I would like to ask a unibody engineer some questions

Shocks, Springs, Brakes, Frame, Body Work, etc

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mk e
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Re: I would like to ask a unibody engineer some questions

Post by mk e »

pdq67 wrote: Sat Jan 20, 2018 7:58 am Mark,

Don't know if it is possible, but can you use ceramic fiber or block insulation in the arch of your traveling pizza ovens?

Light as a feather, but I don't know about any health issues due to ceramic fiber, "dusting"? Make the hearth/floor out of brick because they naturally will have cracks due to their joints! A strong lightweight insulating firebrick might be used instead of dense brick to make the ovens lighter?

Might install a suspended corrugated stainless steel, "shroud", in front of the ceramic fiber to catch any dusting?

"Corrugations", just to handle the constant heat cycles is all here.

Back when I was with AP Green Refractories, I designed a lot of furnaces using all of these types of material and construction techniques.

pdq67
I don't know much about ovens, that's my bother's thing. I know we talks about thermal mass being important so he's generally not interested in reducing the weight of the oven itself, but he have talked about materials that have higher thermal storage/lb. I think the whole mobile setup weighs just under 6k lbs with the heavy 4x4x1/4 frame.
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Re: I would like to ask a unibody engineer some questions

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exhaustgases wrote: Sat Mar 21, 2015 2:54 am 1st question has to do with choice of jack points especially the use of the knife edge of a pinch weld, ie very small surface area and very little side stability.
Many vehicles these days are no longer equipped with either a spare tire or a jack, so no worries about where to jack. And if a road service guy changes a tire, he's going to put the jack where it's most convenient for him. Doesn't matter whether it's a factory dimple in the pinch weld or the solid keyed jack point, him crawling around on his belly in the mud and snow looking for whichever ain't gonna happen anyway.
All cars should have solid keyed jack points and not a flimsy little thin strip to jack on. By keyed I mean a pad with a hole in it for a post on the jack to fit in or something on those lines. And have the top of the jack with a plastic pad incorporated on it, am I the only one that cares about protecting stuff?
I asked one engineering team why the above weren't universally used. The answers:
1. Only an insignificantly small minority of owners ever jack up their own car, so their convenience is not cost-effective.
2. The design goal is to reduce pieces, reduce welds, reduce assembly time.
3. Once a unibody is designed and tested, the team is then asked to go back over it and find ounces which can be removed from every area without compromising strength.
4. Our cars don't have spare tires or jacks.
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Re: I would like to ask a unibody engineer some questions

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Back in the mid-seventies, my pal had his new GMC van recalled because they discovered the jacks wouldn't fit under the trucks if the tire was flat.. #-o
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Re: I would like to ask a unibody engineer some questions

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They went on the develop the Hubble telescope...
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Re: I would like to ask a unibody engineer some questions

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mk e wrote: Sat Jan 20, 2018 7:31 am It funny because I just had basically this same discussion with my bother last weekend but talking about pizza ovens. He builds masonry wood fired ovens and has people who want them on a trailer from time to time. .... Masonry cracks so the frame had to be very stiff, beam strength wise if could carry 20 ovens but we just wanted 1 oven with no cracks.
Interesting. I've always had an interest in truss and structural design and won several bridge building contests in college. I've also noted how BAD many competitive old racecar structures are when you consider triangulation. The "Petty Bar" would seem to be an obvious thing to do and alone, it's not that great.

Regarding concrete pizza ovens! :D
Are you aware of basalt fiber and basalt rebar? You can buy basalt cloth which could have some very interesting automotive applications involving extreme heat resistance with nearly the strength of carbon steel. Basalt roving could be incorporated into the dome of a pizza oven. In masonry, basalt rebar:
- Is a fraction of the weight of steel.
- Can't rust - it's a rock
- Nonmagnetic, nonconductive
- AND, it's organic! (like high-end pizza!)

https://www.basalt.guru/use-basalt-reba ... -concrete/
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Re: I would like to ask a unibody engineer some questions

Post by MadBill »

mk e wrote: Sat Jan 20, 2018 7:31 am..
It funny because I just had basically this same discussion with my bother last weekend but...
Is it a Freudian slip mk e that in two posts in a row you call him your bother? (I've got a pair of those myself...) :lol:
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Re: I would like to ask a unibody engineer some questions

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MadBill wrote: Thu Feb 01, 2018 5:28 pm
mk e wrote: Sat Jan 20, 2018 7:31 am..
It funny because I just had basically this same discussion with my bother last weekend but...
Is it a Freudian slip mk e that in two posts in a row you call him your bother? (I've got a pair of those myself...) :lol:
If that's the worst typo in 2 posts that pretty good for me :lol:
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Re: I would like to ask a unibody engineer some questions

Post by Ratu »

mk e

You write, " I did a formula car and started by designing a structure all triangles with no opening for a cockpit of anything else and did the fea. Then I removed only the 1 diagonal across the cockpit area and the chassis torsional stiffness dropped by 75%! A lot of care is required around openings. The final design in steel was on par with the late 80s- early 90s F1 carbonfiber stuff stiffness/lb wise so it can work very well."

That is fascinating. Can you post some pictures/drawings of it?
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Re: I would like to ask a unibody engineer some questions

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Ratu wrote: Fri Feb 02, 2018 7:36 pm mk e

You write,.....

