Body on frame stiffness question

[jaybee]

I'm trying to understand the stresses involved when trying to increase stiffness of a body on frame car. What are the advantages of a frame center section of the style which effectively adds a second set of frame rails near the center of the car http://www.artmorrison.com/55-57chev.php as opposed to the lighter style of multi-tube center section popular in the street rod world? http://www.speedwaymotors.com/Deluxe...-Kit,6815.html

I'm not trying to pick at either style, just wondering how each handles the stresses involved. The style used by the Morrison GT-Sport and Max G chassis should be considerably heavier and has a lot less space through which to run things like exhaust, so I presume that extra material must be gaining strength in some important dimension.



This is of interest to me because I'm starting to do indepth planning of my own project. At this point I can go with anything I can build at home and can choose whatever will work best.

[raustinss]

I'd contact the manufacturer of the frame and ask for their engineering information regarding that and, if the can't or won't run...and run fast

[j-c-c]

I disagree, first they will as any company will only reference the areas that enhance their product, seldom will you ever get a truthful forthcoming response if you ask the simple straightforward question like "what is the biggest downside of your design?" or "where is the weakest link?" etc. Second if they give you real engineering calculations, if you understood them and how they were arrived at, you probably would not be asking the question in the first place. What i think you are asking and trying to find out, is how can i get the most stiffness or "bang for buck", and "buck" in this context can be described as cost, size, weight, volume, complexity or any combination. However there is one concept almost anyone can understand, and all the magic in the world has yet to overcome, and that is the "deeper" a member is, the stiffness increases dramatically ie why a cage, which is nearly 48'? tall (deep) is the holy grail, anything less, is much less. No way around that. However what the OP is kinda saying is, for example, if I use one 2x4 on edge, and add a second 2x4 along side, the result will twice as stiff, but if I use one 2x6 on edge, I'm shooting from the hip here, it might more then twice as stiff, but weigh only 50% more. There is of course much more to this but, you as a buyer need to define in your own mind what exactly your goals first, and then you can target them. Sometimes the goal is just to buy the newest and latest and coolest, and that's fine, keeps a lot of companies in business. lol.

[jaybee]

Sort of, but not exactly. I'm not actually buying a ready-built solution (mostly because I don't have $13K to drop on the outstanding AME chassis) so there's no one to call and ask for engineering specs. Given that, I can either play monkey see, monkey do and imitate something, or I can try to understand what that thing is actually accomplishing to avoid missing one or more important aspects of the concept.

[mikedc]

IMO the practicalities of packaging frames have a way of reducing the relevance of theories. Some designs look like crap on paper/theory but they work pretty well in practice because the "little stuff" can be done better. Thoroughly welded joints, etc.

But theories are still relevant to know.

The best way to torsionally stiffen anything (besides growing the height of every frame member) is to X-brace the middle of it. The X bars should ideally be at 45 degree angles. Moving the angles in either direction away from 45 degrees reduces the effectiveness. The point of the X-brace is that when the two diagonals are totally joined in the middle it turns the torsional stresses on the frame directly into beaming stresses on the diagonals.


The "inner frame" designs are just more indirect ways of X-bracing the frame. IMO the Art Morrison frame is a better frame at least theory.

[raustinss]

Yes adding x bracing works as a torsional member, things can, will and do change once you change the plain...ie a arm mounting location, rear suspension pick up points. Having said that younsir are still on the right track and probably right that art morrison makes some of the best frames

[raustinss]

I disagree, first they will as any company will only reference the areas that enhance their product, seldom will you ever get a truthful forthcoming response if you ask the simple straightforward question like "what is the biggest downside of your design?" or "where is the weakest link?" etc. Second if they give you real engineering calculations, if you understood them and how they were arrived at, you probably would not be asking the question in the first place. What i think you are asking and trying to find out, is how can i get the most stiffness or "bang for buck", and "buck" in this context can be described as cost, size, weight, volume, complexity or any combination. However there is one concept almost anyone can understand, and all the magic in the world has yet to overcome, and that is the "deeper" a member is, the stiffness increases dramatically ie why a cage, which is nearly 48'? tall (deep) is the holy grail, anything less, is much less. No way around that. However what the OP is kinda saying is, for example, if I use one 2x4 on edge, and add a second 2x4 along side, the result will twice as stiff, but if I use one 2x6 on edge, I'm shooting from the hip here, it might more then twice as stiff, but weigh only 50% more. There is of course much more to this but, you as a buyer need to define in your own mind what exactly your goals first, and then you can target them. Sometimes the goal is just to buy the newest and latest and coolest, and that's fine, keeps a lot of companies in business. lol.

