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Old 10-02-2018, 01:35 AM   #1
VolvoGuy123
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Default Need advice! rear spring rate for a 940 T Sedan?

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! ;)

Last edited by VolvoGuy123; 10-02-2018 at 04:26 AM.. Reason: Because spelling!
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Old 10-02-2018, 01:40 AM   #2
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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.
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Old 10-02-2018, 03:16 AM   #3
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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?
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Old 10-02-2018, 03:23 AM   #4
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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?
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Old 10-02-2018, 04:09 AM   #5
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Quote:
Originally Posted by Sjeng View Post
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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Old 10-03-2018, 02:40 AM   #6
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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!
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Old 10-04-2018, 08:20 AM   #7
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Don't forget the motion ratio of the rear arms. Any spring behind the axle will give a higher wheel rate.
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Old 06-14-2021, 11:17 AM   #8
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Im 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/sh...ad.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 Im totally wrong? I know springs from IPD nivo delete kit should be ~92 lb/in, which looks incredible soft...
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Old 06-18-2021, 11:10 PM   #9
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I can work on that when i get home from this tow/road trip.
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Old 06-21-2021, 09:56 AM   #10
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Thanks Mike, that would be nice
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Old 06-21-2021, 11:11 AM   #11
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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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Old 06-22-2021, 04:17 AM   #12
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Its mk1 irs, so there is no transverse spring.
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Old 06-22-2021, 11:57 AM   #13
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450 divided by 650 is .69 squared is .476. So for 200# wheel rate you need 400-425# spring

Last edited by mikep; 06-22-2021 at 12:15 PM..
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Old 06-22-2021, 03:30 PM   #14
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Wow, that is pretty stiff in compare... but should be equal, so I will try it.
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Old 06-22-2021, 03:55 PM   #15
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What do you have in front?
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Old 06-22-2021, 04:23 PM   #16
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Also Lesjofors lowering springs, should be ~130lb/in. Paired with Bilstein HD.

Last edited by petercz; 06-22-2021 at 04:36 PM..
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Old 06-22-2021, 08:04 PM   #17
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Then get 275lb/in springs unless you want more or less rate than 130.
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Old 06-23-2021, 11:18 AM   #18
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Doesn't the 91 940SE sedan use the same rear suspension that you are trying to setup?
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Old 06-23-2021, 12:37 PM   #19
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It does, but always with nivomats...
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