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From digest.v7.n584 Thu Nov 13 15:45:31 1997
From: Pete Read <>
Date: Thu, 13 Nov 1997 11:00:40 -0500
Subject: Re: <E28><E24> Shock Damping Numbers (Bilstein)

Ed Mellinger writes:
>we can either talk about religion, or we can talk about shocks...
>Does anyone know of a source of actual numbers that would allow
>an intercomparison between, say, Bilstein, Boge, and KYB shocks
>for an E28? I mean, there should be a spec for compression, and
>one for rebound, in some kind of units like newtons per meter per


Yes, I have some actual damping numbers from Bilstein and I'll attempt some rough estimates on the Boge and Koni shocks.

My Bilstein information is better than the other brands because I purchased Bilstein Sports (and Dinan springs) and both Bilstein reps have been extremely helpful.

East Coast Rep (CT), Scott MacDonald 203.265-2854 West Coast Rep (CA), Cleve Hardaker 619.453-7723

Shock Damping Numbers, Dyno Speeds

Shock damping is the force required to move the shock shaft at a particular (shock piston) speed. Increase the speed and the force increases, so specific damping numbers are always measured at a specific speed.

A complete shock damping curve can be plotted by measuring the force at various speeds with a shock dyno. Damping force is measured in newtons (N), kilopond (kp), or pound-force (lbf). N = 0.101972 kp = 0.224809 lbf.
Shock piston speed is measured in cm/sec or inch/sec.

Until reading Paul Haney's shock dyno description in his "Inside Racing Technology" book, I was confused by the different ways of specifying shock speed. Some references talked about speed in terms of shock dyno stroke and rpm, while most used cm/sec or inch/sec. I couldn't correlate the two using mean shock piston speed (i.e. mean speed = 2 x stroke x rpm).

Haney's explanation of the convention used for shock dynos confirmed that both methods work out to the same cm/sec or inch/sec speeds.

rpm x stroke x pi = speed

So for a shock dyno at 100 rpm and 100mm (10cm) stroke:

100 rev min 10cm x 3.14

  • x ------ x ----------- = 52.36 cm/sec min 60 sec rev

Typical shock dyno rpm, stroke, and resulting speed:

        Stroke   Speed    Speed
  Rpm     mm    cm/sec   inch/sec
  ----  ------  ------   --------
   10     100     5.2     2.1
   25     100    13.1     5.2
   50     100    26.2    10.3
   75     100    39.3    15.5
  100     100    52.4    20.6
  200     100   104.7    41.1       

  100      25    13.1     5.2
  100      50    26.2    10.3
  100      75    39.3    15.5
  100     100    52.4    20.6  

E28, E24 Bilstein Numbers

Bilstein shocks usually come in three valving choices: Comfort = Original ride quality
Heavy Duty = Firmer than original
Sport = Firmest (shorter shaft for shorter/stiffer springs)

The part numbers work like this:
P36: P = Strut insert, 36 = piston diameter in mm. B46: B = Shock, 46 = piston diameter in mm.

The last four numbers (e.g. P36 0243, for E28 sport front strut) give the model applicability and valving.

All the Bilstein reference damping numbers are for 52 cm/sec, a relatively high speed. Obviously one number can't convey what the whole damping curve looks like, but it gives some feel for relative values between the Comfort, Heavy Duty (HD), and Sport valving.

          Bilstein E28 5 series 82-88, E24 6 series 83-on
            rebound / compression force at 52 cm/sec
                      Newton        pound-force
                        N            lbf (0.2248 x N)
                    ------------   ------------------
  P36 0243 Sport      2900 / 950     652 / 214 
  P36 0233 HD         2860 / 650     643 / 146
  P36 0239 Comfort    1790 / 650     402 / 146


  B46 0607 Sport      1605 / 805     361 / 181    
  B46 0608 HD         1605 / 805     361 / 181
  B46 0610 Comfort    1590 / 770     358 / 173

Notice how close the numbers are between the HD and Sport, while the Comfort front damping is significantly softer. The difference varies between car models.

General Bilstein Info, '02 Numbers

Bilstein seems to have changed their philosophy through the years. The earlier cars, such as the 'O2, E12 528i, and E21 320i, have much stiffer damping on the Sport shock, while the later E30 and E34 models have the same damping numbers for the Sport and HD. The E28 is somewhere between with little difference except for front compression damping.

Taking at look at the 'O2 numbers as an example of an older model: <02> 1600, 2002

                      Newton         pound-force
                        N            lbf (0.2248 x N)
                    ------------   ------------------
  P30 0025 Sport      2150 /1110       483 / 250 
  P30 0023 HD         1660 / 780       373 / 175   


  B46 0118 Sport      2351 / 664       529 / 149 
  B46 0803 HD         1210 / 575       272 / 129   

The 94% increase in rear rebound damping jumps out at me. That may help explain why many experienced BMW owners (probably former '02 Sport users) say to never use Sports on the street. They probably assume that all BMW Bilstein Sports are equally stiff, but the later models are much more reasonable for street use.

Comparison to Boge and Koni

The original Boge shocks on most E28s should be close to the Bilstein Comfort valving (E28 M5s use front gas charged Boges and rear load-leveling Fichtel and Sachs shocks. The E28 535is uses Bilstein Mtechnic shocks with damping somewhere between the Comfort and HD).

Boge turbo gas shocks should be a little stiffer than the standard model.

Koni shocks are rebound damping adjustable only, so I assume the compression damping is similar to the Bilstein Sport. The typical starting rebound adjustment is 0.75 turns out of 2.5 turns total, so they probably can be adjusted for stiffer rebound damping than the Bilstein.

E28, E24 Shock Summary in order of increasing price

  1. Standard OEM Boge. Twin tube (inner and outer tube),

    non-adjustable shocks. Similar to Bilstein Comfort valving. p/n front 32 836 0, rear 32 912 2

  2. Boge Turbo Gas. Low pressure gas charged, twin tube, non-adjustable

    shocks. Upgrade from standard Boge, firmer damping. p/n front 32 839 F, rear 32 279 F

  3. Bilstein. Monotube (single tube), high pressure gas charged

    non-adjustable shocks. E28 rear HD shocks have three height adjustment grooves 10mm apart, while Sports have six adjustment grooves. Sport shocks have 0.75 inch shorter shafts so shorter/stiffer springs remain seated when the suspension is unloaded. p/n (see numbers above in Bilstein table)

  4. Koni. Low pressure gas charged, twin tube adjustable shocks.

    Rebound adjustment only. Probably can adjust rebound stiffer than Bilstein Sport. E28 rear shocks have two height adjustment grooves. Front rebound damping can be quickly adjusted. E28 rear shocks must be removed for rebound adjustment (remove spring, push shock rod down all the way and turn to adjust). p/n front 8641-1080, rear 8240-1010

Take your choice. Any of these shocks should be fine. Remember that Boge has been BMW original equipment for years.

Bilstein and Koni are the highest price and highest quality. Koni puts the extra cost into making the shock adjustable while Bilstein uses the technically superior monotube design.

Pete Read
'88 M5
Arlington, VA

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