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
Steve Laifman wrote:
>
> Randy, Mayf, Tigers
>
> On breaking loads, the l bending loads at the root between the bushing
> and the pin (where the crack will be) is much greater in a slow,
> reverse, full lock, parking maneuver. The bending loads are
> significantly higher than at speed because of the extreme angle the
> wheel are in. In some cases one tire is not rolling, but is being
> dragged sideways while not rotating. The load is greatest just before
> the tire starts to drag / skid.
>
> Check it out, as this is not typical in car suspension geometry, and
> only applies to this terrible design. The bent steering rack arms are
> the example of Shelby telling his engineer / mechanic " just make it
> fit at an angle", from which all trouble then flows.
>
> I have an acquaintance who taped George Boskoff's dinner speech, but
> he has yet to get it transcribed.
>
> Steve
>
> ___
> Steve Laifman
> Editor - TigersUnited.com
>
>
>
> Randy Smith wrote:
>
>> Maybe I missed this, but the obvious question is: Do Alpine Fulcrum
>> Pins break too? Or as often? Or is the stress a result of the bad
>> Ackerman angle in the Tiger, putting stress on the pins in a
>> full-lock turn?
>>
>> -Randy
|