Originally published in Concrete Wave Magazine 2011 Longboard Directory Issue
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 & 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 & 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-pos-sessed 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 the wobbles.
Steering System Analysis
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 shopping-cart 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 self-righting properties of trailing systems. Negative Trail is the reverse, an extremely sensitive system where the pivot is behind the wheels and 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 Feedforward systems), which utilize a controlled counter-action to the undesired frequency. Luckily, we are the ultimate feedforward 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.
Riding Out The Death Shimmy
I asked Adam Colton a few questions about his wobble experiences to hear from someone who’s rolled over it all. “Speed wobbles happen for a variety of reasons: bad weight placement, pavement quality, lack of confidence, wrong gear for the situation…”
This is the most common advice by far, and also the most useful. It’s difficult to relax if you don’t know what you’re doing, so the first rule of Wobble Prevention is to “Skate within your limits, and don’t get going faster than a speed you are comfortable slowing down from.- Experience helps you relax because it gives you confidence in your abilities; so build your speed skills slowly. By relaxing you’re also maintaining your mass traveling with the overall direction of for-ward travel — so you will not get as involved in feeding the hunting oscillation, but rather act as a stabilizing force. Adam also says, however, that “hitting bad pavement on my distance trips has led to board convulsion and wobbles no matter how confident lam.” Sometimes there’s just no avoiding it, so it really comes down to how you smooth it out.
Lean Forward Slightly
Wobbles feed on sudden movements, so make any weight shifts with a slight and confident manner. Leaning forward shifts your center of gravity closer to the nose, thus emphasizing weather vaning stabilization, as well as lowering your center of gravity a little and helping coun-teract negative caster. You are part of the board, and your forward placement creates a slight trailing system, where the rear truck is farther back from the center of gravity than the front truck. It’s almost like dropping a little fin in the back to help stabilize the overall vehicle. This also puts more weight on the front truck, acting as a type of steering damper. Plus there is less board flex nearer the front truck, so chassis flex won’t feed as much energy into the system. And finally, leaning forward keeps you from engaging the rear wheel oversteering.
Initiate A Carve
What you’re doing here is attempting to impose an oscillation of your own, one that you control, over the hunting oscillation. Adam says, “If I [were] a beginner and rode my exact same truck setup down the hills I do now, I totally would wobble out. But since I have confidence and know how to place my weight, I can charge through turns and down straightaways without wobbles on pretty loose trucks.
Tweak Your Ride
There are many opinions as to what the best board setups are, and each rider will have his or her own valid preferences. Here are just some properties I’ve noticed as I’ve explored truck performance, and how they affect stability at speed. Some will have a negative effect for certain styles of skating and other styles will benefit from it, so don’t take any advice blindly; test some of these adjustments so you can know them experientially and judge for yourself.
Length greatly improves stability, as it increases the Steering Radius and results in a higher critical speed before wheelset hunting starts.
REMOVE SLOP Free play in the system is not something we can control, so removing any slop from our trucks will help prevent certain wobbles and increase critical speed and control. Pivot cups that fit your pin snugly and no free play in the bushing ad-justment are a good place to start.
By wedging a truck you tilt the kingpin so that it increases trail and stiffens the extremities of rail, providing some extra dampening. But this also increases the steering axis, thereby reducing the turning radius and decreasing the critical speed it takes to induce wobble. Bushing durometer can be used to compensate for certain wedging changes. De-wedging, by contrast, loosens the rail extremities, but also decreases the steering angle, leading to a softer, less responsive truck.
Loose trucks do not in themselves cause wobbles. It’s more about the combination of the right speed with the turn properties of the board. Skilled riders generally prefer looser handling for more sensitive control – the “sharp knife is safer” approach – but adding some bushing tension does increase damping, which is aptly defined as “any effect that reduces the amplitude of oscillations in a system, helping control harmonic motion from becoming excited.- Softer bushings tend to be bouncier and can further incite hunting by providing a deeper rail, which rebounds even further in return. Stiffer bushings provide more dampening but can make it more difficult to course-correct as well. It’s a series of tradeoffs that you will want to experiment with to find what works best for you.
It takes more energy to swing the wheels around the bigger arc of a wide hanger when it turns, and this lagging effect acts as a type of natural dampening. Plus the wider track of-fers finer rail-to-rail control because of the increased leverage against the bushings.
My Favorite Setup
Double trailing / front truck steering
I’ve been riding a kind of wedged Reverse Kingpin truck with soft bushings on the front and a Standard Kingpin truck with soft bushings on the tail for years because I prefer the combination of nimble turning and speed stability that is the hallmark of Front Wheel Steering (see “Double Trailing truck set” diagram at right).
Both trucks trail, so there’s added stabilization, which further allows the front to be quite maneuverable while maintaining good stability. This setup is directional, because when ridden backwards it becomes a rear-wheel steering system, and so it accentuates oversteer and wobble tendencies. If you like to slide, or switch board directions at speed, then you’ll likely prefer a more symmetrical truck setup.