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      02-04-2017, 04:25 AM   #1
R.G.
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Suspension math is fun!

Hello to all the suspension gurus that visit this forum.

This is the suspension question that needs your insight to help me avoid hours of time in the garage trying to solve:
Can you tell me the ideal rear spring rates for this vehicle?

-e92 335i / M3 rear spring carrier arms
-Corners weigh in at approx. 815 lbs
-Rear fenders lip will be ~25" high from center of lip edge to ground
-Looking for 40/60 bias towards compression/rebound available spring travel
-shock travel is 6" and would like static height at 2-2.5" compressed


The most difficult part for me is to get the measurements I need while it is planted on the ground. While not impossible, still a very tedious chore for the DIY garage guy when the car is that low
Insight would be MUCH appreciated!
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      02-04-2017, 12:24 PM   #2
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Spring rates are all preference...

Motion ratio is close to 1:3 so I personally like to run springs with a neutral natural ride frequency f:r.

I chose 4k 12k spring for roughly 1.7hz/1.8hz. The test coilovers I got came with 6k/12k and the front feels unnaturally stiff (oversteer).
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      02-04-2017, 01:13 PM   #3
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For measuring suspension settings on my buddies dirt car we made metal stands that were the same height as his scales. About 3" tall. We could measure and adjust then roll car onto scales without touching a jack. Was a very good system and made it much easier to get measurements.
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      02-05-2017, 09:14 PM   #4
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What are your intended uses for the car

The actual spring rates don't matter(to an extent) it's the frequency that matters. Even the wheel rate doesn't matter, you just need that to plug in to the formula to get the frequency

By my calculations, 1,000 rear spring rate gives you 1.99 HZ, that's with the rear weight being 782#'s unsprung

A front rate of 310# should give you about 1.78 HZ. 400# is at around 2.14 HZ
A rear rate of 725# gives you about 1.79 HZ.
These are off of the weights being 889 lbs front, 782 lbs rear (sprung mass)

You generally want a rear frequency that's 10% (or maybe its 15) higher than the front for a street car, to make the car not feel like its pitching back and forth.

http://www.e90post.com/forums/showth...t=85206&page=2

http://eibach.com/eibach/img/ers-14s...nworksheet.pdf

Take the number you get from the formula on eibach's site, and divide it by 60 to get the frequency.

It takes maybe 20 minutes to read this thread, plug your #'s into the formula on eibach's website, and calculate what spring rates are acceptable for your needs.
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      02-05-2017, 10:28 PM   #5
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08Nj - pretty clever

This particular suspension set up will be biased for maintaining rear traction on high hp dig and roll launches
I'm trying to allow the rear to squat then rebound in a controlled fashion while keeping the shock in its ideal travel area. The shocks are DA and the front springs will be pre loaded ~300#
Rebound will be set stiff with compression soft
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      02-06-2017, 05:26 PM   #6
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Quote:
Originally Posted by shirtpants_ View Post
What are your intended uses for the car

The actual spring rates don't matter(to an extent) it's the frequency that matters. Even the wheel rate doesn't matter, you just need that to plug in to the formula to get the frequency

By my calculations, 1,000 rear spring rate gives you 1.99 HZ, that's with the rear weight being 782#'s unsprung

A front rate of 310# should give you about 1.78 HZ. 400# is at around 2.14 HZ
A rear rate of 725# gives you about 1.79 HZ.
These are off of the weights being 889 lbs front, 782 lbs rear (sprung mass)

You generally want a rear frequency that's 10% (or maybe its 15) higher than the front for a street car, to make the car not feel like its pitching back and forth.

http://www.e90post.com/forums/showth...t=85206&page=2

http://eibach.com/eibach/img/ers-14s...nworksheet.pdf

Take the number you get from the formula on eibach's site, and divide it by 60 to get the frequency.

It takes maybe 20 minutes to read this thread, plug your #'s into the formula on eibach's website, and calculate what spring rates are acceptable for your needs.

