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From digest.v7.n214 Wed Sep 3 16:48:02 1997
From: albert jenab <>
Date: Wed, 03 Sep 1997 16:22:01 -0400
Subject: Re: Shock construction questions (long)

Was going to tackle this myself, but then I thought I'd defer to the experts. A good friend of mine designs and builds custom racing shocks for a living (also has been known to revalve Bilsteins...), among other things. He's also a Ph.D. mechanical engineer from MIT, hangs out with some CART & SCCA FA guys and major US auto manufacturer's suspension development groups. He's forgotten more about shocks than I'll ever know. I asked him if he'd mind writing up a layman's explanation of the difference between monotubes and twin tubes. Anyway, here's his response to some of your questions. He's not a subscriber to the digest, but f eel free to contact him with further questions. Enjoy.

  • -Al PS: New E36 M3 Bilsteins are monotubes. All other E36 Bilsteins are twin tubes.

Just as the name implies, monotube shocks use just one tube with a piston that fits snugly inside and moves up and down. Historically, there have been some monotube designs that use an oil-air emulsion as a working medium (aircraft landing gear use this concept), but most modern monotubes use the deCarbon design which uses a floating piston to separate the shock oil from a small chamber of high-pressure gas (usually dry nitrogen at 200 - 400 psi). As the rod moves into and out of the shock absorber, the floating piston moves slightly, compressing and expanding the gas to accomodate the volume of the rod (remember that shock oil is incompressible). Why use gas at such high pressure? Elimination of oil foaming and cavitation that would make the performance erratic when the shock get hot.

The real secret of monotube shocks is in the piston. Obviously, for the piston to move up and down inside a snugly fit cylinder, the oil has to flow through the piston. To control the force produced by a monotube, you have to control the flow of oil through the piston. The piston has two sets of relatively large holes that go straight through. Each of these sets of holes are covered on one end only by a number of valve shims which regulate the flow in one direction and completely block the flow in the other. The end of the holes that is not covered by the shims has "pockets" machined in the piston around them to allow oil to flow past the shim stack on the opposite side of the piston and into the holes freely. So we have two sets of valve shims, one to regulate flow in compression and one to regulate flow in extension. And when the shock is in compression, the extension shims block their holes, when the shock is in extension, the compression shims block their holes.

Monotube shocks are "tuned" by selection of valve shims, and different shim packs can be specified for extension and compression (or rebound and jounce, if you prefer). If a large number of relatively thick shims are used, the shim pack would be considered "stiff" and would provide a lot of force at a given shaft speed. In the same way, a single very thin shim would give a very soft shock. The art and science of coaxing desired performance out of monotubes is in the design of the shim packs and literally millions of combinations are possible. For example, a complete stock of shims for re-valving Bilstein shocks and struts ranges from 0.15 to 0.5 mm in thickness, and 20 to 38 mm in diameter. This range of shims in their various combinations would suit any application from a street Neon to a full-race Coupe deVille!

Design variations abound in twin tubes, but what they all have in common is two tubes mounted inside one another, with the piston moving within the inner tube. As the piston moves up and down, hydraulic fluid is forced either into or out of the space between the two tubes (the annulus). There is always a "compressible element" in the design, located in the annulus, which may be compressible closed cell foam or simply a gas volume. No current twin-tube design uses a floating piston or any other method to separate the compressible member from the shock oil. Twin tubes typically operate at lower pressures than monotubes, in the range of 10 to 50 psi.

The valving or flow regulation hardware is placed at two separate locations: at the bottom of the inner tube and within the piston. Perhaps the most common design of modern twin tubes places the compression valving in the piston and the extension valving in the bottom of the inner tube. The valving in twin-tubes is of the spring-plunger type, and tuning is accomplished by changing springs. Given the extremely wide range of springs available, a similarly wide range of damping characteristics is also available.


  1. Monotubes virtually eliminate oil foaming and cavitation by use of high pressure (see above).
  2. The deflected disk valving scheme used by virtually all manufacturers of monotubes allows finer tuning of damping characteristics with better consistancy, particularly at low speed.
  3. Twin-tube shocks have a "dead-band" around the zero-velocity point (where the motion of the rod changes direction) that is simply not present in monotubes. [Try it yourself...see if there isn't a little "play" in your twin-tube as you alternately push and pull on the rod!]
  4. Generally, you can't run twin tubes in an inverted position. Inverting the shocks (not applicable to struts) is good practice to reduce the unsprung weight of your suspension.

The highest performance shock absorbers in the world including Penske, Ohlins, Koni racing shocks and Bilstein (with a few exceptions) are all pressurized monotubes. All teams in F1, CART, IRL, etc., etc. use high pressure monotubes exclusively. This by itself speaks volumes.

If there is a downside to monotube shocks, it would have to be a SLIGHT cost disadvantage. Bilstein monotubes list in the range of about $139 to $259 (M3 front strut). Compare to Koni, Carrera, and the others. There is probably a price difference, but not as much as you may think.

Michael G. O'Callaghan
227 Hathaway Street East
Girard, PA 16417

814-774-8808 VOICE
814-774-8734 FAX

At 02:04 AM 9/3/97 -0400, you wrote:
>From: (Barrett Nicholas)
>Date: Tue, 02 Sep 1997 21:24:50 GMT
>Subject: Shock construction questions
>Reading Duane Collie's review of the new E36 M3 Bisteins prompted a
>couple of questions:
>1. If I remember right, only the new E36 M3 Bilsteins Sports are
>mono-tube low pressure shocks, which had lead me to believe that
>mono-tube low pressure shocks are probably superior to dual tube
>shocks, but then Duane wrote "Not willing to spend $2,000 + for the
>custom built twin-tube Konis" - All other things being equal, which
>provides better dampening, high pressure or low, mono or dual tube?
>2. In the ultra spendy penske coilovers, what type of design is used?
>How about H & R?
>3. How big of a difference is there between the e36 non-M3 sports and
>the M3 sports? What would it take to be able to use the M3 shocks in a
>non-M3 e36?
>Barrett Nicholas
>Houston, Texas
>BMWCCA# 130,000
>'95 325i - CeeKay, the SuperCar
>JimC powered

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