RC Suspension Tuning Guide: Part 3 (2023)

How to get the Most Performance from Your Traxxas Vehicle

Part 1 of our Suspension Tuning Series covered camber and toe-in, and how these wheel angles affect handling. In Part 2, we’ll dig into the role the shock absorbers play, and how they can be powerful tools in your chassis-tuning arsenal not only for smoothing your car or truck’s ride, but altering the relationship of the chassis and suspension.

RC Suspension Tuning Guide: Part 3 (1)Whether you're racing at the track or just bashing in the backyard, proper shock tuning puts more power to the ground and keeps you on the winning line.

How Shock Absorbers Work

Everyone knows what a “shock absorber” does—it’s right in the name. But how does it actually absorb shocks? There are two essential parts to any shock absorber:

The Spring
Just about every RC car and truck uses traditional coil springs as the suspension component that absorb jolts, stores the energy, then releases it to return the suspension to its starting point. In the full-scale world, leaf springs are also common, and even compressed air is sometimes used as a spring. But for RC cars, it’s all about coil springs. A spring’s stiffness is called its rate, and optional springs with higher (stiffer) and lower (softer) rates are available.

RC Suspension Tuning Guide: Part 3 (2)All Traxxas shocks are “coil over”—a coil spring placed over a damper.

The Damper
If cars and trucks were outfitted only with springs, they’d constantly bob up and down with every bump, and wallow in every turn. The scientific term for that bobbing and wallowing is oscillation, and the term for reducing or restricting oscillation is damping. And so, the part of a suspension system that prevents the springs from oscillating is called a damper. For every Traxxas model, damping force is created by a piston displacing oil in a cylinder—the classic device most of us envision when we hear “shock absorber.”

RC Suspension Tuning Guide: Part 3 (3)An oil-filled shock uses a piston moving through the oil to create damping force. Holes in the piston allow oil to flow through.

When spring rate and damping force are properly matched to a car or truck, the suspension will support the vehicle without being too stiff (resulting in a bumpy ride and reduced traction) or too soft (resulting in excessive chassis roll, and “bottoming-out” too easily). When the suspension must absorb a bump or jump landing, well-tuned shocks will quickly compress to absorb the jolt, extend to return the suspension back to its normal position, and resist any further oscillation until the next bump comes along. Traxxas carefully chooses springs and damping settings specifically for each model, optimized for versatile performance in all types of terrain. But, if you feel the need (or like to experiment), you can change damping and spring rate to suit your terrain and driving style.

How to Adjust Damping

If you remove the spring from a shock and move the shaft in an out, you can feel the damping force created by the piston and oil inside the shock. To alter the amount of damping force, we can change two things:

Piston Openings
The shock piston has holes that allow oil to pass through. Installing pistons with more holes or larger holes will allow oil to flow through the piston more easily, reducing the shock’s damping force. Fewer or smaller holes will increase damping force. Traxxas Ultra Shocks can be equipped with 2-hole or 3-hole pistons to tune damping. GTR shock pistons have two holes, and you can choose pistons with various openings ranging from 0.95 to 1.6 mm.

RC Suspension Tuning Guide: Part 3 (4)GTR pistons are labeled by hole size. Use 1.5mm to increase damping, 1.6mm to reduce it.

Oil Viscosity
“Viscosity” refers to a fluid’s resistance to flow. The higher a fluid’s viscosity, the more slowly it flows. For example, molasses is a high-viscosity fluid, and water is low-viscosity. Filling a shock with higher-viscosity oil will increase damping force, and vice-versa if you choose a lower-viscosity oil. Most Traxxas models have 30-weight silicone oil in their shocks, and use pistons selected specifically for each model to provide optimal damping.

RC Suspension Tuning Guide: Part 3 (5)Most Traxxas models use 30-weight silicone oil in their shocks.

This Traxxas Support video explains how to properly fill Ultra Shocks, and covers all the steps for a full rebuild if required.

More Damping or Less?
As with all suspension settings, damping is a compromise. The optimum setting is somewhere between light damping—which allows the suspension to react more quickly and offer a smoother ride, but also increases body roll and makes it easier for the suspension to “bottom out”—and heavy damping, which reduces chassis roll and can improve handling in smoother conditions, but gives a less stable ride in rougher terrain. Your model’s factory settings are always the best place to start, and you can experiment from there. If you feel you need a bigger change than one step in piston size, or more than a 10 wt difference in oil viscosity, it’s likely you’ll also need to change springs as well. More on that next.

