Effects of preloading frame during SFC install
Increase responsivness by preloading frame during SFC install Here's my idea: If one were to preload the frame on 4th gen f-bodies by pulling the two opposing subframes apart and then welding in subframe connectors in. I would think that a great increase in the responsiveness of the launching characteristics of the car would occur.
Here is why:
After welding in the subframes, a compression preload would exist in the sfc's, the neutral axis of the "truss", basically all of the structural components of the car, would move up and the subframe connectors would begin to share the compression load of the lauching car with the structual floor members. This could cause serious problems because the roof/body structural components take a higher tension load.
I am assuming that the neutral axis of a normally install sfc equipped car is very close to the floorboard/structure. The floorboards on the car take the compression load. but when sfc's are installed the neutral axis moves down which transfers the compression load to the sfc's instead of the floorboards.
Here is a picture. Please copy this add whaterver and then repost with your thoughts/comments.
Basically the green arrows are before preloading the frame and the purple is after preloading and installation
Here is why:
After welding in the subframes, a compression preload would exist in the sfc's, the neutral axis of the "truss", basically all of the structural components of the car, would move up and the subframe connectors would begin to share the compression load of the lauching car with the structual floor members. This could cause serious problems because the roof/body structural components take a higher tension load.
I am assuming that the neutral axis of a normally install sfc equipped car is very close to the floorboard/structure. The floorboards on the car take the compression load. but when sfc's are installed the neutral axis moves down which transfers the compression load to the sfc's instead of the floorboards.
Here is a picture. Please copy this add whaterver and then repost with your thoughts/comments.
Basically the green arrows are before preloading the frame and the purple is after preloading and installation
Last edited by squealingtires; Apr 27, 2006 at 01:33 PM.
TECH Enthusiast
Joined: May 2005
Posts: 555
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From: Blairsville, GA
Are you talking about stretching the subframes further apart from each other??? I'd almost think that'd have to be such a tiny amount that you'd gain almost nothing. The only reason popping in my head to do such is to offset the welds drawing up tight as they cool, which, if done sloppily, could pull the subframes closer together and cause problems as mentioned. I'd suggest tweaking your SFCs to give a have-to-beat'em-in fit to fix that problem.
Of course, I may have just went WAAAYYY off into left field... even though the ballgame's over.
Of course, I may have just went WAAAYYY off into left field... even though the ballgame's over.
Correct, basically pull the subframes apart, weld in the SFC's then releave the stress on the subframses. By doing so you done exactly what you said, "taken the slop out" I'm not saying things are "sloppy" on normally installed subframes by any means.
Heres a perfect example: Spring/Mass system, you car is a perfect example with the engine/front end being the majority of the mass and the spring comprising of all of the components between the front end and the lower control arm connections.
OK so take you mass and spring and apply a step input force-->the spring will compress some amount before you notice any signifigant acceleration of the mass. Now take the same spring and tie it up in a compressed position which makes the spring effectively stiffer, the spring constant didn't change just the preload. Do the same thing with the loaded springs and you'll notice the mass doesn't have as much lag now in response to the step input.
You could say from this that the preloaded spring/mass system is more responsive the an input force.
Heres a perfect example: Spring/Mass system, you car is a perfect example with the engine/front end being the majority of the mass and the spring comprising of all of the components between the front end and the lower control arm connections.
OK so take you mass and spring and apply a step input force-->the spring will compress some amount before you notice any signifigant acceleration of the mass. Now take the same spring and tie it up in a compressed position which makes the spring effectively stiffer, the spring constant didn't change just the preload. Do the same thing with the loaded springs and you'll notice the mass doesn't have as much lag now in response to the step input.
You could say from this that the preloaded spring/mass system is more responsive the an input force.
TECH Enthusiast
Joined: May 2005
Posts: 555
Likes: 0
From: Blairsville, GA
I see your point, now. It makes good sense. Only thing is the SFCs are bent at the ends where they weld on. If they were straight, they'd virtually eliminate the compression load on launch, but then the bends aren't that extreme. The idea makes perfect sense, just the process could be risky. You might try shimming your SFCs with a flat strip of 1/8" metal tacked on one end or the other... it wouldn't take much to have an effective preload, considering you'd probably have to pound your SFCs in with that extra 1/8" of material.
the relation of stress to strain (stretch) is linear for elastic deformation therefor deflection will be equal for the same stress regardless of prestressed condition. example, add 100lbs to a spring and it stretches 1 foot, add another 100lbs and stretches an additional foot total 2 feet. Stress/strain is 100lbs/ft.
