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      05-16-2014, 04:10 PM   #209
ajsalida's Avatar

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

iTrader: (2)

Hard to tell what a spring will do WRT ride height by looking unless you have some background. A shorter spring is not necessarily going to lower your car.

You can vary only a few things on coils and still fit into factory spring perches in front BMW type struts. # of coils, wire diameter, dead length. Hard to vary coil diameter due to form factors. Coil springs actually work like a torsion bar, just all wound up. The spring force comes from twisting the metal all along the coil as it compresses. Those parameters determine how stiff the spring is but also what the static ride height will be when loaded. You can have a shorter dead length spring but thicker wire and not lower at all, or even be raised.

Because of the way the front strut is set up, you can only get a small number of spring coils into one. This is why the top diameter is huge, to get more length of metal in there. On BMW it is like 4 total (active coils). If you reduce the number of coils to make it shorter you stiffen it, but you get into a situation where now there is too much torsional load on the metal and may go past deformation limits in full compression. You can trim maybe 1/4 to 1/2 coil off the top and that is it. You'll get some lowering but a lot of stiffening. The bottom diameter tapers to fit the lower perch and there is some progressive effect, but you can't trim off that end.

So the way Eibach makes a lowering spring in front is to make slightly thinner wire and same number of coils, slightly shorter overall dead length. If you did the compressor yourself you probably noticed how much stiffer the stock XI spring was even though shape is nearly the same. That is nearly all only from thicker wire. Stiffness (torsional) goes proportional to diameter to the 4th power on wire, so a small increase is a lot stiffer.

I measured the wire diameters and the Eibachs were definitely thinner than OEM XI. Other people have measured them on spring testers and found Eibachs to be about same spring rate as stock non-XI, just lower. Both front and rear.

In back there is not as much constraint fitting into stock form factor. The rear Eibachs are shorter dead length, fewer coils, and thicker wire than either OEM XI or non XI. OEM XI is much thicker wire in the rear than OEM non XI. But here is where looks are deceiving, Eibach has same linear spring rate as stock non-XI in lbs/in but lower ride height. Unfortunately I think you will find you get a larger drop in back than you want by a lot.

Part of this is due to the heavier car at both ends than non XI. Part is due to the fact that in front the lower spring perch on an XI is HIGHER above the axle than on non XI, so while the drop from stock XI height is whatever it is, even more than the non XI, it is still higher up than non XI. XI rear suspension is same as non XI but the car is heavier so rear will drop MORE than non XI and you get a funny rake effect with rear looking too low. That is even without the unusual higher than normal front gap you got. So I am afraid you might be disappointed when you put the rears on.

One thing to check, just to make sure. Did you in fact get XI Bilsteins? Since they do not, as you have said, have the lower part that sticks down past the lower hub, just want to make sure on that.

Also on the pic, is the lower-most end of the installed spring flush up to the notch in the rubber pad? Can't tell. It is on the top pad not sure on the bottom.

Lowering an XI evenly without true coilovers is very tricky.

edit: one thing bilstein advises WRT install of their shocks is to fully compress them by hand before you install. In front this helps to get the air out of the housing. As far as driving a lot sure, esp try driving slowly over things which tend to cycle the suspension in front fully, like speed bumps. Not to bottom out but to compress the front shocks as much as you can maybe fart some of that air out. Your plan on the rears is good, install it all see how it looks. Take it from there and replace if you need to. Any ZSP rear should work in case you need to find some. Read the thread "i springs on XI" in this section for more info.

Last edited by ajsalida; 05-16-2014 at 04:22 PM.