30 replies to this topic

[Earthling]

As I mentioned in my previous post (the one about push/pull rod damper actuation), I want to build my own sports car (from scratch), and again, the emphasis is on low mass, PMI and CG, and excellent handling characteristics. Drivetrain layout is FR....

Like I said before, I'm not terribly fond of tubular spaceframe construction. It uses cheap stock material, it's easy to design and repair but that's about it. Its stiffness/weight ratio is inferior to that of either aluminum extrusions or aluminum honeycomb. It requires skilled labor(ers) to fabricate, not to mention a jig etc... And despite all of that, tolerances for chassis pickup points etc.. are usually no better than 3-4mm compared to 0.5mm for bonded aluminum. Big difference...

So there is no doubt in my mind that I would rather use a bonded aluminum chassis of some sort.
But there are a few questions I need answered before I can commit any ideas to paper. Firstly,
what are the pros and cons of bonded extrusions compared to bonded aluminum honeycomb? Why would one use one in place of the other? I would imagine that honeycomb would be more expensive, but would be stiffer and lighter yet... I'm also guessing a combination of both would be best? Like honeycomb for the bulkheads and extrusions for the "frame rails"? Any books or references on the matter would be highly appreciated.

Secondly there's the matter of economics. How much more expensive would extrusions and/or honeycomb construction be compared to a tubular steel spaceframe? With the former and please feel free to correct me at any point, you've got an expensive raw material, but a cheap manufacturing process (less labour intensive, easier and faster to make) whereas with the latter, raw material is (significantly) cheaper, but production is slower and more labour intensive. To sum up, approximately how much more expensive would a bonded aluminum chassis be?

Then there's the issue of durability, safety, and ease of repair.

Regarding durability, I'd like to bring up a good point imaginesix brought up,

With regards to the frame, I must point also out that Lotus were unable to adapt the bonded unibody concept to the engine/gearbox/rear suspension cradle of the Elise, because there were too many mounting points that needed to be located in close proximity, that caused insurmountable structural problems. The engine bay for the Elise is a steel spaceframe bolted to the aluminium 'tub'.

I would imagine that this would be more of a problem on the mid-engined Elise, with its more cramped engine/transmission layout, and their proximity to a multitude of mounting points. I don't think would be as much of a problem for my setup (FR remember) , especially if I were to ceramic coat the headers and not install catalytic converters That would solve our problems wouldn't it?

Safety should be at least as good if not better than a similar steel chassis, and if I were to design in replaceable front, rear, and side impact modules (a la Panoz Esperante) the chassis should be very easy to repair in the event of a prang!

That's pretty much it I guess, I'd like to hear your opinions on this, see if there's anything I left out or didn't cover...

[Ben]

The leeds FSAE team use a cut and fold method to create a chassis from a large sheet of honeycomb skinned with carbon one side and aluminium the other.

The idea is that you cure the panel flat and then rout slots in the carbon side. The ductile aluminium can then be folded into the desired shape. The slots in the carbon are then filled with foaming adhesive and patched over with wet layup carbon.

Leeds use metal subframes bonded in to locate the suspension, but they are working to FSAE cost report restrictions. If you wanted to go the whole hog with this approach you could bond in bulkheads of similar material and bond in solid inserts (tufnol or aluminium) into the panel as you make it. These inserts would then allow you to bolt on brackets and feed the load into the bulkhead.

The advantage to this as I see it is that you can cure the panels with vacuum and heat in their simple flat form (a flat plate is about the simplest tool I can think of ). All the actual jigging of the final chassis could be done using cut stations that you could clamp the panel in as the joints cure.

I don't think aluminium extrusions are realistic for a one-off build.

Ben

[Earthling]

Thanks for the reply Ben, I'm going to have to check out the Leeds FSAE site later for pics, as its 2.30 in the morning and I don't quite get what you're saying This isn't going to be a one off, I was thinking production will be between 300-500 units a year... Why aren't extrusions an option for a one-off? Is it because creating an extrusion die is expensive? With that out of the way I'd like to hear more ideas... keep em coming gents!


