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Yes, absolutely!

And that's the reason that major manufacturers are obsessed with torsional stiffness, these days: it's nothing to do with handling (they're relatively softly sprung, and the likes of the Caterham and Mazda's Mk. 1 Miata prove that you don't need that much stiffness to handle well), but you need incredible levels of stiffness to prevent the squeaks and rattles that modern customers find so unacceptable.

I'm assuming that we're not too worried about such things for the cars we're building on this forum, though?

If we are, then we definitely need to abandon spaceframes, as the level of stiffness they are capable of delivering on open cars simply isn't anywhere close to what the mainstream manufacturers would expect these days.

Noah Writes:


We've covered this several times and mention it as a guideline when it comes up. You can't help but notice this if you are actually doing any FEA work on a frame, you're trying to get the frame deflection down to 1/16" for an inch of coilover movement.

Sam writes:


Your confusing relative and absolute numbers here. If a car weighs several times what a Locost weighs, it needs several times the stiffness also. There is no magic here. Spaceframes have no problem delivering adequate stiffness. If a Locost or Seven does not provide the stiffness you expect or desire, it just means it wasn't the same priority of it's designer that you expect. or desire.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

No, I'm not: even when you take into account the weight difference, most spaceframes (particularly on open, 'Seven'-type cars) just couldn't cut the mustard compared to the standards modern manufacturers demand for good NVH.

A good modern saloon might be 20-25,000 lb.ft/deg, which makes it (roughly) 10 times as stiff as a typical 'Seven' spaceframe, whilst it's kerb weight might typically be 3 times as much.

Or to look at it from the opposite direction, you'd need a Seven spaceframe to deliver 8,000 lb.ft/degree to match the stiffness:weight of a good modern saloon.

I guess the post which contains this quote is what sets me off. The only reason you show up in our forums is to market your carbon fiber project. Your post states that we clearly are not concerned with rattles and then further seems to imply that because the frames you mention don't meet a stiffness standard for rattles, that such a thing is not possible. It's easily possible, if you want a 8000 ft./lb. per degree frame, go ahead and design one. The tools to help with the math for this are free these days.

The stiffness of our frames decreases very rapidly with weight. It takes very few ponds of additional metal to increase the diameter of tubing or to provide some additional bracing. For many of the common space frames really high stiffness simply wasn't their design goal.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Perhaps you need to read the full post, slowly and carefullly, before going off on one?

If you did so, you'd realise that the whole point of the post was to say that you don't need very high levels of stiffness to make a car handle: I was agreeing with Noah's statement that the reason that major manufacturers are obsessed with stiffness is nothing to do with handling, it's to minimiise squeaks and rattles and thereby increase perceived build quality... which is irrelevant on our kind of cars.

My post that preceded it was pointing out that the current fashion for virtually all the roll stiffness being at one end makes the torsional stiffness of the chassis even less relevant.

As to 'marketing my carbon fibre project', it's a long way from the public domain and even further from requiring marketing for production. The commercial project I'm at the prototype stage with at the moment is a spaceframe; a Seven-type one, moreover, with no torsion structures down the sides (although it incorporates a few other tricks and novelties). We're in a topic discussing monocoques, if you hadn't noticed - maybe you need to read the thread title as well as individual posts. Yes, monocoques interest me, and I don't think it's inappropriate to be discussing them in a thread that someone else started with, guess what, the purpose of discussing monocoques. Live with it.

I doubt you'll offer an apology, but at least wind yer neck in a bit, eh?

I have to take issue with this comparison, because it's misleading. A good modern sedan is generally under 20,000 lb.ft/deg in torsional stiffness. Numbers for this in modern cars are generally given in Nm rather than lb.ft. 25,000 Nm is about 18,500 lb.ft. 20,000 Nm is just under 15,00 lb.ft. There are certainly modern sedans with over 20,000 lb.ft/degree of torsional rigidity, but an awful lot of very high-end coupes and sedans by German, Asian, British, and American makers are a good deal lower.

Also, A Seven space frame should not be compared to vehicle with a roof. That's apples to oranges. Look and any of those modern sedans, and then look at the convertible versions; the verts are generally only 1/2 to 2/3 as stiff.

If you look at modern convertibles, and compare lb.ft to lb.ft, the Seven actually comes closer than you might think in terms of stiffness:weight.

Also, I think modern vehicle manufacturers are very much beyond where stiffening the chassis, as such, would make much difference in the extraneous noises. Localized stiffening, isolation, damper control and tuning, have a much larger effect on that once you get to a certain point. The seat squeaks because of the seat, not because of the chassis.

Looking at the numbers from lots of manufacturers, I just don't see them chasing stiffness that much. Maybe in a few cars, like the Audi R8.

My contribution, anyway.

-Graveyard

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Aedifico ergo sum.

