Larry,
Build four "blocks"out of 2 by 4s. Make them 2 X 2 X 2. Put one under
each corner. They will never shift !!!!
At 05:03 PM 8/15/01 -0700, Larry Colen wrote:
>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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