Spaceframe Chassis Construction

GregF · 06-29-06, 08:51 PM

Getting closer to a decision now, but a question for you engineers out there:

All the spaceframe chassis I've seen so far seem to use box section, as opposed to using tubular construction. I guess using box sections, it would be easier to work with, cheaper and a hell of a lot easier to mount things on, but I am curious as to why many race car chassis still use tubular and not box.

My guess is that tubular construction would use less welds, and would produce a stronger and stiffer chassis, and may be lighter - am I on the right track?

Also, other than cost, is it worth considering stainless instead of mild steel? Would stainless be stronger or heavier? I guess you would not have to worry about internal corrosion with a stainless chassis.

Having inspected an RF chassis a few years ago, I was impressed at the apparent strength of it, and if photos are anything to go on, the DRB chassis looks (to a layman anyway!) even stronger and stiffer. Either way, I intend to use a full roll cage, which should add a lot of torsional rigidity.

Thanks in advance for suggestions etc. can't believe the depth of information and expertise on this forum!

Cheers
Greg

Chris Martino · 06-29-06, 09:22 PM

I'm not an engineer but I do know that MDA make it's chassis with square steel tube, tig welded in a jig. A few pics-

I'll be starting to load good pics of the MDA product over the weekend for those who wants to take a better, longer look.

Chris

GregF · 06-29-06, 10:02 PM

Thanks Chris - I'm very familiar with this type of chassis - I'm more interested in the pro's and cons of the two different construction methods ie: box section Vs tubular.

Cheers
Greg

Phil · 06-29-06, 10:27 PM

Greg
Although I would consider round tube stonger than square...the advantages of square tube are as you mentioned the ease of connections. Round tube can be connected rather nicely, but requires a fishmouth connection to another round tube. If you have the equipment to do this quickly and accurately it is a breeze, square tube is usually butted up at the square or angular cuts, and these connections are pretty easy to make up after a little practice. Its just simpler to match up square tubing. Also, when fishmouthing round tube you may need several sizes of cutters to correspond to the sizes of tubing you are using. If you are interested in strength I would not hesitate to use the square tube, and if you feel you need it most square and rectangular sizes available in 4130 alloy for added strength, and usually 4130 can be employed in thinner wall sizes to subtract some weight. One last thing is that when sheeting a chassis, it is much easier with a square tube unit as the panels sit flat and fastening is accomplished relatively easily. I did my chassis in square and rectangular tubes, with a few round tubes here and there, and it is very ridgid. Adding an integral roll bar to your chassis can also stiffen it substantially.
Hope this answers some questions
Cheers
Phil

Mark Worthington · 06-30-06, 01:43 AM

Strength / weight ratio is what you're looking for. I doubt that stainless would have an advantage over other materials simpy because you don't see it used in structural applications in aircraft like ou do with 4130. The aircraft guys sweated the engineering details many decades ago, and their design goals are light weight and reliability. I'm not aware of any aircraft fuselages that were fabricated from mild steel space frames or that used square tubing - they used round 4130 tubing until they were replaced by the aluminum monocoque and, more recently, more exotic and composite materials.

That said, I'm not sure the strength/weight benefits of round 4130 would offset the the increased cost and fabrication time. I know a guy who designed and built a mild-steel tube-frame Mustang and he said once that if he had gone with 4130 it would've only saved him 30 pounds on a 3,000 pound car.

Jac Mac · 06-30-06, 02:12 AM

If my memory is correct the Trans Am guys went with rectangular tube for the majority of the spaceframe for both strength & ease of fitting internal panelwork, much the same as the 40 kits. They still used round tube for the rollover & side intrusion bars as these had to be bent without deformation beyond certain limits. Also back around 1984 I can remember a lot of publicity about a spaceframe kit for a Corvette of that era that used stainless round tube with some clever aerospace tech fittings that the s/s tube was a push fit into. These were used @ the tube clusters & the stainless was either brazed or welded into the fitting. I think ' Mr Gasket'?? or a Company similar was behind it. Might be worth looking into if your serious about stainless.

Bob Putnam · 06-30-06, 07:33 AM

Almost all (A500 and A513) square and rectangular tubing is shaped from low-alloy ERW (electrical-resistance-welded) tubing that has not been work-hardened much. It has a relatively low tensile strength. Round tubing generally used for race-car chassis is DOM (drawn-over-mandrel) steel with higher carbon-content and has a higher UTS. Steel, however, has the same approximate stiffness, no matter what the UTS, and stiffness is ultimately what determines the quality of the chassis.

Russ Noble · 06-30-06, 02:55 PM

Gregg,

As I understand it, for a given size and wall thickness of similar tube, the round tube is about 30% lighter and more resistant to torsional deflection than square tube. Square tube has more resistance to bending loads. Generally I use square tube for convenience where I want to attach panels, round tube everywhere else. In the ideal spaceframe design, no members should be in bending anyway.

Regards

Bob Putnam · 07-01-06, 08:19 AM

In an ideal spaceframe, all tubes are in tension or compression. Any difference that an individual tube has in advantage in torsion or bending is simply not a factor. The small number of tubes that aren't in an "ideal" situation shouldn't drive the total design if it makes construction much more difficult.