That is fascinating. Can you post some pictures/drawings of it?
Couple.....it was many years ago. I did a couple designs for a mid 911 engine speedster I was contemplating, 1 with working doors and one that was just silly rigid but no doors.

On the formula car I started the work with a pretty simple rectangle chassis, that was the version I did the with/without functional cockpit test thinking the engine bay which is also not properly triangulated many times is easy enough to solve with bolt-in bracing but the cockpit needs to be open. After confirming it was a major problem that became the focus of the work and the solution I settled on moved the front roll hoop forward about 6 inches and I added a couple members that go above the drivers shoulders and form a "v" in the cockpit opening. Very stiff and strong and worked very well. The speedster design is a pair of the formula chassis siamesed together.
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Re: I would like to ask a unibody engineer some questions

Post by mk e »

I'll add this. Way back when I was doing the analysis on the formula chassis I noticed that a quick and very accurate way to tell how well optimized the design is was to look at what happen when the tube diameter (or wall thickness) is changed. If the design is perfect all the loads will be tension and compression so the change in stiffness will be directly proportional to the change in tubing cross sectional area. If the design is not perfect there will be be bending loads so stiffness will also be a function of I (moment of inertia) of the tubes. IIRC the finished formula chassis was 90%+ proportional to the tubing CSA,

Playing with the thought of adding an upstairs to my new garage and wishing I'd been smart enough to order the trusses with a room up there I've been running a bunch of numbers on modifying the trusses. The trusses as-is are triangles and I get within 5% the same answer if I call the joints rigid or flex which means there are basically no bending loads. Restructuring to have a room means remove the center triangulation, which means bending loads and a 40% difference if I treat the corners as joint vs fixed, so the same result I saw with the formula chassis design just showing up a different way.

I found this free on-line program, its only 2D but you'll see pretty quickly the difference between bending and properly triangulated

click "Deflections"
original truss fixed joint.
http://en.sopromat.org/2009/?id=45833

original flex joint
http://en.sopromat.org/2009/?id=45844

modified flex joint (where applicable)
http://en.sopromat.org/2009/?id=45846
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Re: I would like to ask a unibody engineer some questions

Post by pdq67 »

pdq67 wrote: Thu Feb 04, 2016 6:49 pm
exhaustgases wrote:No matter what, its still dumbness to design a jack point like that. Just have someone inexperienced like most people driving on the roads are, doing the jacking, and once the jack area gets folded over all strength in it is lost and it will just keep folding over after its been straightened, been there done that. All cars should have solid keyed jack points and not a flimsy little thin strip to jack on. By keyed I mean a pad with a hole in it for a post on the jack to fit in or something on those lines. And have the top of the jack with a plastic pad incorporated on it, am I the only one that cares about protecting stuff?
As soon as paint is removed with a metal on paint edge a nice place is made for rust to start its job, especially in the snow belts where salt is over used.
Good come-back about the dedicated jack points and plastic jack body protector!!

I forget what car it came from, but the jack laid on the ground and raised the car up at a dedicated point in the undercarriage. Kinda like a sheet metal mechanical floor jack.....

Maybe it was from one of the little BOP compacts back in the early '60's?

Oh, my '67 Camaro has a hole at each "corner" of both bumpers just inside the bumper bolts to insert the bumper jack stud into..

pdq67
I was just re-reading this again for whatever and remembered that my '67 Camaro had a recall letter sent out because the spare tire hold down hook wasn't strong enough. it would bend and come loose from its mount thus allowing the spare tire to bounce around in a bad wreck. I think they sent(?)/gave(?) me a hook that was like a 1/2" in dia. vs 3/8" or so, I forget? Might have been 7/16" in dia., I would have to go and look...

pdq67
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Re: I would like to ask a unibody engineer some questions

Post by Dan Timberlake »

Packard V8 said - "body-on-frame car with a bumper jack and saw doors which wouldn't open, doors which flew open, windshields or rear windows cracked by body twist, bumpers which twisted into the body panel".

Some 60s BoF cars claimed to have fancy body mount biscuits, in strategic locations to isolate the frame from the body for NVH improvements.
http://oldcarbrochures.org/index.php/NA ... ll-Size-24
"node point body mounting"

Certainly there are times when controlled flexibility is required for a great design

I always silently ponder the reverence with which some folks say "and it's got a full frame ."
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Re: I would like to ask a unibody engineer some questions

Post by pdq67 »

Isn't an old '53 Chevy convert. frame one of the most rigid because it was a full frame plus an "X" frame in the middle??

Frame was used for the 'Vette's up to'62 before they went IRS. It also had the old style center steering design in the frames front end.

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Re: I would like to ask a unibody engineer some questions

Post by Brian P »

Open cross-section frame members (C-channel) mean they would have had little torsional stiffness. The "X" triangulates against fore/aft distortion of one side relative to the other, but it does little to stop the front from twisting relative to the rear (the whole structure deforming by warping).

Boxing in frame members greatly improves torsional stiffness but you are ultimately still dealing with a relatively flat structure. A large cross-sectional tube, even with thin walls (i.e. a unibody) is much stiffer than a small tube. The stiffness goes up exponentially with the cross-section.
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Re: I would like to ask a unibody engineer some questions

Post by pdq67 »

Brian,

Are you saying to 100 percent box the '53 Chevy convertible frame and it will be made about as stiff as possible?

For all channel formed steel, box the forth side and for any I-Beam formed steel, box both sides.

pdq67
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