Art & Inspiration Billet Aluminum Chassis by Kirkham Motorsports

Thanks!

And, please call me David!

David


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I'll answer the question #1 first.

Thank you for your questions. The chromoly question is complicated to answer. Funny, I just had the same question asked on another forum. This is the answer I posted. I know it is long, but I really think it is worth reading.

"Chromoly:

I know this is long, but for those who are interested in material science, I think this is well worth your time.

I'm glad you brought up this point. Chromoly is a most interesting alloy--and greatly misunderstood material in racing circles.

If we go back into history, chromoly tubing (generally 4130) really came into being for use in aircraft. The alloy was invented by Henry Royce and Charles Rolls (Rolls-Royce) to reduce the cracking on their frames when they were subjected to the monstrous "Shaking Machine," (the first 4 post automotive engineering test rig). The literally shook their cars to death, observed what broke, fixed it, and tried to break it again. There is a reason their pre-war cars were so highly regarded. They found by adding a little chrome and molybdenum to their steel they could greatly increase its tensile strength, yield strength, and fatigue life...in the the heat treated condition. Amazingly, they actually heat treated their entire ch***is. There is a phenomenal book about the early history of Rolls-Royce. I'll post the ***le if I can ever find it discussing all the things I am posting here.

Come along WWII airplane manufacture became a life-and-death race, literally. The loser in a dog fight--died. Rolls-Royce became heavily involved in the aircraft industry during the war (even today Rolls-Royce makes jet engines). Their alloy--4130 chromoly--became critical to the engine cages of airplanes which were subjected to horrendous vibration and G's while flying. In a race to the death, the guy with the highest horsepower and lightest weight craft generally wins as long as his airplane stays together. So cost of manufacture became of second concern to England--not speaking German being their first. It was truly one of the only times in history a manufacturer had an unlimited budget for an unlimited race--the race to survive.

Chromoly has fantastic properties--when it is heat treated. (When it is not heat treated properly, including by poor welding techniques, it has some rather nasty properties--not to mention chromoly steel is notoriously bad for corrosion resistance in either condition). To get the nicer properties of chromoly, you have to heat it up until the austenite turns to martensite (somewhere around 1525-1550 degrees F if I remember right). Then, it must be quenched to retain some of the hard martensite. As you can well imagine, heating up an entire race car ch***is to over 1500 degrees F requires some very large furnaces and quenching baths. Also, when the steel is that hot it will rapidly oxidize on the surface and the thin tubular sections most of us make our ch***is out of will be severely affected by scale if the heating is not done in an inert atmosphere--further complicating matters. To make matters even worse, the ch***is will then need to be heated up again and tempered to bring the material back to a reasonable hardness so it won't be brittle, crack, and fail in service (can you say John Force??? YouTube moment). When your country is being invaded, you don't mind doing all these things. When you are making a race car...well the theory sounds great, but in practice no one heat treats their ch***is because it is simply not reasonable to do so.

Now, this leaves us with a serious problem. If we use tubes, we must of necessity weld the ch***is together. When we weld, we certainly heat the steel up much past 1525 degrees. When we quit welding and take the heat off, we "quench" the weld because the rest of the tube we didn't weld on is cold and "****s" the heat out of the weld, thereby hardening the weld (and making it brittle). We might as well put a sign on the weld, "Break Here" because that is surely where it will break if we don't stress relieve the weld by heating it up again after welding to "even" things out. (We have not even got into the problems the higher susceptibility of the weld to stress corrosion cracking, intergranular corrosion, internal stress formation, and the formation of inclusions). When is the last time you saw anyone do that? Leaving hard areas next to annealed areas is asking for trouble.

