18 replies to this topic

[Monstrobolaxa]

Well I'm in a project where we will design and possibly build a spaceframe sports car, I was wondering what alloy is usually used in spaceframes?

Would a CRW 1000N alloy be any good? yield strength of 790N/mm2 Tensile strength of 980N/mm2 enlongation of 10%!

[Greg Locock]

Beats me. What are your design requirements? That looks like a moderately high strength steel, tho I can't see it on matweb, how are you going to join it together? isn't stiffness more important than yield stress? what happens to the material properties when you weld it? Is it notch sensitive? 10% elongation is about what we get out of aluminium castings, ie not very much.

[McGuire]

Originally posted by Monstrobolaxa
Well I'm in a project where we will design and possibly build a spaceframe sports car, I was wondering what alloy is usually used in spaceframes?

Around these parts, SAE 1010 or 1020 DOM steel tube.

Door bars and roll bars 1.75" x.125" min, all bend radii 3.5xOD minimum.

[dead_eye]

chromemolys the best option due to the weight advantage but it is a fair whack more expensive than normal steels.

Id talk to a decent chassis builder first if i was you, theyl also be able to give you a few bits of help on triangulations and loading etc.

[Joe Bosworth]

From your question I have to conclude that you have never built a space frame before.

If that is the case definetly don't use chrome moly, stick with mild steels in the range that McGuire mentions above. Stay in the range between 1010 and 1025 of which 1020 is in the sweet spot of adequated elasticity and ease of welding.

Modulus of Elasticity is far more critical in space frame design than is ultimate tensile strength. You will design around E.

Chrome moly needs to have its weld areas heat treated which involves technology beyond that of the first timer unless that first timer has consideable experience in related matters.

I safely predict that if you don't stay in the 1010 to 1025 range that you will either have brittle welds prone to cracking or a frame that is far from square and true. (Or both).

Also, don't think in terms of going with wall thicknesses less than 18 gauge and probably use thicker depending on size of vehicle, use and experience of builder.

Regards

[turbolimac]

CRW 1000N is a new stainless steel with very high strength on par with 15CVD6 (which is currently used for high end steel spaceframe aplications such as WRC and similar) but both are VERY expensive, arpund 6 times more than 4130, and CRW 1000 is not really widely available still..

I went for 4130 (in fact 25 CrMo4 ) but mostly because I could not get Mild steel DOM locally and had to import from abroad, so I bit the bullit and bought 25Crmo4

[mariner]

I am sure you can use one of many hi strength steels. However ,as you may know, in the long lost days of spaceframe racing cars simple electric resistance welded (ERW) mild steel tube was used in conjunction with brazing not welding.

This may all sound very low tech today but it worked . For example Lotus built sports racing cars with 250 bhp 2.7 litre engines using 16gauge ( 1.6mm ) tube or lighter of 20 to30mm diameter for most of the frame.

Brazing was used because it avoided the need for stress releiving the frame and also allowed multiple tubes to be joined at one point without stess fracturing the tubing. One example an early Lola sports car had eight tubes joined in one pyramid. This gives an architectural benefit of closeley acheiving the "pin joint" theory of a pure space frame which probably offsets some of the theoretical benefits of high tensile tubing with bigger tube offsets to allow safe welding.

There was an additional theory ( I think after the event) that the brazing approach gave a better fatigue resistance long term than welding because the frame could flex very slighlty and thus redistribute high shock loads across the whole structure without sudden local failure.

Unitary ( monococqe) construction is better but spaceframes worked fine until tire grip began to regularly exceed 1.0g. You only need to be a fixed % stiifer than the wheel loads for the frame to work.

Agian its very old but there is a book " racing and sports car chassis design" by Costiin and Phipps which was published in about 1961 which is a "bible" of spaceframe design. The fact it the Costin is Mike Costin the co founder of Cosworth engines and the ex chief engineer of Lotus racing demonstrates the books credentials.

[zac510]

I've heard of some spaceframes made with 25CD4S. How does this compare to some others being thrown around in this thread?

[McGuire]

25CD4S is a chrome-moly-vanadium alloy fairly similar to 15CDV6, and used pretty much like 4130 or 25CRMO4... though each has its own properties, and pros and cons in welding, formability etc.

Needless to say all these metals are considerably more expensive than mild steel, and more difficult to work with. Some years ago a true craftsman named Ron Fournier wrote a book about race car fabrication. In the chapter on materials he wrote, "one word about chrome moly: DON'T." That pretty much sums it up for me too, unless you truly need it for a specific application.

