F1freak

Actually you don't have to be an engineer. Any info would be appreciated. Does anyone know how much of a difference a targa top vs. a fixed roof hardtop (ZO6) makes in the torsional rigidity of the respective vehicles. Is it marginal/negligible or is it substantial in respect to the performance of the vehicles? If it is substantial does anyone how much % wise. Ballpark figures? Anything? :thumbs:




[Modified by F1freak, 12:22 PM 2/18/2004]

Scissors

The difference is less than 2 Hz of torsional rigidity. The Z06 is 24 Hz and the C5 Coupe with the top on and latched is 22.x Hz. ("X" because I don't remember the exact number.)

F1freak

Thanks Scissors. So I'll assume that's a marginal amount. Anyone know what it is with the top off?

drew priest

I've read that the rigidity of the FRC versus the coupe is 10%; which is by definition a 'significant' amount. That is, whenever a number is changed by 10%, then it is considered a significant change, i.e. 350hp...385hp is a change by 10%.

Hopefully Duke can double check me on this but I think the equation you want to use here is omega natural W_n=sqrt(k/m), where k is the stiffness, and m is mass. solving for k yields k=(W_n)^2*m, therefore an increase of 2 hz should mean that k has increased by nearly 16%, well clearly when one has a value that is being squared the numbers past the decimal become important. So assuming that 24hz is correct I calculated a value of 22.77Hz for the coupe's value.
wait, did I over think things again?
Drew

Jim 47

With the top out the Coupe's rigidity is 20.5 Hz and the Vert's rigidity is 20.0 Hz. So the FRC and Z06, at 24 Hz, are 20% more rigid than the Vert. The C4 Coupe was only 13 Hz

Jim

F1freak

Thanks for all the help everyone. Great info here. Seems that the difference is substantial. :eek:

SWCDuke

Looks okay to me. I've driven both a Z06 and Z-51 Coupe over a specific wavy bumpy road that I use to get a SOTP feel for a car's rigidity.

You can feel that the Z06 is stiffer. I would say the improvment is "significant", but the Coupe is near the top of the heap in both torsional and bending stiffness, especially for an "open" car. There's a slight difference top in - top out, about 0.5 Hz as I recall.

Duke

Scissors

Especially when you consider that most open top cars are in the 8 Hz to 16 Hz range.

Of course, no low-slung car with a ladder frame can compete with unibody sedans and SUVs, which have torsional rigidity numbers of closer to 36-44 Hz.

SWCDuke

I don't think any body on frame SUV's are particularly stiff because with the body mounted on cushions it doesn't contribute that much to stiffness. Unibody vehicles are usually much stiffer than body on frame types.

The current generation of GM Pickups and SUVs have hydroformed frame rails and are much stiffer than earlier generations.

I'm not aware of any cars or trucks than claim greater than about 30 Hz first mode bending or torsion frequencies.

What's out there that is over 30 Hz?

Duke

F1freak

If the C5-ZO6 measured in at 24hz of torsional stiffness I think the C6-ZO6 has a legitimate shot at breaking 30hz. Can't wait to see.


[Modified by F1freak, 6:17 PM 2/18/2004]

Scissors

VW Touareg is over 40 Hz torsional:
http://www.google.com/search?hl=en&l...al+%2Bvw+%2BHz


Audi TT is 50 Hz bending:
http://www.google.com/search?hl=en&l...di+%2Btt+%2BHz

WhiteDiamond

GM's initial C5 measurements are some of the highest ever done at the time of the C5 introduction. The only other cars I saw discussed as being as stiff where a Mercedes and the Oldsmobile Aurora. I am not sure what these measurements posted for the VW actually represent, but I don't believe the numbers are directly comparable to the numbers GM is releasing. Does anyone have actual information on how the various companies measure the various aspects of body structure?

Todd

drew priest

Im just speculating but it may be that these companies are using rev/sec as opposed to rad/sec both would be in units of hertz but rev/s is larger by a factor of 2*pi or about 6.28. If that us what they are doing and they are actually only 50/2pi= 8hz then that woud seem very dishonest and disreputable. Maybe they really are that stiff, they are after all large squarish boxes which may help alot with the stiffness as compared to a thin box like the vette. Im not sure... :confused:
Drew

Scissors

Yes, some of the highest for a Convertible and for a low-slung car.