I don't know what the hell your saying. I had said to contact the engineering department and ask for numbers and info on the frame such as torsional ridgity. comparing 2 different frames can be tough but the overall dimensions should be darn near the same. Allowing you to put solid info regarding that frame on paper vs another. Phoning a company and asking where the weak points are is ridiculous. Just ask for the pertinent information that's it. If said company cannot or will not provide necessary info I said run. Reasons being its your life and or your family members, the cost ask me to drop 13-20k$....i want info,pride any frame manufacturer should be proud of their product. As for "stacking " steel on top of each other is sorry to say moronic. The weight alone of place two 2x4 rails on top of each other far outweighs any strengths they would have together. Hence why you see some frame manufacturers making side "rails" out of round tubing and building it like a sideways ladder. Incredibly strong and yet light weight. It's a similar idea as adding a gusset a small piece of tubing vs a piece of plate steel...

[j-c-c]

I don't know what the hell your saying. I had said to contact the engineering department and ask for numbers and info on the frame such as torsional ridgity. comparing 2 different frames can be tough but the overall dimensions should be darn near the same. Allowing you to put solid info regarding that frame on paper vs another. Phoning a company and asking where the weak points are is ridiculous. Just ask for the pertinent information that's it. If said company cannot or will not provide necessary info I said run. Reasons being its your life and or your family members, the cost ask me to drop 13-20k$....i want info,pride any frame manufacturer should be proud of their product. As for "stacking " steel on top of each other is sorry to say moronic. The weight alone of place two 2x4 rails on top of each other far outweighs any strengths they would have together. Hence why you see some frame manufacturers making side "rails" out of round tubing and building it like a sideways ladder. Incredibly strong and yet light weight. It's a similar idea as adding a gusset a small piece of tubing vs a piece of plate steel...

Yes, I agree, you have no clue what I was saying. If you would have asked, I would have gladly shared what I interpret as your weak point was in understanding my blurb. You could rectify that somewhat be reading it again.

[mikedc]

I don't think he was literally suggesting stacking a pair of 2x4 rails. Just using it as an engineering example.

Asking several frame manufacturers for specific stiffness data might be reasonable but that doesn't mean you are likely to get it. Just the way it is measured can affect the numbers dramatically.


Yeah, it's your family's safety. Yeah it's $13-20k. But brakes are your family's safety too. So is steering. Shocks. Wheels. Tires. Drivetrain. Wiring. Seats. Etc. That thinking (It's your family's safety so how can you possibly want to skimp on it!?!?) can be applied to most of the stuff on the whole car. Budgets still have to apply somewhere.


I don't think it's likely that any aftermarket frame company is going to literally sell you a very unsafe frame. Not if you buy from the known respected top builders, use your head about what you are building, carefully check over what they deliver before using it and then install & sort it out well. There will certainly be differences, but all of the frame options are likely to be tolerably strong and it's more question of stiffness. Stiffness does not become a major safety issue for street cruising until the chassis gets pretty damn floppy. I'm saying the wrong frame is more likely to be disappointing than unsafe IMO.

[jaybee]

Not unsafe, but as Colin Chapman said "add lightness." Of course one can't compromise stiffness or any other important function in the process. That's where it gets tough, especially without extensive engineering facilities, equipment, and testing.

Here's what I think I have figured out. It's very difficult to get that perfect X due to the need to accommodate a roomy cabin and of course the driveline runs right up through the middle of that intersection. At the same time the suspension and driveline all stress the chassis in different ways. The engine and third member can really put a lot of torsion into the chassis. Different sorts of suspension will stress the chassis in different ways, with leaf springs trying to twist the frame members while 3 links, 4 links, and truck arms need a really solid crossmember to push against. Uneven traction is going to try to "diamond" the frame.

That's a lot going on. The "extra frame rail" style would do certain things well. It would stop the front of a leaf spring suspension from twisting the rails, and provide a solid place for 3 link/4 link suspensions to mount. The diagonals up to the front frame kick up/torque box area would share those stresses to the opposite frame rail through the center section. Potentially there's room for an exhaust X pipe right around the rear of the transmission where an X Member would intersect. It's going to need to be beefy, though, because the longitudinal part of the unit wouldn't be loaded in beam and would twist in torsion.