On Wed, Aug 15, 2001 at 05:39:46PM -0400, Barrie Robinson wrote:
>
> Larry,
>
> I think you may find your mechanics slightly flawed. Blocking a rear wheel
> does not stop the car from collapsing sideways, forwards or
> backwards. Three point blocking (and by that I assume you mean jacking up
> and putting on stands) is more unstable than four especially when done at
> the furthest corners you can manage. You also need bloody great foot
> prints on the stands. Your triangle only works in the useless (in this
> case) horizontal planes (car bottom and ground)
It's been 20 years since I took any mechanical engineering courses, so
I'll respectfully request any M.E.'s on the list to referee.
For the sake of clarification, and to make sure that we are both
discussing the same thing, I'll refer to configuration:
Q as the car on four jackstands, two at the leafspring mounts, two at
the subframe behind the front wheels.
T as the car on its rear wheels and two jackstands under the front
subframe with the wheels chocked/e-brake on.
T' same as T, but without the rear wheels blocked.
I will conceed that they all have approximately the same stability
when subjected to a lateral force applied normal to the longitudinal
axis of the car (shove on the front fender towards the engine).
When subjected to a lateral force applied parallel to the longitudinal
axis of the car (pushing on the headlights toward the rear bumper), I
contend that the stability of Q and T' are approximately equal (my
guess is that they differ by a factor of 2) and that configuration T
is much more stable.
I will define a collapse as the condition when the point of contact
between the jackstand and the car is no longer over the base of the
jackstand. At this point the downward force of the weight of the car
will push the jackstand over, rather than attempting to right it.
(Is this assumption correct?)
In configuration Q collapse will occur when the force applied is
enough to lift the weight of the car the height of the center of the
base of the jackstand when it is tilted to the point of
instability. In configuration T', since only half of the weight of the
car is supported on the jackstand, this force should be approximately
half as in configuration Q.
(Question, does the height at which one pushes on the car matter or
does the force effectively act at the Cg of the car?)
Since there is nothing to prevent the jackstand from rotating in
relation to the car, no additional force is required to achieve
instability. Someone that knows what they're talking about would be
able to list degrees of freedom and explain how they relate to the
problem.
In configuraion T, in order to achieve the condition of instability,
one must overcome the force of the rear tire skidding across the
floor, and/or rolling over the chock, in addition to the force
required to lift half the weight of the car the height of the center
of the base of the jackstand at the point of collapse.
Therefore it is my contention that it is actually much safer to have
the car on two jackstands rather than on 4, if the wheels on the
ground are chocked, and twice as dangerous if they aren't. (Assuming
that both jackstands are at one end of the car. If both jackstands are
at one side of the car then the force required for instability is that
for lifting the weight of the car the height that is the delta between
the car resting on the jackstands, or the center of gravity over
either the tires or the jackstands, plus the force required to
overcome friction).
Larry (Am I still a pedant if safety is involved?) Colen
--
I can't go back and change time, but I can make up for lost time.
lrc@red4est.com http://www.red4est.com/lrc
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