Torsional numbers from any manufacturer are directly comparable with torsional numbers from any other manufacturer. Quite simply, it is a measure of stiffness of a frame against a twisting force at both ends. (Image a huge hand at each end twisting in opposite directions.)

Bending stiffness is also directly comparable between manufacturers. It is the measure of stiffness of a frame against a bending force at both ends. (Imagine a giant hand pressing down in the middle of the car.)


All of the companies use similar methods and the same measure: Hz. The natural frequency of a frame for first order torsional and bending is the measurement they all use. Just like how a Kilogram for GM is exactly the same as a Kilogram at VW.

WhiteDiamond

Unless you know for a fact, these statements are not true. I have personally seen many examples where the same "catagory" of information is presented, but measured or calculated in much different terms. Also, if the frequency of resonance is similar to other frequency equations, then a step from 20Hz to 40Hz is an incredible move. Given the sheer size of a SUV frame, I don't think VW is reporting the 40Hz in the same respect as GM.

Todd

WhiteDiamond

Here is something interesting I found. I have several articles here on the release of the C5 saying its torsional stiffness is 4 times better than the C4's chassis. I also have the C5 coupe listed at 23Hz, the number they compared to the C4. NOW, here is what I don't understand and hope Duke or somebody can help. The C4s torsional stiffness is listed at 16Hz in one of the articles I remember reading(I think Automobile) during the C5 press haydays. If this is true, then a jump from 16Hz to 23 Hz quadrupiled the stiffness of the frame.

Todd

Timmy's C5

Maybe this scale is exponential like the db scale for sound. There an increase of just 3db (say from 93 to 96 db) represents not just a small increase, but an actual doubling of the sound energy.


[Modified by Timmy's C5, 9:30 AM 2/20/2004]

SWCDuke

Yes, those first mode frequencies jibe with what I've read, and since the natural frequency is a function of the square root of stiffness, to double the natural frequency you have to quadruple stiffness! (BTW, it's not logarithmic like "gain".) The advantage of high frequencies is that it decouples the structure from the suspension hop and tramp modes and many road input frequencies, so there is no resonance. You end up with a car that is quiet, rattle free, and doesn't "ring" after you run over a pot hole.

The industry's ability to substantially increase chassis stiffness and mode frequencies without huge increases in weigh is due to the sophisticated structural analysis tools they now have available - finite element modeling and analysis, and cheap, fast, computers that allow very fine grained finite element models - the more elements, the more accurate the model. Development and proliferation of these sophisticated design tools in the mid eigthies and nineties has resulted in substantially better vehicle structural design - a quantum leap from the seventies and early eighties.


Of course, they usually test prototypes to compare with the predicted numbers from modeling, and the actuals are usually within a few percent of the predictions. Corvette's hydroformed frame rails and "backbone" center tunnel structure has a lot to do with the Corvette's ability to achieve high natural frequencies in an open car without excess weight. It was a real innovation and is still a unique structural architecture in the industry.


Duke

F1freak

SWCDuke what exactly do you mean when you say "open car"? :confused: Do you mean open top car?


[Modified by F1freak, 2:08 PM 2/20/2004]


[Modified by F1freak, 6:57 PM 2/20/2004]

Scissors

Well sorry, but they are. Your belief and reality are two different things.

But these vehicles are not calculated in "much different terms" (whatever that means, as it doesn't really make any sense.) Their stiffness is measured in the same ways and represented by their natural frequencies. One Hz of torsional at one company is the same as one Hz of torsional at another, no matter what you want to believe.

Well they are. Anyone with even a High School level of architectual knowledge knows that if you have a ladder that is supported at both ends, you can make it much stiffer (both against bending and against torsion) by building and attaching a frame above and to that ladder. This is literally the same kind of difference between the C5 (a ladder frame) and some SUVs (a unibody or a larger, non-ladder frame) such as the VW.