One more thing, I'd like to make a reference to a paper on honeycomb construction... Very interesting reading, I suggest you take a look. Here is the link:www.tech.plym.ac.uk/sme/innovation/chass.pdf

[Cory Padfield]

Earthling,

A die suitable for an aluminium extrusion is not expensive (~ US$1000). The problem is finding an extrusion company willing to use their high-productivity equipment to produce a few meters of final product. If you were able to use same-sized extrusions for many frame members, then 300-500 vehicles could mean hundreds of meters of product, which could be enough to interest an extruder.

When you ask for the "pros and cons of bonded extrusions compared to bonded aluminum honeycomb", I am unclear how you plan to use the honeycomb. By itself, honeycomb is not suitable for frame construction. Do you intend to bond surface panels (polymer matrix composite, aluminium?) to the honeycomb to complete the structure? Based upon the questions you are asking, I would recommend against any composite structure and encourage you to look at the benefits of a space frame melded with the benefits of lower density materials and alternate joining methods.

Cory

[AdamLarnachJr]

What about titanium spaceframes? I mean the stuff is dirt cheap in Asia, almost cheaper than steel.

Or perhaps what R&S has done with their new LMP, they used a relativley simple spaceframe structure but increased its rigidity by riveting carbon fiber panels to it. Simple, cheap, and reliable.

[david_martin]

An alternative to extrusions, which are not really viable on such a small volume, might be to use hydroformed aluminium or steel sections. Hydroforming is a reasonbly cheap and increasingly widespread technology and it is realtively straightfoward to set up for a small size production run. Using hydroforming, it is possible to produce stiff and lightweight long, hollow sections in complex shapes and the same joining technologies that are suitable for extrusions will work with hydroformed parts.

Additionally hydroformed sections offer another approach to spaceframe construction which is becoming increasingly popular in the auto business for aluminium vehicles. The space frame is assembled out of a relatively small number of large cross-section, hollow hydroformed beams, using light metal cast or machined locators to fix the beams during assembly. This requires much less complex jigging than the traditional welded tube space frame.

[AdamLarnachJr]

I know the vette uses hydroforming it its structure, but could you somehow hydroform a monocoque like structure? Perhaps have a rigid body shell with the window pillars and roof molded in, and simply use spaceframe subframes that bolt to the body shell? I don't know how rigid a hydroformed frame rail would be, but in high torque applications they need to be braces... much like the C5R which uses the stock frame rails.

[Ben]

Hydroforming dies would make extrusion does look like small change for something on the scale of a large chassis rail/monocoque.

I think the advantages of a spaceframe are often ignored. Monocoques were originally designed for aircraft where they are excellent at dealing with the large distributed loads. In a car we're talking point loads and a well designed spaceframe linking these point loads with members in pure tension and compression. I was suggesting the Leeds approach because I thought you meant a one-off. If you're gonna make 300-500 a jigged spaceframe will be an excellent solution.

Ben

[AdamLarnachJr]

Couldn't it be easier to subcontrac the work to a factory who could produce a good hydroformed body structure? Welding and whatnot is very labor and $$$ intensive, so couldn't cost be saved, as well as body and interior mounting time with a possible formed body? Almost like what Audi did with the A8. Design the body and mounting points in a CAD program, have em run a few hundred production peicies of it, weldin in the floor pans, attach teh subframes, and you have a chassis.

All more simple than it really is, but with a hydroformed aluminium spaceframe, wouldn't the cost be cheaper and the end product lighter than a full chromemoly or steel space frame?

Also, anybody seen or heard of a titanium spaceframe?

Thanks

[unrepentant lurker]

My understanding of titanium is that it is cheap to cast but fantastically expensive to machine or weld. Aside for jet engines and other super-high performance stuff, its never caught on. I can only assume that it is down to cost.

[AdamLarnachJr]

Can't you TIG weld titanium? I know tubing isn't that expensive.

[MRC]

Adam, yes you can TiG weld titanium. It requires that are thorough in your gas shielding of the welds. Any oxygen contamination of the weld, will lead to a weak and brittle weld. I don't know about casting, but at least everything else with titanium is expensive.

Earthling, I think that you'll find that 4130 (Cr-Mo) tubing (of which many spaceframes are made of) isn't that cheap. Cheaper than Ti, I guess.

