does the fully-framed chassis get a bad rap?

[mikedc]

This whole post is just a big theoretical B.S. session.



Of course, in practice, most of the car unibodies that we deal with in this hobby are much better platforms for PT cars. And even in theory it's MUCH better to build a frame in the third dimension that just length and width. Advantage: unibody again.

But once in a while I look at fully-framed cars and trucks and think the performance car hobby totally writes this setup off a bit too quickly.


------------------------------


It's just that the VAST majority of the perimeter/ladder frames I have ever seen under anything, they aren't even starting to try to be stiff. It's like the designers just crossed off that whole category when they were evaluating their priorities.

I see tons of open C-channel rails and crossmembers in places that obviously call out for boxed materials. Not to mention open-sided stamped-steel crossmembers that are only welded (or even just bolted) to only one plane of those side channels.

I mean, if they had been building most of their unibodies like this for the last 40 years then we'd probably be calling that design a piece of sh*t too. Some of the same "difficult compromises" needed to stiffen up the body-on-frame setups look like stuff that would be treated as foregone conclusions on a unibody chassis.

I don't see a ton of 3rd-dimension structure in most unibodies either. At least not on anything older than the mid-1980s. They've got some roof pillars and doorjambs and stuff to help a little, but I see very little additional structure over and above what a BOF car from the same era would have had in its separate body.

And yet we consider these older unibody shells pretty stiff compared to a body-on-framed setup. It makes me think our overall perceptions about the stiffness of a BOF setup must be pretty damned bad.


--------------------------------


The weight issue fails to impress me too. Just eyeballing things, I see maybe 100 pounds of extra weight on a fully framed car in relation to a unibody car from the same era & type.

And then let's compare that unibody chassis again AFTER we're done adding our beefy subframe connectors and rocker box plating over the underside. (This is stuff that we need to do in order to "stiffen up the floppy factory chassis" on these old unibodies, remember?) Now the difference is even smaller still.


--------------------------------


Bushings? Tightness of the chassis versus isolation? "Road feel?"

I've heard truck designers saying that they've had to soften up the control arm bushings on truck platforms as they switch them over to unibodies. Not only that, but how many modern unibodies (and not just the luxury-floppy ones) are using rubber-isolated subframes at one or both ends? The unibody's supposedly huge-foregone-conclusion advantage slides again.


--------------------------------


The whole picture I see is a small advantage to the unibody setup but not the radical difference that we think of them having. It just doesn't look to me like we've ever seen many apples-to-apples comparisons of these two setups. Maybe in the truck world now and then but not much in sporty cars.

[oestek]

Interesting stuff. I agree that the unibody adds tremendous stiffness in the vertical dimension. I remember in the mid '90s when Buick did the last Riviera they were advertising that the body structure had a resonant frequency around 33hz or so.. it was extremely rigid. Body on frame cars probably can't be compared in the same manner 'cause I'd imagine the frame to body bushings making the assembly much more "loose" than that. But that's what they are supposed to do. Personally, I don't like the feeling from small unibody cars in a daily driver situation, even little tar strips in the road make the whole car thump like a bass drum inside. I like the isolation of a body on frame for a daily driver.

[FirstGenZq8]

corvettes are still body on frame...and they're world class sport cars (base) and arguably super cars (Z06/ZR1)...

[mikedc]

What I am really getting at is this:

How bad would a separate-framed car still be, if they actually TRIED to make it stiff and relatively light with modern methods?


After looking at a modern C5 and C6 chassis, I wonder.



There's plenty of stiffness to be had with a backboned chassis at the trans tunnel, that much is well known. But I wonder about how much better it might get just from putting some effort into doing a traditional ladder/perimeter frame correctly.

Increase the size (diameter) of the rails larger than Detriot traditionally ever did it on midsize cars, particularly in everything between the axles. Then use equally larger crossmembers like at the tranny & the rear axle/susp area, and actually make them boxed/round and fully attached to the side rails.



I also wonder about a firewall-area crossmember. (Like maybe up & over the bellhousing.) I can't recall ever seeing it done before. But I wonder if that might provide a decent percentage of the same gains as boxing/backboning the entire tunnel. My logic here is that it certainly does loads of stiffening work when they put a crossmember at the backseat/axle kickup (like on a 1980s Regal or something). Doing one at the firewall would just be the same thing at the other end.

[dipren443]

Can you really consider the C5/6 Vette body on frame?

https://static1.pt-content.com/images/noimg.gif

[mikedc]

Not really, no.

But with the C5/C6 we're looking at a chassis that is loosely rooted in a flat-plane ladder frame.

The important detail is that the roof structure is never loaded even on a hardtop car. Without that, the floor structure under the center of the car is forced to handle all the torsional and bending loads. Which is also the core area where all the effort is concentrated when you're trying to stiffen up a ladder frame.

[Norm Peterson]

What I am really getting at is this:

How bad would a separate-framed car still be, if they actually TRIED to make it stiff and relatively light with modern methods?


After looking at a modern C5 and C6 chassis, I wonder.



There's plenty of stiffness to be had with a backboned chassis at the trans tunnel, that much is well known. But I wonder about how much better it might get just from putting some effort into doing a traditional ladder/perimeter frame correctly.