So, let's say you do everything right. Good weld preparation, good welding technique, stress relieving of the weld after welding...what have we gained in stiffness? Nothing. Yes, nothing. Heat treating steel doesn't make the part any stiffer. 4130 steel, heat treated or not, has the same Young's modulus (stiffness) as plain old mild steel. Yes, that is not a typo. So, what do we gain by heat treating 4130? In short, (if everything is done properly and that is a BIG IF) we gain tensile strength (not too important in a race car). We also gain yield strength and fatigue life (extremely important in a race car because we want the part to tell us it isn't happy by BENDING before it BREAKS so we have some small chance of fixing it before we DNF, see ourselves featured on YouTube, or die)! Whereas a mild steel part and a 4130 part have the exact same stiffness, a properly heat treated 4130 part will have a MUCH higher yield strength and fatigue life (so we can bend it further and more times than we can a mild steel part). With a ch***is, stiffness is critical. If the stiffness of 4130 (heat treated or not) is the exact same as the stiffness of mild steel then why use it? If everything is done properly, you can save a small amount of weight with a 4130 tubular ch***is because the yield strength of the modern, normalized 4130 tubes is higher than 1020 tubing. However, if we use thinner, stronger tubes, we must inspect our Chariot of Fire more often for cracks, fatigue, gremlins, and other nastiness that somehow works its way into all of our lives. In the end, for most of us mortals, 1020 DOM (mild steel) is a far better choice. I don't want my customer dying today, tomorrow, or in 20 years. Who knows what will happen with my super duper 4130 ch***is in 20 years? John Force pays people to check his ch***is after EVERY race. Most of our customers don't even know what to check; fewer have any inclination to do so. Better safe than viral.

So, why do some race rules require it? VERY good question. The only reason I can think of is most of the racing rules came out of the dry lake and Salt Flat racing that was done right after the war. There was a good supply of high quality 4130 tubing at the time because the manufacturers existed with the relevant quality control to insure good product. In the mild steel world...well, not so much QC was needed on your water pipes. Welded tubing (or water pipe) wasn't made very well and QC was pretty poor. So, when guys showed up at the track with a water pipe car and the tubing split open (along with the driver's head) the stuff was banned. NOT because 4130 is better than mild steel but because the QC controls were far more strict on the 4130 aircraft tubing as planes falling out of the sky is a real bummer--especially for the pilot and p***engers. Today 1020 QC is highly controlled and most of it is a very high quality product. Modern 4130 tubing is even better than its counter part of years past and is now somewhat stronger than mild steel. We can save a little weight if we are conscience in our manufacturing techniques by using 4130--if we know, understand, and are willing to accept its drawbacks. However, instead of spending time time checking and rechecking our 4130 welds, I maintain most of us could save a LOT more weight in our rides by skipping a burrito or two and stepping on the treadmill."

I know some on this site aren't too thrilled about all the billet stuff...that is cool with me. The real reason I am here is to hopefully help out with a few answers to questions like this that are universal in car fabrication.

David

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Myself, being a HUGE fan of all cool cars and good craftsmanship would like to extend a welcome to you on the HAMB. I agree that billet stuff might not be so popular here, but I don't really think it's the point of your post anyway. Billet accessories are a little bit different than a WHOLE CH***IS. I understand why you posted and think the work is pretty awesome as well as the cars being produced by Kirkham. I agree with the people that say Cobras were hot rods in themselves. In fact, I think there is a good following of vintage racing around here to begin with, so as long as Cobras don't become topic specific then you will find warmth here. The fabrication going on with these cars is awesome (not like the gl*** stuff) and I would love to see more on the actual fabrication.

NOW THAT I HAVE YOUR ATTENTION... I have just two words that need to be said...

DAYTONA COUPE!

OH! And "Please".

Thanks for sharing and welcome to the HAMB!

 

The exact weight number escapes me right now. It was an error in the book that is now starting to really bug me

From the best I remember (and I could look it up, I just haven't had time yet) the billet aluminum ch***is car is 35 pounds lighter than one of our normal cars. FEA showed the ch***is to be 300% stiffer, but that number is too high as we didn't model every bolt in the car. If we would have tried that, we would still be entering the data (and probably still be waiting for an answer from COSMOS)! I imagine the ch***is is at least 200% stiffer than one of our ch***is. Test driving on the track with very experienced drivers confirmed the ch***is was much stiffer than an original car. The difference is quite pronounced when you drive the car.

