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      05-20-2014, 02:14 PM   #211
ajsalida's Avatar

Drives: 95 M3, 02 R1150GSA, 09 335xi
Join Date: Aug 2010
Location: SW USA

iTrader: (2)

I have not installed any Bilsteins on e9x but on other McP strut cars (like my e36 M3) they always said in the instructions to compress struts fully by hand before assembly/install. Below is a pic of the way a front bilstein monotube looks like inside, you can see where air gets trapped inside at the bottom, you want to get all that out if possible. Based on what you're saying about settling you are probably burping that air out already.

I would do what you suggest and just install the rear bilsteins for now, see how it goes. If the front settles a lot re-evaluate. Installing rear springs is mostly unrelated and you will not need to re-align after just shocks. The only thing that may be a little off is the spring rate in back is going to be stiffer than ideal WRT front, but it shouldn't be an issue and besides you'll not need a bigger sway bar.

BTW in the pic below, the high pressure gas chamber is the orange shaded region at the top inside of the strut on the left. There is a floating piston there that separates the gas and oil volume, it has to move in order to compensate for fluid displaced by the piston rod movement inside the shock body. You can see the other "rod" visible from the outside that slides up and down on a monotube strut is actually the shock body with the fluid. The high pressure gas is what causes the additional ride height in monotube struts and shocks. You'll get some of that in back too. The air trapping thing is unrelated to that.

You can calculate the offset force due to the high pressure gas pretty easily, it is merely the lower piston rod cross sectional area in sq in times the gas pressure in PSI. Last time I did this for another car it was 40-50 lbs per corner pressing UP. So like nearly 200 lbs whole car of static force in the vertical direction, enough for 1/4" or so additional ride height depending on each corner's effective wheel spring rates (with motion ratios etc.). The gas chamber acts like a mini air shock IOW. But the benefits usually far outweigh the small increase in ride height vs other types of shocks in same price range.
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Last edited by ajsalida; 05-20-2014 at 02:54 PM.