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Speed Wobble Hunting – Avoid? How? Why?

Submitted by admin on April 4, 2011 – 10:33 PM
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SPEEDWOBBLE HUNTING

When I was asked to write about speed wobbles for the magazine, it made me realize that while I had spent the better part of my life on a skateboard, I didn’t really know that much about the wobs. Sure, we all have our stories, complete with tales of how we survived them. And from these we’ve created all kinds of homespun theories, many of which seem to contradict each other. One skater I asked said he crouches down and touches the nose of his board to gain control during wobble onset; another said not to crouch or make any drastic body movements. So how do we know what is good advice and what is fiction? I decided to explore the wobble phenomenon from two perspectives, alternating between the scientific and the experiential, in hopes that between the two a better understanding of wobbles would emerge.

I start my inquiry into the science behind wobbles by looking at the laws that govern the  phenomena we’re experiencing. But the more I look into the physics of wobbles, the broader and more complicated the story gets. There’s the kinematic analysis of the geometry of motion, the harmonic motion of sine waves and the properties of self-exciting oscillation. Then on the mechanical side there are steering systems, dissected into the basic geometries and their performance characteristics. At first I wonder how much of this will actually relate back to my skateboarding speed wobbles, but the more I read, the more I’m amazed at how wobbles occur in everything, from vehicles and wings to electrical systems and the stock market. They’re the gremlin in the works of everything.

TRAINS, TRACKS AND TRUCKS – CLASSIC WHEELSET “HUNTING OSCILLATION”

One of the first recorded examples of wheel wobbles occurred in locomotive trains. Around the 1890s, trains started traveling fast enough to reach the critical speed of 140 mph, which brought on a form of oscillation that sent their cast-iron wheels shimmying back and forth. If unchecked, this phenomenon would escalate into a violent motion that damaged track and wheels and caused derailment. Trains were jumping their tracks due to wheel wobble! Surprisingly, it took us until the late 1960s to figure out dampened suspension systems that can allow rail travel at speeds of 180 mph or more without wobble.

Ironically, the name  truck” as it’s applied to skateboarding is derived from the locomotive’s truck, which is a cast and sprung steering mechanism that holds the axle and wheels, like our trucks. So in a way, our own wobble woes are hereditary.

The instability observed in those long-ago locomotive trains is but one example of a phenomenon called Hunting Oscillation, which shows up in a variety of forms, including guitar feedback, wing flutter, fin hum and steering shimmy. Even the stock market is prone to hunting oscillation, when bull markets tend to create ever-increasing prices until a precipitous fall, or when bear markets expect further losses and quickly fall, only to rebound sharply. It appears that our Great Recession is simply a case of living too fast and getting financial speed wobbles. Every system has a speed at which it fails, and when it does, it jumps the tracks, goes broke – or pitches you from your board. And if speed is the accelerant, so to speak, that ignites the fire of wobbles, we need to find out more about speed’s relation to steering systems and what types are more prone to wobbling.

CRITICAL SPEED AND SELF-EXCITING OSCILLATION

The wobble is a phenomenon known as a Self-Exciting Oscillation, a snake-like sine wave that grows in intensity as the amplitude of the wave grows. Once it reaches a critical speed where performance hovers at the edge of control, the waveform begins to feed off of energy from the forward motion, siphoning that energy into increasing the amplitude of the sine wave in a closed loop circuit until it exceeds the limits of the system and it fails. As I read the bit about feeding off the forward momentum I’m struck by how sentient it seems – as if the board takes over in an energy-possessed trance and begins gorging on power until it darts off the road.

But what about the way the board handles, and how that can play into this parasitic phenomenon? I turned to automotive and bicycle steering geometry analysis to learn more about the fundamentals of steering systems to see how they applied. The dissection of a skateboard’s steering geometry turned out to illuminate quite a bit about how it can, and does, feed OG Skateboard Truck, circa 1890. the wobbles.

STEERING SYSTEM ANALYSIS AND THE SKATEBOARD

Steering is the system of linkages that allows a vehicle to follow a desired course.  Stability, or the lack of it, is the one of the main topics of steering engineering. What becomes clear after some study is that the interconnectedness of all the components makes the tuning of steering properties a series of gains and compromises. All of the different  geometries of basic steering mechanisms are somehow responsible for controlling or  exacerbating wobbles, but each one is inextricably linked to the others. Finding a  combination that works best for one application, like speed or maneuverability, is the key to improving the system.

Understeer and Oversteer are the two polar tendencies of a steering system. Understeer needs more energy to stay in a turn; oversteer easily dives into a turn. Understeer is often referred to as “tight,” or “plowing,” and requires more effort to stay in a turn. This tends to feel stable but also unresponsive. By contrast, oversteering rides are referred to as “loose,” “twitchy” and even “ill-tempered,” and have an instability mode that occurs at a lower critical speed. As this speed is approached along a relatively straight course, the steering becomes progressively more sensitive. Any slight movement can send the vehicle out of control. As we will learn, skateboards have serious oversteer, making them an ill-tempered steed.

Steering Ratio is another aspect that can affect stability. If a truck has a steep steering angle it will describe a tighter arc of turn, and therefore any turning will send it off course more quickly. Because of this it will have a slower critical speed and tend to oversteer, even at lower speeds. By contrast, less steering angle will produce less-sensitive handling.

