740/940 Stock Spring Rates w/ Analysis
Alright, since I was curious about the stock front spring options on this car and confused about where my new springs come from, I did some research.
On the 1329826 front spring I have fitted, the catalog on GCP.se says as follows:
14.8mm wire, for chassis code 161, 278, 279
This number only appears in the 85-87 740/760 catalog. Perhaps it was superseded but the following information tells me it likely wasn't and is a three year only part.
If you look up p/n 1329826 on the Volvo Parts US Web Store no results are returned. What gives? A quick look at GCP's 85-87 catalog reveals that it is specifically *NOT* for US/Canada market cars. So, it's a RoW only front spring, for 760 with any of the available engines (D24/B280/B230FT). At least, originally. It seems like the Kilen/Lesjofors equivalent was probably available everywhere seeing as I was able to pick up a set with basically no issue. Maybe I'm just lucky.
Looking at my build plate, the car has front spring code AC which cross references to 1329824, the fabled "diesel" front spring. If you take the Kilen applications list as law my car should have the 14 mm x 385 mm 1329823 front spring, instead of the 14 mm x 435 mm spring the Volvo catalog claims the car has. So who's right? Upon disassembly the manufacturer, it turns out, was correct about the car they produced.
At least the rear springs are simple. Sedan or wagon? It sure looks like 240 rears will fit if you really wanted to get weird.
Next up, some fun with stock spring rates.
I figured I'd start with something RWD of comparable weight to figure out a wheel rate target, and found this forum thread on the similarly-weighted BMW E36 M3.
https://www.bimmerforums.com/forum/showthread.php?2014016-E36-M3-Spring-Rates-and-Ride-Frequencies
So to compare, I need to know the ride frequency front and rear, and to know that I need to find my corner weights and motion ratios front and rear. First, the corner weights as measured by Turbobricks user Magnum TE in a thread started by... Me, age 13!
http://forums.turbobricks.com/showthread.php?t=252873
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.