Increase the size (diameter) of the rails larger than Detriot traditionally ever did it on midsize cars, particularly in everything between the axles. Then use equally larger crossmembers like at the tranny & the rear axle/susp area, and actually make them boxed/round and fully attached to the side rails.



I also wonder about a firewall-area crossmember. (Like maybe up & over the bellhousing.) I can't recall ever seeing it done before. But I wonder if that might provide a decent percentage of the same gains as boxing/backboning the entire tunnel. My logic here is that it certainly does loads of stiffening work when they put a crossmember at the backseat/axle kickup (like on a 1980s Regal or something). Doing one at the firewall would just be the same thing at the other end.
I think you end up being constrained by requirements for things like clearance, powertrain and exhaust space requirements, ingress/egress, etc. By the time you've made a perimeter frame rigid enough without building it out of excessively thickwalled steel tubes, it's picked up a pretty substantial section height. The alternatives with deeper, thinwall tubing (letting the frame hang visibly below the body, building deep quasi-sills in the body to clear the frame rails) don't seem very attractive either, at least not to the buying public at large. By the time the frame gets that bulky, and gets surrounded by bodywork on three of the four sides, and any solutions for incorporating crossmembers of similar rigidity become more complex than achieving the same in a unibody platform - the OE question would be "Why didn't we just merge the frame into the body in the first place?".

Any time that you have two structures bolted together at only a few points, you're going to give up something in terms of overall stiffness even if the bolted connections are fairly rigid (IOW, not with the usual resilient rubber body bushings).

I'm assuming that this discussion concerns street driven cars, for which roll cage structure or even most roll bars represent hazards to unhelmeted heads.


Just how much torsional stiffness are we looking for here?


Norm

[mikedc]

Oh, just street-car fun. I wasn't thinking SCCA or NASCAR kinds of stress levels.

The whole thing is more of a bull session than anything else. I think about how much trouble we go through to restore and beef up 40yo unibodies, and I always come back to thinking about how much easier our lives would have been if some of these cars were separate-framed.

Unibodies work, but they're basically disposable items. The factory just didn't intend for this stuff to be fixable once it got rusted or hit hard.




Meanwhile, look at those Art Morrison 2x4-tubed separate frames they sell. Those are intended to be welded into an old unibody and replace everything that stands in its way, sort of like a plastic model's body shell and flat chassis sections get glued together.

https://static1.pt-content.com/images/noimg.gif


I look at these things and wonder why you couldn't just mount one of those AM frames (or any other custom-fabbed frame similar to it) on rubber donuts instead of a rigid weld-in job. That thing isn't anything crazy difficult to fabricate or fit into the car, and it looks stiff as HELL compared to any of the old factory frames. It looks like it would easily work as a standalone chassis, without a rollcage or the upper half of a unibody welded to it just to hold it in shape.

You're already supposed to be fabbing a whole new floor/lower firewall/trunkfloor in the process of that Art Morrison job anyway. And the necessary air gap between most perimeter frames and their body shells from the factories isn't much. It only looks like 1 or 1.5 inches on most cars. Seems like building something as a whole new perimeter-framed setup would be a pretty short jump to make from here.


---------------------------------

[Rhino]

...why you couldn't just mount one of those AM frames (or any other custom-fabbed frame similar to it) on rubber donuts instead of a rigid weld-in job...

The first reason I could think of is ride height. The level most people like to see their cars simply couldn't be attainable by grafting the chassis under most unibody shells.

[Young Gun]

I am sure you could fab up some mounts for the MaxG chassis but you would not get the low aggressive stance that most people here want. An realistically how often would you remove the body from the frame? So why would yah really want to deal with the high ride hight?

[Norm Peterson]

Oh, just street-car fun. I wasn't thinking SCCA or NASCAR kinds of stress levels.It's not the stresses I'm concerned with as much as it is the torsional stiffness, as that's a major part of how "tuneable" your chassis is. IOW, how sensitive it is to changes in springs/bars/shock damping. A really flexible chassis stubbornly resists attempts at tuning the handling because the amount it flexes can significantly reduce the shifts in lateral load transfer that you use to tweak the handling in the first place. I've seen where 1960's unibody cars might have been 2500-4000 ish ft-lb/deg. The bad news is that by itself you'd be lucky to get half that from any reasonable 4" deep tubular frame taken by itself. I'm open to correction on either of those numbers, but I don't think I'm way off either.


The whole thing is more of a bull session than anything else. I think about how much trouble we go through to restore and beef up 40yo unibodies, and I always come back to thinking about how much easier our lives would have been if some of these cars were separate-framed.Probably because most (if not all) of the commercially available "fixes" aren't much more than band-aids. That's all that the factory reinforcements for convertibles were. That these things are better than nothing is absolutely true. But they're nowhere near what a clean sheet of paper redesign could give you. Think way out of the current box here, as in beyond the SN65. I think there has been one (aborted?) study concerning the conversion of the '78 - '87 G-body to unibody (and no, it wasn't me).

IIRC, a severely bent frame (in addition to the accompanying sheetmetal damage) was cause for scrappage.



Meanwhile, look at those Art Morrison 2x4-tubed separate frames they sell. Those are intended to be welded into an old unibody and replace everything that stands in its way, sort of like a plastic model's body shell and flat chassis sections get glued together.

