Russell,

On Mon, 25 Sep 2006 16:56:42 +1200, Russell McMahon wrote:

> I recently discovered in a very niche situation (stupidly overloaded 
> vehicle with largish rear bias)(at 2am fwiw) that speed wobbles in a 
> car could be instantly corrected by applying a ~ 2 Hz oscillation to 
> the steering wheel, approx 30mm movement range on a reasonably low 
> geared steering system.
> 
> As I don't have vehicles / situations which have speed wobble 
> available every day I can't retry this in a hurry. I was wondering if 
> anyone else had heard of this "cure" or of how universally applicable 
> it was.
> 
> Results were "like magic" - at one moment a slowly building (slow in 
> this case) oscillation that required intelligent ongoing correction 
> and the next moment instant stability. No head shaking or slowly 
> coming right but an instant transition to stability.
> 
> The friends who I have asked about this all lecture me about 
> overloading, which is fine, but fail to comment on the "wobble cure".

I think you may have been experiencing "Pilot Induced Oscillation", somewhat familiar to flyers!  :-)  Once it started your attempted correction input 
was just at the right phase to add to the problem, and doing something to distract you from that (the deliberate steering-wheel wiggling) stopped 
that, and it sorted itself out.  Did you try letting go of the wheel?  That may have worked too!  (Or you may have ended up in a ditch...)

Overleading the rear is a very Bad Thing because it reduces steering authority.  I was once in a seriously overloaded van (I and two other mechanics 
at the garage where I was working had just loaded up a large stack of wet asbestos-cement sheets - it was a long time ago!) and it was possible to 
turn the steering-wheel half a turn each way with no change of course at all.  The only way to get round corners was to brake, so transferring some 
weight to the front wheels.  Looking back, I sometimes wonder how I made it out of my teens...  :-)

Cheers,


Howard Winter
St.Albans, England


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