Mayf,
While your numbers seem enlightening, and STATICALLY accurate, there are
some grievous flaws. Your static weight distribution has nothing to do
with the peak side loads on the fulcrum pin outer loads. While you have
static weight distribution, you do not consider the lever, in the
applied load transfer, that multiplies through the unequal arm
suspension levers. With an offset teeter totter, you can lift the moon
with little force, provided you have a place for the fulcrum.
While that is an extreme example, it does illustrate the influence of
leverage.
Use the existing multiple lever design, and recalculate the maximum
fulcrum pin bending loads at the thread relief.
Steve
___
Steve Laifman
Editor - TigersUnited.com
drmayf wrote:
> Steve, I am sorry, but I do not believe that backing loads are the
> culprit. Here is why. The car has a weight distribution of about 50/50
> and it weighs around 2500 pounds. That puts 1250 pounds on the front
> suspension and about 625 pounds on the indicidual front tires. Even if
> the coefficient of friction was 1, which it isn't, then the maximum
> force that the tire can transfer to the spindle is 625 pounds:
> otherwise it just slides. If that force acts perpendicular to the A
> arm centerline and all the load is on the lower arm then the torque
> couple at the arm bushings is about 390 ft lbs. on each end of the
> fulcrum. Remember, that the upper arm also take a significant portion
> of the side loads and the real force is much lower. Taking the force
> couple and resolving it into forces, the force near mid point of the
> bushings on the A arm is about a 1000 pounds. If the car is being
> backed then the car is going slowly and that is about the max force
> that can be applied. However, if the car is making a turn around a
> corner which has a dip in it for drainage (like just about every
> corner on the planet) then the car is diving into that outer wheel and
> that adds more down force to the tire. More down force means more
> friction force that the tire can transfer to the A arm. I suspect that
> since we go forward more than we go backwards that this is far more
> likely to add to the failure. I am sure thee any number of other
> engineers on the list who can sit down and generate a free body
> diagram and do a far better analysis than my back of the envelope.
>
> Never the less, the real issue, is not whether they fail in backing or
> driving forward, it is that they fail unexpectedly and sometimes
> catastrophically.
>
> mayf, out in pahrump
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