By Daniel Gesmer
printed in the Spring 2000 issue of International Longboarder Magazine
Allot of good info. Enjoy!
"High-speed wobbles are an extraordinarily complex phenomenon that even leading professors of mechanical engineering can’t fully explain. They’re simply a fact of life for any type of flow-motion sports gear.
The only skateboard that might be immune to speed wobbles is one that is
completely incapable of tilting, turning, and flexing. Beware: Any manufacturer who claims that a steerable skateboard product is totally resistant to shimmying is either lying or ignorant.
Any skateboard that can turn or flex will also be subject, under certain conditions, to speed wobbles. For extreme velocities, it is always recommended that you tighten your trucks, or use stiffer bushings or springs. If your trucks make tight turns easily, you should also consider slowing down their steering response with angled risers, or switching to trucks made specifically for high speeds.
Speed wobbles can occur whenever something starts a vibration that matches a resonant frequency of your skateboard. A resonant frequency is one at which your skateboard will vibrate very easily; a particular skateboard may have multiple resonant frequencies. The impetus may be a bump in the riding surface, a rough patch in the road, an unlucky movement by you, or some combination of these factors. Other potential contributing factors include the small torques resulting from wheel rotation and the tiny lateral oscillations that spinning wheels make if they’re not aligned with absolute perfection. If you start wobbling at speed, the standard advice is to crouch and try to grab the deck. Touching the deck presumably lets your upper body absorb some of the vibrations and probably also changes the resonant frequency of the skateboard, since you’re "connecting" your upper body to it.
In 1979 racing legend John Hutson assisted in a presentation made by the Stanford Mechanical Engineering Department, in which the stability of skateboards at various speeds was compared with that of aircraft. What emerged is that like aircraft, skateboards go through various "zones" of oscillatory stability and instability as they accelerate up to their highest speeds. One might even compare this to musical notes or octaves of relative vibrational resonance. For example, a particular setup might be stable up to 30mph, prone to wobbles at 40mph, but stable again at 50mph. However, this absolutely does not mean that if you start wobbling just go faster and things will smooth out! It may be the case that a given skateboard will be unstable at all speeds beyond a certain threshold. Be careful out there.
Skateboard design factors which effect vulnerability to speed wobbles are numerous and extremely complex. Obviously the trucks’ steering geometry and control system play a central role. Trucks which steer more slowly, with stiff suspension elements, are better at rolling with the punches and absorbing a wide variety of vibrations. Trucks with neutral or even "trailing" caster, as opposed to "leading" caster, may also help fend off wobbles. Without going into excruciating technical detail, caster relates to the position of the wheel axle relative to the steering axis. Trailing caster means that the deck’s center-of-gravity actually has to rise a bit in order for the deck to tilt. Thus gravity itself lends a hand in stopping wobbles before they get out control. Your deck’s torsional flex characteristics also play into the skateboard’s overall vulnerability to speed wobbles. There are at least two factors to consider here: torsional flexibility and torsional resilience (the "snap" with which your deck rebounds from a twisted state). Others may have more to say about this, but for pure balls-out speed the theoretical prescription would be a deck that offers good torsional stiffness and excellent torsional dampening (the ability to absorb deck-twisting forces without throwing the energy back in your face). The most skateboard engineers can do is minimize a specific deck-truck-wheel combination’s vulnerability to wobbles on a specific downhill course. It may be theoretically possible to "tune" a skateboard to wobble at certain speeds and not others, to increase its stability at higher velocities. However, the mathematical models needed to drive that sort of R&D would have to be extraordinarily sophisticated and would need to account for not just the skateboard’s numerous design parameters but also specific riding surface characteristics and the rider’s body mass distribution and racing technique. Good luck!"
VIEW ARTICLE HERE
or
VIEW MAGAZINE HERE pg. 18.
Any skateboard that can turn or flex will also be subject, under certain conditions, to speed wobbles. For extreme velocities, it is always recommended that you tighten your trucks, or use stiffer bushings or springs. If your trucks make tight turns easily, you should also consider slowing down their steering response with angled risers, or switching to trucks made specifically for high speeds.
Speed wobbles can occur whenever something starts a vibration that matches a resonant frequency of your skateboard. A resonant frequency is one at which your skateboard will vibrate very easily; a particular skateboard may have multiple resonant frequencies. The impetus may be a bump in the riding surface, a rough patch in the road, an unlucky movement by you, or some combination of these factors. Other potential contributing factors include the small torques resulting from wheel rotation and the tiny lateral oscillations that spinning wheels make if they’re not aligned with absolute perfection. If you start wobbling at speed, the standard advice is to crouch and try to grab the deck. Touching the deck presumably lets your upper body absorb some of the vibrations and probably also changes the resonant frequency of the skateboard, since you’re "connecting" your upper body to it.
In 1979 racing legend John Hutson assisted in a presentation made by the Stanford Mechanical Engineering Department, in which the stability of skateboards at various speeds was compared with that of aircraft. What emerged is that like aircraft, skateboards go through various "zones" of oscillatory stability and instability as they accelerate up to their highest speeds. One might even compare this to musical notes or octaves of relative vibrational resonance. For example, a particular setup might be stable up to 30mph, prone to wobbles at 40mph, but stable again at 50mph. However, this absolutely does not mean that if you start wobbling just go faster and things will smooth out! It may be the case that a given skateboard will be unstable at all speeds beyond a certain threshold. Be careful out there.
Skateboard design factors which effect vulnerability to speed wobbles are numerous and extremely complex. Obviously the trucks’ steering geometry and control system play a central role. Trucks which steer more slowly, with stiff suspension elements, are better at rolling with the punches and absorbing a wide variety of vibrations. Trucks with neutral or even "trailing" caster, as opposed to "leading" caster, may also help fend off wobbles. Without going into excruciating technical detail, caster relates to the position of the wheel axle relative to the steering axis. Trailing caster means that the deck’s center-of-gravity actually has to rise a bit in order for the deck to tilt. Thus gravity itself lends a hand in stopping wobbles before they get out control. Your deck’s torsional flex characteristics also play into the skateboard’s overall vulnerability to speed wobbles. There are at least two factors to consider here: torsional flexibility and torsional resilience (the "snap" with which your deck rebounds from a twisted state). Others may have more to say about this, but for pure balls-out speed the theoretical prescription would be a deck that offers good torsional stiffness and excellent torsional dampening (the ability to absorb deck-twisting forces without throwing the energy back in your face). The most skateboard engineers can do is minimize a specific deck-truck-wheel combination’s vulnerability to wobbles on a specific downhill course. It may be theoretically possible to "tune" a skateboard to wobble at certain speeds and not others, to increase its stability at higher velocities. However, the mathematical models needed to drive that sort of R&D would have to be extraordinarily sophisticated and would need to account for not just the skateboard’s numerous design parameters but also specific riding surface characteristics and the rider’s body mass distribution and racing technique. Good luck!"
VIEW ARTICLE HERE
or
VIEW MAGAZINE HERE pg. 18.
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