Therefor a pre-stressed frame will deform exactly the same amount as the unstressed frame under the same loading conditions.
Most fabricators would want the frame stress-releaved in order to preserve the design assumptions for expected loading. Should a joint have been prestressed too much any additional functional load may cause a catastrophic failure.
Thrust is were the rubber meats the road, and only the weight of the vehicle keeps the car from turning over backwards (wheel stand). prestressing the frame will not change this dynamic.
I would suggest that prestressing will change the shape of the frame and nothing more, stiffening the frame to resist the applied loading is a different mater.
but of course I could be wrong.
Therefor a pre-stressed frame will deform exactly the same amount as the unstressed frame under the same loading conditions.
Most fabricators would want the frame stress-releaved in order to preserve the design assumptions for expected loading. Should a joint have been prestressed too much any additional functional load may cause a catastrophic failure.
Thrust is were the rubber meats the road, and only the weight of the vehicle keeps the car from turning over backwards (wheel stand). prestressing the frame will not change this dynamic.
I would suggest that prestressing will change the shape of the frame and nothing more, stiffening the frame to resist the applied loading is a different mater.
but of course I could be wrong.
billc5, you are correct in your assessment of the mechanics in a spring. Using your example: pre-strain the spring with 100 lbf load and it will deflect some distance "x". now adding 100 lbf more will bring the total up to 200 lbf will give a deflection of "2x", assuming we are still operating in the "elastic region". I've never read anything on the stiffness of a car frame but I'm sure it is not linear, it is far to complex of a design.
In my example of the car structure, we are operating with a fixed amount of mass, if we preload through the SFC install and then launch the car. Deflection in the structure of the car should be less, assumining non-linearity of the stiffness of the car structure.
The other thing I was after was the neutral axis of the car structure/truss. Basically the imaginary line in which no stress or strain is evident. Here is where my concept might prove to be more useful than my spring mass example, because it does have it's limitations as you pointed out. By moving the line of action away from the upper and lower truss members you've effectively created a stronger structure. It's like an example of an "I" beam. By increasing the distance of the top and bottom plate the beam becomes stiffer because of the increased moment of inertia. Now, at this point we haven't moved an structural members in the car so would this do anything?
In my example of the car structure, we are operating with a fixed amount of mass, if we preload through the SFC install and then launch the car. Deflection in the structure of the car should be less, assumining non-linearity of the stiffness of the car structure.
The other thing I was after was the neutral axis of the car structure/truss. Basically the imaginary line in which no stress or strain is evident. Here is where my concept might prove to be more useful than my spring mass example, because it does have it's limitations as you pointed out. By moving the line of action away from the upper and lower truss members you've effectively created a stronger structure. It's like an example of an "I" beam. By increasing the distance of the top and bottom plate the beam becomes stiffer because of the increased moment of inertia. Now, at this point we haven't moved an structural members in the car so would this do anything?
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Originally Posted by squealingtires
billc5, I've never read anything on the stiffness of a car frame but I'm sure it is not linear, it is far to complex of a design.
Deflection in the structure of the car should be less, assumining non-linearity of the stiffness of the car structure.
Deflection in the structure of the car should be less, assumining non-linearity of the stiffness of the car structure.
Anelastic deformation occurs past the material yield points and only during increasing load until failure, it is not recommended to design for this condition.
But as usual, I could be wrong.
Last edited by billc5; May 1, 2006 at 06:58 AM.
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Verdad Gallardo Makes perfect sense. I've been thinking about how to pull the subframes apart, not much just a little. What if the front of the car and the rear of the car were suspended behind the usual points of where the suspension connects in then have a couple of people sit in the car while the sfc's are welded in. the extra weight shoudl cause the car to sag to some small degree. What do you all think