Chassis Stiffening for Alfa Spiders ???

[RonHorowitz <RonHorowitz@xxxxxxxxxxxxxx>]

From: "Paul  Misencik" <paul@domain.elided>

>This is probably taking "hack engineering" to a new low,

Hey, if it works, it works, but I've got to admit this isn't quite what I
had in mind!

>Raise one corner of the car - even drive one wheel up onto a curb or
>something - then open one of the doors and measure the relationship of the

If everything is exactly the same every time, this _could_ work, but
precision, accuracy, and repeatability may be problematic.

>The narrower the angles, the stiffer the chassis, right?

Yes, I'd say probably the best way to measure the chassis stiffness is to
think of it as a torsional spring.

Kind of what I had in mind would be to remove the suspension, rigidly
anchor three corners, fit an accelerometer to the free corner, hit the
chassis with a train of impulses, and do a windowed "FFT" or fast Fourier
transform of the impulse response. I'm not a mathemetician, but
essentially, since time and frequency are conjugate variables, what this
does is to convolve an impulse response from the time domain into its
correlating frequency response in the frequency domain. Off the top of my
head, I'd say the spring rate is proportional to the inverse of the square
root of the product of the equivalent mass and compliance, where compliance
is the inverse of the stiffness. Since the mass is relatively constant, and
since any added mass from additional bracing would tend to decrease the
resonant frequency anyway, increased stiffness will exhibit itself as an
increase in resonant frequency.

Sounds so simple in theory, doesn't it? Unfortunately, actually doing the
work instead of just thinking about it is the bitch! My guess is that this
is a month or two worth of work and maybe two or three hundred dollars
worth of steel and miscellaneous bits and pieces before getting the work
done. Now let's see.... (40 hours a week) x ( 8 weeks of work) x  (uh,
what's minimum wage?) /  ((total sales)-( fixed +variable cost) and uh,....
Hmmm....  I should just take a job at Burger Hut instead....

Ah, well.....

RON
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