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home made sway bars

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6,363
8,187
Ran some numbers in case anyone feels like building their own bars..

Swaybar maths...

According to my trusty swaybar calculation spreadsheet for a 36" bar with 6" arms:
1" Hollow (.188 wall) = 753 lbs/in
1" Hollow (.095 wall) = 505 lbs/in
1" Hollow (.083 wall) = 458 lbs/in
7/8" Hollow (.095 wall) = 325 lbs/in
7/8" Hollow (.120 wall) = 376 lbs/in
19mm (Solid) = 278 lbs/in
 
Sorry for being unclear. I was just throwing that out in case someone wanted to experiment on their own. I'm just not sure what that equation assumes for what type of steel is being used.
 
I have read several threads on this and tried to understand it. Th issue on production cars you need more smarts to calculate all the weird bends and angle effects. Or build a test rig. I'm going with test rig, we don't calculate spring rates we measure them, so testing them seems like the best approach for normal folks. Mass produced bars are less exotic and are tempered / hardened into a specific property. Some of the higher grade materials are a good base to home build a bar. It would get expensive to have a shop professionally treat the bar post shaping.
 
6,363
8,187
Well, yes...sort of.. when I was racing MG midgets I discovered the way the Brits "hardened" something was not really reliable. At that point I built several bars out of different materials, mostly chromoly and played around with them. They were straights bars, welded ends, then reheated the welds in an attempt to normalize the welds. They worked pretty good, although I couldn't ensure longevity.
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
http://www.gtsparkplugs.com/Sway-Bar-Calculator.html

Not sure what modulus is assumed for that link. Or the accuracy for that matter.
I know this is an older thread . . .

I went through the math some years ago and came up with Fred Puhn's formula using 12E6 psi for the shear modulus G. That's maybe a little on the high side for most steels, assuming that for most steels that the Young's modulus E is 29E6 psi and Poisson's ratio (symbol = Greek lower case nu) being 0.3.

Fred's formula only considers torsion along the center portion and cantilever bending in the arms. There are a number of smaller effects that end up softening the Puhn-calculated bar rates by around 5% (it varies a bit depending on the other dimensions).

It's hard to say what the effect of bends actually is, relative to this simplified model, other than that's likely to be somewhat variable as well. At least one example I threw at a piping structural analysis software that supposedly knows how to calculate elbow effects ended up slightly stiffer with the bend.

Incidentally, Fred's formula is for the bar's stiffness taken end to end rather than from one end to a neutral position at the center of the torsional segment.


And there's this one other thing . . . using any of these bar rates reflected out to "wheel rates" as direct contributions to roll stiffness leads to results that may be a bit suspect. As in a much greater reduction in roll per lateral g when you stiffen a bar and touch nothing else than passes a sniff test. There is a better approach.


Norm
 

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