<snip>


I look at these things and wonder why you couldn't just mount one of those AM frames (or any other custom-fabbed frame similar to it) on rubber donuts instead of a rigid weld-in job. That thing isn't anything crazy difficult to fabricate or fit into the car, and it looks stiff as HELL compared to any of the old factory frames. It looks like it would easily work as a standalone chassis, without a rollcage or the upper half of a unibody welded to it just to hold it in shape.The body's contribution to stiffness is considerable in the case of body-on-frame cars. Unless the car is a show car or strictly a boulevard cruiser, I don't think you want to give up any stiffness considering the effort involved. I sure wouldn't.


Norm

[mikedc]

Hey, I'm not suggesting that a body-on-frame setup would work well if you're building something that will get 1.3" swaybars and 35-series sidewalls. I agree that a unibody + decent cage is the only way to go if we're trying to run in those circles.


My inspiration for the whole idea is more about the practicalities of restoring bad-condition cars than trying to make a better mousetrap for racers. The factory 1960s/70s unibodies are such a pain when they're compromised at all. Whereas there is a big market (the "Street-rodification of muscle cars") for restomod options that just work pretty well for a reasonable amount of trouble and cost.

I'm talking about maybe restomod cruiser usages. Better handling than the stocker, but nothing so maxed-out as to compare evenly to a rollcaged old unibody (or an uncaged unibody that was designed in the last 10-15 years). Most street cruiser musclecars these days run 55-series tires or taller and the ride height isn't very far below stock.


---------------------------------------------


As for the frame's torsional stiffness:

I agree that even an old unibody would usually blow a bare frame from the same era out of the water.


But what about an old unibody, with the subframe rails missing and setup for a moderized Morrison-esque separate frame? I fail to see how that would be worse than a stock 1960s framed car. It should be a lot better.

You'd get a stiffer stand-alone frame than a stock factory frame of the era. And it would be carrying a body that's still got at least some of the stiffness of a separate unibody on top of it. (Yes OF COURSE, I know you'd lose a big chunk of the shell's stiffness when the factory unibody rails came off. But would that resulting shell actually be any WORSE than the non-unibody 1960s body shells? I doubt it. Maybe even still a bit better depending on the specifics of what you did.)


I guess the most directly relevant question for this area would be, "How stiff can you realistically make a Morrison-style 2x4 frame just by itself?"


-------------------------------------------


As for the ride height issues:

Remember that you're ditching the factory floors and subframe rails as part of the standard Art Morrison-style installation. (Some of us outside the southwest can just kick that stuff out the bottom of the rusty unibody project cars we find these days.)


Q. So what's the difference in height between a typical Morrison installation (which most of the handling crowd seems generally pleased with), versus the idea I'm kicking around?

A. Probably a grand total of maybe 1-1.5 inches, like I said above. The only real significant addition is the air gap for the rubber body mounts.

(And we're not even talking about 1.5 inches more than the car's original stock height, either. We're talking 1.5 inches over whatever height the Morrison deal would have produced with a normal welded-in installation, which usually looks pretty low in the big picture.)


----------------------------------

[Norm Peterson]

As for the frame's torsional stiffness:


I guess the most directly relevant question for this area would be, "How stiff can you realistically make a Morrison-style 2x4 frame just by itself?"

Really quick and dirty - I think a basic 2 x 4 x 1/8 rectangular shape with a single X connecting the diagonally opposite corners might be good for 2500 ft-lbs/deg over a 108" length (i.e. wheelbase). Any bends, hole cuts, or reduced sections that you need to clear things like the driveshaft and exhaust will reduce that figure, although it is possible to compensate at least partially for these effects with careful detailing and the addition of reinforcements.

I don't expect Matt (AME's engineer) to either confirm or deny the above number except to perhaps comment that it's either way off the mark or (if I'm lucky) that it's good enough for discussion purposes here in this particular thread. Any real number would be proprietary information.


Norm

[parsonsj]

Norm,

Can you comment on the value of the X bracing? Does it add much torsional stiffness? I've always thought it wouldn't since it is in the same plane as the frame rails...

jp

[oestek]

John, my '62 Galaxie ragtop has a factory X as part of the convertible frame... and it's literally an I beam X that looks as like it was once part of a bridge. Combined with the boxed perimeter frame, this chassis is stiff as hell. In fact, we once supported the car on jack stands under the front and rear bumper mounts at the far ends of the car, and were still able to easily open / close the doors with no top and full driveline installed. It's crazy stiff.

[parsonsj]

it's literally an I beam X that looks as like it was once part of a bridge. Funny! But good information. Thanks!

jp

[Twentyover]

Kevin- your describing beam stiffness, not torsiona stiffnes. While both are increased w. the X frame, they really are functions of 3 dimensional frame features

[Norm Peterson]

Norm,

Can you comment on the value of the X bracing? Does it add much torsional stiffness? I've always thought it wouldn't since it is in the same plane as the frame rails...

jp
The difference is that the torsional loads mostly resolve into bending loads that act against the strong axis of the rectangular tube, which directly opposes the tendency for the plane of the frame rails to warp out of "flat". The closer you can get the X-bracing to 45°, the better this approximation gets.

I'd guess that the 2 x 4 X-brace here might contribute about half of the total torsional stiffness.


