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Discussion Starter #1
Unfortunately, the PDF won't be availalble online for about a month, but a very good article if you get the magazine.

Automotive Engineering is an SAE (Society of Automotive Engineers) publication.

If someone wants to buy it and post it... just kidding - you're not supposed to do that (funny thing, those copyrights).

AutomotiveEngineering said:
"GM Explores Sheet Hydroforming
It is a slow way to make them, but for low-volume vehicles such as the Pontiac Solstice, GM believes it is effective for producing large, complex, and blemish-free body panels. "
http://www.motorsportsengineering.sae.org/automag/current.htm#features

http://www.motorsportsengineering.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=1-112-7-94
 

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I read a bit about the Chevy SSR awhile back, and that artical said that one of the challanges of the SSR design was producing deep draw sheet metal components that no one has done for decades. I guess they had to create a new way of stamping to do them, but my understanding was that they were still stamped. Why didn't they use hydroforming for the SSR?
 

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AztekzRpurty said:
That's true, old cars used to have deeply curved body panels. The original Beetle, all the fenders on the 30's era cars, the 40 ford. So what makes it so hard now?
A quick and probably not well thought out guess would be the steel they're using. In the 'good old days' didn't they use something like 1/2" (ok, slight exageration) thick sheet steel?

They could make the deep draws and the metal would stretch but it never got as thin as the metal they use today. (Not knocking it. It is what it is.)
 

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Discussion Starter #5 (Edited)
AeroDave said:
I read a bit about the Chevy SSR awhile back, and that artical said that one of the challanges of the SSR design was producing deep draw sheet metal components that no one has done for decades. I guess they had to create a new way of stamping to do them, but my understanding was that they were still stamped. Why didn't they use hydroforming for the SSR?

AztekzRpurty said:
That's true, old cars used to have deeply curved body panels. The original Beetle, all the fenders on the 30's era cars, the 40 ford. So what makes it so hard now?
AeroDave:
Don't know why the didn't use hydroforming for the SSR, and Aztekz, what makes deep-draw stamping so hard now? I can speculate what might be happening. Let's take a trip to "Dr. Solsticeman's" classroom. :lol

Traditional stamping ideally takes two heavy dies made out of tool stamping metal. The parts aren't just formed in one stroke, they usually require multiple stages of forming. The ideal is to do it with only two or three strokes/stages. Each of these stages can be thought of as a separate die itself - the stages put together in a progressive stamping press can be called a "die-set"(can't exactly remember the terminology, but this works). These die sets cost money, and the cost of them are multiplied by the progression stages.

The old days, it would not be unheard of to see five stage progressive deep-draw stampings. The deeper the stamping, in general, the more stages it takes to form steel or aluminum without tearing it.

The demand for more efficiency in the 80's and 90's in vehicle mfg. processes led to some interesting conclusions (from people like Jim Harbour): the Japanese cars and body stampings required less stages and therefore saw significant increases in production volume and lower tooling costs. This meant the ability for more vehicles with less money (can we say "profit"). The American mfgrs. would use more stages in their dies, and if they needed more vehicles, they just doubled the presses.

These die sets had to be able to produce as many as a million vehicles - so the mfgrs. could spread the cost over a bunch of vehicles. This means making the dies out of tool steel. Which makes them as expensive as they are.

The other big difference between american and japanese cars back then is the japanese were "boxes" where we had more curvature (like on doors for the old corolla vs. cavalier, as an example). They used to say things like "slab-sided" and "flat". More curvature means more die stages... more money... less throughput... see where I'm going?

So the question is: knowing all this, how would you make a niche vehicle with deep-draw classical styling?

I suspect the SSR is using "sort of" traditional stamping, and judging from the shape must have progressive dies in 5 or 6 stages. How could they do this without making an $80,000 niche vehicle (like the Prowler or the Viper)? One way is to take advantage of 'niche'. If the dies only have to live to make 75,000 vehicles, then they could make the dies out of less durable material - like the prototype dies made before they cut the real ones out of tool steel. I think these dies are kirksite and they only last for 75,000-100,000 parts before they break down and can't make consistent and dimensionally stable parts. And, if I remember correctly cost half as much as regular dies. BINGO: tooling for half the cost of normal tooling for a vehicle intended for, say, 15,000/year for 5 years of it's life cycle.

