Sway bar calculations (check my math)

[JMortensen]

I need to replace my swaybar I'm trying to figure out the math here and just want to verify that I'm doing this right, because I usually **** it up. Plan is to move the sway bar attachments on the LCA inboard for brake duct clearance.

I measured the LCA length at 11.5 from inner pivot to ball joint, the distance from the sway bar attachment to the inner pivot at 8 and a new further inboard spot at 5 inches. Drew it out on graph paper, calculated for 3" of travel which I think is probably more motion than I get at my spring rate, and found that the outer pivot moved 2 1/8" and the inner moved 1 3/8". Then divided to get a motion ratio multiplier of 1.54.

Using a sway bar rate calculator here

 

http://www.gtsparkplugs.com/Sway-Bar-Calculator.html I input the width of the center section of the bar at 32". This is a stock bar with a bend in it, so I measured the full length with the bends, not just the width to the arms. The arms, which angle out are 11" long and the longitudinal distance from the control arm pivot to the control arm is 10. The bar diameter is 1". I'm getting 297 in/lbs.

Then using hollow sway bar calculators, I'm trying to hit about 1.54x the spring rate, so call it 450 in/lb. Same calculator with 1.25" OD, 1" ID, 32.5" wide, 10" arms no splay is giving me 448 in/lb.

Does all of that sound right to the more technically proficient?

[JMortensen]

Just FYI, the above methodology is wrong. Good thing I asked.

"Got this response elsewhere and doesn't make sense to me, but probably right and I'm probably wrong:

"If that's the case, I think the 1.25" with .125" wall mounted at 5" out vs. your 1" solid mounted 8" out will still be a fair amount softer. Even a .25" wall would still be softer. Something like a 1.375" bar with a .188 wall would seem to be pretty close wheel rate wise to your 1" solid."

When I asked what part of the calculations I messed up, I got:
"Not sure -- I just used own spreadsheet I have. For the raw bar number I think my numbers same as yours. But when I look at wheel rate of the bars I think you need to use motion ratio squared. So for 8" point that is 8/11.5^2 = .484 and for the 5" point it is 5/11.5^2 = .189. So you really need a bar that is .484/.189 = 2.56 times stiffer. Hope that makes sense."
 

and

 

"Yes, you need to square the motion ratio of the swaybar to convert it to a wheel rate.

In addition, the calculated rate of the swaybar is really in lbs/0.5". This is because the analysis assumes that one arm of the swaybar is held fixed and the tip of the opposite arm is moved 1". This is like rolling the car only 0.5". So if you want to do any math that compares the swaybar rate at the wheel to the wheel rates from the springs on the car, you need to double the calculated swaybar number so that it is in lbs/inch. Virtually every swaybar number you will ever see published is really lbs/0.5". If all you care about is making a new swaybar that has the same rate as the existing swaybar, none of this will matter."

[260DET]

Eeeeek!! You should know by now Jon that once you get into calculations everyone else shakes their heads and walks away.