Norm

[parsonsj]

I'd guess that the 2 x 4 X-brace here might contribute about half of the total torsional stiffness.Interesting! And counter-intuitive, at least to me. Kevin's description of beam deflection in his old Galaxy is not torsional, but is related. So maybe these x-members do matter. I may have to revise my thinking...

jp

[Norm Peterson]

Kevin- your describing beam stiffness, not torsiona stiffnes. While both are increased w. the X frame, they really are functions of 3 dimensional frame features
Blodgett (of Lincoln Arc Welding Foundation note) provides simplified approximations for estimating the torsional resistance of a couple of different diagonal bracing schemes. These quickie formulas are in fact based on the bending moment of inertia of the bracing members.

R = 3.54 x [bending MOI] for single diagonal bracing (not X-bracing)
R = 10.6 x [bending MOI] for X-bracing


Norm

[novaderrik]

oems build unibodies becasue they are cheaper to build- end of story. not because they are better than full frames, but because it takes less time to build and requires less parts.

[mikedc]

The OEMs are probably still doing everything with unibodies today because of weight, crashworthiness, and stiffness. In that order. Given their current priorities, they would be crazy to go back to perimeter frames on much of anything that weighs under 4500 pounds.

But the original switchover in the 1960s-80s was purely to save money, I agree. Anything else they said at the time about ride quality or handling or safety was mostly marketing B.S.




For the '62 Galaxie example, you would need to put one jackstand at each end of the car, but have them under opposite corners side to side. THEN try to open the doors. That's a real test of torsion.

[JEFFTATE]

Why can't you get the ride height low on a chassis / frame car ??
I read another thread that discussed the inability to get a low ride height.
But why ??
Aren't they adjustable.
Don't they all have adjustable coilover shocks ??

[mikedc]

A true ladder framed chassis has the rails running directly underneath the passenger compartment. That is a detriment to getting the car very low. They were builing cars like this up through the late 1950s. Trucks are still done this way today.




By the 1960s they were building the car lines with "perimeter" frames where they bent the frame rails outwards and around the passenger compartment. This was so they could set the rails up into the rockers and bring the sheetmetal floors lower in between the rails.

There's no real issue with trying to lower these cars. They'll go about as low as a unibody car will.

[Roadbuster]

Why can't you get the ride height low on a chassis / frame car ??
I read another therad that discussed the inability to get a low ride height.
But why ??
Aren't they adjustable.
Don't they all have adjustable coilover shocks ??

If it is an Abody there are several things that start to show up as it gets lower. It is not impossible, just not "throw in new springs" easy. The driveline angles get messed up, clearance issues with the fenders, driveshaft, rear end, frame to name a few. Lowering these cars to the point where the center of the wheel is above the rockers causes ripple issues that need to be addressed.

Jon

[JEFFTATE]

I'm talking about an aftermarket full-frame chassis in a First-Gen Camaro.
Specifically the one by Muscle-Up and Schwartz.
I understood , from the other thread , that the car owner couldn't get the low ride-height he wanted.
The car was actually 3/4 inch higher than his stock '67 Camaro.

This thread :
https://www.pro-touring.com/forum/showthread.php?t=55885

He's saying that the rear suspension geometry limits the ride height of the car .

[Roadbuster]

I'm talking about an aftermarket full-frame chassis in a First-Gen Camaro.
Specifically the one by Muscle-Up and Schwartz.
I understood , from the other thread , that the car owner couldn't get the low ride-height he wanted.
The car was actually 3/4 inch higher than his stock '67 Camaro.

This thread :
https://www.pro-touring.com/forum/showthread.php?t=55885

He's saying that the rear suspension geometry limits the ride height of the car .

Oops! Thats what I get for posting when I can't sleep!

Jon

[mikedc]

That thread is discussing a practical issue, not a theoretical one.

They're putting an aftermarket frame under the sheetmetal floors of a camaro that was a factory unibody in that area. And then they're sticking an aftermarket linked rear suspension onto that frame instead of the stock leaf springs.


I see no theoretical problems being demonstrated. Not if the aftermarket frame had been actually shaped to fit that linked suspension, and then the car's flooring was shaped to fit that frame correctly.

[JEFFTATE]

Ohh , Sorry.
I was chiming in about a bad rap for the full frame.
This thread is actually talking about the theoretical frame stiffness.
So , I'm a little off topic.
I was just talking about any bad raps , associated with full frames , that I had heard of.

[mikedc]

No prob. Sorry if I sounded jerky and dismissive about it.

[mikedc]

Hey Norm -


Earlier, when you were talking about the ultimate stiffness figures for a 2x4" frame, you threw out the rough figure of 2500 lb/ft @ 1 degree of twist. Well now I'm curious - what would some dimensional increase of the frame members do to that? Like, what if you made it out of 3x6" or something instead of 2x4" metal? What about thickening the connection points or something? What about switching to round tubing (same ultimate height) instead of rectangular?



I can totally see your basic reasoning that no frame only 4" high could approach decent stiffness numbers w/o the body shell's help. That makes sense.

But the frame thoughts haven't stopped gnawing at me. I wonder what it actually would take to get 4000 or 5000 twisting pounds out of a separate frame.