Is this what they did? I dunno, but it makes the most amount of sense, and how I might do it if I were king of GM for a month and looking for creative ways to produce niche vehicles.

So, it's not hard now, AztekzRPurty. Just expensive to do with traditional dies and able to blow the business case out of the water. My guess is the Solstice panel curvature is unique and intended volumes lend itself to sheet hydroforming.

Sheet hydroforming is much lower investment cost. You only need to buy one die that doesn't need to be class-A surface. Problem is: traditional stamping makes parts at line assembly rate or faster. Sheet hydroforming is much slower - either multiple dies or limited production runs of complex shapes with the same basic attachment are the answer. When you're making stainless steel kitchen sinks, both aspects (limited production of different sink styles fitting in the same hole, and for the standard double-bottom sink, multiple dies) are important, especially when there is limited volume (when was the last time your family bought a stainless steel kitchen sink? A new car?).

So, there's my speculation. I dunno if it's accurate, just full of fun information just for your entertainment.

I'm not really that much of a geek :wink , but I am rooting for Ken Jennings - the man is a machine!!!
 

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Discussion Starter #6
A couple of tidbits from the article:

"...hydroforming is slow...one or two panels a minute vs. a dozen a minute for a conventional press doing a complex part and up to 18 per minute for something simpler..."

"...The Saturn roadster (a new design, not like the Curve concept showns at the 2004 North American International Auto Show in Detroit early this year) will be made in the same plant as the Solstice... [and] also is likely to use hydroforming for the deeply drawn panels..."

"...Sheet-metal hydroforming is hardly brand new. GM used it more than 40 years ago to produce very small parts -- metal headlamp reflectors -- because the process provided such a high quality, distortion-free surface..."

"...The first level of appeal for the technology is the 10-50% lower cost of the dies. In fact, there's normally just one primary forming die per panel..."

"...The dies to stamp the panels can be as much as 25% of the investment in a new vehicle program..."
 

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solsticeman said:
The old days, it would not be unheard of to see five stage progressive deep-draw stampings. The deeper the stamping, in general, the more stages it takes to form steel or aluminum without tearing it.
Actually I read that in the 50's, when we were really doing creative things with sheet metal, it took up to ten stages to do those sort of things. Now all of the stampings are done in two stages. Huge cost savings. They acheived the reduction of stages in part by making simpler shapes. I had also read that it is estimated that if Chevy wanted to make an exact replica of the classic 55, they would have to charge over 100k to make a profit due to the complexity of construction.

Cost saving measures have forced us into some pretty boring cars. Designers have been limited with what they could do and still be production fiendly. Hydroforming promises to allow designers a more free hand, at least on a limited scale. Hopefully the future will bring more efficient hydroforming so that mass produced cars can benifit from greater design flexibility. I still don't understand why they didn't use it for the SSR.
 

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Discussion Starter #8
AeroDave said:
...I still don't understand why they didn't use it for the SSR...
They had to do a lot of learning on the Solstice, my guess is that sheet hydro was not even an option to think of for the SSR. Probably not justified, because the announced price point was over twice the "Lutz line in the sand" for the Solstice.

Official early estimates of SSR cost were "around $40,000" or "Low 40's". GM never tried to pretend you could get an SSR for any less. Not that there wasn't wishful thinking and whining about the cost on many of the forums, but at least they didn't over-promise.

At that cost, the added program time to learn how to do Sheet-Hydro was probably not worth it - also ASC had a lot to do with the design, and I'm sure if they had any shared engineering responsibility, the lowest risk thing to do is head toward a variant of traditional manufacturing. At least that's how I reason it.
 

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If you had an infinite number of monkeys with an infinite number of hammers beating on infinite amounts of sheet metal over an infinite amount of time, would one of them accidentally create a Solstice?
 

