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From digest.v7.n585 Thu Nov 13 16:41:44 1997
From: Pete Read <>
Date: Thu, 13 Nov 1997 11:13:37 -0500
Subject: Re: <E28><E24> Spring Rate Numbers

Ed Mellinger (E28 528e owner) writes:
>I wonder if anyone who's upgraded their E28 suspension has managed
>to get spring length and spring rate data from one or several of
>the popular suppliers?... all I can get are testimonials!
>If you have any hard data on springs, or know here it can be
>gotten, please let me know.


I've been trying to measure every E28 spring that I can get my hands on, including stock springs from my former '87 535i and present '88 M5, and the E28 Dinan springs installed on my M5.

You're also in luck on the stock front spring rate for the 528e. I managed to run out of talent (or judgment) at a driver's school and damaged my left front. Thinking that the M5 may be too fast for me, I decided to slow it down by modifying it with a used 528e left-front strut housing and lower control arm <grin> (the junk yard parts came with the original spring, so I measured it -- E28s all have the same strut housing and control arms).

The 528e mods to my M5 seem to have worked as I haven't been off the track since. However, I seem to get passed more often, hmmmm...

E28 Spring Measurements

Here's what I wrote for the June '97 M5/M6 MPower newsletter. I've added the 528e front spring data to the table and the aftermarket spring summary below.

Since writing my original M5 Suspension article (MPower, 12/95), I've learned more about M5 spring rates. <same article can also be found at the sites below:>

  1. Mformation M5/M6 list archives Mformation #22

  2. BMW Digest Home Page Faq 11.5.2 M5 Suspension

My original calculated rates were a little too high. After talking with Dinan about spring rates, I've changed my method to better fit the actual spring rates of my Dinan springs.

Reducing the measured wire diameter to allow for paint thickness and increasing the number of active coils slightly makes my calculations fairly close to Dinan's specified rates (164 lb/in front, 220 lb/in rear). I've used the same method to calculate the stock spring rates for comparison (see the table below). Thanks to the people at Dinan, particularly James Leong, for patiently answering all my questions.

Notice that the M5 and the 535i have the same calculated front spring rate. Checking the parts fische, I found that they are exactly the same part number 31 33 1 126 024. That was surprising to me. After that revelation, I measured the stock rear springs from my previous '87 535i for comparison to the stock M5 load-leveling springs. Even though the physical dimensions are different, they calculate out to the same spring rate.

Therefore, both the E28 M5 and 535i use essentially the same stock spring rates! The big difference between the M5 and 535i suspensions is the sway bar sizes (25mm/18mm vs 19mm/15.5mm), different shock valving, and larger wheels on the M5.

Calculated Spring Rates for E28 M5, 535i -- Stock vs Dinan

                (K)            (D)   (W)           (N)  
                Calc   Spring  Coil  Wire  Total  Active 
                Rate   Length  Dia   Dia   Coils  Coils 
               (lb/in)  (in)   (in)  (in) 
               ------- ------  ----  ----- -----  ------ 
 <528e          124      14.0  5.08  0.511    7      6>     
  M5, 535i      137      14.0  5.08  0.523    7     
6                    Dinan, M5     174      12.0  5.08  0.531    6     



  535i          140      15.0  3.39  0.436   10.5    9.5 
  M5 LL         140      13.0  3.99  0.472    9      8  
  Dinan, M5     221      12.5  3.39  0.468    9      8                
  Dinan, M5 LL  226      10.5  3.99  0.500    7.25   6.25

(LL = Load-leveling rear spring)

Method for Calculating Spring Rate

W^4 x G Spring Stiffness K (lbs/in) = -------------

8 x N x D^3

W - Wire diameter of spring (inches, subtract 0.004 for paint) G - 11,500,000 for steel springs
D - Spring coil center diameter (inches). Measure to

middle of spring coil, not outer diameter. N - Number of active coils -- total coils minus one.

E28 Aftermarket Spring Summary

H&R rep Roland Graef supplied me with the H&R spring data as well as the Eibach rates from his notes (he wasn't positive about the Eibach rates). Note that both the H&R and Eibach are progressive rate in the rear.
         Front    Rear    Lowering
         lb/in   lb/in    inches 
         -----  -------  ------- 
Dinan    164      220      0.75
H&R      200    175-230    1.25

Eibach 175 150-175 1.25 (rate is estimate from H&R rep)

Personally I like my Dinan springs. The greater increase in rear versus front spring rate seems to balance out some of the understeer. They only lower the car 0.75 inch versus the 1.25 inch of the others. Notice how the 0.75 inch drop works perfectly with the 0.75 inch shorter Bilstein Sport shocks.

After living with a 0.75 inch lower car, I wouldn't want to go any farther as there are too many things to hit in everyday driving. Also, BMW stock suspension geometry is very good. Too much lowering, without changing suspension pick-up points, can actually hurt suspension performance.

Pete Read
'88 M5
Arlington, VA

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