-----------------------------------------------

[Norm Peterson]

It may help to think of torsional stiffness as being more of a 3-D problem that requires a 3-D solution (IOW, lots of "enclosed volume"), while bending stiffness is closer to 2-D that can be increased substantially just by making the beams deeper. Big tubes, deep-section diagonal bracing, space frames designed for torsional stiffness (many cages are not all that good, torsionally, as they have a different primary function and may be limited by competition class rulesets) - those or combinations of them are the theoretical solutions. Making them fit into a workable, liveable vehicle is the separate "practical" part of the problem where you have to make compromises.

If work is slow enough this coming week (I just finished up one little job last Friday, not sure what's going to turn up right away) I may try tossing this problem at a 3-D space frame software that can directly handle rectangular shapes. But for really rough talking purposes (no calculator, mental math only) - if 2500 is actually achievable with 2 x 4 x 1/8, figure that [cube root of two] times the 2 and the 4 ought to get you to 5000-ish ft-lb/°. So, 3 x 5 x 1/8 ought to be enough, at a weight penalty of about 1.7 lb for every foot of that tubing if you're keeping track of that sort of thing (an extra 100 lbs is not an unreasonably high guess).

Round tubing is better if the loading is purely torsion, but may not necessarily be so where you have combined bending, vertical shear, and torsion. End connection details do matter, but the members in between all the "nodes" need to be of reasonably constant stiffness over their entire lengths. Holes cut in any member need to have reinforcement to compensate for the material (and strength & stiffness) removed, and I'll note that it's much easier to recover the lost strength in that member than it is to get back its lost stiffness. The numerical effect of any single compensated hole cut on overall chassis stiffness is probably less than it is on the individual member that has it, but I wouldn't attempt to guess by how much.


Norm

[Tig Man]

You wouldn't believe how much a winston cup chassis moves around with all the structure they have! I'd like to see some of these chassis and street cars on a shaker!



Mark

[mikedc]

yeah, there's a lotta cars and trucks that would be interesting shaker-rig tests.

NASCAR chassis are dripping in frame weight, but most of it isn't really designed around stiffening the suspension mounting areas. The ratio between torsional stiffness versus overall frame weight on those things would be totally ass.


-------------------------------------------------------


Norm -


3x5 might do 5000, eh? That would be even less than I thought.

If 5" would do a given # strictly on paper, then I'm thinking a 5.5" or 6" tube might cover the additional compromises inherent in building a real-world frame?

This has been my gut feeling about the flat-frame idea since I started this thread: Let's see if the torsional stiffness is still such a deal-breaker after you make the main side rails out of round tubing the size of a coffee can.




Of course anything with tubes this size also cracks open the weight issue in a big way.

I assume that with a large-sized set of frame tubes then you could feasibly thin out the walls of the tubing at least a little bit. Maybe take it from 1/8th down to 0.95 or something. It's not exactly gonna get the whole mess down to unibody weight or anything, but I don't know of any other realistic way to reduce the weight penalty at all.

The weight issue would remain a big downside to actually doing anything like this, no matter what is done to the tubing thicknesses or grades of steel used.


----------------------------------------------




So what does all this B.S. produce?

A frame with 5-6" round main rails, and the whole thing weighs another 150-200 pounds above the total weight of a unibody on a given vehicle?

All this stuff wouldn't be much to cry about on a short-wheelbase fullsize pickup truck, though. It sure might produce something that rides & handles a lot better than a stocker.



---

[Norm Peterson]

Pretty sure I've posted this link in another PT thread but since it's easier to find the paper than the thread . . .

NASCAR Chassis Stiffness (http://www.ces.clemson.edu/~lonny/pubs/journal/sae983053.pdf)


Norm

[Norm Peterson]

DISCLAIMER - THE MODEL USED DOES NOT REPRESENT ANY KNOWN FRAME, IT'S JUST A PERIMETER FRAME WITH A SINGLE "X" BRACE THAT FITS INTO A 108" WHEELBASE WITH ROOM PROVIDED FOR TIRES AND POWERTRAIN AND A KICK-UP OVER THE REAR AXLE. NO HOLE CUTS CONSIDERED.

IOW, THESE NUMBERS ARE JUST FOR TALKING PURPOSES MOSTLY WITHIN THIS THREAD.



3x5 might do 5000, eh?A little better, actually, since 3" is greater than [cube root of 2] times 2 . . . closer to 6000, perhaps.


Let's see if the torsional stiffness is still such a deal-breaker after you make the main side rails out of round tubing the size of a coffee can. Make the main rails only out of 4 x 4 x 1/8 square tube and keep the diagonals 4 x 2 x 1/8 and 4000 might be possible. 4" OD x 1/8 round tube main rails instead, slightly less (3500?)



Of course anything with tubes this size also cracks open the weight issue in a big way.

I assume that with a large-sized set of frame tubes then you could feasibly thin out the walls of the tubing at least a little bit. Maybe take it from 1/8th down to 0.95 or something. It's not exactly gonna get the whole mess down to unibody weight or anything, but I don't know of any other realistic way to reduce the weight penalty at all.

The weight issue would remain a big downside to actually doing anything like this, no matter what is done to the tubing thicknesses or grades of steel used.Stiffness is more sensitive to diameter or width & depth than it is to wall thickness, so a larger section of thinner wall can be of equal or greater stiffness with less weight. Eventually, the mid-portion of such a chassis might become stiff enough in just the rails such that you could do away with the diagonals entirely, leaving you with a central passenger cell 'tub' vaguely similar to what I suspect lies underneath David Pozzi's Lola. Of course, as the metal gauge gets lots lighter there are other limits that you then have to take a lot more interest in (such as buckling). And you'd be back to a unibody, more or less, since the metal thickness would be too light to reliably bolt much of anything up to.


