Death Wobbles

Saul O. Ruiz

Physics is found all around us. When we walk, ride our cars, mow the lawn, in every aspect of our lives there are physics involved. Due to the resent growth of longboarding I wanted to show one of the biggest fears of many riders. Longboards are just the same as skateboards but are longer between 34-60 inches long and usually having bigger wheels almost up to 90 mm in diameter. The components of the board are reverse kingpin trucks, wheels, bearings and grip tape. The truck’s main components are the bushings, pivot cup bushing, axle, hanger and base plate. Most of the riders love going fast but there is a phenomenon that riders call speed wobbles, which takes away confidence from going faster. A speed wobble occurs when the rider is going fast usually downhill, and starts losing stability and the board starts wobbling and the board causes the rider to lose control and in most cases causes the rider to fall. Physics has a way of analyzing this phenomenon. What riders know as speed wobbles in physics it is called hunting oscillation. Hunting oscillation occurred long before skateboards where invented. Hunting oscillation occur in any system trying to find an equilibrium. That is why it is called hunting oscillation, it is an oscillation created when a system is not in equilibrium and it tries to hunt for equilibrium. One of the first recorded hunting oscillation occurred on the train tracks. Locomotive trains were reaching high velocities and started shaking side to side. It is an interesting note to mention that some locomotive trains started forming oscillations which made the locomotive shake from side to side and it would turn into a violent motion that damaged the track and the wheels of the locomotive train which caused derailment. And in later years they would create dampening suspension system that would allow the locomotive trains go faster without wobble. The actual wobble is a phenomenon called self-exciting oscillation which creates a sinusoidal wave which the amplitude grows with in reaching top velocities. What happens during longboarding at high speeds is the same as the locomotive trains it starts shifting from side to side after receiving the initial wobbles the system hunts for equilibrium and instead of being derailed you get bungeed out of the board. When riding longboards the two main reasons for wobbles are the actual rider and the lack of dampening. The way the board turns and the geometry of the trucks speed wobbles might always occur. Ways to defend against the speed wobbles is dialing up a correct setup with no bushing or pivot cup slop. Having the correct bushing hardness for your weight also helps. Even when having the “right” setup, you might still get speed wobbles. That’s when the rider comes into play; the rider is the most essential part of the problem. The usual problem with speed wobbles is the distribution of weight of the rider. Ways to correct the problem is that you want most of the rider’s weight in the front of the truck. A lower stance of the rider will also lower the center of gravity making him have more control and stability. Even when all this fails if you still get wobbles do not fight them, join them, what you want to do is relax, lean forward and ride them out creating a sine wave which returns the system to equilibrium.