Which Company makes "best" replica?

[ItBites]

Silverback (and others):

With frames, we usually care about torsional stiffness, not bending stiffness. Ix or Iy is normally the cross-sectional moment of inertia with respect to the neutral bending axis (what you calculated).

Ip is typically the abbreviation for torsional cross-section moment of inertia. I think if you look at torsional stiffness, you will find the 4" round tube is superior to any other shape with dimensions less than 4 x 4 (given similar wall thicknesses). The .120 wall thick 4" round tube is still likely better than the SPF .160 wall thickness, due to the 2" dim on the SPF frame (and with less mass).

Have a nice day

[Silverback51]

Originally Posted by ItBites

Silverback (and others):

With frames, we usually care about torsional stiffness, not bending stiffness. Ix or Iy is normally the cross-sectional moment of inertia with respect to the neutral bending axis (what you calculated).

Ip is typically the abbreviation for torsional cross-section moment of inertia. I think if you look at torsional stiffness, you will find the 4" round tube is superior to any other shape with dimensions less than 4 x 4 (given similar wall thicknesses). The .120 wall thick 4" round tube is still likely better than the SPF .160 wall thickness, due to the 2" dim on the SPF frame (and with less mass).

Have a nice day

And that is why we don't have square axles.

But the majority of the torsional stiffness comes from the rest of the frame design.

[RICK LAKE]

DinoByte Kit Car Mag had talked about trying to do something like this. The Big problem was ringers, getting a prodriver to test the cars and be consistent from car to car. You have to find owners who will let some one abuse them. You would need to get the same springs, shocks, wheels and tires, rearend ratios and motors to really make this an honest test. Who is going to swap motors from car to car? This is why the Run&Gun was started back in 90. It is a test of car and driver to see who had the best car. In the last 90's FFR was doing well and winning alot. SPF then got into the game and was winning alot of classes. Shell Valley and Backdraft are now getting there turn in the sun. Others have shown up and done well. Some Companies will not back a car and driver for fear of lawsuits if hurt, ERA is one. Sure there are others. IMO the Windsor strokers guys will beat the FE guys about 65% of the time on most tracks without long straights. It comes down the wieght/ power. I have a 482 motor now and can't get the car to hook up now. I may redegree the cam to take some of the torque out of the lower rpm range to stop this problem. If you look at alot of the guys here with BBF motors, the 482 strokers are the new kid on the block. ERA cars also weight about 2-300 pounds more that SPF, FFR, Backdraft, Shell Valley. I have seen these cars weight at R&G. IMO this test will never be run, at least not by Kit Car Mag. This Mag is just holding on with the small number of people who buy it. If you look at stories and pictures, the editor does everything alone. Some tech write up are done by owners. Rick L

[RICK LAKE]

Silverback 51 From all the reading of the old days with cobras, the issue was about frame flex. I think the issue was that round tubing could bend and TWIST easier than rectangle tubing of say 2" by 4". This is my understanding of the issue. Is there a formula about this? IMO this is why Nascar builds there cars with round tubing for the roll cages. It's easier to work with cheaper to buy, lighter the rectangle, and it bends to absorb an accident easier than rectangle and still hold its strength. I believe that the main frame on nascar is still rectangler tubing. I also think that alot of the round tubing built cars have gone to a thicker wall tubing to cut down on the issue we are talking about. Twist and flex. Rick L.

[ddcobra]

You can't compare a 2 rail chassis to a bird cage style like Kikham,CSX,and D&D.
Don,

[Silverback51]

Originally Posted by RICK LAKE

Silverback 51 From all the reading of the old days with cobras, the issue was about frame flex. I think the issue was that round tubing could bend and TWIST easier than rectangle tubing of say 2" by 4". This is my understanding of the issue. Is there a formula about this? IMO this is why Nascar builds there cars with round tubing for the roll cages. It's easier to work with cheaper to buy, lighter the rectangle, and it bends to absorb an accident easier than rectangle and still hold its strength. I believe that the main frame on nascar is still rectangler tubing. I also think that alot of the round tubing built cars have gone to a thicker wall tubing to cut down on the issue we are talking about. Twist and flex. Rick L.

Just so you don't think I'm ignoring you, I will answer your questions when I get home after work.

[Silverback51]

Originally Posted by RICK LAKE

Silverback 51 From all the reading of the old days with cobras, the issue was about frame flex. I think the issue was that round tubing could bend and TWIST easier than rectangle tubing of say 2" by 4". This is my understanding of the issue. Is there a formula about this? IMO this is why Nascar builds there cars with round tubing for the roll cages. It's easier to work with cheaper to buy, lighter the rectangle, and it bends to absorb an accident easier than rectangle and still hold its strength. I believe that the main frame on nascar is still rectangler tubing. I also think that alot of the round tubing built cars have gone to a thicker wall tubing to cut down on the issue we are talking about. Twist and flex. Rick L.

The strong point of a round tube in torsion is that the stress value is the same along it's outer most surface. That is why we have round drivelines, axles, half shafts, etc.. With a square, rectangular, or other shape of structural material the stresses will be concentrated at the outermost fibers where the shape is changing.

Most of the strength of a round shaft is at it's most outer surface. That is why drivelines are always hollow. The closer you get the center, the less strengh is gained by the material in the center. Why are axles solid? Generally because there is less packaging room for a large diameter hollow shaft.

Here is a link to the formula for a round shaft. The formula changes for different structural shapes.

http://www.instron.us/wa/resourcecen...rm.aspx?ID=173

Needless to say, the design of a frame is very complex. And the most efficient design will be a combination of different structural shapes that are choosen to satisfy the stress loads that it's being subjected to.

Take a look at the NASCAR frames. As you pointed out they consist of rectangular main rails and the rest of the frame is mostly different sizes of round tubes.

Now take a look at a top fuel dragster. It's all small diameter round tubing. Different requirements dictate different material shapes and designs. The dragster is a truss design that also allows a great deal of flex in one direction to allow maximum weight transfer for straight line traction. The NASCAR frame is designed with different requirements, so that design is completely different.

Hope that answers your questions.