Spring Rate

Spring rate is the measure of a spring’s stiffness. The greater the rate number, the stiffer the spring. You may also hear spring stiffness referred to as pounds; for example, a “4-pound” spring. This is also a reference to spring rate, since rate is a measure of how many pounds of force are required to compress a spring one inch. A “4-pound” spring would compress one inch if you placed a 4-pound weight on it. The more weight it takes to compress a spring one inch, the stiffer it must be. But all you need to remember is “bigger rate number = stiffer spring.”

RC Suspension Tuning Guide: Part 3 (6)Traxxas springs are offered in a variety of rates, and color-coded for easy identification—note the green marks.

Which Spring Rate is Best?
Spring rate and damping go hand in hand. If you’ve increased your shocks’ damping beyond what the springs can quickly overcome, the shocks will “pack up” in bumpy terrain and limit suspension travel unless you install stiffer springs to compensate. Or, if the damping you prefer is so light that the suspension is bouncing with each bump, you’ll need to install softer springs to restore control. The best combination of springs and damping varies with each track and conditions, which is why a selection of springs and shock oils are an essential parts of any racer’s pit box. But the stock settings of your Traxxas model will be close, and a good starting point.

Ride Height and Spring Preload

A model’s ride height is the distance between the chassis and the running surface when the model is at rest. All Traxxas models allow you to adjust ride height via the shocks, either by placing spacers above the spring retainer, or by threading the spring retainer up and down the shock body. You may also hear this adjustment referred to as “spring preload,” because the springs must be compressed or “pre-loaded” a few millimeters to support the weight of the model. Beyond that, adding spacers or threading the collar farther down the shock body only serves to raise the chassis relative to the running surface.

RC Suspension Tuning Guide: Part 3 (7)The distance between the model’s chassis and the running surface is its ride height.

How to Adjust Ride Height
Many new drivers assume a suspension system only needs to allow the wheels to move up to absorb bumps, and will “jack up” the suspension to maximum ride height believing this will provide the “most suspension.” And this will indeed give the suspension the most “up travel,” but a suspension must also have “down travel.” In addition to allowing the wheels to move up when the model hits a bump, the suspension must also allow the wheels to drop down when the model hits a pothole, rolls over a dip, or drops off a ledge. And so, ride height is best set close to the middle of suspension travel.

RC Suspension Tuning Guide: Part 3 (8)Ultra Shocks use clip-on spacers to set ride height.

RC Suspension Tuning Guide: Part 3 (9)Check your model's parts bag for a selection of spacers.

RC Suspension Tuning Guide: Part 3 (10)GTR shocks have threaded bodies for precise adjustment without extra parts—just turn the spring retainer to adjust its position. Tip: fill in one of the ridges with paint as shown, so you can easily count each turn as you adjust ride height.

For maximum performance in smooth and/or high-grip conditions, lower ride-height settings will lower the model’s center of gravity and improve cornering performance. For rougher terrain and looser conditions on or off the track, higher ride-height settings will improve ground clearance, allow more up-travel for big hits and jump landings, and transfer more weight to the outside tires when cornering. Most off-road drivers set ride height by eye, relative to suspension arm angle or driveshaft angle rather than actually measuring the distance between the chassis and the running surface. As with all suspension tuning options, experimentation and testing is key. Adjust, test, and repeat.

RC Suspension Tuning Guide: Part 3 (11)“Driveshafts level,” and “arms level” are typical ride-height settings, and racers may go up or down a few millimeters from each to find the best setting. Note that the wheels’ camber angles are changing too—we covered that in Part 1.

RC Suspension Tuning Guide: Part 3 (12)“It’s common to set front ride height 1-3 mm lower than the rear ride height. This helps improve steering and counteracts the tendency for the chassis to lean back and shift weight rearward under acceleration.

That marks the end of the second installment. Click on the links below to see other installments of our Suspension Tuning Guide series

  • Click here to see part 1: Camber & Toe-In
  • Click here to see part 3: Shock Position and Roll Center
  • Click here to see part 4: Sway Bars and Tuning Tips
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