Russ Noble · 07-01-06, 06:49 PM

"The small number of tubes that aren't in an "ideal" situation shouldn't drive the total design if it makes construction much more difficult."

True Bob, from a manufacturers perspective, but depending on the goal or end use some people will willingly trade a more difficult construction (and possibly higher costs) for resulting gains in other areas. Car construction is a marriage of conflicting requirements. The major advantage of doing ones own build from scratch is that you decide which level of compromise you are prepared to accept in any one aspect in return for advantages elsewhere. One is not bound by decisions made by other people to suit their particular design parameters.

However since Gregg is looking for info so he can make a decision over which kit to choose, knowing the underlying basics and understanding why something is built a certain way, or why it should really be built differently, means he can make a more informed decision over what is the best compromise for him. For any individual it is a matter of determining which is the best kit for his purposes. It will not be the same kit that someone else with a totally different set of requirements will choose.

Regards

Kalun D · 08-21-06, 09:28 PM

""All the spaceframe chassis I've seen so far seem to use box section, as opposed to using tubular construction. I guess using box sections, it would be easier to work with, cheaper and a hell of a lot easier to mount things on, but I am curious as to why many race car chassis still use tubular and not box.""

pound for pound square tube is stronger but only in a directional fashion, with the force aligned with the planes of the square tubing's surfaces. Round tube is stronger on average with force applied in any direction. That and the afore mentioned availability issue, commonly available DOM round is higher quality than ERW square.

When scratch building saving some time is really an important issue. Square is way easier to bolt up to and then round where you need ultimate strength and bendability in the roll cage. Unless your competitive racing at the very top level square is more than adequate.

""Also, other than cost, is it worth considering stainless instead of mild steel?""

Forget about stainless unless your a glutton for punishment. It's way harder to work and weld. It requires more expensive tools and it goes through tools and grinder wheels at twice the rate of mild steel. You have to tig weld it unless you want to buy an expensive specialty mig stainless welder. Most of these cars are garage kept anyway so rust shouldn't be an issue unless you live by the beach. Properly sealed and painted mild steel should last.

Forget about chrome-moly also. It's fine in airplanes that aren't designed to survive an accident, and that was what it was originally designed for. Cars are different, they should be able to survive a hit. Chrome moly gets brittle at the weld joints, unless it's normalized and heat treated which is not practical with a chassis size piece. Chrome moly cages fracture at the welds with a really hard hit.

CliffBeer · 08-22-06, 03:28 AM

While it's easy to rain on the stainless parade from a cost and PITA during construction standpoint, I really do believe stainless will be a better material long term.

If you've spent any timing doing chassis repair/restoration I think you end up with a keen appreciation for a more corrosion resistant chassis material. Take a look at some of the complex rocker box sections of "restored" cars (as in, have a look at one cut open) and it all looks great on the outside but there's inevitably a bunch of rust and swiss cheese in the middle. True, while these cars are generally garaged/covered, over enough years the rust bugs will get a bite. I plan on keeping my CAV stainless monocoque for many years so I'm a big fan of the chassis design/material.

TimB · 08-22-06, 09:05 AM

It just dawned on me that the entire front subframe of an e-type jaguar is welded square tube. The whole front assembly is then bolted to the monocouque tub. It is a special alloy that can't be repaired without embritlement (reynolds 3130 if I remember correctly)

TimB · 08-22-06, 09:06 AM

Here is a better pic of the subframe

aji · 08-22-06, 11:45 AM

On the 4130 what is the big deal with Normalizing

http://www.lincolnelectric.com/knowl...hrome-moly.asp
Q. Do I need to heat treat (stress relieve) 4130 after welding?
A. Thin wall tubing normally does not require stress relief. For parts thicker than .120", stress-relieving is recommended and 1,100ºF is the optimum temperature for tubing applications. An Oxy/Acetylene torch with neutral flame can be used. It should be oscillated to avoid hot spots.

heck If I could afford it I make the frame out of aluminum round tube and weld flats where needed. If you have a tig welder and know how to weld no issue. I cannot see not having a Oxy/Acetylene torch if you are doing metal fab work.

The 1020 mild steel square tube costs less and it is strong enough for what I'm doing.. so what is the big factor (cost). What about Titanium tubing for the build... somehow I do not think so! I can weld it just fine however!

I will coat the 1020 steel with Paint for protection, stainless would be good however! cost prohibitive for me!

best

Hemipanter · 08-22-06, 01:24 PM

Just a short reply concerning tube frames. As allready stated, tubs should only recieve straight forces. This means that the important factor is construction, the right way of tubing layout. This is hard to describe in a few lines in a forum, so a good book about the subject is recomended.

As an example from my own car. The monocoque as original had a twisting number of 5000 fp/dgr. A simple X bracing in the proper location rised the number to 15500fp/dgr! And the tubing was extruded aluminum 25mm diameter, which is not exactley the strongest in the book.

Safety is another matter.

Goran Malmberg