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Tuning With Shock Fluid

Shock fluid is all too often overlooked when tuning a RC vehicle, but the slimy, messy stuff dramatically changes the handling of your vehicle. Follow these simple guidelines below and you will be on top of your game every week!

The first thing you must do is get a base shock setup. Use the factory setup and tune from there–after all the engineers and pro drivers that made that baseline setup know the vehicle best. After starting with the baseline setup (and making sure you have the right tires for your track), you should only make one change at a time.

Without shock fluid, your RC truck would look more like a pogo stick than a race truck when it lands from a jump. The shock piston passing through shock fluid coverts the energy of the moving suspension into heat. Heat? Technically, yes, but an easier way to get the concept is to just know that shock fluid controls how fast the piston can travel from point “A” to point “B” in the stroke of the shock. This impacts jumping (as described), but it also impacts handling during cornering, acceleration and braking.

With very few exceptions, the shock fluid used in RC is silicone fluid–that’s why we try to remember to call it fluid and not oil (hard habit to break). Silicone fluid available in different viscosities or thicknesses. The thickness of shock fluid is listed in one of two ways: WT or CTS. WT is more often used in American and with American brands. In either case, the lower the number, the thinner the fluid.

CTS measurements are linear, so 200 CTS is twice as thick as 100 CTS–pretty straightforward. That is not the case with, say, 60WT and 30WT. More importantly, the difference in thickness between 25WT and 30WT is actually significantly greater than 55WT and 60WT fluid.

Most club racers mix and match different brands of shock fluids and for most of us that’s just fine. You should always build your shocks in pairs to avoid any weird problems from using two 30WT fluids that aren’t exactly the same.

A higher fluid number indicates a thicker shock fluid, and the thicker the fluid, the slower the piston will move throughout the stroke of the shock. Thicker fluids makes the vehicle more stable and feel smoother, and it also helps the truck jump and land better.

If the track is bumpy or blown out it can hurt the handling of the vehicle. This is where you need to start tuning with your shock fluids. If the track is bumpy and blown out, you will want the rebound to be faster and this can be done by using a lighter or what is also often called a thinner shock fluid. This will keep the tires in contact with the track thus making the vehicle handle better. You want to make small adjustment and make sure you write the setups that work. If you’re really ambitious, you can take notes of how your truck responds to all of the changes you try.

The temperature impacts shock fluid. The higher the temp, the thinner the fluid is going to feel and the colder it is, the thicker it’s going to feel. Your shock fluid may change or “fall off,” so on hot days you’re going to want a thicker fluid than on a cold day. If you find the perfect setup early in the season when temps are right around 60F, you will probably have to go up 5WT when summer is at its peak and temps are in the high 80’s or low 90’s.

Another tuning aid is the shock piston. If your piston’s holes are too small the shock will have too much “pack” and the shock will be very stiff and almost lock up in rough sections on the track. Small holes are good for smooth tracks, with big jumps or jumps with harsh landings. Big holes provide less pack and are good for bumpy tracks. Bigger holes will make the vehicle more stable and have more traction in the bumpy sections and will allow the suspension to soak up the bumps. A piston change is a much larger change (more noticeable) than shock fluid and can make the car good in one section of the track and bad in another. Use shock fluid changes before changing pistons. A good time to use a piston change is if the track as a whole is blown out or is noticeable and consistently smooth.

Because of their simple design, most RC shock pistons offer the same amount of resistance on rebound as they do on compression. Ideally, a shock should be able to rebound faster than it compresses. This allows the suspension to be better able to absorb a rapid succession of bumps. Aftermarket piston such as RPM’s Two Stage pistons the best of both worlds. These two-piece pistons pull apart during rebound and allow the shock fluid to flow easier and the piston to move faster. RPM’s Two Stage pistons require tuning and experimenting just like ordinary pistons, but with some work you can have a truck that can smoothly land jumps and fly over sections of small bumps. MIP recently announced its own take on shock pistons with its Bypass1 pistons which are also adjustable and also provide different compression and rebound characteristics.

There is no set-in-stone shock setup. It’s all about feel and what the vehicle is doing, and I cannot stress enough how important notes are. Setup sheets and notes make setting up the vehicle week to week or track to track 100% easier and much more consistent.

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