[AdamLarnachJr]

Chromemoly is roughly much more expensive than steel, the only advantage I see in it is weight over steel as well because you can use less of it to get the same strength as steel and save about 25% in weight. But just like the titanium, you have to TIG weld chromemoly, where as you can MIG weld (or any other weld for that matter) steel.

However, I'm really interested in seeing what the production cost of building an aluminium body would be, something done by perhaps Alcoa or Cooper.

Anybody have any idea of the cost of around 300 production runs?

[MRC]

Adam, while I agree that TiG welding is the best way to go in most instances, there are many people that will disagree with you, that you must TiG weld chrome-moly steel alloys. Gas welding and brazing have been used for many years with excellent results. Gas welding & brazing has been succesfully used very well in the aircraft industry for the welding of 41xx alloys, for many years.

mark

[imaginesix]

Originally posted by Ben
I think the advantages of a spaceframe are often ignored. Monocoques were originally designed for aircraft where they are excellent at dealing with the large distributed loads. In a car we're talking point loads and a well designed spaceframe linking these point loads with members in pure tension and compression. I was suggesting the Leeds approach because I thought you meant a one-off. If you're gonna make 300-500 a jigged spaceframe will be an excellent solution.

Yup. And don't think that a bonded chassis would require any less skilled labour or processes.

Originally posted by AdamLarnachJr
But just like the titanium, you have to TIG weld chromemoly, where as you can MIG weld (or any other weld for that matter) steel.

What do you mean 'you have to TIG', 'whereas you can MIG'...? That's like saying, "If you get the sport model, then 'you have to' get the manual gearbox, 'whereas you can' get an automatic with the luxury model". TIG welding gives you much more control, and can result in stronger, lighter, better finished welds. MIGing is for the shadetree mechanic.

[AdamLarnachJr]

chromemoly, titanium, aluminium should not be MIG welded for certain reasons, I forgot who told me this but it has to do with the weld strength and penetration into the materials. As you said, TIG welding is more for high end applications, while MIG is rather easy on the pocket book, but still provides great strength for mild steel applications. I have never seen, nor heard of anybody TIG welding titanium, aluminium, chromemoly, or any other higher end materials for chassis design other than production line junk.


----------
Another reason why you should rather TIG the above materials is the large investment of a 230V pulsed MIG welder, a 230V source, most likely a pricey $2000USD generator, Argon and Helium, whereas with a TIG welder which is rather pricey, you only need Argon to weld titanium and aluminium and the prep time you spend on the metals with a MIG welder is non existant, no more scraping and scuffing aluminium every half minute or so, you get better penetration, and anybody who has a 230V pulsed MIG welder has got to be welding some time, and I would assume would be proffesional enough to invest in a TIG welder. This is all blah blah heresay, but you are correct you can MIG weld titanium, aluminium, and chromemoly, but if your building a high end sports car; It's not worth it.

[MRC]

Adam, I never mentioned MiG. I said brazing and gas welding. I assure that many a fine sports car have been built utilizing these methods. Fine (chrome-moly) bicycles have been brazed for years. MiG welding is a bad idea, unless it's in a purge chamber. I have heard MiG welding chrome-moly is not a great idea for various reasons, but I don't really know why. Aluminum can be welded quite using Mig. Given a properly set up mig welder, anyone can lay down an aluminum bead. It will still be crap. A properly skilled operator can do just fine. SST is also something normally associated with tig, but this can also be welded quite well with mig, again skill being the issue. I don't agree with mig being used just by hacks. In the hands of a skilled operator in can do just fine, and doesn't take the time that tig does. Also, just FYI, helium is some time used with argon while welding aluminum to gain more heat and penetration. You can get a good square wave entry level tig/stick combo with pedal, for about $1500 USD.

[Cory Padfield]

This discussion is filled with much anecdotal material, but less factual matter .

In one post, Adam said "I mean the stuff [titanium] is dirt cheap in Asia, almost cheaper than steel". This is untrue - the economics of producing high-quality titanium alloy base material and finished products hasn't changed substantially in the past decade, and certainly isn't available in one part of the world at steel-like prices. If someone is offering this, then buyer beware.

