I would like to ask a unibody engineer some questions

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

Moderator: Team

Brian P
Guru
Guru
Posts: 1611
Joined: Wed Jan 02, 2008 7:35 pm
Location: Toronto, Ontario, Canada

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

Post by Brian P »

Sorta (I'm on the tooling - mfg - automation side of it). If I knew what the question was, I could direct it to someone who is, if I don't know the answer myself.

One of my customers is the world's largest Tier 1 supplier for automotive bodyshell components, stampings, and weldments. I'm going to visit their automation HQ on Monday. Fire away.
JCR
Pro
Pro
Posts: 290
Joined: Sat Mar 04, 2006 12:13 pm
Location: H-Town, Tejas

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

Post by JCR »

I guess you posted the same question here: http://www.eng-tips.com/viewthread.cfm?qid=380311
Brian P
Guru
Guru
Posts: 1611
Joined: Wed Jan 02, 2008 7:35 pm
Location: Toronto, Ontario, Canada

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

Post by Brian P »

Answer on the pinch weld is the same here as it was there. Whether the jack applies load to the actual bottom edge of the pinch weld or to the surfaces immediately on either side of it, either way the jack has a notch that locates the top of the jack against the bodyshell so that it doesn't slip, and it's making use of the beam strength of the pinch weld in the vertical direction to minimize local deformation. (I have a customer that makes scissor jacks, too ... They're all built pretty much the same ...)

As for the beam strength between the front and rear of the car in a convertible ... it's all up to the rocker panels and the central tunnel, because that's all there is. The beam loading is only part of it; torsion is a biggie because a one-wheel bump tries to twist the bodyshell, first up front, then in back.

The rocker panels have inner and outer structures welded together that essentially form a tube, which is resistant in torsion, and being at the outer edge of the width of the car, that gives as much resistance to bending and twisting as they can get. The central tunnel is an open structure, which means not very much resistance to twisting, but at least the vertical depth gives some resistance to bending (like a beam). The connection between the central tunnel and the firewall needs to be more than just a firewall if this is to accomplish anything. Nowadays there needs to be a strong load path between the passenger cabin (rockers at the outer edges and central tunnel in the middle) and the front rails (halfway in between those structures, either side of the engine but inboard of the wheel wells) because of crash requirements.

The only vehicle that I've worked on that has had a convertible variation has been the new Mustang, and unfortunately the parts that I had something to do with were all up front, not the floor structure and not bodysides.

Everyone has been strengthening the "door ring" - the inner structure that goes all the way around the door opening - because of the new low-offset front crash test. No can do, with a convertible. I don't know how they are handling that, or even if IIHS has tested a convertible.
Brian P
Guru
Guru
Posts: 1611
Joined: Wed Jan 02, 2008 7:35 pm
Location: Toronto, Ontario, Canada

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

Post by Brian P »

Also, some unibodies have a multi-layer construction around the central tunnel, in an effort to stiffen it. The top and bottom layers again form something of a tubular cross section, which is stiffer in both torsion and bending, and it gives the structure up front something to tie into in the footwell area.
User avatar
panic
Guru
Guru
Posts: 2295
Joined: Sat Dec 11, 2004 12:04 pm
Location: Ecbatana
Contact:

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

Post by panic »

the inner structure that goes all the way around the door opening

Requires a "B" pillar, so 2 dr ht is also ng.
Brian P
Guru
Guru
Posts: 1611
Joined: Wed Jan 02, 2008 7:35 pm
Location: Toronto, Ontario, Canada

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

Post by Brian P »

Yep, hard-tops of the style seen through the 1970s are gone. No floor to roof B pillar = side impact failure, in addition to weakening the door ring, which has now become very important for low-offset frontal crash.

This is why the little triangular side window on the Challenger and Camaro doesn't roll down the way it did on the 1960's - 1970's predecessors.
peejay
Guru
Guru
Posts: 1946
Joined: Tue Aug 10, 2010 9:16 pm
Location:

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

Post by peejay »

The jack point often has a straight vertical panel sandwiched inside it for jacking strength, as people who jack "any old place" on the pinch rail often find out!
pdq67
Guru
Guru
Posts: 9841
Joined: Thu Mar 04, 2010 8:05 pm
Location:

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

Post by pdq67 »

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
HotRodRay
New Member
New Member
Posts: 23
Joined: Wed Aug 16, 2017 12:29 am
Location:

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

Post by HotRodRay »

Remember, most of the design "quirks" are required by the bean counters controling the production.
Cars are designed for a limited lifespan, then recycling.
That is why WE add chassis upgrades.
mk e
Guru
Guru
Posts: 5482
Joined: Mon Sep 17, 2012 3:19 pm
Location: Elverson, PA

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

Post by mk e »

Brian P wrote: Sat Mar 21, 2015 10:26 am The beam loading is only part of it; torsion is a biggie because a one-wheel bump tries to twist the bodyshell, first up front, then in back.
I did the chassis design for an fsae car years ago and dug up some reasearch from the 60s that said its a waste of time to worry about beam strength becausev any design that had adiquate torsional strength also has plenty of beam strength. The was true for all the design iterations i ran through fea and i remember referencing the work in the report.

Today I think you more hit the nail when you mensioned crash testing and safety. I'm pretty sure that any design that will pass the crash test standards will have both adequate beam and torsional stiffness, at least for street use, racing applications change things a bit.

So the question I have for the original poster is what exactly is the concern with beam strength as it's normally only important in things like truck load capacity and mot very interesting if you're talking about racing?
Mark
Mechanical Engineer
PackardV8
Guru
Guru
Posts: 7619
Joined: Sun Jul 30, 2006 2:03 pm
Location: Spokane, WA

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

Post by PackardV8 »

Today's unibodies are wonderfully stiff, yet deform to spread crash impact. Those of us who've been wandering through wrecking yards for fifty years can really appreciate the difference.

