Why is the roadster lighter than the coupe?

[Yal]

but I'm an engineer and I know what I'm talking about it.

Thanks, I needed a good giggle tonight. I've worked with engineers and scientists (mechanical, chemical, electrical) for the last 25 years. Bachelors, Masters, PhDs, every one of them was right 100% of the time, even when they were wrong.

[Dammmittt]

Originally Posted by Yal

Thanks, I needed a good giggle tonight. I've worked with engineers and scientists (mechanical, chemical, electrical) for the last 25 years. Bachelors, Masters, PhDs, every one of them was right 100% of the time, even when they were wrong.

That's impressive. But, I don't go around telling other people that I know more about their business than they do. For instance, I'm sure whatever you do for a living Yal, you know much more about it than I do. And if you ever gave your expert opinion on something, than I wouldn't argue with you pointlessly.

I'm just a little sick of it. Or maybe I just can't stand to lose an argument, especially when I know I'm right.

[MMMazing07]

this thread is hilarious.

its just dammit ranting on about how the coupe is better and faster around the "track" than the roadster. be careful, hes getting technical

[Yal]

I'm a manager now, so therefore, I know nothing. Actually, I know that I need to listen to my experts. I'm really sorry Dammmittt, i just couldn't resist the shot, but I know that I should have. Even my realtor has a masters in something to the effect of high temperature effects on materials from Stanford.

I used to have Dammmittt on my ignore list because of some of his rants, then i realized that i wanted to hear what he had to say.

[Dammmittt]

Originally Posted by MMMazing07

this thread is hilarious.

its just dammit ranting on about how the coupe is better and faster around the "track" than the roadster. be careful, hes getting technical

[rc'Z]

Dammmitt, In the world of automotive stiffness, how stiff is stiff to the point that it no longer matters, like when auto stiffness approaches that of a bolder, how much more stiffness matters. Corvette gets it's stiffness with out weight by using high tech materials. The coupe could have used high tech instead of less expensive materials. How much difference does twice the stiffness make in a car that already has minimal flex, no cowl shake, no rattles. Where does it stop other than in an argument???? (Serious question, not arguing with you.)

[Dammmittt]

I think the argument stops when the other side concedes that there is more to doubling the stiffness than simply putting a roof on top.

[xg3]

Originally Posted by Dammmittt

I think the argument stops when the other side concedes that there is more to doubling the stiffness than simply putting a roof on top.

does it matter?? really? does it??

[enfield]

I have a Coupe and may I say it so bloody rigid that it is uncomfortable for anything over 400km in a day. I cannot wait for the lease to end. Don't get me wrong. The Coupe is a bloody good sports car but it sucks as a daily driver. I have my Infiniti FX45 for proper long drives....

[CalRick]

Originally Posted by Dammmittt

I think the argument stops when the other side concedes that there is more to doubling the stiffness than simply putting a roof on top.

I dunno - evidence to the contrary is pretty sketchy. Having to depend upon phantom structural elements (fudge factors?) that don't make much engineering or economic sense isn't a particulary good way of validating one's assumptions. Most engineering analyses at least attempt to represent reality rather than simply proving that one is "right".

[CalRick]

"As far as the roof goes, it's glued bolted into place, so it's not adding as much stiffness as the parts that are welded/riveted to the frame. "


Bit of a leap of faith there, particularly since BMW is using bonded subframes on some of their upper models (that cost a few GDP to repair.) Not that I am a big fan of the fatigue resistance of bonded joints, but to dismiss them as insignificant, particularly in combination with other fasteners, misses a good deal of structure - the car is a monocoque/unibody after all, not a space frame.

Hmm, I wonder if the contribution of a nice shear web located at its maximum contribution to the moment of inertia (like a roof panel) would really be negligible. For that matter, that slick double-bubble roof profile may be more than just style and added headroom.

[KevinC]

Originally Posted by rc'Z

Dammmitt, In the world of automotive stiffness, how stiff is stiff to the point that it no longer matters, like when ....