Norm

[mikedc]

Yeah, that's another thing I was thinking too. Try to optimize the separate flat frame too much and you just start turning it back into a 3D space frame again.

But the bit about using tall door sills to allow the elimination of the diagonals is interesting.

I've always thought the factories have been under-utilizing the benefits of growing the heights of the door sills over the years. Of course a mass-marketed passenger vehicle demands those kinds of concessions to comfort like low step-in heights. But at the same time, the door sills haven't really changed in decades while they're going to increasingly extreme measures trying to regain stiffness elsewhere in the structures.

----------------------------------------


Here's where your figures really surprised me:


How the F does a 4x4 square-tubed frame produce a greater stiffness number than 4" round tubing used in the same way?


I could understand 4" round tubes losing out to 3x5" square or something. But I don't quite understand how square tubing is gonna surpass the stiffness of round tubing when the overall heights & widths of the tubes are both the same. Would the square tubing still be the stronger stuff for the main side rails if the diagonals were going to be the same rectangular 4x2s in both cases?

[Norm Peterson]

Partly because the corners of a 4 x 4 square are more than 2" away from the neutral axis and partly because there's more than just torsion happening.

What a circular tubular cross section gives you is essentially uniform torsional shear stress, so as far as torsion is concerned it's a more efficient shape stress-wise relative to the weight involved. But this definition of 'more efficient' does not necessarily translate to 'stiffer' in a frame where you also have bending moments.

BTW, the diagonals and laterals were left at 4 x 2 x 1/8 in both cases. Only the perimeter rails were changed.


Norm

[mikedc]

I'm trying to picture a cardboard tube that's round, and another one that's squared, both of them the same height/diameter/whatever. Somehow I can't imagine the squared one having more torsional resistance than the round one.

The corners being slightly farther out on the squared tube doesn't seem to make up for the fact that they're also creating an uneven surface to be twisted. Does the square's gain come from the inherent work-hardening that has occurred at the corners of a mass produced metal square tube or something? Or is this a truly theoretical advantage that the square tube has?


I mean, we don't see a whole lot of square-tube driveshafts in the real world. Even in places where the speed of the shaft is slow enough to keep balancing/vibrations from being factored into the equation, we still always see round tubing be used just by default.





I get what you're saying about the bending loads being applied on a set of side rails as well as torsional loads. Perfect sense there.


------

[Norm Peterson]

I'm trying to picture a cardboard tube that's round, and another one that's squared, both of them the same height/diameter/whatever. Somehow I can't imagine the squared one having more torsional resistance than the round one.Try drawing a circle inside a square such that the circle touches the square only at the midpoints of the four sides. Then realize tubes in structural service have reasonably normal wall thicknesses and rounded corners. If you could duplicate that in cardboard, it would have much better local bending resistance at the corners than the taped-together collection of cardboard panels that probably comes to mind (the latter is really nothing more than four long, narrow panels with piano hinges connecting the adjacent panels, neither rigid nor particularly strong). Fabrication details matter . . .



I mean, we don't see a whole lot of square-tube driveshafts in the real world. Even in places where the speed of the shaft is slow enough to keep balancing/vibrations from being factored into the equation, we still always see round tubing be used just by default.A driveshaft is a prime example of a structural element essentially loaded only in torsion. Shear and bending effects due to weight are negligible, and if the shaft isn't straight you'll know all about it from the vibration long before any bending stress resulting from the inertial loads matters. Not to mention that it's got to be lots easier to get the U-joint yokes centered or when you put the thing all together and go to balance it. I can't, for example, think how you'd get a balancing machine to correct the amount of weight to be used at an unknown radius (this would vary over nearly a 40% range) at the azimuth where you need the weight.


Norm

[Twentyover]

Norm said it better (and before) me

[jaybee]

If you replace the body mount bushings with something of high durometer like polyurethane or UMHW how much might the (now rigidly mounted) body contribute to the torsional stiffness of a full frame car?

[mikedc]

It would probably help quite a bit. At least it would help once you get past that first little bit of rotation that would be easily allowed by the fasteners & tolerances between the frame & body.



Poly is nearly a solid bushing though. Not much give.

(I can't believe the aftermarket bushing industry is still only offering either stock rubber or poly all the time. Why don't they offer a harder durometer of rubber that's not as stiff & rigid as poly? The OEMS are fine-tuning the hardnesses of specific bushings on their platforms all the time.)

[Norm Peterson]

IIRC, there is a choice in body bushing Durometer. I seem to remember getting 88A body bushings for the Malibu, which was the stiffer of two. And Maximum Motorsport has (or at least used to have) a 3-piece poly bushing arrangement for Fox Mustang rear control arms that featured pieces of two different stiffnesses.

Offhand, I'd say that the OE's are far pickier about tuning for NVH than most enthusiasts who are looking for better location without too much regard for how much stiffer that a "slightly stiffer ride" actually measures. OE has to satisfy perhaps 100,000 customers per year per model. Individual enthusiasts, one.


Norm

[mikedc]

Agreed.