Yes, many saloons are lower; but manufacturers are continually aspiring to higher figures, so I deliberately chose figures toward the current upper range and qualified it by saying that they related to a good modern saloon. I disagree that they don't chase stiffness much: look at the figures for (say) a Mercedes saloon or even a Saab convertible (to use an example that ought to be worst case) and you'll see numbers that certainly aren't achieved casually. Even the Saab convertible - which is half as stiff in convertible form as the sedan variant - is about 4 times as stiff as a typical 'Seven' spaceframe.

And yes, mainstream sports cars are lower (which again, is why I used the word 'saloon'). The first generation Mazda Miata and the BMW Z3 were particularly dismal, being only about double what you'd expect from a 'Seven' spaceframe. The Miata weighed slightly less than double what a typical Seven weighs, so the stiffness:weight was almost on parity.

...but the Mk.1 Miata and BMW Z3 are about the worst mainstream production cars of modern times, and both were notorious for shuddering, squeaking and shaking like a [PooPooing] spaniel with constipation. Their manufacturers made efforts to improve them substantially for later generations.

I've owned a Mk.1 MX5 (Miata), and it's the only modern car I've driven where I felt the need to fit aftermarket bracing in an attempt to reduce "scuttle shake".

Sevens get away with it mainly because they don't have doors or complicated folding hood assemblies, or complicated separate dashboard assemblies that generate relative movement when the main structure flexes. It is these assemblies that set up the noticeable rattles and squeaks in a Miata.

...Doesn't stop it from handling perfectly well, though.

Yes, I get what you're saying about the Miata, and I agree about that car. The C4 Corvettes have the same problems with the targa removed.

However, there are other cars that defy this rule. A Porsche 911 Cabriolet (991, from 2012) has a torsional stiffness of under 12,000 Nm, and weighed 3500 pounds. These cars are known for being pretty 'tight' for convertibles. The 996 verts are less stiff than the 991s, and they exude quality. Modern Porsche cars also stay tight with lots of miles.

The Seven replicas also have the advantage, when it comes to the effects of torsional flexing, of a smaller cross section than just about anything else. The shortened distance means that twisting the same number of degrees is less overall movement. This would make for much less noticeable, noise-making movement than you would get for the same number of degrees of twist even in a Miata.

Also, The torsional numbers for a common Seven space frame are not tested in the same way as they are for production cars, in my experience, and I think this difference tends to show higher numbers for those cars. For example, a car can have a much higher torsional stiffness with seats and gas tank installed. (A BMW E46 coupe or sedan is dramatically less stiff with the fold-down seat option. A LOT less stiff.) Even the weight of the driver in the car makes a difference, a very big difference in a very light car. These variables would make a proportionally larger difference in a lighter, less-rigid car.

Also, we have to look at sprung vs. unsprung weight, as not all weight is the same in the stiffness/weight equation. Shouldn't we be looking at sprung weight/stiffness? This being considered would tend to make the Seven look even better in comparison. Sevens usually have a pretty high percentage of unsprung weight, especially with a solid axle.

The low center of gravity also comes into play here, as roll resistance is reduced and therefore roll resistance differential between front and rear is reduced. Less torsional stiffness is asked of the chassis.

The Seven would also have less of a tendency to 'loosen' over time, as it does not depend on spot welds but full welds, the tightness of the door latching is not in play, and the mounting of the glass contributes much less.

In short, once all of the variables are considered, the Seven would drive a lot stiffer than the numbers (as usually figured) would suggest. Stiffness/spring rate would probably be more accurate than stiffness/weight, especially if cars were tested for torsional stiffness in full street/race trim. 2500 lb.ft/degree sounds pretty flexible, but in this case I think it's deceptive, and if one is worried about it the stiffness can be doubled with about 20-40 lbs. of extra bracing.

Interesting discussion, here. I hope it continues.

-Graveyard

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Aedifico ergo sum.

One reason for that is that there isn't the straightforward and linear relationship between mass and required torsional stiffness that Horizonjob suggests. Amongst the other factors involved are wheel rate, damping stiffness and - pertinent to the 911, because it's a bit of a freak in this respect - weight distribution. All the 911's weight - and therefore inertia when it hits a bump - is at the back. As anyone who has ever driven one will tell you, this means it tends to pitch (with a slight odd 'bobbing' feel to the front end that's characteristic of the 911) more than twist, proportionate to most cars. There are further complications with respect to sprung:unsprung mass, and these tend to work against lighter cars, if anything (relatively speaking, you need stiffer damping to control the unsprung mass, so proportionately more shock is transmitted to the sprung mass).

All we're ever talking about here is rules of thumb.

12,000 Nm is still pretty damned stiff, incidentally: that's 8,600 lb.ft/degree, so upwards of 4 times as stiff as most 'Sevens'.


