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Shimmy - Around Steer Axis - What do you know?

Shimmy - Around Steer Axis - What do you know?

Shimmy - Around Steer Axis - What do you know?

Hi all,

I'm working on a project that on initial shake-down has shown symptoms of shimmy through the steering system. No data was gathered, and observation/driver accounts are not that great at the moment. We can't correlate it with any situational constants just yet (vehicle speed, a certain "bump" input etc). We hope to explore it in more detail within the next few weeks or so.

The vehicle has no compliant suspension, has a minimal, reasonable amounts of castor trail and castor angle, same for kingpin angle (zero scrub radius/kingpin offset - "centre-point steering").

I suppose I just wanted to gather people's general thoughts/knowledge/experience on the subject. I've got the Millikens' "Chassis Design: Principles and Analysis" book arriving tomorrow, which as I remember covers the subject in a large amount of detail.

I've been aware of this phenomenon in the past, mostly through university. I still remember one of my lecturers, very experienced with MSC.ADAMS work during his studies and work in industry saying that shimmy is one of the hardest phenomenons to recreate in simulations.

I've experienced it a few times in my VW beetle, and it occurred pretty much always on the same stretch of road, at the same speed, intiated by the same man-hole cover! This occurred in spite of my Beetle being fitted with a steering damper (no it isn't leaking!). I'm also aware that Land Rover Defenders, even the new ones, also have steering dampers, presumably to reduce shimmy which is a problem due to the presence of a solid front beam axle.

One of my initial thoughts then, is that shimmy may be reduced/removed if the wheel is able to laterally translate in relation to the body. Even though "independent suspension" is present on a beetle, the wheels cannot laterally translate, due to pure trailing arms and solid joints. Similarly, a Defender, or any solid front beamed-car can also not allow the wheels to translate inwards/outwards. Conversely, a car with a "wishbone" can allow lateral translation during vertical bump (wheel moves closer to body along "y-axis"/lateral axis of the car). Also, more strongly, wheels on more modern cars can laterally translate thanks to bush deformation, without any vertical bump movement needed.

My interpretation, based upon initial (internet) research, is that shimmy seems to be an un-pure form of gyroscopic precession (as opposed to gyroscopic nutation). The front wheel's rotation axis (hub spindle) cannot follow it's own complete circle when "shimmy" occurs, as classical "precession" theory seems to describe. Instead, the spindle axis almost seems to just oscillate back and forth about the kingpin's axis, hence the steering system movement. Perhaps this does make it nutation, not precession? Either way, this doesn't really get me anywhere, as I don't yet know what causes the oscillation, nor how it can be mitigated. If new Land Rover's still have dampers on, perhaps the problem can't actually be removed without adding in compliance in the way of a suspension system.

So, what do you know?


RE: Shimmy - Around Steer Axis - What do you know?

Hi Chris,

How well balanced are the wheel tire assemblies?
Residual imbalance, especially couple imbalance, is good at exciting resonances or taking advantage of component joints with clearance.

The classic Den Hartog's "Mechanical Vibrations" describes the phenomenon nicely for beam axles.
Probably still less than $20 bux for the Dover reprint.

"perhaps the problem can't actually be removed without adding in compliance.. "
Did you mean damping?


Dan T

RE: Shimmy - Around Steer Axis - What do you know?

Hi Dan, cheers for the reply. I'll check out that Den Hartog material, it's been mentioned to me by a few sources now.

I wonder if I could be a pain and refer to my duplicate thread regarding this issue, in the Automotive Suspension Engineering section of the forum - http://www.eng-tips.com/viewthread.cfm?qid=364677

I've just update it with some progress I've made on this issue.

" "perhaps the problem can't actually be removed without adding in compliance.. "
Did you mean damping? "

I meant spring compliance when I said that, with appropriate damping levels if need be. But to be honest, it was just a shot in the dark. My train of thoughts on this subject have progressed significantly since I last posted, this is reflected in the other thread.



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