[RichardTS]

Originally Posted by CalRick

"As far as the roof goes, it's glued bolted into place, so it's not adding as much stiffness as the parts that are welded/riveted to the frame. "


Bit of a leap of faith there, particularly since BMW is using bonded subframes on some of their upper models (that cost a few GDP to repair.) Not that I am a big fan of the fatigue resistance of bonded joints, but to dismiss them as insignificant, particularly in combination with other fasteners, misses a good deal of structure - the car is a monocoque/unibody after all, not a space frame.

Hmm, I wonder if the contribution of a nice shear web located at its maximum contribution to the moment of inertia (like a roof panel) would really be negligible. For that matter, that slick double-bubble roof profile may be more than just style and added headroom.

Ferrari uses a lot of bonded chassis parts and, if I am not mistaken, Ferraris have pretty good stiffness.

I am firmly convinced that you have a very valid point about adding a shear web. Perhaps Dammitt can tell us why and how a simple four bar square structure with no plate has the same stiffness as a plate with a square bar assembly attached to it. Oh, I forgot, the internals are not important to stiffness because all the internal forces are in static equilibrium and therefore not important.

[RichardTS]

Originally Posted by Dammmittt

Where do I begin. Ok, first of all let me explain to you what you're looking at. You're not looking at a stress color plot, you're looking at a displacement color plot. So remember that F = k*x and similarly M = k*phi, and since I'm torquing the car with the same forces and boundary conditions in both cases the ratio of the displacements is going to be equal to the ratio of the stiffness in the two cases. Got it?

Next, realize that the Finite Element Method takes care of all the internal forces stresses and strains by way of the interpolation function (element type) and by using any of a variety of methods, either the direct method (force balancing) or by using the method of minimum potential energy or galerkin's method, etc. For the end user it doesn't really matter, as long as your boundary conditions and your external loading is right, it'll all work out assuming you have no issues with the mesh. This why you usually mesh the thing multiple times until your solution converges. So, unless you're trying to build an FEA model by hand or you're looking at it analytically, like I said...IT DOESN'T MATTER. The software, provided you're using it correctly, does it for you.

As far as the amount of material added and extra stiffness goes, like I said it probably has more to do with a change in bending moment of inertia. Just from looking at the pictures of the skeleton you can see that the frame has box beams in the coupe and a tube or cylindrical frame in the roadie. That can make a big difference in moment of inertia without changing the mass of the frame significantly.

Why do you think most buildings are made with I-beams? They have a high moment of inertia and they save weight.

Any other questions? Or should I give up?

Oh and sorry for the sarcasm...I tend to get that way when people argue with me and they don't really know what they're talking about. And if that offends you fine...but I'm an engineer and I know what I'm talking about it. And I get a little tired of defending my opinions from people who have a limited understand of mechanics.

Where do I begin, Dammitt?

“Ok, first of all let me explain to you what you're looking at. You're not looking at a stress color plot, you're looking at a displacement color plot. So remember that F = k*x and similarly M = k*phi, and since I'm torquing the car with the same forces and boundary conditions in both cases the ratio of the displacements is going to be equal to the ratio of the stiffness in the two cases. Got it?”

This is rather obvious from the plots. However, you might note that I referred to high stress AREAS and NOT high stress. It is also obvious that the plot is showing displacement – why else would the scale be in meters? If we were talking stress we would be in kg/m2 or psi or ksi, or whatever your favorite flavor is. And don’t forget that Hook’s law for elastic materials states that strain is directly proportional to stress. So, since we are only concerned with proportions and not exact quantities it does not really matter if we call it displacement or stress anyway.

How do I know you are torquing the car with the same forces and boundary conditions? You did not include the load and restraint information that is normally included with an FEA. All I can see from your plots is that the force configuration shown on one plot is different from the other plot. Besides that I am not too sure if I agree with you configuration – the only external forces come solely from the interaction of the wheels with the road.

Are you so smug as to believe that I haven’t ever seen one of these before?

“So, unless you're trying to build an FEA model by hand or you're looking at it analytically, like I said...IT DOESN'T MATTER. The software, provided you're using it correctly, does it for you.”