To tell you the truth, I suspect that most normal car buyers wouldn't see any problem if every car/truck they ever buy rides like a '59 Caddilac. I think the average car-buying market has just been conditioned to think cars shouldn't be that soft because of the influence of gearheads on the automotive press.


Just look at the SUV & fullsize pickup truck trend in the last 15 years. As soon as people started ignoring the "right" car purchases and just bought what they actually liked, they went back to buying full-framed battleships that can roll through a deep pothole and barely know it was there.


---------

[Norm Peterson]

My take on the SUV/pickup trend is that it's been a delayed reaction to minivans. Same (or higher) seating position in a vehicle that nobody ever associates with the word "meek". Not exactly cornering-friendly in either case though (not that this matters much to the majority of drivers). As long as you can still shut the doors with one wheel sitting in a hole, the fixed glass doesn't break when you hit bumps, and not too many rattles and buzzes develop the chassis is stiff enough. :rolleyes:


I may eventually try to put a more complex model together to simulate the inclusion of a separate body and see what happens when you then tie the frame and body together using different approaches. Maybe some day there will be a use for the information beyond satisfying a little academic curiosity.


Norm

[mikedc]

I totally agree about SUVs in the last 15 years - It amounts to a minivan that doesn't make the buyer feel completely emasculated.

But I also think it's a notch deeper than that. I don't think we would have seen anything like the same move towards SUVs that we originally had in the 1990s if the SUVs of the time had been the unibody FWD jobs that they're doing now. The BOF design, the torquey drivetrain feel, the big wide flat hood out in front of the driver . . . I think that masculine, industrial, mechanical, "ride the bull" feel is more like what the buyers want at some deeper level. The govt regulated it away from cars with CAFE + crash testing rules between 1974-1990, so eventually the buyers sought it out elsewhere. The only difference was that SUVs sat people up a foot higher than the old classic-era cars used to.



I still happen to like the perimeter-framed principle even now because it's so much more abuse-able than unibody cars could ever be. Bang the front bumper into things, tow something with the rear end, blast down a gravel road hitting every bump on the way, let snow/rain/mud get caked all over the underside without worrying about structural rust, etc. I just love how idiotproof a pickup truck is compared to a fragile unibody sports car.


It's not that BOF vehicles don't get hurt by the same things. But the consequences of these kinds of things are so much less traumatic & permanent & difficult to fix than on unibody cars.

When you think of the most fun car/truck owning experience you've ever had in your life, it probably wasn't the highest-performing car on the list. It was probably some POS that you abused the hell out of as a teenager.

[Norm Peterson]

When you think of the most fun car/truck owning experience you've ever had in your life, it probably wasn't the highest-performing car on the list.
That gets a lot harder to answer when one of your main conscious approaches to buying a car is a satisfactory answer to the question "Do I honestly expect to be happy enough with this car three months after the payments have been completed to still keep it?".

So far, we've never bought a car that turned out to be a disappointment in that respect. Of the six new cars that we've bought (plus zero used cars, and that's since 1971, BTW), we still have four, three of which are in at least semi-regular service. And of the two that we no longer have, I wish I could have one of them back and my wife would take the other one (in more reliable mechanical form, that is).


A little closer to topic, most of my cars have been unibody. Not particularly by choice, just the way things worked out (see above). Only two full-frame cars out of nine cars total that have I owned (the 1979 Malibu that I still have and a used 1957 Pontiac that lasted all of 3 months), plus one of my folks' cars that I learned on and got to drive after getting my license.


Norm

[Twentyover]

When I was growing up, Mom had a Country Sedan.We could hook the boat or the trailer and drag it cross country all day long.

The SUV deal is the modern station wagon, able to haul the family toys cross country or pack 3.7 kids in for a trip to the beach.

[mikedc]

I don't mean to be slamming the entire unibody concept on principle. It's the better design than BOF in a ton of ways. (Not the least of which is crashworthiness and stiffness/weight effeciency.)



I guess I just have a soft spot for the sledgehammer-grade durability of trucks and older BOF cars. It would be beautiful to be able to just wave a magic wand and toss out the weight & stiffness drawbacks of the BOF setup.

The police & taxicab fleets still seem to prefer BOF platforms, for example. The departments have crunched the numbers and they're willing to use unibodies sometimes. But the individuals actually behind the wheel of the cars every day tend to prefer the BOF cars. Like, I've heard cops complain that their unibody cruisers are disposable crap compared to the Crown Vics & older Caprices. Whack a curb two or three times while chasing bad guys and the car's whole suspension is already trashed. They have similar complaints about the unibody cars when getting into accidents or going fast on grass & rutted gravel roads. I'm sure some of the issue is modern stuff/tight/low unibody design details, but the raw BOF/unibody difference is probably still a good portion of it.




I just think the BOF design has been declared obsolete a little prematurely, that's all.

[The Stickman]

By the 1960s they were building the car lines with "perimeter" frames where they bent the frame rails outwards and around the passenger compartment. This was so they could set the rails up into the rockers and bring the sheetmetal floors lower in between the rails.

There's no real issue with trying to lower these cars. They'll go about as low as a unibody car will.


This is what my car has. The frame rails are up in the rockers and it's a full boxed frame. I have had no such problems getting it low. It's also plenty stiff.

[TnBlkC230WZ]

To the round v.s. square tube: Round tube is a stronger geometric shape. It has not flat sides to flex. Not sure why TCI is the only one building a round tube frame for Camaros and Novas.

