Found this chart on a 911 site. Numbers should be the same for 914's. Suprising how much the rate increases with each mm. The difference between the two numbers has to do with where the value of the bar is measured, at the wheel or the bar itself. I did not think that my 21mm bars would have such a high value (227/249)

dr

Front Rates
Torsion Kw (lb/in) Ks (lb/in)
Dia (mm) (Wheel) (Spring)
16 77 90
17 97 112
18 122 138
18.5 137 154
19 152 170
20 187 207
21 227 249
22 273 298
23 327 355

rate increases with the 4th power of diameter, all else being equal.

there's also a formula for hollow bars -- the center of the bar weighs something, but does essentially no work. somewhere i have my Carroll Smith and Paul Van Valkenburgh books but i haven't unpacked them yet. gotta do some of that tonight in the process of making room in the garage for the cars before what's left of the hurricane hits sometime Friday morning ...

The numbers looked higher to me because of what most people tend to run on their cars. Seems like many people run 21mm up front with only 140 or 180 springs in the rear.

My experience with 911's and my mid engine sport racer tells me that the spring rate at the rear should be higher than the front, not lower like our cars seem to be set up.

I would think that for a road race setup, 21mm bars up front should match with 225 => 250 in/pound rear springs, and then a heavy front sway bar.

dr