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Need advice! rear spring rate for a 940 T Sedan?

VolvoGuy123

Iron Man.
Joined
Mar 17, 2016
Location
CT
So I have a 1995 Volvo 940 (turbo) sedan. I have Bilstein HD's up front and Bilstein "tourings" in the rear along with a set of (40mm) lowering coils (Proparts 43435807) and I recently purchased a "rear adjustable spring perch kit" (kaplhenke) but it is made to use rear springs without "the pigtail" on top (2series?) so I am left to buy another set of (Eibach 5" universal) coil springs for the rear but I don't know what spring rate to go with? Now, I am not picky with the suspension and I am not planning on much track use but I do want it to ride and handle well! Now, I came to my own conclusion of (5"OD x 15"L) and around 200-250lbs (spring rate) for the rear? Anyone have any quick advice before I order these tonight? Thanks, I appreciate any insight! ;)
 
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*FYI; I have stock sway bars, most new bushings, ball joints, wheel bearings, IPD Chassis brace, IPD Panhard bar, etc. Just in case any of that matters? lol.
 
My estimates show that a 200lb rear spring, gives a ride frequency of 2.1Hz on a 940. Already quite firm. (Assumptions: 290 kg sprung corner weight, 1.43MR^2)

As a general rule of thumb your rear frequency should be approximately 0.3Hz higher than the front. If we want 1.8Hz in the front you need approximately 300lb springs.

So what are you running in front? And what dampers are you going to use?
 
One other thing, I see stock 940 rear springs are 375mm. Your 15" are 381mm, and stiffer. So 15 appears to be a bit much if you do not want your car looking like a tractor.

How much adjustability do you have with the adjustable perches? What ride height do you want?
 
One other thing, I see stock 940 rear springs are 375mm. Your 15" are 381mm, and stiffer. So 15 appears to be a bit much if you do not want your car looking like a tractor.

How much adjustability do you have with the adjustable perches? What ride height do you want?

All the info I have is in my post. (kaplhenke) gives a choice between 11" or 9.5" (length) rear springs for the "adj. spring perch kit" but I measured my stock springs and they were 16" and my "lowering springs" I have on the car now I haven't yet measured. I honestly don't know much about suspension setups, hence the reason for this post. Maybe the (Eibach 5" x 11" x 200lbs) is the way to go? I don't know.
 
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think i figured it out? lol. If I did and all goes well I will post specs so other 7/9 series owners don't have to go thru the same situation with this or a similar kit. Thanks, ttys!
 
I?m trying to calculate spring rate for IRS mk1 axle. On my 940 with rigid axle, I have Lesjofors springs, which should be ~120 lb/in ( http://www.forums.turbobricks.com/showthread.php?t=57184 ) and the motion ratio of trailing arms is 1.23. On IRS arms, motion ratio is 0.69, so if my calculations are correct ( https://www.hypercoils.com/spring-rate-calculator/ ), I need ~200 lb/in springs like equivalent. Is this about right or I?m totally wrong? I know springs from IPD nivo delete kit should be ~92 lb/in, which looks incredible soft...
 
Square that solid axle MR, so 1.5. Multiply that by 120, you get 180 lb/in. in vertical (less in roll). Not a bad start.

Now we not only need to check the MR of the IRS (and square it), we need to figure out the transverse spring rate (if applicable).
Do you have a pic of your rear suspension?
I have notes from a 960 from a while ago, if I can't find those I'll need some measurements.
 
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