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.


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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.


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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.


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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.


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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?

[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.




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.


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[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.


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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?"


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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.)


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[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