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      02-13-2008, 09:21 AM   #10
stressdoc
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Dominica
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Drives: BMW i8; Toy 4runner TRD pro
Join Date: Jun 2005
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Orb's point is that although the stiffness formula is usually framed in terms of a %, derived from the ratio of difference in diameter to the 4th power, one needs to consider the value that is being increased. 10% of 100 = 10, whereas 10% of 10 is only = 1. Because the rear bar is so much smaller than the front, an equal % increase in each results in much more absolute stiffness increase in the front.
Switching to H&Rs results in the following change:

% increase in stiffness = (27/26.5)^4 = 1.07763468413, i.e., a 7.8% increase in front bar stiffness (if we believe the measurements, and if the materials have the same tension resistance).

For the rear, % increase = (20/13) ^4 = 5.60204474631 i.e., a 460% increase.

So just looking at the %s, it looks like rear stiffness has increased much more than the front change, a % ratio of roughly 460/7.8 or about 60:1. But if you look at the change in actual stiffness, it is more even, because the front bar was so much bigger to begin with (10% of a big bar is a lot more than 10% of a little bar). The change ratio of actual stiffness is roughly 13/4, or about 3:1.

Note that this formula does not work for hollow bars like the Hotchkis, nor adjustable bars. Note further that there are a lot of factors that enter into the end result. You really need a sophisticated modeling program to sort the effects of the variables out, and then a ton of road time to fine tune the art. This is why some folks are hesitant to mess with suspensions piecemeal.

My bottom line: if you want a simple moderate increase in roll stiffness and reduction of understeer, opt for a M-sport rear sway. If you want a major increase in roll stiffness, opt for the H&R set. But be wary that you have significantly changed the balance that BMW engineered, and you may need to adjust other components (e.g., get adjustable coilovers). As noted above, there are traction issues (inside wheel will lift off ground more with increased roll stiffness), and having a LSD puts another complex variable into the mix.

No-nos in my book include: lowering w/ stiffer springs (and doing nothing else), or making a big change in one or the other sway (small relative increase in rear being the desireable change).
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