Thread: Structure vs mass

Tweet

How about a thread to discuss the old engineering quandary of structure vs mass as it applies to building a chassis? We have some folks on here with engineering degrees, and a whole host of others with credentials of designing, building, and driving some very good working cars that excel in street and or track performance cars.

My '57 is about due for a chassis, lots of mods done to the sheet metal and lots of sheet metal eliminated already so it's time to start designing the chassis. The criteria is quite simple, from outward appearances (with the exception of ride height) the car will be a 'gasser' style, complete with straight axle, long ladder bars, etc. The mechanical and chassis will be newer technology, including a 12 point cage, not state of the art but practical and affordable. The car will be dual purpose, mostly street with a blast or two at the drags from time to time.

First consideration is of course the frame, I'm considering either 2" X 3" rectangular tubing, or 1 5/8" DOM tubing.... Perhaps round moly, if I can justify the additional expense and convince myself the weight reduction is important. I can't see good enough to do much more than tack with a TIG anymore, so I'd also have to hire the welding done if I use moly. If I go rectangular tubing, I'm considering using 14 ga. (.083"), if it's round tube DOM probably .110" for the main rails and main cage hoop, then .083" wall for everything else.. Rectangular tubing for the main rails is easier to fabricate, but not near as 'racy' as a round tube car.

Weight is always a big consideration with me, 100 pounds extra weight adds 1/10 second to the ET. So, am I thinking too light, too heavy, or about right on materials?

I'd love to follow this thread----------Dave, first thing is to check the rules of the class that you would want to race in because I think more of them that allow DOM also require 1 3/4 od and thicker wall than 4130 moly which can be 1 5/8 x .083?????????

The main rails for a street driven gasser would be a lot easier to build with the 2x3 tubing as far as mounting brackets,etc--------also there are lots of places that you can get rails bent for various cars so you just need to add the cage in round tube.

Seems to me when I was researching rule books for guidance on a roll bar for my car, that .120 was the minimum wall thickness for mild steel.

I know you said weight is important, but for the added hassle of getting moly welded properly, I would just use mild steel. Much simpler to weld, cheaper, and more than adequate for the use you described.

Like Jerry said, there are places that can hydro form rectangular tubing with smooth flowing bends. It would be worth checking into. Art Morrison comes to mind, but I'm sure there are others with less of an advertised name that could do it just as well for less money. I would use 2 X 4 due to the weight of the car.

Sounds like a fun project. There is an episode of "RoadKill" on YouTube that shows Mike Finnegan getting the pro built chassis for his 55 Gasser. You might get some inspiration and ideas from it.

Found it! The actual chassis stuff starts at around 19 minutes in. Chassis is by Jim Meyer Racing.

https://www.youtube.com/watch?v=lATwQ7wkhTs

I just looked at Meyer's website. He gives a pretty detailed description of his chassis complete with tubing sizes and thickness.

http://www.jimmeyerracing.com/produc..._chassis.shtml

[QUOTE=Hotrod46;578637]Seems to me when I was researching rule books for guidance on a roll bar for my car, that .120 was the minimum wall thickness for mild steel.

I know you said weight is important, but for the added hassle of getting moly welded properly, I would just use mild steel. Much simpler to weld, cheaper, and more than adequate for the use you described.


Nice car, thanks for the link! It's for sure a stout chassis, when you're building for John Q. Public one has to do the overkill on tubing sizes just to try to make them idiot proof so some fool doesn't try to pull a trailer with the car or some other poorly thought out plan the car wasn't intended to do! I had to do the same thing when I had my shop and sold to the public. Building my own, just for me, I can saw off some pounds and go minimal on some things.... I know .120" is the standard wall thickness for most frame rails, just planning now so it's easy to 'lighten up'!

Yeah, moly is a pain in the butt to work with and weld, but it's light and strong. I guess I'll have to decide how much extra time and $$$$ the weight is worth to me. I've taken a few nasty rides in moly cars and walked away from all of them just a bit dazed and confused!!!