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solsticeman said:
Let's take a trip to "Dr. Solsticeman's" classroom. :lol
Another great lesson Solsticeman! :cool Thanks for the well thought out explanation!

I have a wild guess about the SSR using stamped steel and not hydroforming. It may be possible that they just did not consider hyrdroformed body panels to be a viable option at the time. They have taken a lot of time with the Solstice to perfect it for use on body panels, but maybe when they were developing the SSR, they just did not see it as something they could get perfected in time, or at a reasonable enough cost to make it worth their while. Or maybe they just never really considered it.

Another option they could have used with the SSR would have been composite panels. They have plenty of experience with these in the Corvette, XLR, and Saturns. The downside is larger panel gaps with them, but they can mold basically any shape they want.
 

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Discussion Starter #11
Composite was probably an option that was quickly jettisoned. You can't really have a car with a "Truck Heritage" and then make the body out of composite.

It's made out of metal. Real trucks are made of metal. Just like the old 50's trucks. Steel. Good, strong, steel, uhg-argh-arrrgh-arrrgh-argh!

JMHO.
 

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Fformula88 said:
Another option they could have used with the SSR would have been composite panels. They have plenty of experience with these in the Corvette, XLR, and Saturns. The downside is larger panel gaps with them, but they can mold basically any shape they want.
What I remember is that the planners for the SSR dedicated themselves to the concept of making a modern hot rod and so insisted on steel fenders rather than plastic. Hot roders have a real bias against plastic. To them a real hot rod is steel, period. They will go out of their way to find original steel '32 Ford bodies to chop up rather than use easily obtainable plastic reproductions. Reproduction steel stampings aren't as good as originals either because the factory had better presses.

For someone like me who spent years of my life trying to save old cars and return them to absolute stock condition, it used to drive me nuts to see hot rod guys buying cherry low mileage originals to cut up into something that may never even be finished and probably have a very short life span with little value in the future.

Anyhow Chevrolet was trying to impress the traditional hot rod folks and so plastic was right out. I think they did a pretty good job and if they could lower the price a little I think they would have a really big hit on their hands.
 

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But back to the Soltice, why not plastic? Or metal and plastic? I'm talking about the body skin not that convoluted floor pan. I can see why it's hydroformed. But why not fiberglass. It's the rock that nitch cars have always stood on.

Sounds off topic but bear with me,
How I spent the last few days:
My son went out a week or so ago and bought his first car ('99 Explorer.. oh well). Now the car I got him when he got out of high school is mine and I'm getting it ready to sell. It's a '94 B4C Camaro (that's like a Z28 but made for the police). One of the things I noticed that would influence its value was that the driver's side door skin was loose and in danger of falling off. When we bought the car 4 years ago, I saw that probably some city mechanic with little bodyshop experience had done a rotten job of glueing it on. I carefully chipped all the old glue off, sanded it and glued it back on with body panel/ doorskin epoxy I got from a auto paint shop.

What I found was the door skin is fiberglass, as well as most of the rest of the door. I got out a magnet and the only metal body parts on that car is the hood and the two rear fenders. Both front fenders, the doors, the roof, both sail panels and the trunk lid were glass. Did you guys know that the Camaro was just a hood and a couple of fenders away from being a Corvete with a back seat?

Why the big investment in a new technology (on this scale anyway) when plastic works so well, it can go undetected for years? It's a sure thing the front and back bumpers will be poly anyway. Do you really think the first metal bodied Saturn will be a Kappa? Or is hydroforming the only way to make just the frame pieces?
 

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I see your guys point about using steel on the SSR to be true to the hot rod hobby and to make it a "real" truck! The irony there is that it makes for a terrible "real" truck, but thats another story! :lol I could see buyers being turned off by composites out there, and your right, hot rodders would be appalled!

Maybe the SSR is about to take off. Aren't they supposed to be dropping in an LS2 400 HP V8 for 2005? I don't know what kind of price increase it will bring, but it should make it more of a serious performer and not just another pretty boulevard tourer.