In several other posts, the terms TIG (an acronym for Tungsten Inert Gas) and MIG (an acronym for Metal Inert Gas) are introduced and debated. These two acronyms are non-preferred - the preferred terms are Gas-Tungsten Arc Welding (GMAW) and Gas-Metal Arc Welding (GMAW).

MRC mentioned gas welding, which could refer to quite a few processes - can you be more specific with what process you mean?

Adam said "chromemoly, titanium, aluminium should not be MIG welded for certain reasons...". This is untrue. Quoting from ASM Handbook, Volume 6: "All commercially important metals, such as carbon steel, high-strength low-alloy steel, stainless steel, aluminum, copper, and nickel alloys can be welded in all positions by this process if appropriate shielding gases, electrodes, and welding parameters are chosen".

Each process has its merits and disadvantages, and should not be excluded based upon the anecdotal "evidence". For example, singling out titanium, the following is also from ASM Handbook 6: "Gas-metal arc welding is used to joint titanium and titanium alloys more than 3.18 mm thick. It is applied using pulsed current or the spray mode and is less costly than GTAW, especially when the base metal thickness is greater than 13 mm."

Cory

[AdamLarnachJr]

Originally posted by Cory Padfield

In one post, Adam said "I mean the stuff [titanium] is dirt cheap in Asia, almost cheaper than steel". This is untrue - the economics of producing high-quality titanium alloy base material and finished products hasn't changed substantially in the past decade, and certainly isn't available in one part of the world at steel-like prices. If someone is offering this, then buyer beware.

While I will grant you that all my information may not be 100% accurate regarding welding... Have you ever been to Russia? You ever seen Titanium shovels, screwdrivers, etc. etc. that is cheaper than the food you eat? Titanium is an abundant resource in Russia, its very cheap... we have food, they have alloy's we need, kinda funny. In my USCAV catalog they sell a titanium crowbar from Russia, solid titanium and its around 16" long and almost an inch think, it retails for only $45USD.

I have learned all the information about TIG and MIG through my experience starting out welding back in the 11th grade, I have conversed with many people at speed shops who use both MIG and TIG for their purposes, the reason I singled out titanium and aluminium is because we were discussing them in chassis design. You can MIG weld anything, provided you have a 230v mig welder that supports high amps (150+). I have tried to MIG weld aluminium quite some time ago with a MIG welder, wasn't the easiest way to do it, nor the funnest having to prep the damn thing every few minutes. When the shop teacher showed me with the TIG, it looked worlds easier., but why waste your time with all the prep work? Anyway, its a moot point, you are all verily correct, but I was simply stating regarding chassis design, because thats what this post is about, if your building a high end sports car why use a lesser method? If your doing it with mild steel, sure np, but other exotic metals requires a bit more attention in weld quality.

And again, because no body has answered yet... what would it cost to do a short run of hydroformed vehicle bodies from manufacturers such as Alcoa or Cooper run per chassis?

[MRC]

Cory, when I mentioned gas welding, I was thinking of oxy-actelene. While GTAW & GMAW are proper terms, I think that you will find that tig and mig are more common terms used by even the peofessional welder. I have rarely heard a welder use GTAW or GMAW, even they all know what it is. Mig for titanium, like I said is done, but you still have to have proper shielding.

As to the manufacturing costs of hydroformed bodies, I imagine most of the cost would be in the dies. Beyond that, no idea.

[Engineguy]

This is a hoot.... I've heard this, I've heard that, my shop teacher said...

I MIG and TIG weld just about every day. Properly done with the same amount of joint prep and care, the right settings and shielding, etc. , a good weld is a good weld. TIG is more dependent on welder skill, so a bad weld is just as likely. The opportunity to go to scarey thin wall thicknesses using chrome moly rather than mild steel drives you toward TIG in that situation on a RACE car. If Michael Andretti bumps me at Laguna Seca and my left front wishbone buckles, I go off the track into a tire barrier, get out, and shake my fist at him. If I hit a chuckhole on the highway and my left front wishbone buckles, I have a headon collision with a 12 ton truck. There are limits to even very high-end road cars... race cars are by comparison pampered, constantly inspected, and parts are carefully lifed. I'll take nice thicker wall, MIG welded, safely ductile mild steel parts on my road car. It's a different environment. If I must drive around lighter I'll just have to leave my briefcase at home.