David


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Excellent answer, David! And plenty of food for thought (so to speak). I love your writing style, too - easy on the sensibilities...
I don't trust my skills enough to do moly for customers. Plus I don't even have my own TIG.
But for what I do myself, power-to-weight ratio is still priority one (it's the only way I can afford to go fast). And I don't mind checking for cracks - everyone needs a hobby, right?

 

Hmmmmm A lot of the blokes who really appreciate this car are older hot rodders, many describe themselves as engineers or machinists.

I wonder how many of those who don't appreciate it ,and are anxious to tell everyone they only like trad rods, were riding around in VWs until recently???

 

Hi David, any word on how this project might be coming along?





Hope all is well in your world. Nice tea party you had . Tell Thomas and the Family I say Hi!

 

You are not "nit-picking" anything. Please ask all the questions you like. I am happy to answer them. Maybe someone will find something wrong with what we have done or what I am saying and then we can all learn together.

Yes, the "ribs" are parallel and somewhat evenly distributed. They were designed in the place they are in because they had to do some dual function. For example, the cross brace right in front of the differential held the drive shaft safety loop, supported the front differential mount, provided the seat belt anchors, and had the holes in it to mount the seats.

Bird cage cars carry loads down tubes so they have to put tubes where the loads are. A monocoque structure is somewhat different. It is much like an insect. The loads are mostly carried by whatever m*** that is the furthest away from the neutral axis of the bending load. That m*** in an airplane (and our car) is the sheet metal of the floor pans, under body trays, belly pan, and tunnel. By spreading the loads across large sheets, the uneven forces are even more easily controlled than with a few tubes that of necessity must be placed in a few distinct areas.

Our frame is actually rather difficult to describe. A bird skeleton is like a space frame. A monocoque frame is more like an egg shell or an insect. Our ch***is we kept the flavor of the ladder frame and used monocoque principles to increase the torsional rigidity of the frame by stressing the sheet metal in the ****pit area. The ****pit is the the weakest (I should really say, least stiff) part of the ch***is and we had to pull out all the stops to increase the stiffness. The front of the ch***is, where the engine sits, is a ladder structure that has been stiffened up with tubes, plates, and the motor itself. The motor mounts are quite stiff--we made them from urethane and there is very little play in them.

David

 

billet aside, that is incredible. my personal feelings are not opposed to billet as a material in and of itself, just the m*** production factors that go along. it is these m*** quan***ies of similar parts that makes cars start looking like cookie cutters. i think that ch***is looks amazing.

 

Thanks for the kind words. We just finished digitizing a Coupe, interior, exterior, panels, tubes, frame, everything. One day.

David

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Mike,

Thomas is moaning and groaning and complaining...that means he must be itching to make a Salt Flat Racer

Yes, we have put on a few (OK 8) Tea Parties this year--so far. Thomas moans and groans about the Salt Flats...I moan and groan about my memories of the lights out in Poland's defense industry.

David



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I need one of those someday!

 

Yeah, they have been using it in aircraft for decades, which makes my point for me.

Aircraft parts, especially those made from aluminum have a finite life span. Which once elapsed, the part is taken off the machine and summarily destroyed to make sure it never goes back into service.

This is done because of fatigue. There is no stress threshold that will give an infinite fatigue life in aluminum (most steels have this property, but not aluminum), so no matter if the factor of safety is 0.9 or 6.8, that aluminum piece is going to fail eventually, and usually at a load far below it's yield point.

That's why airplane parts have a finite service life. Over the years, engineers have been able to estimate the life of a part through calculations, and then follow it through service to check their ***umptions. We're at the point now where they've got a pretty good idea how long a part's going to live.

David - That's the most informative post on 4130 I've read yet. The notion that the entire structure would be quenched and tempered is one I hadn't heard before. I know that a couple of ch***is makers in the sprint car world used to (and maybe still do) oven normalize their completed ch***is. But I'd never heard of someone doing a quench and temper on one. That's cool (no pun intended).

Where did you learn all this Rolls-Royce history? Just curious. I like to see where the technology we use came from. Lets me better understand how to use it effectively.

 

David Kirkham ...

