There’ so much engineering HP here I figured I’d ask. In my misspent youth (slide rules and drafting tools) auto chassis stiffness was expressed in degrees deflection per ft/lbs of torque, determined by clamping one end down and twisting that sucker from the other end. These days it’s expressed in Hertz. I’ve never been able to find any way of correlating the two and am curious if anyone knows if that’s even possible?
Not an expert in this area but it sounds like (pure) stiffness would be independent of mass. Adding mass into the picture you would get a natural frequency of vibration like a spring+mass system. I’m guessing the absolute stiffness is less relevant than stiffness relative to weight when it comes to cornering or other dynamic behavior.
That’s where I get confused. What’s the point of reporting stiffness in Hertz(frequency) if it can be altered by adding/subtracting mass with no change in stiffness? I think the main requirement, mainly in racing where big $$$ are spent to pare every tenth of second from lap times, is the chassis must be stiff enough to never act as a spring in the suspension system, other wise you wind up chasing your tail in set-up.
Think of it this way, suppose you drive a Chevy Suburban around a curve, compared to a Porsche at the same speed. The suburban would feel much worse but that doesn’t mean the suspension is weaker. It might even be stiffer in absolute terms but softer in terms of stiffness per weight.
Suspension stiffness and chassis stiffness are two totally different things. I think more descriptive would be calling chassis stiffness torsional rigidity and suspension stiffness the resistance to movement of unspring weight, i.e. weight not supported by the suspension system.
The materials don’t know they are in a spring or a chassis. The chassis is acting like a spring when it flexes.
If it’s well designed it doesn’t flex, you can’t really choose the right spring rates, shock valving etc. if it does. Although current thinking in GP motorcycles is longitudinal stiffness with controlled lateral flex.
Everything flexes.
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Even when the forces applied are below the threshold of flexing?
There is no threshold of flexing. Everything is a spring, its just a matter of degree. Deflection is proportional to force and inversely proportional to stiffness. Maybe the deflection is imperceptible but there is always flex.
Imperceptible is the magic word in chassis design. It’s like saying something is perfectly flat but you can always disprove that, just depends on how far down the scale you need go to measure it, big difference between a dial indicator and an electron microscope.
I don’t know shit about this topic. But apparently Google does:
Hz stands for hertzs and ft *lbf stands for foot-pounds . The formula used in hertzs to foot-pounds conversion is 1 Hertz = 4.88713811368439E-34 Foot -Pound. In other words, 1 hertz is 2.0461873119565E+33 times smaller than a foot -pound.
Apparently it’s just a constant relation between both, couldn’t get any easier 
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I appreciate the input, my continued confusion not withstanding. 
One hertz is a unit of frequency of one cycle per second, I’m still a little foggy on how frequency has this constant relationship with force. Somewhere I’m missing how a cycle has a constant relationship with a force, I did find something saying frequency is related to the square root of stiffness but if mass affects frequency and not necessarily stiffness it would seem you’d have two different frequencies for the same level of stiffness.
I might be wrong, but I’m under the assumption most if not all but two of the SI units are interdependant. Meaning you can get from one to another with a bit of math.
So I guess it might even be possible to talk about stiffness in amperes or in volts.
At some point it doesn’t make much practical sense.
I’m not sure why it’s been said here that frequency is affected by mass. For instance, church bells pitch (basically its resonnating frequency) doesn’t change much with mass, but essentially changes by the bell’s physical dimension. So here, the fact that a low frequency bell is heavier than a high pitched one is more like a consequence of it being physically larger, it’s not the cause.
Seems like frequency is essentially affected by material stiffness, while the sound amplitude and durability in time (the echo if you will) will be affected by its mass (and also the force of the initial impact).
Here is a very ugly webpage that describes a bit the bell’s design and casting process, I think it’s quite interesting to read: https://www.moz.ac.at/sem/lehre/lib/pd-sounddesign/acoustics.html
Another, maybe more practical example: take a guitar. You can tune any chord to get the same pitch as the one next to it, what will change is the sound amplitude and deepness, but the frequency itself will be the same, even though one chord is smaller and lighter than the other one.
If we agree that everything deflects a nonzero amount (even if imperceptible) then I think it’s reasonable to say that if the mass is doubled, the load (force) will be double when going over a speed bump or something. So if stiffness is fixed, then twice the mass will cause it to deflect twice as much. If the mass were twice and stiffness were also twice, then the deflection would be the same.
So the frequency is not constant relative to stiffness. Frequency is constant relative to deflection, or in more general terms, frequency is constant relative to behavior. True, suspension stiffness is not the same as frame stiffness but I think it’s a reasonable analogy. A heavier car needs to be proportionately stiffer (in absolute terms) in order to maintain the same behavior.
Yes, so that’s more amplitude, not a different frequency.
This is part of where my understanding goes missing. I used to see comparisons of different chassis expressed in force/degree of twist, made it kind of easy to picture how large a gain/loss of stiffness has been made. Then they started reporting in hertz only. I’m missing why if frequency is essentially affected by material stiffness, why two chassis made of the same material can have completely different stiffness expressed in hertz.
But just say the word and I’ll apologize for bringing this up. 
Ok, well a bit more googling and it seems that the difference occurs essentially because while the first unit is appropriate to measure static loads, using Hertz allows to get a grasp of the dynamic loads, which are probably more useful in a car.
That never happened in my entire life until today, but I red an actually decent answer to a question in reddit:
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At this point I feel like I’m almost at the edge of a genuine AHA! moment. Nothing would please me more than sitting up at about 3AM and saying Aaaaah, so that’s it! 
The old way to measure stiffness is easy to understand, and measure by your shade-tree mechanic. It offers a metric by which you can see if something actually helps, or does not. For example, my Supra, lots of owners swear by strut braces. Steel braces that run between the top of the suspension towers. I wasn’t sure, so I measured my car with and without them. Then I promptly removed the strut braces and sold them to someone who was very happy with the upgrade. I found that there was no difference whatsoever in deflection, and it was 1.5 lbs of metal above the CG of the car. Of course I have no way of knowing if there was a difference in the frequency of the flex, as I do not have the equipment to measure it on the time axis, so it’s possible that those people measuring the improvement by the seat of the pants are right, but I didn’t find it convincing.
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