Tooling and process development for hydroformed frame rails and/or door opening hoops is $millions; not a low volume solution (Corvettes are not really very low volume... 650 per WEEK, every week, with many years between chassis changes). Semi-low volume aluminum extrusion/aluminum cast node "semi-space-frame" cars like the Prowler and Ferrari 360 are mostly MIG welded (hand and robot) as are most aluminum frame crotch rockets. Large cross-section rectangular aluminum tubing (standard sizes, no tooling) and aluminum sheet all MIGed together can be kept simpler and will have fewer tubes and weld joints than a traditional steel (mild or CrMo) tube space-frame. Doubler plates to create long welds keeps the loads per unit weld area down.

Titanium crowbars are readily available in industrial supply catalogs (i.e. McMaster-Carr p/n 3500A1, 21", $46.08), as are titanium wrenches, etc., mainly for their non-sparking characteristic. But a titanium crowbar has as much in common with high quality thin wall alloyed titanium tubing as a $6.00 steel crowbar has in common with a $100 steel alloy Carillo connecting rod. And for all critical applications, titanium must be welded inside an inert gas chamber, and even then strong welds are not assured because of the metallurgy involved in the alloys.

[AdamLarnachJr]

Cooper does low volume runs with hydroforming, you could perhaps create several small extrusions that could take the place of tubing, and weld them together (MIG ) as a much cheaper substitute to the rather large sizes of the vettes frame rails. They do it quite cheap too.

Aside from that... engineguy, you seem to have more experience in welding that most of us here, have you ever ran into trouble joining different metals such as titanium, aluminium, and steel together?

As far as titanium goes... check out the following links if you have time.
http://www.amm.com/i...98/rk121198.htm
http://www.amm.com/i...98/rk100998.htm
http://www.amm.com/i...98/rk072498.htm
http://www.amm.com/i...98/rk050898.htm
http://www.amm.com/i...98/rk012398.htm
http://www.boeing.co...ease_980311.htm
http://www.russiajou...e.shtml?ad=5230
http://www.tradeport...sia/trends.html
http://www.bikepro.c...dlebarover.html
http://minerals.usgs...nium/670494.pdf

"Russia has recently been identified as a possible source of low-cost, high-strength titanium alloys. The appeal seems to be twofold:

First, in theory, Russia's costs of labor and electricity are lower than the West's. However, costs are also lower because those manufacturers offering tubing for sports applications have not invested in up-to-date equipment and processes for optimum quality.


Second, Russian producers reportedly have a more extensive array of high-strength alloys. This, however, is a misunderstanding that arises from Russia's labeling system for its 200 alloys. In fact, many Russian alloys are similar to U.S. alloys, but carry different names or slightly different formulations. For example, Russia's equivalent to 6-4 is called VT-6. The properties of these alloys are nearly identical. And Russia's VT-5 alloy has similar performance specifications to 3-2.5.
In 1993, the Raleigh Cycle Company began distributing a frame featuring tubing manufactured in Salda, Russia (the frame is welded in England). This tubing, called BT01, is a Commercially Pure titanium approximately equivalent to U.S. Grade 4, or Russian grade VT1-1 (64 ksi yield) The yield strength is roughly 70,000 psi, an increase of 40,000 psi over U.S. Grade 1. The tubing is strengthened to this level through oxygen induction (or-oxygen hardening); oxygen content tolerance is 2.6 times higher for Grade 4 than Grade 1. Nitrogen induction is also employed in BT01 to increase yield. Although yield does increase with oxygen induction, ductility is reduced by about 80%; that is, elongation falls from 27% to 6%, creating a much more brittle structure. Fatigue strength is also reduced.

Merlin has worked with a few groups from Russia for the past four years, but so far the quality of their products has been unacceptably low. Raising the quality will require heavy investments in tooling, processing and equipment, which in turn will increase costs, probably to levels equal to or greater than those in the U.S.