DreamerDave said:
But back to the Soltice, why not plastic? Or metal and plastic? I'm talking about the body skin not that convoluted floor pan. I can see why it's hydroformed. But why not fiberglass. It's the rock that nitch cars have always stood on.

Sounds off topic but bear with me,
How I spent the last few days:
My son went out a week or so ago and bought his first car ('99 Explorer.. oh well). Now the car I got him when he got out of high school is mine and I'm getting it ready to sell. It's a '94 B4C Camaro (that's like a Z28 but made for the police). One of the things I noticed that would influence its value was that the driver's side door skin was loose and in danger of falling off. When we bought the car 4 years ago, I saw that probably some city mechanic with little bodyshop experience had done a rotten job of glueing it on. I carefully chipped all the old glue off, sanded it and glued it back on with body panel/ doorskin epoxy I got from a auto paint shop.

What I found was the door skin is fiberglass, as well as most of the rest of the door. I got out a magnet and the only metal body parts on that car is the hood and the two rear fenders. Both front fenders, the doors, the roof, both sail panels and the trunk lid were glass. Did you guys know that the Camaro was just a hood and a couple of fenders away from being a Corvete with a back seat?

Why the big investment in a new technology (on this scale anyway) when plastic works so well, it can go undetected for years? It's a sure thing the front and back bumpers will be poly anyway. Do you really think the first metal bodied Saturn will be a Kappa? Or is hydroforming the only way to make just the frame pieces?
Actually, they are not fiberglas, but a type of plastic called reaction-injection moldings. GM first used the materials on a large scale on the 84 Fiero and 84 Corvette (both new designs). They then adopted it to Saturn (all side panels are composite, roof, hood and trunk are steel), and have used it there since, as well as the 1990-1997 GM minivans, and the front fenders and doors of the Camaro and Firebird. The RIM composite is a little more durable (think bendable, plyable, etc) than straight fiberglass, and it finishes smoother which makes for a better paint finish.

GM didn't use them on the Solstice due to cost. Ms. Queen, the GM small car VP (thats her name right?) was quoted in one article I had read on the hyrdoformed body panels talking about the different options they had. After doing a cost analysis, they found that although the composite could be made to the correct shape it would cost more to use plastic body panels than it would to make hydroformed steel panels. The plastic would have been dent resistant which would have been nice, but they never would have hit their pricing targets on the car.
 

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Body gap panels with plastic is a problem as well as noted by the press with regards to Saturn brand..

Something about the need for expansion and shrinkage room ...causing larger than normal body gaps...something that the consumer cares about..

Plastic body panel benefit never materialized for the majority of the GM lineup..

Corvette being the exception...
 

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Although the inside of the door skin didn't show the expected print through of mat or roving, my itchy arms suggest the it has fiberglass reinforcement but perhaps with a non-epoxy matrix. The front fenders have a lot more flexibility than the other non-metal body parts. Just a guess there's a mixture of multiple "space age" materials.
 

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Discussion Starter #17
Plastice also has the tendency and potential to increase the weight of production vehicles. Corvette started out non-metallic, and part of it's image is to maintain that. But it could be made lighter if the body was made out of metal and corresponding changes to the structure are accomodated and re-optimized.

The key is allowing the body panels to become part of the structure, instead of just keeping the shape. If you use a composite material such as CF - you can get strength AND lightness, but also a 6-figure+ car.

Yes, plastic panels are dent resistant, but they are also a pain to paint, don't have the greatest acoustic qualities, and are generally non-recyclable (at least RRIM, which stands for Re-inforced Reaction Injection Moulding).

The thermal expansion is a real issue. The old saturn S-series had huge gaps that grew by (no sh*t) 2mm in the winter. On a hot day, it was not unheard of for some gaps to close up and have body panels actually touch.

Also RRIM and other compounds are prone to heat load warping - ever wonder why the horizontal panels on the saturns are metal? Now you know.

The itch you mentioned comes from the small glass fill that they put in the RRIM that Fformula88 mentioned. This makes the plastic more rigid and stronger (but still a fraction of the strength of Carbon-fibre reinforced composite).