And don't get me started on bumper jacks - a tool of the devil if there ever was one! By comparison, a scissor jack on an underneath pinch weld is sanity itself. Those of us who jacked up a 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; and that's if the bumper jack actually held up the car during the tire change. We don't miss the bad old days.
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
Kenova
Expert
Expert
Posts: 504
Joined: Wed Sep 01, 2010 9:05 pm
Location: Ontario, Can.

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

Post by Kenova »

PackardV8 wrote: Thu Jan 18, 2018 1:07 pm ..... Those of us who jacked up a 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; and that's if the bumper jack actually held up the car during the tire change. We don't miss the bad old days.
I once had a '66 Beaumont convertible (Chevelle). I used a bumper jack to jack up a rear corner to change a tire. I swear that thing had more articulation than some rock crawlers. The next time I used a scissor jack out of my brother's Toyota Carolla and put it under the axle.

Ken
Over the hill but still learning!
Retaining it is the hard part.
j-c-c
Guru
Guru
Posts: 6545
Joined: Sat Sep 03, 2016 9:03 pm
Location:

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

Post by j-c-c »

mk e wrote: Thu Jan 18, 2018 9:48 am
Brian P wrote: Sat Mar 21, 2015 10:26 am The beam loading is only part of it; torsion is a biggie because a one-wheel bump tries to twist the bodyshell, first up front, then in back.
I did the chassis design for an fsae car years ago and dug up some reasearch from the 60s that said its a waste of time to worry about beam strength becausev any design that had adiquate torsional strength also has plenty of beam strength. The was true for all the design iterations i ran through fea and i remember referencing the work in the report.

Today I think you more hit the nail when you mensioned crash testing and safety. I'm pretty sure that any design that will pass the crash test standards will have both adequate beam and torsional stiffness, at least for street use, racing applications change things a bit.

So the question I have for the original poster is what exactly is the concern with beam strength as it's normally only important in things like truck load capacity and mot very interesting if you're talking about racing?
Would be nice to review that 60's beam strength understanding mentioned above. And i take from that, most typical sub-frame connector end user designs that focus on vertical beam strength vertical rectangle tubes are misdirected?
mk e
Guru
Guru
Posts: 5482
Joined: Mon Sep 17, 2012 3:19 pm
Location: Elverson, PA

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

Post by mk e »

j-c-c wrote: Fri Jan 19, 2018 8:53 pm Would be nice to review that 60's beam strength understanding mentioned above. And i take from that, most typical sub-frame connector end user designs that focus on vertical beam strength vertical rectangle tubes are misdirected?
I remember is was all based on ladder fame stuff, which was the standard of the day...and it makes a lot of sense if you thing about it.
A stamped channel is pretty strong up and down, as long as id doesn't twist which is why they add the cross members. If you make it big enough it works just fine, but big enough means it gets heavy and heavy costs money and reduces performance which is way the factories were looking for more efficient designs.

I was working mainly with space frames but modeled a basic ladder frame and the previous design I was replacing was a monocoque so I also modeled that specific design for comparison but made no attempt to improve it.

Subframe connectors serve a couple purposes depending on the car. If the subframes are rubber mounted they are moving relative to each other in every which direction which is very unhelpful to repeatable setups. Adding subframe connectors, the bigger the better, basically converts it to a ladder fame, but then the whole body above that was designed to add stiffness handles the torsional loads and it works better.

On something that was stamped and welded subframes, like a foxbody mustang the connectors work a little different. Again the bigger the better applies but this time you really have a monocoque structure and you are reinforcing it. stock the tunnel and rockers form the structure and the roof adds the torsional strength, but now you are adding 2 additional beams which can dramatically stiffen the floor and the whole chassis along with it.

Adding metal pretty much always helps stiffen things up, but getting the stiffness /lb as high as possible is the goal. Monocoque structures are about the best at stiffness/lb, really had to beat.....but cars need doors and engine bays and such which mean cutting big wholes in the skin and that then requires frame like shapes be added to support those areas. When you look at something like an F1 chassis they call it a monocoque but really its a ladder frame with HUGE side rails and sear panels added here and there.

Space frames were the subject of my research and work very nearly as well as monocoque structure when they are designed properly for torsion, but are great for cars because they give you "hard point" to mount drivetrain and suspension too. The problem is most examples I found were not designed very well. To work right a space frame needs to be made from triangles, but stupid engine bays and door opening bugger that up so a lot of structure needs to be add just like on the monocoque but most examples I found simply removed the diagonal to create the opening without beefing up the structure which absolutely destroys the torsional stiffness of the whole chassis. 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.

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. When he did the first one like 10 years ago we designed a a ladder fame using 4x4x1/4 steel tubing and that oven is going to events all over the state and happy as can be. 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. That frame was friking heavy and the welding shop talked him into 3x3x3/16 on the next one which was 1/2 the weight and still "plenty" strong...but that oven now has odd cracks in it. So the talk was about can he do something like cars do where all the storage space is structural and helps replace the heavy frame......about an hour later explaining the design concerns the frame will be 4x4x1/4 because its a frikin masonry oven and nobody but the welding shop cares how heavy the frame is and anything you do to lighten it for them adds WAY more cutting and welding which drives the price way up. Sometimes simple is good.
Mark
Mechanical Engineer
pdq67
Guru
Guru
Posts: 9841
Joined: Thu Mar 04, 2010 8:05 pm
Location:

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

Post by pdq67 »

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
Post Reply