Montreal Subaru Club

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The formula for calculating the resistance of a solid sway bar, according to Fred Puhn, is:

Code:

                           500,000 D^4
K (lbs/in) =  -------------------------------------
               (0.4244 x A^2 x B) + (0.2264 x C^3)


               B
       _________________
A|    /                 \  C
 |   /                   \

A - Length of end perpendicular to B (torque arm - inches)
B - Length of center section (inches)
C - Length of end (inches)
D - Diameter bar (inches)

This formula does not take into account the flex of the bushings used to mount the sway bar, which can be significant. I've measured the stock sway bars (roughly, not precisely) at:

Stock Front sway:
A: 5 in
B: 32 in
C: 7 in
D: 0.79 in
hence, K = 467lb/in

Stock Rear sway:
A: 7 in
B: 42 in
C: 7 in
D: 0.79 in
hence, K = 205lb/in

Now, what matters isn't the spring rate of the bar, but the spring rate at the wheels. The motion ratios front and rear in the STI are about 0.75 and 0.875, respectively. To get the wheel rate of the sway bar, we multiply by the spring rate (K from above) by the square of the motion ratio.

WHEEL RATE = K * Mr ^ 2

Thus, the final wheel rates from just your sway bars are:

Front: 267lb/in
Rear: 157lb/in

Add this to your coil spring rates to get your total wheel rate. The stock front springs are 224lb/in with a motion ratio of 0.97 (=210lb/in at the wheels), and the rear springs are 194lb/in with a motion ratio of 0.98 (=190lb/in at the wheels). A stock STI rides on its bump stops up front, which causes these spring rates to hike tremendously, but I'm going to ignore that for this conversation and assume a suspension with adequate suspension travel.

FINAL STOCK WHEEL RATES:

477lb/in front (267lb/in from sway bar, 210lb/in from coil spring)
347lb/in rear (157lb/in from sway bar, 190lb/in from coil spring)

The distribution of these total spring rates match the car's weight distribution almost exactly.

Anybody want to measure the Whiteline sway bars? If they're the same as stock, except for the diameter, you'd get spring rates (not wheel rates) of:

Whiteline Front:
24mm: 955lb/in
27mm: 1877lb/in *this is not accurate because this bar reduces to 24mm prior to the bushings

Whiteline Rear:
24mm: 419lb/in
27mm: 671lb/in *this is not accurate because this bar reduces to 24mm prior to the bushings

You can see that just minor bumps in sway diameter increase resistance quite a bit- it actually goes up by a power of four! So, the question I'm arriving at: is a 24mm sway bar too big? What about a 27mm sway bar?

Javid (of 6gun Racing) has said his car, which uses high spring rates and big Whiteline sway bars, is slower than the competition running without sway bars (but having more body roll). This actually makes a lot of theoretical sense. Quite simply, the car's spring frequency becomes too high for optimal use of the car's tires, which most racing literature suggests takes place around 2.0 to 2.5hz. (Further reading about how to determine your spring frequency based on your wheel rate.)

I think that big sway bars are useful on cars with soft springs to reduce body roll, but on cars with firm springs (400lb/in front, 350lb/in rear or higher), smaller (closer to stock) sway bar sizes should be used for optimal grip. This assumes the car has sufficient camber and enough suspension travel- in other words, no other pre-existing, more significant problems. It also looks to me like running a larger sway bar up front (especially the huge 27mm bar) doesn't make much sense (in theory) for anything other than A-stock autocrosser.

Comments appreciated.

EDIT: I spoke with an engineer who assured me my numbers (from Puhn's book) were wrong in regards to body roll. Since you have twice as much bar flex in a body roll situation (inside and outside wheel move roughly equal amounts in opposite directions), the sway bar rates I calculated should be DOUBLE what I originally said.

In other words, take the number from the original equation and double it.

My original equation only works for a one wheel bump. I thought the doubled force was divided among the two wheels (thus you wouldn't double the rate), but it isn't. That's where I was wrong. The sway bar force is equally applied to both wheels.

So, again- as extreme as the sway bars sounded earlier, double that.