I've been a lurker for some time now, and I've seen lots of discussions about swaybars.

Doses anyone know how to calculate the relative stiffness (%) of aftermarket bars(hollow and solid) as compared to the OEM?

 

Swaybar effective rate is usually given in lb./in. or "pounds per inch". To find this you need to know the length of the arm upon which the bar acts, the outside diameter and inside diameter of the bar itself, and the modulus of steel in use.

If bar A is 150lb./in. and bar B is 225 lb.in. then bar B is 50% stiffer than bar A.

Don't know that I've seen the stock bar's effective rate published, measured or calculated.

 

Assuming that the bars are the simple U shape (and the same material), then it's pretty simple.

1) The stiffness is proportional to diameter to the 4th power. To find the stiffness ratio due to diameter: calculate the diameter to the 4th power for each bar. For a hollow bar, calculate the outer diameter to the 4th power, then the inner diameter to the 4th power, and subtract. Now divide the number for bar 2 by the number for bar 1. This is how much stiffer the shaft of bar 2 is.

2) The stiffness is also inversely proportional to the square of the right-angle distance of the endlink attachment hole to the axis of the shaft. Divide this number for bar 1 by the number for bar 2. This is how much stiffer bar 2 is due to the position of the mounting holes.

To determine how much stiffer bar 2 is overall, multiply the result of step 1 by the result of step 2.

If the materials are different, then there's an additional factor of E2/E1 (the ratio of the Young's moduli of the two materials) to multiply in.

 

Roger, you could also check out the following links:

www.theherd.com/articles/swaybar.html

www.spswebpage.com/tech/swaybars.html


Pierre

 

Thanks all!

Based on the principal that the stiffness of a swaybar is proportional to the 4th power of the diameter, I have compiled the following information of stiffness relative to the

 

At 31.75 OD and 4.75 WT, the Comptech should be closer to the Mugen... +48% after playing the calculator game.

 

....and the Spoon closer to -16%....

GREAT REFERENCE!

 

According to the specifications provided with the Comptech co-axial adjustable front sway bar, here are the ranges of stiffness in the 5 bolt positions. In discussions with a representative from Comptech, they claimed that these rates were relative to the "00, '01 OEM bar.

Original Comptech Adjustable Sway Bar - +27%, +44%, +67%, +97%, +124%
corrected values
New Design Comptech Adjustable Sway Bar - +62%, +82%, +102%, +136%, +178%

incorrect values from original post
New Design Comptech Adjustable Sway Bar - +162%, +182%, +202%, +236%, +278%

These values seem very high compared to the other bars listed. Does anyone know the effective rates of the Gendron bar? I'm using the new design bar in the "full stiff" position for autocrossing with Kumho V700s on concrete, and there is no understeer concern. I haven't tried the new bar on asphalt yet, but the old bar understeered a bit on its full stiff setting.

 

I suspect that you need to add 100% to the original Comptech values to make 'em the same. I don't think that the redesign added THAT much stiffness.

 

[QUOTE]Originally posted by jguerdat
[B]I suspect that you need to add 100% to the original Comptech values to make 'em the same.