I would like to ask a unibody engineer some questions

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

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exhaustgases
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I would like to ask a unibody engineer some questions

Post by exhaustgases » Wed Feb 18, 2015 3:35 am

Is anyone here an actual engineer for automotive body's and chassis? And not belittling anyone that isn't and likely knows as much or more. Its just that some people we deal with will only believe an engineer.

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

Post by Brian P » Wed Feb 18, 2015 4:41 pm

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.

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

Post by exhaustgases » 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.
2nd is beam strength in between front and rear axle, what are the primary members that are loaded the most? Think convertible as well.

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

Post by JCR » Sat Mar 21, 2015 3:25 am

I guess you posted the same question here: http://www.eng-tips.com/viewthread.cfm?qid=380311

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

Post by Brian P » Sat Mar 21, 2015 10:26 am

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.

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

Post by Brian P » Sat Mar 21, 2015 10:29 am

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.

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

Post by panic » Sat Dec 19, 2015 7:08 pm

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

Requires a "B" pillar, so 2 dr ht is also ng.

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

Post by Brian P » Sat Dec 19, 2015 8:24 pm

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.

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

Post by peejay » Sat Dec 26, 2015 4:01 pm

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!

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

Post by exhaustgases » Thu Feb 04, 2016 5:17 am

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.

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

Post by pdq67 » 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..

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

Post by HotRodRay » Tue Sep 12, 2017 9:02 pm

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.

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