wobble me dites-vous?

D'intérêt général...

"THE LOW-DOWN ON SPEED WOBBLES"
By Daniel Gesmer
printed in the Spring 2000 issue of International Longboarder Magazine

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 flexural 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!