Explain? Torsion figures these days are usually taken from hub to hub, with rigid links installed in place of the suspension. The track of a 'Seven' isn't dramatically narrower than any other car (at least here in the UK... I appreciate that you guys tend to supersize things).


Lest we drift too far off-topic, bearing in mind the title of this thread:

Nobody is disputing that Sevens are stiff enough to handle adequately. And tantrum-based misunderstandings aside, nobody is suggesting that NVH is an even remotely important consideration for this sort of car.

The point is, however, that a monocoque chassis is capable of not only giving you twice the stiffness of a spaceframe (if you want it...), but properly designed and in the right materials will pretty much halve the structure weight at the same time.

Is there anyone out there who thinks that weight reduction isn't advantageous to performance?

The amount of actual movement in the parts moving against each other, e.g. the cowl moving one way, the floor the other is less when the cowl and floor are nearer to one another. Sevens are very narrow and very short, compared to just about any car except maybe classic two-seat MG-like cars. This is not relevant for performance, but it would make a difference for the amount of cowl shake and other undesirable rattles and such.

I do agree that a monocoque structure can be made 2x as stiff as a normal 'Seven', but so could the space frame. The monocoque would be lighter in that case, certainly, if it was constructed of aluminum or composite. However, the actual 'frame' of the chassis is what, 10% of the overall weight? How many actual pounds would one actually expect to lose by going to a monocoque?

Virtually all of the monocoques I've seen actually executed in road cars are not entirely monocoque, but have a central tube in the monocoque design with space frame type assemblies that bolt on to the front and rear. That does address very nicely the central structurally weak area, but the weight savings would be less than a full monocoque.

Maybe another thing to consider is how much of the weight savings is construction method and how much is material. Aluminum space frames are possible, though I'd never build one, and they would save maybe 1/3 of the weight while being stiffer because the tubes would have a larger cross section.

I just feel that outside of an all-out racing environment it would be hard to justify the extra expense and effort of a composite or aluminum monocoque, unless one had a particular interest in this approach. If you want more stiffness, it's easily achievable with the steel space frame for a very small weight penalty. If you really want something lighter, you could also go to an alloy steel and save 25%. Most people wouldn't because it's not worth it, but it would probably be far simpler than constructing a monocoque. In other words, one could make the frame much stiffer and also lighter than a typical 'Seven' using a space frame approach, but no one has really 'maxed out' the space frame (at least that I've seen). When there's 'meat on the bone' so to speak, why change approaches?

Once in a racing environment, all of the rules are different and it would depend entirely on exactly what racing with exactly what sanctioning body.

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Aedifico ergo sum.

Ah, ok. I understand now. Probably a moot point, since you can design the scuttle so that it's bonded and riveted directly to scuttle hoops welded to the main frame, in any case, so it's not so prone to shake. As mentioned above, the problem with mainstream production cars is that they have opening doors and large, heavy dashboard assemblies that are relatively loosely attached to the main monocoque.


The actual frame (paneled, which is how they are torsionally tested) can be closer to 15-17% of the overall weight.

To use the example of the FW400 as a benchmark (I daren't mention that designs offering even better stiffness:weight are possible, because someone will get upset ), the weight saving versus a fully paneled steel and aluminium spaceframe of half the stiffness was around 100lbs. Double the stiffness of the spaceframe to match, and oh, I dunno... maybe you're looking at another 50lbs weight penalty; I'm not sure I've ever seen an uncaged Seven spaceframe that stiff, so it's difficult to benchmark?

The difference is basically similar to that of carrying a passenger, therefore, which has a pretty noticeable effect on both performance and handling on this sort of car.

Certainly, the additional expense of the FW400 was substantial (again, I won't mention that it's possible to do it in a much cheaper way... possibly even cheaper than a spaceframe, when you factor in the man-hours for a skilled welder. ). Whether it's worth is it is a matter of personal judgement (and affluence). Can you ever truly say that an impractical 'toy' like a Seven offers value for money, so it's surely a matter of individual judgement where the line is drawn?

150 lbs is a lot, but that difference would probably be almost cut in half if the space frame were actually optimized and you also included panels in the weight of the monocoque. You could loose 30% of the weight of the steel with 4130, and probably a bunch more by optimizing panel thickness/alloy. Still, 75 lbs is nothing to sneeze at.

A full 4130 frame with an awful lot of extra bracing could be built for $500 in materials and consumables at home if you are a decent welder and have decent tools. Mild steel could cost 1/3 of that. I really don't know what it would cost to construct a monocoque.

I think a composite monocoque Seven would be very cool, and definitely offer weight advantages, especially if you require much higher chassis stiffness. I'd probably go a little heavier and spend my extra money on really good dampers, but I could see going the other way as well.



-Graveyard.