Depending where you live, who you know or maybe where you work, sometimes you can find these alloys or similar really cheap, except without their paperwork. That's fine, if you really and truly know the back story -- what the steel is and where it actually came from. If you don't know, I would be very careful about "bargains."

[NRoshier]

Having visited a couple of fabrication shops recently I was surprised to see the chrome moly being welded to mild steel. I was told the metalurgy has changed since many of the books have been published. Has anyone insight into this?
Mariner, nickle-bronze welding (it is a form of brazing, but was always called welding for some reason) is very effective as you say. I think the fatigue resistance was partly due to the care taken in the method and also that it is a very forgiving process to the steel as it never actually melts it. In addition a reasonably large fillet should be used and apparently this is advantageous for the transfer of forces. Perhaps others could comment on these words of wisdom from an old chassis builder.

[Ray Bell]

A comment from chassis builder Brian Shead back in the days:

"They use nickel bronze for welding chair legs together, but that's not using it the way we do. We leave a nice solid fillet there to give it some strength."

[rgsuspsa]

Mariner: The Costin who cowrote the referenced book was Frank Costin, brother to Mike Costin.

rgsuspsa

[mariner]

I am not trying to be a smartass but just in case anybody tries to locate the book Racing and sports car chassis design the authors are one Michael Costin and David Phipps according to the frontpeice on my copy.

It was published by Batsford books of the UK in 1961/62 (long before ISBN's!)

[desmo]

What Americans call brazing seems frequently to be called welding by Brits.

[Wolf]

Shouldn't there be a fundamental difference between the two? The way I figure it, brazing should be more similar to soldering than to welding*. ISTR that the only difference between brazing and soldering is melting-point of added material.

* welding requires that the material to be joined should melt in the process, while with brazing/soldering that material should not melt.

[McGuire]

Depending upon the particular operation and materials, the processes of soldering, brazing and welding overlap each other. Silver soldering is really more like brazing; nickel-bronze welding/brazing on steel alloys has as much in common with welding as brazing, and what we call welding is sometimes a brazing process. For example, when gas welding cast iron with nickel rod you are are really brazing, if you want to get technical about it. In the end these are just words and the definitions are not as mutually exclusive as one might wish.

One way to look at it, I suppose: in "true fusion welding" two or more pieces are in effect melted together with no filler material added to form a homogeneous structure, or additional material of exactly the same composition is added. If the filler material is complementary rather than identical we are departing slightly from the concept of "true fusion welding" as it were...because the join will be of notably different composition than the parts themselves. Semantic distinctions...

[Greg Locock]

and of course the main attraction of brazing in the land of the Poms was that the filler rod wicked into the rusty crap that you laughingly might refer to as the base structure.

Which might well be why they banned it.

[NRoshier]

Still used quite a lot in Australia (e.g. FF etc), but did they ban it overseas? Done badly it is as dangerous as bad fusion welding.

[McGuire]

Brazing was never banned here in the USA, in fact it is pretty common... or was... now being obsoleted by the availability of cheap wire welders, which perfom a faster, more consistent join with near-zero operator skill. Here brazing has been most commonly used for less-critical structures on autos... one-off fabrication and field repair on exhaust pipes and systems, brackets, body panels etc. Brazing rod with white flux coating is standard bronze alloy (often called "brass" here) while the blue-coated brazing rod is nickel-bronze (typically).

I think there is some misunderstanding of how brazing works... as though it is a form of soldering perhaps... not really. First the steel pieces to be joined must be heated with the torch up to red-orange hot to get the brass to flow and stick. So we are not hot-gluing; we are producing a molecular change on the surface of the metal. This creates a thin layer of material in the join that is not quite bronze or nickel and not quite steel but allows the dissimilar metals to bond without quite coalescing in the conventional sense. Then a rather thick fillet of filler material -- rather plastic, evidently -- reinforces the thin bonding layer. The beauty of it is we are not altering the structural portion of the parent metal in any way so no heat treatment is required. Done right the join is damn strong, over 80,000 which is usually greater than the steel itself.

That said, while bronze welding has held together various Loti etc. adequately, the stuctural loads involved there are similar to a fat lady sitting on lawn furniture, no offense. When I see it holding semi tractors together I could be more impressed. Around here, when we want to bodge together two great hulking pieces of rusty steel we get out the stick welder and crank up the knob. Known as "farmer welding."