Welcome to the H.A.M.B. !!!

Awhile back I started a "pictorial history" thread about the Markley Brothers belly tank Lakester (click HERE) ... and in that thread, Michael LeFevers (Dynoroom on the HAMB) posted some January 2007 photos of the belly tank (currently owned by Dennis Varni) that he had taken at your Provo, UT shop.

I'm really curious as to the current status of that project ... Will you be offering the belly tank "bodies" to the public? ... or was this a one-time deal between you & Dennis?

Thanks in advance for your reply (in this thread and/or in my "The Markley Bros BELLY TANK &#8211; A Pictorial History" thread).


EDIT: D'oh! ... I see now that Dynoroom already posed the question to you regarding the Salt Flat Racer project ... he must have posted while I was still reading the first 7-8 pages of this thread

 

that ch***is is amazing!

 

I had the pleasure of setting up next to Kirkham at a Good Guys show (I forget the state or year) but I asked them many many questions throughout the weekend and the 2 guys that were there never once made me feel stupid about the questions asked and were quite happy to explain the process of how they build up their cars...........

I know many people may not dig what they do - but a dream car of mine would be a Cobra

2200 pounds and 427 HP - sounds like an adreneline rush to me!

 

Thanks for Posting,

Thanks for sharing the Free book ( very, very cool thing to do )

Thanks for taking it to another level - and letting the word see how you did it.

Anybody who has a Problem with the word ( Billet ) as far as this project goes, can just substatute the phrase ( Bar Stock Monocoque ).

What A MIND BLOWING amount of design, fabrication & machine work.

 

Thank you very much for your careful replies. With respect to the bird, I was thinking of the cross section of the bone, not the skeletal frame. The ribs follow the lines of force. In a traditional monocoque the stressed skin is an even isotropic material. In yours you can thicken it wherever you wish, as is is machined. I was wondering if, in your continual quest to reduce weight (you show a number of rejected components from earlier generations), you had aligned the ribs with the lines of force (and across them to keep these from buckling) this would have made the structure more efficient?

 

What else can I say,
It's very cool to be able to view such a book on line, I'm sure that most of us here would in no way be able to afford the " billet book" . Thanks once again for your generosity.

 

That's a bold statement for a guy that hasn't even done an introduction yet.

The man made a car out some blocks and sheets of aluminum. He took almost nothing but raw material and made a car. You obviously cannot even fathom the amount of skill that takes. He used a CNC machine for the frame work. SO WHAT?! Could he have used "traditional" mills and lathes? Probably for most of the work. But it simply wouldn't have been practical, time or cost effective. He's not using his CNC to make 10,000 valve breather caps to sell to street rodders. He built an entire car!!!

Chip Foose couldn't make a car out of a block of aluminum even if you threatened to embed said block of aluminum in his skull. His name doesn't even belong on the same website as David's.

And is an ****og mill/lathe proud of its work? I see you're an engine machinist, I ***ume all of your equipment is ****og then right? 'Cause digital read outs aren't exactly traditional either. I bet the guys that programmed the CNC were proud as hell to see that frame come out.

Many of David's techniques and methods could be used to build traditional rods. The bodywork section I dare say puts nearly everyone on this site to shame. How many people here use MIG/TIG welders to ***emble their frames or patch sheetmetal? How traditional is that? Where are all the parts in your car made? Got any from China or Taiwan? How traditional is that? Did you order ANY of the parts on your car from a catalog? Is that traditional?

There is NOTHING more traditional in hot rodding than taking nothing but an idea and raw material and building a car from SCRATCH. Perhaps not all of David's methods are "traditional" in the strictest sense. But I dare say that if you held every car on this site under that microscope you'd have A LOT fewer cars on here.

Even as a former engineer, I've learned a TON just from David's posts here, let alone his book.

 

Wow, do I ever agree with this statement! Including the Thank You to David.

 

You are exactly right. Aluminum, unlike steel, really doesn't have an endurance limit. It just keeps fatiguing until it breaks. Fortunately, most of us don't design anywhere near yield when designing with aluminum.