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Aedifico ergo sum.

Those figures are based against the Caterham, which is a design that has been 'optimized' over half a century of development. It's one of the best '7' spaceframe designs on the market, as a result.

The truth is that the Seven spaceframe form, in particular, simply isn't very structurally effective. The engine bay and cockpit area being the two glaring weak spots, of course; you can improve the former - as I have with the spaceframe design I'm currently working on - but there's not much you do about the latter, short of caging the whole lot (heavy, restricts access, potentially unsafe for road use), triangulating the transmission tunnel (sensible, but small 2nd moment of area so not that efficient), or adopting the patented Costin 'perfect spaceframe' approach (which dictates internal ergonomics more than most people are willing to countenance). Even with the monocoque FW400, all the designer could achieve was a 'bodge' solution of double thickness (2") sandwich construction down the sides, to add stiffness to the cockpit section.

I don't include panels on the weight of the monocoque because you don't need them - that's kind of half the point! The structural tub of the FW400 formed both internal and external 'panels' of the whole lower half of the car. No further trim or aesthetic cladding panels were required.

In fact, I'm being generous by comparing with the Caterham (which is sort of a semi-monocoque already, with stressed external panelling); If I drew direct comparison with the 'normal' Westfield:
a) the torsional stiffness of the (internally only) panelled tub is rather lower than the Caterham, and;
b) I could legitimately include the weight of the lower (cosmetic) tub moulding, which you need aesthetically to get you to the same level of 'finish' offered by the bare tub on the monocoque.

Proof of the pudding: the complete, 'finished' Westfield FW400 weighed 433kg wet and fueled in prototype form (as measured by a UK road test magazine), with a pair of astonishingly heavy, adjustable seats for press purposes, and a monocoque that used lots of individual mouldings glued together (which would have been substantially simplified had it progressed to a full production run). Developed for production, the designer was confident of hitting the 400kg target weight. But even at 433kg, that would be 43kg (95lbs) lighter than the 476kg. quoted dry weight of the Caterham Superlight R, a car which was fully and extensively developed and used an identical Rover K-series engine. And remember that apart from being >95lbs lighter, the Westfield was also twice the stiffness of the Caterham.

The Caterham frame is not nearly an optimized space frame. They are just started to get into double-butted tubing last year, for example. I don't think they even sell one of those yet.

It is quite a bit more optimized than lots of garage builders end up with, but only because of the people building them, not because of the possibilities. There are guys out there building welded titanium tube structures in their garages. It could also be done in double/triple butted aluminum tubing if someone who builds those types of bikes had an interest.

Yes it could also be done in carbon fiber tubing, which would be lighter and stiffer than any of those. But, maybe once you get to composites monocoque makes more sense.

Double-butted (and triple-butted) tubing has been around, and used in cars, for a very long time. You'd have to ask Caterham why they didn't start using it many years ago. My only guess is that they had no incentive to get significantly lighter or stiffer with the frame. Maybe that says a lot.

-Graveyard

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Aedifico ergo sum.

Really? I must admit that I'm struggling to take this post seriously: prove me wrong.

To the best of my knowledge (and more importantly to Reynolds knowledge; their partner on the project and a world leader in the technology), Caterham are the first company anywhere to use butted tubing for an automotive spaceframe, but the results are fairly marginal: they are claiming 10% reduction in chassis weight for no loss of stiffness, whereas a monocoque has been proven to offer the potential for 50% weight reduction with 100% increase in stiffness. If you know of previous butted automotive spaceframes, then I'd be interested in more information. The costs are proving prohibitive for the simple reason that you need so many different variations to the butting within the tubes, on something as complex as a car spaceframe.

Caterham have spent a lot of time and money on professional analysis to optimise their spaceframe, over the years, so I don't see how you are expecting major improvements from an amateur.

If you know of stiffness:weight figures for a Seven type spaceframe that demonstrates substantially better optimisation than the Caterham, then show us.

I've been close to the Caged/Aerial project that fabricated a titanium spaceframe for the Aerial At-om, so I know what was involved. All I can say is that if there are people out there building titanium welded tube structure in their garages, then I hope to God that those structures are not safety critical. Lotus tried it when they were in F1 and abandoned the technology due to weld embrittlement. Caged managed it only by enclosing the entire structure under fabrication in an argon tent that took a great deal of effort to purge, and again the actual improvements were not that impressive (I'd need to check my notes, but from memory they achieved about 30% reduction in weight, but with an attendant small reduction in torsional stiffness). The fabrication process was an absolute nightmare, and certainly not cost-effective for production.

I ought to add that I'm a member of the UK Niche Vehicle Network - the organisation that funded the Reynolds/Caterham and Aerial/Caged projects, so my information is first hand from attending their project presentations and talking to the people involved; though I've also had business dealings with Caged, too.