The next size up from 14 ga. (.083" wall) is all the way to .125" (advertised) that averages out at about .120" wall, I don't recall right off the difference in weight per foot but it is a considerable amount. DOM has always been my choice after moly, more strength then mild steel and welds like a dream with a MIG, maybe round tube 1 5/8" at about the .110" wall thickness and a round tube chassis would be a better choice? Lots more labor, but stronger and lighter then the usual 2 X 4 .120" that seems to be the norm. I'm in the process of doing some significant weight reduction on the body (ie aluminum door skins and quarters, 'glass hood, etc.) which is my rational for lightening up on the chassis.... My initial calculations (round tube DOM frame and cage) bring the car in at about 2600 pounds....


Thanks much for the input! Asking knowledgeable people the questions for me is still the best way to come up with a workable plan! I certainly appreciate the help!

Originally Posted by jerry clayton

I'd love to follow this thread----------Dave, first thing is to check the rules of the class that you would want to race in because I think more of them that allow DOM also require 1 3/4 od and thicker wall than 4130 moly which can be 1 5/8 x .083?????????

The main rails for a street driven gasser would be a lot easier to build with the 2x3 tubing as far as mounting brackets,etc--------also there are lots of places that you can get rails bent for various cars so you just need to add the cage in round tube.

For sure the rectangular tubing is much easier to build a chassis with, I guess I'll have to get in the books and see what the weight difference is from round moly to rectangular mild steel... As for the cage, yeah it would have to be moly at .083 wall to be rule book legal, but my track time would be grudge night stuff. Locally if the car has a properly built cage they tend to overlook some of the "details" if your not class racing or points racing...

lol, Besides, first, last, and always I'll be a street racer and use the '57 to settle a few 'discussions' on cruise night at Sonic!

As always I appreciate your input, I'd be a fool not to listen to someone with a Doctorate in going fast and surviving the ordeal! I'm old enough to remember you on the dragstrip!

After the great advice on here, plus a bit more research I've decided to go DOM round tubing on the frame. With the advice here I've gone up to 1 3/4", .134" wall for the main rails, 1 5/8" .110 wall for the cage main hoop an halo, then 1 5/8" .083 wall for the rest of the cage. It will increase weight, but minimally and the added beef will be good for that possible day when things go terribly wrong!

Yeah, I'm making a lot of extra work for myself by building a round tube frame vs. a rectangular tube frame but the difference in weight (over 1 lb. per ft.) makes it worth the extra effort in my opinion. Nothing is final yet, the tubing isn't ordered yet, so further input is much appreciated. I really wanted to have most of the chassis and cage done by now, but just too many other issues to deal with this summer. Oh well, no time limit on building your own stuff, right? Hopefully I'm able to get these other projects complete and enough room and time to get going on the '57 this fall.

You should look at the moment of inertia for each type of tubing. That's what tells you the tube's resistance to bending. Round tubing would typically only be used in a truss design.

For example, a car like the factory five '33 has a truss frame made of mostly square tubes.

https://www.factoryfive.com/33-hot-r...u-get-stage-1/

A 2x3 tube has about half the resistance to bending as a 2x4 tube.

If you look at beam deflection formulas, the deflection for a given load is inversely proportional to the moment of inertia (I). An I value that's half the amount means twice the deflection.

Was watching RoadKill last night and they were building a '55 Chevy Gasser with 426 hemi, using a Jim Meyer chassis. You might take a look......
Jim Meyer Racing Products Your Hot Rod Suspension Specialists
.

Here's his latest video dynoing it with the fresh engine.

https://www.youtube.com/watch?v=u8BEhGwei9o

Nice chassis, way beyond my meager financial means! Looks like it would weigh too much for me, numerous body mods already done that would mean a lot of modifications needed to make it work for my '57, I've just never been much on the store bought stuff. Thanks for putting up the information, even though I don't like the rectangular tube design there are still a lot of good ideas I can steal from other's design!