping range suitable for your model's springs. For fine tuning, you can combine piston and shock fluid changes. As with all suspension tuning, experimentation is key. But for play driving, all that's is essential is that the shocks work!

FOR GENERAL FUN DRIVING, ALL YOU NEED TO DO IS MAKE SURE YOUR SHOCKS ARE FULL OF OIL AND FUNCTIONING PROPERLY. THE MANUFACTURER-SPEC'D SETTINGS ARE NEVER FAR OFF.

PRELOAD AND RIDE HEIGHT

Clip-on spacers (left), threaded collars (center) and clamping collars are the three ways preload can be set. Most RTRs use clip-on spacers, which are goof proof.

Shocks allow you to adjust the amount of static tension or “preload” on the spring. This allows you to compensate for the amount the spring compresses under the car's weight, and also effects the car's ride height (the height of the chassis over the ground). Depending on the design of your shocks, preload may be set by clipping spacers onto the shock body above the spring, or by threading a collar along the shock body. Less common are collars that clamp around the shock body tightened by a screw, but some models still use them. When experimenting with preload, be certain to set the left and right side shocks equally, or else your car won't handle properly (at the very least, it will turn more tightly in one direction than the other). Clip-on spacers make it easy to set preload equally—just put the same size and number of spacers on each shock. For threaded and clamping collars, measure the distance from the shock cap to the collar to make certain the left- and right-side preload settings are identical.

UNDERSTANDING SPRINGS

The top coils of these springs are color-coded to indicate their rates.

What's there to understand? It's a spring! True enough, but what we're talking about is spring rate, or “stiffness.” Spring rate is measured by the amount of weight required to compress the spring one inch. For example, a “2 pound” spring would compress one inch if you placed a 2-pound weight on it. A “3- pound” spring would be stiffer, since it requires more weight to compress it the same amount. Some brands state actual spring rates, other simply label them “soft,” “medium,” and “hard” or something similar. For severe terrain and bigair action, installing stiffer springs can help your car or truck better cope with those big hits. Check the accessories section of your model's manual for optional spring sets.

LEAKY SHOCKS? REBUILD!

No matter how thrashed out your shocks are, a rebuild kit can make them perform like new.

The shocks are arguably the hardest-working parts on any RC car. Even on smooth terrain, the shocks are constantly compressing and rebounding, which eventually wears out the O-ring seals that prevent oil from leaking past the shock shaft. Running in dirty, dusty conditions speeds up this process. If you notice your shocks are coated with a film of dirt and oil (or worse, they're wet with oil), it's definitely time for new seals. All brands offer replacement shock seals, and many offer convenient rebuild kits that include the seals as well as shaft guides, rod ends, pistons, and other parts. You'll find the part numbers for seals and rebuild kits in your manual. Lost it? You should be able to download a copy online.

WRAP-UP

Racers spend a lot of time getting the shocks and springs just right for the conditions of the day, but for general fun driving, all you need to do is make sure your shocks are full of oil and functioning properly. The manufacturer-spec'd settings are never far off for fun-running, but if you want to set up for bigger jumps, rougher (or smoother) terrain, or alter handling, changes to damping, spring rate, and preload/ride height can dramatically affect your ride. Experiment!