1992 940T Sedan
Total: 3220
Front : 1780
Rear : 1460
FL: 900 - FR: 860
RL: 740 - RR: 740
One model year off but it'll do.
For the sake of simplifying things I averaged out the front axle as 880 lbs on both sides. I won't be staggering spring rates so this will do. I estimated my unsprung weights as follows:
Wheel and tire 40 lbs
Rear axle 131 lbs
Half lower control arm 3 lbs
Half the shock 3 lbs
Half tie rod negligible
Front brake 20 lbs
Spindle/hub 6 lbs
Rear brake 16 lbs
And that gives me sprung corner weights of:
Front: 808 lbs
Rear: 612.5 lbs
Now we move on to the motion ratios. With a McPherson strut setup the motion ratio is simple, equal to the cosine of the strut angle. I put an angle finder on both struts compressed to ride height and averaged it out since I didn't trust that the car was on level ground. This returned a measurement of 12.45 degrees. Cosine of 12.45 degrees is (almost, I hope I'm not getting graded on this) 0.976.
In the rear, I measured from the center of the trailing arm bolt to the center of the wheel, and to the center of the rear spring.
Trailing arm to wheel 665 mm
Trailing arm to spring 810 mm
810/665 = 1.218 motion ratio in the rear.
Wheel rate = spring rate * motion ratio^2
I calculated the following wheel rates based on the various wire diameters I found in the parts catalog, assuming (oops) they would all work out to have the same number of active coils as I measured on my car, that being 7.5 rear and 4.25 front.
Front springs:
1329822 13.6 mm wire, 126 lb/in spring, 120 lb/in wheel rate
1329823 13.7 mm wire, 130 lb/in spring, 124 lb/in wheel rate
1329824 13.8 mm wire, 134 lb/in spring, 128 lb/in wheel rate
1329825 14.4 mm wire, 161 lb/in spring, 153 lb/in wheel rate
1329826 14.8 mm wire, 181 lb/in spring, 173 lb/in wheel rate
IPD front spring measured by mikep 145 lb/in, 138 lb/in wheel rate
A company called Rallyboutique sells a 16.1mm front spring for rally use, that works out to a 261 lb/in rate at the spring and 249 lb/in at the wheel.
Rear springs:
1273759 9.8 mm wire, 51 lb/in spring, 76 lb/in wheel rate
(???) 10.2 mm wire, 60 lb/in spring, 89 lb/in wheel rate
1273977 11.3 mm wire, 93 lb/in spring, 138 lb/in wheel rate
1359710 11.9 mm wire, 117 lb/in spring, 174 lb/in wheel rate
1273978 12.1 mm wire, 125 lb/in spring, 185 lb/in wheel rate
IPD rear spring measured by mikep, 95-140 lb/in spring, 141-208 lb/in wheel rate
If you have more spring information please let me know!
The formula for ride frequency in Hertz is (1/2pi)*(sqrt(K/M) where K is the spring rate in N/m and M is the sprung mass in kg. Plugging in previously mentioned values provides the following results:
1329822 13.6 mm wire, 1.206 Hz
1329823 13.7 mm wire, 1.235 Hz
1329824 13.8 mm wire, 1.254 Hz
1329825 14.4 mm wire, 1.374 Hz
1329826 14.8 mm wire, 1.457 Hz
Rallyboutique 16.1 mm wire, 1.747 Hz
IPD ??? wire, 1.306 Hz
1273759 9.8 mm wire, 1.095 Hz
(???) 10.2 mm wire, 1.192 Hz
1273977 11.3 mm wire, 1.485 Hz
1359710 11.9 mm wire, 1.661 Hz
1273978 12.1 mm wire, 1.722 Hz
IPD ??? wire, 1.499-1.848 Hz
For comparison's sake, the BMW stock regular springs measure in at 1.02/1.12 F/R and their factory "Sport" springs measure in at 1.47/1.41 F/R. Conventional wisdom says you want a 10% higher rear ride frequency. My car stock comes in at 1.25/1.48 and with the RoW 760 front/wagon rear setup results in ride frequencies of 1.46/1.66.
Neat.