From digest.v7.n584 Thu Nov 13 15:45:31 1997
From: Pete Read <read_at_arl.bna.boeing.com>
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
Bilstein E28 5 series 82-88, E24 6 series 83-on
rebound / compression force at 52 cm/sec
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
Taking at look at the 'O2 numbers as an example of an older model:
<02> 1600, 2002
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
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
E28, E24 Shock Summary in order of increasing price
- 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
- 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
- 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
p/n (see numbers above in Bilstein table)
- 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.
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