How Shocks Work - Monotube vs. Twin Tube, Size Matters, v2.7




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Published on Apr 13, 2013

Let's say you want to move your house. Would you rather hire four weightlifters or four ballerinas? Even if the ballerinas stuffed their overalls to look beefier you'd know something was up when push came to shove. Moral of the story? Packaging can't cover up fundamental deficiencies.

Even more so than in engines, size matters in shocks. Beyond displacement, how that shock (or engine) is tuned will determine it's capacity to do useful work. Informed enthusiasts proclaim their engine size, redline, mods, wheel size and offset, tire width, custom color. Do you know how big your shock pistons are? Do you know how well (or badly) they've behaving?

If you don't know, there's a good chance you're being taken for a ride. In a shock absorber, the result of insufficient shaft displacement and/or poor fluid management is worse comfort and reduced grip. The shock needs to displace fluid both in compression and extension movements. This fluid displacement is based on the area of the shaft, area of the piston and internal design constraints. Usually for most shocks the piston size is big enough to create enough rebound damping force but where shocks vary widely is in compression. Smaller shaft size leads to lower compression forces, or more time delay and non-linear behavior in achieving a desired force. Larger shafts and wider shock bodies are expensive to make and hold tight tolerances on which is why they're usually reserved for more expensive vehicles (or higher-end shocks) and where performance and durability is required.

With a monotube shock such as a Bilstein, in most applications there is a 14mm shaft available to displace sufficient fluid and generate enough compression damping to resist bottoming out. This contrasts to an 11-12mm shaft on the majority of twin tubes that are displacing 30-40% less fluid per inch of stroke. Even if a twin tube has a larger shaft, there is no guarantee there is proper fluid management to prevent jacking down or hysteresis (which is non-linear force behavior).

One downside of a Bilstein shock (similar to the majority of twin tubes we've seen) is that there is often an intrinsic bias toward rebound (extension) damping which can cause poor ride/grip on rougher roads. Fortunately at Fat Cat Motorsports we understand suspension design well enough to determine the optimal damping profile for your needs and specialize in fine-tuning Bilstein monotube shocks and struts. With intelligent fluid management and determining the correct proportioning of compression vs. rebound damping for your application, you will experience an optimally tuned suspension even more capable than delivered from the factory.

We'll explore in future videos exactly what improper fluid management looks and feels like. In general, if you're experiencing a jittery, crashy, un-inspiring ride, unpredictable grip, and a frequent sense of bottoming out, you're probably riding on ballerinas in disguise.

FCM Elite - Torque for the Turns



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