🚀 go-pugleaf

A good treatment, JH.

John Henry wrote:

> Kiki wrote:
>
> > In rec.autos.driving floyd a rogers <fbloogyudsr@accessone.com> wrote:
> > : The ratings are based PURELY upon static measurements:  they
> > : compare the location of the center of gravity and the width of the
> > : rear wheels/tires.  Nothing else, if I read the articles correctly.
> >
> > Which I have a problem with (not to mention that it seems they only picked
> > select cars and I know they didn't have a Pathfinder which means they
> > didn't even test all SUVs... how can they say GM's is the worst when they
> > left at least one SUV out of the picture? And how can they say Accord
> > was the best when they didnt' test all cars? Until they test all cars, I
> > refuse to believe an Accord is more stable than a POrsche or a
> > Lamborghini).
> >
> > Stability of a car does not just involve center of gravity and width of
> > tires. Yeah, that is very important. But also important is the suspension.
> > The problem the Troopers had when CR had a fit about them was that the
> > suspension was soft and springy and would collect momentum so that when
> > you did an S shaped turn the suspension was building momentum from the
> > first tilt to the second tilt and causing the car to have even more
> > momentum from tipping over).
>
> Actually all suspended vehicles exhibit this.  It is often referred to as
> "snap back" (does anyone remember the guy who rolled the beemer right in front
> of the press on flat level ground?).  What happens is the suspension gets
> maximally loaded on one side, say in a sustained right hand slide, if the
> vehicle is steered hard left then, and the speed at which the wheel is turned
> is synchronous with the natural unload rate of the suspension (an over
> simplification, but it illustrates the effect), you end up with the collective
> release energy of the suspension unloading, plus the natural weight shift as
> the car slides back the other way.  This effect basically swings the car
> through its maximum body roll range, kind of like winding up for a pitch.
>
> Suspension unload has basically two components, one is vertical unloading, the
> spring uncoiling, stretching out.  If this takes place on only one side in an
> S-turn transition, and the other side collapses at the same rate (as it
> usually does), the net result is only limit to limit body roll, and no
> actually change in the vehicle height  off the road surface.
>
> The other component is pure lateral load, the springs have little or no effect
> in this.  This is like pushing really hard against the side of the vehicle,
> actually moving it a bit to one side, then letting go.  The car comes back at
> you a bit.  Lateral suspension is due in part rubber bushing and mounting
> points that affix the suspension to the car body, but in greater part it is
> due to simple tire sidewall flex.
>
> So you see as we laterally load a car to the max (think of a steady state full
> side slide), then cut the wheel back (at just the right rate), we get the
> cumulative force of the lateral unload, the weight shift as the body goes back
> the other way, and a maximum body roll, limit to limit.  Like launching a
> slingshot.  Would you like some butter for your roll?
>
> The Govs ratings only attempt to statically determine the torque moment
> created on the leading tires in a steady state slide.  As for *if* a vehicle
> will actually roll, dynamic tire contact patch friction has to come into
> play.  A simple way to understand this is imagine a light house on wheels.  We
> could cut the wheels and spin it on wet ice without tipping it over (of course
> as it spins the rotational moment comes into play, another study altogether).
> Conversely, with sticky enough tires, you could roll a fat bodied 911 on flat
> dry pavement in a steady state lateral skid too  (but tires that sticky  don't
> exist).
>
> So SUVs, have some strikes against them with higher body roll (a better
> "windup"), longer suspension travel (alternating side to side, contributes to
> greater body roll), higher center of gravity (great torque moment placed on
> the outside wheels as we reach that roll limit) and usually a high dynamic
> friction (fat tires, dry pavement).
>
> And as David H says, ad even the slightest bit of "tripping" (curb, pavement
> edge, tire off rim) and all bets are off......
>
> >
> >
> > Tigress
> >
> > --
> > The root cause of problems is simple overpopulation.  People just aren't
> > worth very much any more, and they know it.  Makes 'em testy. ...Bev
> >     |\    _,,,---,,_          Tigress
> >    /,`.-'`'    -.  ;-;;,_     http://havoc.gtf.gatech.edu/tigress
> >   |,4-  ) )-,_..;\ (  `'-'    tigress@havoc.gtf.gatech.edu
> >  '---''(_/--'  `-'\_)         Cat by Felix Lee.
>
> --
> John Henry
> ---------------
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>
> "Stupidity really should be more painful."