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Discussion Starter #1
Since I recently bought an FEA package for my business, I figured I would give a shot at designing a new sway bar for autocross. What would be a good estimate for the maximum inches of deflection from one end to the other on a sway bar? This would be for a LSS springs running A6's and Ohlin shocks.
 

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You are asking the wrong question. It should be now many newton/meters of torsional force to cause 1 inch of deflection, then engineer the bar to survive maximum dynamic force without failing. Start with 1.3 Gs versus the mass overcoming the existing LSS springs will be the total static force, allow for inertia for dynamic force added to the static force, then you will be in the ballpark. BTW I am not joking, or being a smart @$$.
 

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Discussion Starter #4
I have a good starting point as I know the lb/in of the current bar that is on the car. I am looking at what the lbs/in for the new bar will be. To determine the peak stress in the bar I need to know what the maximum deflection will be. 6 inches of vertical deflection between the ends is way too much and no sway bay as short as ours is would survive that. The sway bar connection is even inboard of the shocks so its motion ration is probably around 1/2 of what the wheel travels. I'm thinking maybe 3 inches would be realistic.

I'm attempting to get two things out of the FEA. The lbs/in of travel and the maximum stress at full deflection.

Thoughts?
 

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one hydraulic jack under right wheel (preferably with a load cell) pump jack till left wheel lifts, measure load cell output, measure deflection, and you have a static figure for both measurements (total deflection vs mass of front of vehicle, total pounds of force (load cell value divided by linear (not slanted) length of arm to pivot point) will give your foot pounds figure.
 

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Discussion Starter #6
I do have a set of scales. I can set them up and then using a stack of 1" blocks, jack the car up and place them one at a time under the right tire, set the car down and record the weight. For a bit more resolution I could go 1/2" at a time. If I'm thinking about this correctly, a stiffer bar will have less total deflection so the stress in the bar should actually be decreasing by running a heavier bar (assuming wall thickness stays sufficient enough when going to a larger OD to prevent localized wall buckling failure).
 

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Discussion Starter #8
It depends on the stiffness of the bar. When the sway bar goes up on the left side when that suspension is compressed in a corner the right side suspension is dropping down. The sway bar is try to push up on the right side suspension. If the sway bar is very weak then it would not compress the right side at all and the left side could go to full compression. This would be worse case scenario for the sway bar deflection, but very low sway bar forces. A very stiff sway bar would cause the right wheel to travel up almost as much as the left side wheel resulting in very little sway bar deflection, but very high sway bar forces.
 

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Custom bar

Look at speedway engineeriong. They have a chart of bar rates. The bar rae depends on the outside diameter, since the elasticity of any steel is pretty much the same. There are charts there, also there is a formula in most suspension books. I seem to recall the rate is proportional to diameter to the 4th power....

Anton
 

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Discussion Starter #10
Calculating it by hand is easy. It isn't very accurate though for calculating actual forces applied though as most hand calcs ignore the bending stiffness of the arm. I had looked at going with a speedway type bar, but the out of plane bends on the Elise arms would require custom arms. I have a shop local to me with a CNC mandrel tube bender and a coordinate measuring machine. They took the stock bar and measured it for me and gave me a print out of the coordinates that I used in Solidworks to model up a new bar.
 

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Is it really that difficult? To answer just your question, I would disconnect one side of the sway bar and simply raise the car up on one side. Then measure the distance from the disconnected connection points, add a little fudge, and you have a cake.
 
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