Excuse me? I thought the purpose of this entire exercise was ANALYSIS to determine whether the stiffness difference comes from the roof or not. And it certainly does matter. It may come as a surprise to you but, in many cases, engineering and software do not provide the exact answer and judgment is called for. Your model is a great example of this – you look at the model and where the displacement is at its peak and conclude that the coupe stiffness is only 1.38 times that of the roadster. I look at the model and, in less than 10 seconds, find a place where the same proportion that you are using is 2.7. I note that your response failed to address this discrepancy and again point out that the sill area is inside the roof area and the peak displacement/stress AREA is not. If you were marketing this to people and you had an FEA that looked like this which one would YOU use? Note that BMW, and wrongly so, did not qualify the stiffness claim with an indication of WHERE. Nor did you I might add.

“As far as the amount of material added and extra stiffness goes, like I said it probably has more to do with a change in bending moment of inertia. Just from looking at the pictures of the skeleton you can see that the frame has box beams in the coupe and a tube or cylindrical frame in the roadie. That can make a big difference in moment of inertia without changing the mass of the frame significantly.”

Why don’t you show us the bending moment of inertia for the roadster, the coupe with the skeleton top and the coupe with the roof panel in place so that all of us on the forum can discuss this in quantifiable, meaningful terms? You obviously have the software tools available. And precisely where are the differences in your models? The two FEA images are identical.

“Why do you think most buildings are made with I-beams? They have a high moment of inertia and they save weight”.

The only thing I can even say to this is, well, DUH! Perhaps you can enlighten the group about WHY the moment of inertia is high in an I-beam and maybe even why you stand them vertically and not horizontally (psst, I already know but I’m not sayin’).

“Oh and sorry for the sarcasm...I tend to get that way when people argue with me and they don't really know what they're talking about. And if that offends you fine...but I'm an engineer and I know what I'm talking about it. And I get a little tired of defending my opinions from people who have a limited understand of mechanics”.

Oh, don’t you worry about offending me. I find your attitude condescending and it appears that you have let ego stand totally in the way of clear thought. Oh yes, every bit of your engineering is correct but, conceptually, you don’t have a clue. You might try being a little humble and willing to listen to other points of view. You say you are an engineer but all I see are engineering calculations and computer plots designed solely to prove your opinion (as you yourself said, you are tired of defending your OPINIONS). You even go out of the way to try to baffle people with B.S. to obfuscate their point when it does not agree with yours. If your professors did not instill in you the vast ethical importance of removing your opinions from your calculations, then they failed you (or you failed them if they did).

I may be a little rusty on mechanics but that is only because I don’t do it every day anymore (I am more of a management type now). But I can clearly see that everything you presented is presented solely to bolster your own point of view. And, if you really are an engineer, this is a disservice to the profession.

You really need to get over yourself. You are not the only technical person on this forum and the evidence of this is very clear in other posts, some of which have been made by people who very obviously have some knowledge of engineering but do not go blowing their horn and acting as if they are above everyone else.

And yes, I have been calculating it to 5 decimal places, measuring it with a yardstick, marking it with a piece of chalk and cutting it with an axe for about 33 years now. And, if you don’t know what this means, then you really have no clue as to what engineering is.

And, by the way, you should give up…

[CalRick]

Originally Posted by RichardTS

Where do I begin, Dammitt?



“As far as the amount of material added and extra stiffness goes, like I said it probably has more to do with a change in bending moment of inertia. Just from looking at the pictures of the skeleton you can see that the frame has box beams in the coupe and a tube or cylindrical frame in the roadie. That can make a big difference in moment of inertia without changing the mass of the frame significantly.”

He also seems to assign undue significance to the different representations of the roadie and coupe skeletons on the realoem page. It's rather clear that the roadie pic was created by CorelDraw or other such desktop publishing app by the tech pubs guys while the coupe pic is derived from their CAD/CAE app - Solidworks/Catia/UG or whatever they use in engineering.

Basically what we have is a premise supported by an assumption (with a pretty illustration) with no attempt to validate the assumption, or even assign any boundaries to it. So, what we know now is what happens if a notional roadie chassis is stiffened with a couple of notional tinker-toys between the windshield frame and rear shock towers. Like trying to define a trend with one data point.