In order to get the correct frequency I need the correct WR. I need the angle correction factor to calculate that. Does anybody have that info off hand?
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      02-06-2017, 05:54 PM   #7
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Quote:
Originally Posted by R.G. View Post
In order to get the correct frequency I need the correct WR. I need the angle correction factor to calculate that. Does anybody have that info off hand?
It's pretty insignificant to the overall frequency value from what I remember. There are numbers floating around on the forums that I believe Orb measured. Most people aren't stock and aren't at stock ride height so the value will change. Best to measure it yourself.
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      02-08-2017, 02:49 PM   #8
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Quote:
Originally Posted by R.G. View Post
Quote:
Originally Posted by shirtpants_ View Post
What are your intended uses for the car

The actual spring rates don't matter(to an extent) it's the frequency that matters. Even the wheel rate doesn't matter, you just need that to plug in to the formula to get the frequency

By my calculations, 1,000 rear spring rate gives you 1.99 HZ, that's with the rear weight being 782#'s unsprung

A front rate of 310# should give you about 1.78 HZ. 400# is at around 2.14 HZ
A rear rate of 725# gives you about 1.79 HZ.
These are off of the weights being 889 lbs front, 782 lbs rear (sprung mass)

You generally want a rear frequency that's 10% (or maybe its 15) higher than the front for a street car, to make the car not feel like its pitching back and forth.

http://www.e90post.com/forums/showth...206&page=2

http://eibach.com/eibach/img/ers-14s...nworksheet.pdf

Take the number you get from the formula on eibach's site, and divide it by 60 to get the frequency.

It takes maybe 20 minutes to read this thread, plug your #'s into the formula on eibach's website, and calculate what spring rates are acceptable for your needs.

In order to get the correct frequency I need the correct WR. I need the angle correction factor to calculate that. Does anybody have that info off hand?
Motion ratios are 0.96 front and 0.563 rear
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      02-08-2017, 04:09 PM   #9
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Quote:
Originally Posted by mob17 View Post
Motion ratios are 0.96 front and 0.563 rear
Remember these need to be squared

wheel rate =
Spring rate * .96^2 for front
Spring rate * .563^2 for rear
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      02-08-2017, 08:12 PM   #10
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Quote:
Originally Posted by shirtpants_ View Post
Quote:
Originally Posted by mob17 View Post
Motion ratios are 0.96 front and 0.563 rear
Remember these need to be squared

wheel rate =
Spring rate * .96^2 for front
Spring rate * .563^2 for rear
Yep thats the formula i use. Confused as to why the eibach one divides instead of multiplies.
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      02-09-2017, 02:10 PM   #11
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Quote:
Originally Posted by mob17 View Post
Yep thats the formula i use. Confused as to why the eibach one divides instead of multiplies.
Eibach calculates the cycles per minute (cpm). You then have to devide that (I forget off the top of my head) to covert cpm to hz.

Two different formulas to calculate spring frequency but the same end result.
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      02-09-2017, 02:36 PM   #12
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Quote:
Originally Posted by bbnks2 View Post
Quote:
Originally Posted by mob17 View Post
Yep thats the formula i use. Confused as to why the eibach one divides instead of multiplies.
Eibach calculates the cycles per minute (cpm). You then have to devide that (I forget off the top of my head) to covert cpm to hz.

Two different formulas to calculate spring frequency but the same end result.
Im referring to the wheel rate formula, not the frequency formula.

Formula i use is WR = spring rate x MR x MR
Eibach one is WR = spring rate / (MR X MR)

Clearly these formulas differ. If we take a spring rate of 300lb/in and MR X MR as 0.5

My formula WR = 300 x 0.5 = 150lb/in
Eibach WR = 300/0.5 = 600lb/in

The wheel rate isnt going to be higher, so 150 is right. Only thing i can think of its maybe the MR figure, perhaps Eibach inverses this so 0.5 would be 2 which then works...
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      02-09-2017, 04:13 PM   #13
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I am using: =SQRT((386.4*WR)/Sprung Weight)/6.28

WR=spring rate * motion ratio^2

My suspension doc: https://docs.google.com/spreadsheets...t?usp=drivesdk

Another better doc: https://drive.google.com/file/d/0B9Z...w?usp=drivesdk

Eibach is using different formulas because they are working in cycles per minute... if you work eibach formulas correctly you end up with the same frequency.

Eibach is saying WR=(motion ratio squared)(spring rate)(angle correction factor) same as you are...

http://eibach.com/america/en/motorsp...sion-worksheet

Last edited by bbnks2; 02-09-2017 at 04:24 PM.
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