Reliable delivery is also problematic, in part due to Russia's political situation. With no assurance of a stable supply or guaranteed shipments, the immediate future for Russian titanium seems questionable at best."

Now, Boeing has already contracted with Russian suppliers and has already recieve 2400tons for its products. I'm sure they are working on quality issues, but it is cheaper than anywhere else, and some suppliers must be taking steps to increase quality. Boeing uses this stuff to secure the wings to the fuesalage, and many other stressed components.

[desmo]

TIG welding Ti tubing can successfully be done in an open air environment if the interior is purged of ambient air and replaced with inert gas. I've watched high-end bicycle frames welded up from thin wall 3Al/2.5V alloy tube this way.

[AdamLarnachJr]

Wouldn't that involve a sealed chamber, like a painters booth but completly air tight? Along with some SCUBA gear:) The ultimate in welding.;)

[desmo]

Oddly, a friend who used to be a machinist at Boeing (and helped develop techniques for machining Ti alloy) describes more or less that exact scenario as being fairly common in aerospace (welding Ti inside a chamber filled with inert gas using breathing apparatus) back in the early 60s. I don't know if this technique is ever used anymore, apparently blowing inert gas onto the weld is adequate for most situations.

[Engineguy]

Originally posted by AdamLarnachJr
Wouldn't that involve a sealed chamber, like a painters booth but completly air tight? Along with some SCUBA gear:) The ultimate in welding.;)

More commonly done with a chamber similar to a sandblasting cabinet... built-in long sleeve rubber gloves, and a viewing window you must look through. Sometimes 4 pairs of gloves and 4 windows so you can weld from all sides.

[Earthling]

Hey Guys what's up, I'm kinda in a rush right now and I will post in more detail later, but I'd like to remind you guys that we've gone off topic a bit... This has turned into a post on different welding methods/techniques But seriously now, I was more concerned with the way of making the chassis, and like I said, I'm not too keen on welding (inferior structural efficiency and tolerances) and spaceframes for that matter... I believe that adhesive bonding is the way to go for specialty sports car manufacture... Just give the technology a few more years to mature. There's also the question of how the chassis should be made and layed out... Again, I'll leave that to our resident experts Post away gentlemen!

[H. Eckener]

What are you looking for in terms of structural efficiency and tolerances? Can you quantify their importance to you?

[MclarenF1]

I would have to say that a correctly designed spaceframe has many advantages to bonded aluminum extrusions/honeycomb chassis. The most important of these is cost. The cost of purchasing thin-wall 4130 is going to be much cheaper than making dies for aluminum extrusions. If you are planning on making several of these cars, you can have all of the tubes pre-fish mouthed. A 5-axis laser cutter can do a sweet job of this. Then it is a matter of building a fixture to hold the tubes in place while an experienced welder does his job. If he/she is worth their weight, they can correctly sequence the welding process to minimize the amount of warpage that will take place. Manufacturing a jig to hold tight tolerances on the suspension pickups is not too difficult. And if you are concerned about the welding strength, just ask any aerospace certified welder about their abilities. Secondly spaceframes also can have very high stiffness/weight ratios, obviously not as high as an F1 car but more than enough for a sports car. Our FSAE cars have had stiffness/weight ratios near 30 (ft*lbs/deg)/lb with the addition of a few fiberglass (or carbon fiber) panels added to triangulate areas like the cockpit. Also, why do you need such high stiffness? Are you planning on having a car with radically different front and rear roll stiffness? But if you are set on using aluminum (which would have to be quite thick to resist fatigue), using extrusions is a good design choice. Later.

[AdamLarnachJr]

Composite panels riveted to large openings in the spaceframe can be very beneficial in terms of stiffness as well. I find a spaceframe car much easier to produce than an extruded aluminium car, not to mention very cost effective.

The tolerances might not be as tight, but you can get them pretty damn close, and as far as welding being a rather inferior method... for many race and high end sports cars, its deemed very sufficient... Saleen, R&S, NASCAR, Trans Am, SCCA Formula series, Mosler (Check out the tubbing he did for this, its a relativley cheap car with a carbon fiber monocoque tub), Lamborghini, Ferrari, TVR, Stealth, Ultima, and dozens of kit/race cars.

[AdamLarnachJr]