Four-Wheel Steering is when all four wheels engage in a turn simultaneously. There’s Positive 4WS, in which the front and back wheels turn in toward each other, as our skateboard trucks do. Then there’s Negative 4WS, in which the front and back wheels turn in parallel. This type of steering is used for high speeds because it doesn’t alter the angle of the vehicle as drastically when initiating a turn. There’s less of a tendency for sudden course changes because the amount of turn is very small. The more a vehicle angles away from the direction of forward momentum, the more it will have a tendency to oversteer and dive into a turn. Rear-Wheel Steering is also notoriously unstable because it tends to tighten the turning radius toward serious oversteer. Since our back trucks turn quite a bit, the tail of our board also tends to oversteer, which we now know is prone to oscillation excitation. Once it nears its limits, it turns in the direction opposite of how it is initially steered. A rapid steering input will cause two accelerations, first in the direction that the wheel is steered, and then in the opposite direction, a “reverse response.” This extra “tail squirrel” may explain the common notion that wobbles start from the tail.

But one of the biggest discussions regarding stability deals with Trail and Caster. The easiest way to understand trail is to think of a shoppingcart wheel, and how it trails behind its steering axis during forward motion. This is known as Positive Trail, and it is responsible for the desired selfrighting properties of trailing systems. Negative Trail is the reverse, an extremely sensitive system where the pivot is behind the wheels nd will tend to dive to one side or the other. Because skateboard trucks are generally the same front and back, if the front of a board with Reverse Kingpin trucks has Positive Trail, the rear truck has Negative Trail. That would mean that the back truck is more prone to the onset of wheelset hunting because the negative trail oversteers. In addition, trail does not dampen the system; it only gives it directional stability, so even with sufficient trail the steering system can still succumb to speed wobble.

Caster Angle is the tilt of the steering axis relative to the ground, and this angle affects whether the steering system raises or lowers when you turn. On a skateboard the steering axis is a line straight through the Kingpin. Steering has Positive Caster when turning raises the vehicle and Negative Caster when turning lowers it. A skateboard has negative caster because it lowers into turns; therefore its default preference is to turn because our weight tends to fall to either side. Cars have positive caster, and because they rise slightly when turned, their weight tends to settle downwards, and hence they prefer going straight. That makes cars stable and skateboards prone to diving into turns. Add to this our own elevated Center of Gravity and the tendency to dive into a turn is even stronger.

A WOBBLE THEORY

This recent inquiry into waveforms and steering, along with my years as a trucksmith, has led me to extrapolate that there are two types of wobble that skaters experience. The first kind is of the “loose truck” variety. This is where there is excessive play in an undampened system that allows for a self-exciting oscillation to take over once critical speed is attained. In order to remedy this kind of wobble you want to remove loose play, like under-compressed bushings and pivot cup slop.

The second type of wobble is “us.” We can be the source of oscillation excitement as we attempt to counteract a wobble that may have started with a pebble or a flinch, only to add to it with the energy of our synchronized reactions. This is a situation where it’s a good idea to control one’s over-reactive tendency and hope that you can keep from feeding into the loop vortex.

In order to grok our role in the whole steering system it’s important to see us as the chassis of a human/skateboard unit, because we do function as one. Flexible chassis are shown to feed oscillating systems with the energy from their spring-back, even at lower speeds. Thus there is a need for Positive Feedback systems (also known as Feed forward systems), which utilize a controlled counter-action to the undesired frequency. Luckily, we are the ultimate feed forward system, in that we can use our skill and confidence to counteract the sometimes wayward mechanical tendencies of our boards.

WEATHER VANING AND THE STABLE TAIL

As I searched for solutions to the wobble tendency, I noticed that most of the examples of stable steering systems were ones in which the front of the vehicle turned more than the rear. This configuration creates a sort of Weather Vane Effect, where a trailing wing pivots the vane into the wind – also known as Passive Steering Stabilization. A more stable back truck creates a similar effect, where the tail becomes a sort of trailing stabilizer. The front truck then provides the majority of steering without the propensity for the vehicle to oversteer from the rear. Directionality is a big part of this, so it’s not ideal for riding that involves reversing board direction at speed. If that works with your riding style, then you can easily make a version of this design by using a wedged Reverse Kingpin truck with soft bushings on the front and a de-wedged Standard Kingpin truck with firmer bushings at the rear. You can feel how stable it is when ridden forward compared with when you ride the board in reverse.

USE THE FORCE

Knowing the science behind wobbles has helped me understand why some commonly practiced methods work. What a looseness of body can do, for example, is separate the oscillating board from the mass of the rider so that we don’t feed the oscillation any further, and may even dampen it. Because when we add our body weight to the oscillating system, we provide a lot of mass for the oscillation loop to feed off of. So as it turns out, according to traditional steering analysis, the skateboard is just a twitchy animal that is prone to fits of  instability, exacerbated at every turn by its own design. So why don’t we just fix all these
problems, then? I pondered this question while pumping my board around in front of my house and the answer came to me in a flash of obviousness; it’s all that volatile energy from the system that makes it so much fun. The very essence of the pump comes from our ability to harness the instability, tap into the oversteer, accentuate the positive 4WS, even feed off the negative trail and ultimately jump on the negative caster. All of the things that make a skateboard categorically “ill-tempered” are the exact same things we love about it. It’s good to understand the geometry of our boards so we can fine-tune them to suit our styles, but ultimately it is we who need to adapt to riding this wild horse, and not tame the animal so much that it loses its buck. CW

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