You brought up a very important point regarding fatigue which few people understand. If I take 1 square inch of some material, say 4340 steel heat treated to RC 50 with a tensile of 200,000 psi and a yield of 180,000 psi (the above are numbers off the top of my head so they may not be quite right) and strap a 179,999 pound load to it, it WON'T bend or break. (Didn't meet yield and not even close to UTS--Ultimate Tensile Strength).

BUT,

If we were to load it a 2nd time to 179,999 pounds it would most likely BREAK NOT BEND!

Now, how is that possible? Isn't it supposed to BREAK (UTS) at 200,000? We just loaded it BELOW the tensile and it broke? What happened? It broke from fatigue.

Fatigue is a little understood monster. When designing in steel a general rule of thumb is to use 1/2 of UTS as the endurance limit (the amount you can load a part repeatedly and not have it break). In aluminum, usually the endurance limit is 1/3 of UTS--THOUGH aluminum actually doesn't have an endurance limit.

What on earth am I saying? I'm lost!

Endurance limit is a term used for a specimen that can handle millions and millions of cycles. Typically 10,000,000 cycles. You might think WOW 10,000,000 cycles, I'll NEVER have my part do that...Whoa!

Well, think of a connecting rod. Connecting rods turn say 9,000 rpms in a NASCAR motor. The race is 500 miles long and takes at least a case of Coke (Utah) or beer (everywhere else) to get through. Say 3-4 hours. So,

We take 8000 revolutions per minute and times that by 60 minutes in an hour and times that by 3 hours and now we know why NASCAR has some spectacular YouTube moments. That is 1.6 million cycles. (This is somewhat oversimplified as the rod is actually loaded on the up and down stroke--but bear with me).

Granted, my numbers are very general and maybe not even too accurate. That is not really the point. The point is...what if I drive my car across the country? What if I have 100,000 miles on it? 200,000 miles? What if I run my machine at 2000 rpms in my plant making Taiwanese valve cover breathers for 24 hours/day making 1000 parts/day. You can see it doesn't take long to add up to 10,000,000 cycles. That's why machines are made from steel and iron and typically not aluminum.

So, why did you make if from aluminum. Well, because we could and because it is cool; but we still don't want to kill our customer.

Let's take our ch***is for an example. Will it drive over a road and get cycled at or near its yield 8,000/minute? Wouldn't keep our license too long if we tried that now would we? The fact is we very rarely load a ch***is at or near yield. That is why we can make ch***is parts from aluminum and not connecting rods. Ever wonder why the drag racers used to throw away "perfectly good" aluminum connecting rods after a few runs? Those rods never yielded but they were pulled out before they fatigued and turned into an IED--long before they yielded.

David






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It's think it's safe to say i will never get into a debate with you!!!!! Just reading your posts makes my head hurt.

 

I think my head hurts too! LOL

Thanks guys!

David

 

I'll answer this one in the morning. It's getting a little late!

David

 

Beautiful!

Most concise post on fatigue and endurance limit I've seen posted on the HAMB.

I've been trying to explain fatigue and how it applies to automotive parts (especially on the high end of performance) ever since I joined here. It's a very complex field, but you've summed up in one post what I've been trying to say literally for years now. Very well summarized.

The only thing I worry about with cars is vibration. Unless you hit something, you won't come close to yielding anything in the suspension or ch***is. However, with a very stiff ch***is and suspension, the natural frequency could be upwards of 200-400 Hz (or on an F1 car, upwards of 800 Hz).

It doesn't take long to rack up 10M (ten million) cycles at 200 cycles per second (just under 14 hrs if cycled continuously).

I know of no good way to arrive at a life estimate by calculation and I obviously don't have the means to actually build a rig and test to failure to find out, so I'm basically left to guess, hope, and ask rhetorical questions on internet forums.

But if I ever figure out a cost effective way to do it, I'm gonna.

I'm putting a magnesium quick change under my '36, and I will attempt to do***ent its life very closely. If I have the means available when it decides to fail, I hope to have some testing done, and see what I can learn about the mode of failure and how it fatigued through its life.