To the Camaro has a 4*4 boxed frame: HaHa HeHe. Not even close. it is a stamped steel frame with pieces and parts partially welded together. Just fully welding the parts together helps. A fully boxed frame whould have to be better.

To torsional stiffeness: use solid body bushings and the body shell will add greatly to the stiffness. Isn't this what most protour's do, including myself.

Frames hang too low: Camaros and Novas already have frames hanging down and we add subframe connectors to that. Why not build a one peice system with outriggers to the rockers. To add even more strength, replace the inner rockers with something more substantial. It is no longer bolt-in, but it would be strong

[Norm Peterson]

To the round v.s. square tube: Round tube is a stronger geometric shape. It has not flat sides to flex. Not sure why TCI is the only one building a round tube frame for Camaros and Novas.
All shapes will "flex" under load, just differently.

Round is a more efficient shape where the loads are primarily torsional or compressive. Where axial tension is concerned, it's about a wash (since there are no buckling concerns, only the cross sectional area matters).

But where the main loading is bending, I'd prefer square/rectangular tubing, generally with the "deep" dimension oriented to resist the larger bending moment. Note that it may not be obvious which direction this is.
I doubt that anybody actually plans their design around needing to use the little bits of volume where the corners of square/rectangular tube lie outside the diameter of a round tube of the same nominal size.


Norm

[mikedc]

Looking at this thread again got me thinking again . . .



Has a separate perimeter-style frame made of 4130 steel ever been tried? I would think the weight loss could be substantial when you add it all up from bumper to bumper.




I know chro-mo is more brittle than mild steel. But is this so very much different from a lot of the hi-strength steel alloys that the OEMs are using all over their modern chassis (both types)?

Modern unibodies are getting more and more difficult for collision shops to straighten because the steel can't stand being worked a third time after it's been formed at the factory and then bent in a wreck. That sounds pretty damn brittle right there, and yet the OEMs don't mind trusting it for 200,000 miles as long as it hasn't been crashed very hard along the way.


Most of the brittleness problems with chro-mo seem rooted in the welding of the joints rather than just the properties of the existing tubing off the shelf . . . what if you just gas-welded the joints or something? That would probably make the joint areas soft as hell because you'd basically lose the heat treating entirely. (In fact I think that was what 4130 tubing was originally meant for; tubing to use on aircraft frames and it was expected to be gas welded, right?)


A weld this soft wouldn't be good for the chassis stiffness. At least in theory. But in practice, the tubing diameters are already so large on a perimeter frame that I would expect that to be a big help in stiffening it up no matter how the weld joints were done. We're not exactly talking about 1 5/8" rollcage tubing here, this thread was discussing a hypothetical frame made of very big-ass tubing.

[Tig Man]

A Trans-Am chassis I built years ago!



Mark

[gabe74gt]

This is a very informative thread. Thanks a lot.

[Paul_J]

I believe it was Craig Morrison that made a comment in a similar thread about adding a tunnel formed out of something like .125 plate would make a huge difference. Maybe he can add to this or provide some data.

I took his suggestion and did this to a chassis I'm building.

[Twentyover]

Looking at this thread again got me thinking again . . .



Has a separate perimeter-style frame made of 4130 steel ever been tried? I would think the weight loss could be substantial when you add it all up from bumper to bumper.



Think you are confusing the increased ultimate tensile of ChroMo with the stiffness (Modulus of Elasticity) of chromo. All steels have the MofE of about 30 million PSI. Variations are fractional an minor. The same bending moment applied to a mild steel section and a chromo section will bend them the same amount, until the mild steel section fails. The chromo section will continue to bend further before it ultimately fails.

What this means to me is that if i design to stress failure, I'll use lighter sections of chromo than I would of mild steel. These lighter sections will be more flexible, and the chassis will lose rigidity (is that a real word? You get my drift).

Remember the chassis is supposed to be stiff to let the suspension work.

Colin Chapman, noted for pursuing lightweight design, the guy who said "simplify and add lightness", "Any car which holds together for a whole race is too heavy" also said "mild steel is the clubman's friend."

[jaybee]

I believe it was Craig Morrison that made a comment in a similar thread about adding a tunnel formed out of something like .125 plate would make a huge difference. Maybe he can add to this or provide some data.

I took his suggestion and did this to a chassis I'm building.

Pics please, or a more thorough description of what you're talking about?

[Norm Peterson]

What this means to me is that if i design to stress failure, I'll use lighter sections of chromo than I would of mild steel. These lighter sections will be more flexible, and the chassis will lose rigidity (is that a real word?
I've always assumed so.

So, apparently, does wikipedia (http://en.wikipedia.org/wiki/Rigidity).


Norm

[jaybee]

So, picking up on some recent points from this thread, one of the limitations of the body on frame is the inability to turn a flat structure into a three dimensional structure, and solid body mounts can help in that regard. What if you built a structure into the floor in the same way that subframe connectors are often cut through the floor of a unibody? It could incorporate a heavy tunnel as Paul_J mentioned, perhaps with more structure to close the bottom of the tunnel, turning it effectively into a backbone. The various places where the structure bolts together could be shimmed and cross bolted to limit movement at those points.

Yeah, it doesn't use material as efficiently as a unibody, but that isn't what we're debating here, is it?