A lot of these issues also exist on the Fiero as well - although some of the higher stressed panels are made out of a type of fiberglass (like the rear quarter). But in the end you have a vehicle that is probably heavier than what it could have been (the Fiero was over 2600 lbs and some models weighed into the 2800lb range).
 

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solsticeman said:
A lot of these issues also exist on the Fiero as well - although some of the higher stressed panels are made out of a type of fiberglass (like the rear quarter). But in the end you have a vehicle that is probably heavier than what it could have been (the Fiero was over 2600 lbs and some models weighed into the 2800lb range).
The Fiero actually used 4 different types of plastics in its body.

Sheet Molded Compound (SMC) makes up the hood, roof, rear upper quarter, and rear decklid. This is the stuff that most resembles fiberglass. It is extremely stiff and brittle. It provides enough strength to hold itself in shape on those flat panels, but it will break apart if compromised badly (I know from experience!)

Reinforced Reaction Injection Molding (RRIM) make up fenders, door panels, and lower rear quarters. (the rear lowers were later changed to injection molded nylon). This is what Solsticeman was talking about. It has some reinforcement in it, but not a lot to leave it pliable so it does not suffer damage from minor impacts. It will crack with big impacts (such as an accident), but it can easily absorb door bangs in parking lots, and other lesser impacts (Saturn's commercial hitting the side of the car with all maner of objects).

Plain old Reaction Injection Molded (RIM) Urethane is used for the facias. Its close to the RRIM, except its much more durable. Essentially it allows someone to back into your bumper, and have that pumper return to its original shape and not crack and or break. However, on the Fieros, this matierial has started to sag on many of the cars as they have gotten older. Its not very strong, and the front fascia in front of the hood will start to dip a little.

Thermo Plastic Olefin (TPO) is used for the rocker panels. This is a harder plastic and much thinner. Its basically just a trim piece that coveres the metal rocker panel.

In regards to the body gaps, I think they are so large on Saturns from poor quality control and/or lower levels of engineering. The Fiero has some decent sized gaps too, but the Corvette and XLR bodies are fairly tight and I have not read complaints about either having excessive body gaps. So GM must have a way to make this material and attach it to the car to avoid excessively large gaps.

What makes the car heavy isn't necessarily the body, but the fact that is has to be attached to a complete car underneath. The Fiero has a complete space frame underneath, which could be considered equivalent to a uni-body car with an added body on the outside. The space frame itself is probably about the weight of a similar sized uni-body car, and then all the outer body skins are attached adding more weight.

However, keep in mind this is essentially how the Solstice is being designed, with a complete frame underneath that the body panels are essentially just attached to (like the Corvette). So I am not sure there is much of a weight savings with the Solstice by going hydroformed over composite.
 

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Discussion Starter #19
sorry, too lazy to remember all that.

I think the vette uses mostly SMC which has better thermal expansion properties and better dimensional stability.

It's also heavier than steel when used in the same or similar locations.

Steel is more dimensionally stable than ALL plastics - which is not unimportant. Better dimensional stability means higher quality and better appearance.

The Fiero plant, which I was in several times before it shut down, solved some of the dimensional issues with "the monster", a huge drill/punch fixture that made all the attachment holes for the major body panels in one station. This made acceptable body fits for the Fiero, but the Saturn tried to make cars with plastic panels and get away from a single station - the result is having to make larger gaps to cover the higher variation.

I think the Lotus Elise and all it's cousins are made mostly out of SMC as well. It's light, but I wonder if it could have been made lighter if it was smartly designed out of steel...
 

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Some thoughts about the Saturn my wife used to have; The underlying space frame added weight and reduced interior space. The outside was the same dimentions as a comparable steel car, so they sacraficed interior room for the space frame. I don't remember the gaps in the fenders being an issue at all, and the dent resistant panels were great. Wish we had them now, as her Subaru is covered in small dings the Saturn would never have. We live in a very constant whether enviroment and so never did notice shinking or expanding gaps. Plastic is also corrosion proof, very nice. So plastic has it's ups and downs.
 
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