Speed wobble

Source: Wikipedia, the free encyclopedia.

Wobble, shimmy, tank-slapper,

automobiles. The initial instability occurs mostly at high speed and is similar to that experienced by shopping cart wheels and aircraft landing gear.[2][3]

Theory

Sustained oscillation has two necessary components: an

spring–mass system
, where the mass can bob up and down (oscillate) when hanging from a spring.

If shimmy cannot be designed out of the system, a device known as a

notch filter designed to damp the shimmy at its known natural frequency.[4]

Shimmy is usually associated with the deformation of (rubber) tires. However, it can also be observed in nondeformable (e.g., steel) wheels. The phenomenon can be explained by introducing multicomponent dry friction forces,[5] apart from the usual forces considered in the literature.

Another explanation is that speed wobble is a Hopf bifurcation, whereby a system changes from one state (a stable ride) to a second state, (constant amplitude oscillation), when one parameter (forward speed, or air speed) progresses through a critical point. [6]: 1  [7]: 1 

In two-wheeled vehicles

Wobble or shimmy begins when some otherwise minor irregularity accelerates the wheel to one side. The irregularity may be a wheel which is

out-of-round, out-of-true, or out-of-balance.[6] As the wheel rotates, it will exert a cyclic load to the vehicle frame, which if matched with the system's (vehicle and attached accessories) resonant frequency, can produce a speed wobble.[6] During the wheel rotation, a restoring force is applied in phase with the progress of the irregularity, and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until system failure. The oscillation frequency can be changed by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider is a main component.[2] While wobble or shimmy can be easily remedied by adjusting speed, position, or grip on the handlebar, it can be fatal if left uncontrolled.[8]

Other things being equal, speed wobble is generally less likely to occur in a mountain bike compared to a road bike, because a mountain bike's frame generally has more damping from the suspension system, and the tire knobs also produce some dampening between the vehicle and road interface.[6]: 1 

Since shimmy frequency is independent of bike speed, gyroscopic effects "are clearly not essential to the phenomenon."[2] The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike center of mass, distance of bike center of mass from rear wheel, and cornering stiffness of the front tire.[3][9]

An academic paper that investigated wobble through physical experimentation and computer modeling concludes: "the influence on wobble mode of front tire characteristics, front frame inertia and chassis stiffness were shown. In particular, it shows that [by] increasing front tire inflation, chassis stiffness, and front frame inertia about steering axis and decreasing sideslip stiffness of front tire, wobble mode damping is improved, promoting vehicle stability."[10]

In single-wheeled mobility devices

Speed wobbles occur on Electric unicycles (EUC) and single-wheeled skateboards (such as a Onewheel). "To fix Onewheel wobbles, both feet need to be repositioned by inching them to a position that properly provides a center of gravity. The Onewheel’s controller is designed to only balance the board from front-to-back having no control of the side-to-side movement."[11]

See also

References

  1. ISBN 1-889540-53-6
    . tank slapper: a speed wobble so severe that the handlebars bang alternately against the sides of the fuel tank
  2. ^
    ISBN 0-262-73154-1
    .
  3. ^
    ISBN 978-1-4303-0861-4
    .
  4. CiteSeerX 10.1.1.227.9657. For machines with a stiff front frame, a steering damper is required to stabilize the wobble mode at high speeds, while older, more flexible machines may require a steering damper at intermediate speeds.{{cite magazine}}: CS1 maint: multiple names: authors list (link
    )
  5. ^ Zhuravlev, V.Ph.; Klimov, D.M. (2010). "Theory of the shimmy phenomenon". Mechanics of Solids. 4 (3): 324–330.
    S2CID 122904947
    .
  6. ^ a b c d Empfield, Dan (2015-04-24). "Speed Wobble". slowtwitch.com. Archived from the original on 2015-06-07. Retrieved 2019-08-23.
  7. ^ Zinn, Lennard (2013-11-19). "Technical FAQ: Bifurcation and high-speed shimmy". VeloNews. Archived from the original on 2019-08-24. Retrieved 2019-08-23.
  8. ^ Kettler, Bill (2004-09-15). "Crash kills cyclist". Mail Tribune. Retrieved 2016-07-10.
  9. S2CID 110507657
    .
  10. ^ Mauro, Salvador; Fabris, Davide (May 27–28, 2004). Study of stability of a two wheeled vehicle through experiments on the road and in laboratory (PDF). Modena, Italy. Retrieved 2008-08-31.
  11. ^ "Onewheel Wobbles & Feels Unstable [How-to Fix the Wobbling]". Trailwheel. 7 October 2022.

External links

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