The one big blank area is the loads it will see. I can't afford the instrumentation that would be required to track load at all times, and besides, I don't want to drag hundreds of pounds of instrumentation around with me in my hotrod, that'd be a little ***bersome and impractical (especially considering that it'll take years and years to fail the thing).

It's a very interesting subject though. One that will turn your head inside out if you think about it too much.

I try to keep fatigue in mind when I design something, but since I can't make good predictions, it's really nothing more than a guess (or rule of thumb maybe).

I had just thought that maybe since you guys were really on the ball on this build, you may have spent some time crunching numbers on fatigue life.

The nice things about the automotive field are:

1) When you have a failure, you don't generally fall out of the sky because of it.
2) The forces involved are usually much lower than in an aircraft (excluding intersections with immovable objects, of course ).
3) The failures are less spectacular and more easily traced to their source (because of #1, we don't have to infer things about the failure from looking at a smoking hole in the ground)
4) On racing cars (and very high performance cars like that billet Cobra), the maintenance regimen will usually catch fatigue cracks before a total failure has occurred.
And Finally 5) Because racing cars and other high end cars are bought/built to be driven hard, the owner expects some damage and repair to occur over the life of the vehicle, and further expects that life to be finite (IE, race cars don't last forever, or at least they're not fast forever).

All these points work to the designer's favor concerning fatigue life. We ought to bear it in mind, but it needn't become the obsession that aerospace engineers have for it. Our field just isn't as close to the edge of the envelop as theirs is, so we get more wiggle room to play with.

I am very glad indeed that you dropped by to post this, and stuck around to discuss it with us. It's not often that we find a sharp fellow such as yourself that is willing to stop and really discuss the art like you are here, and I for one greatly appreciate it.

 

-Sorry I don't conform to your narrow worldview, Z.

I discovered long ago that I could learn useful skills and clever ideas from almost anyone. I learned how to use an airbrush from a guy that never used it for anything but taxidermy. I first cranked a handle on a gunsmith's vertical mill- he'd never used a machine tool for anything but gunsmithing. I first learned to weld aluminum from a guy that basically only made boats.

I learned how to smear Bondo from a bodyman, but it's been fifteen years since I used the stuff on a car.





Personally, I'm willing to learn from anyone willing to teach.

Doc.

 

Try not to take it personal Doc, some folks here are very particular with their nomenclature.

They have very specific definitions (and ***ociated connotations) for certain terms which don't necessarily line up directly with the general or mainstream usage of the term.

Rad Rod is a big one.

To most folks, any car that isn't a pastel easter egg on 24" wheels is a "Rat Rod". They don't use the term as a pejorative, but simply to denote a difference between the styling of two cars.

Around here, folks take big offense to the term, because they've got a wildly different definition in mind when they use it.

To them, a Rat Rod is some thrown together piece of **** that is just as likely to break in half as it is to drive in a straight line. No floor, no gl***, 2" of ground clearance, open headers pointed right back into the driver's face, etc. Dangerous semi-drivable cars built more for shock value than anything else. In other words, a terrible waste of old tin, at least in the minds of most guys here.

Call one of their rides a Rat Rod, and you might as well have questioned their mother's honor.

To my notion, it's exactly the same disagreement you would expect to see between a layman and an entomologist with regard to the term "bug", or between a layman and a soils expert with regard to the term "dirt".

These guys consider their genre of hotrod a specialty all its own, which carries with it all the attendant quirks of language you'd expect to find in any other specialty.

And, like any other specialty, you've gotta spend a few days familiarizing yourself with the nomenclature. It doesn't take long, and doing so will avoid most if not all of these moniker based ******* contests.

Other than the terms in use on either side, I believe you and ZMan to be in "violent agreement" about the usefulness of the presented information (Kirkham's Cobra).

 

Psst, Cool. Over here. [leans close]

I know.

Check my join date. The HAMB has been in my daily trawls for over five years and three computers now.

I do know where I am, and I do know what I'm saying.

I also know what Zman is saying. It's basically another version of "Be a nonconformist! Just like the rest of us!" and I find it funny as hell.

Doc.