A couple people have asked me about doing spring modifications... for example, see the email below:

"the 60mm drop springs I have on my 740 Wagon at the
moment are actually about 50-60mm lower in height than stock ones if
you take them off the car and place them next to each other. And at
the moment I`m using a coil spring clamp to clamp 2 of the springs on
the front together, this is why I was asking if I would be possible
to remove 1 coil off each side while its on the car because the
height of the springs compared to the stock ones with it clamped is
probably about 70-80mm shorter. Will cutting the spring shorten its
life seeing as it is shot-peened?

I can see though that I will probably just try to clamp 3 of the
coils together, then jack the car up to make sure the spring isn`t
too short that it slides out of the part it sits into."

I wrote back a response, then decided to post it on the board as some of you may be able to glean some new knowledge about this. I did spend a great deal of time researching (I was a mechanical engineering major @ West Point and did an independent study on Volvo suspensions) so some of the language may be a bit technical but I tried to add layman's translations. A portion of my response to the above email follows:

Just in case you didn't get
the important part, DO NOT CUT THE SPRINGS WITHOUT REMOVING THEM!!!! You
could kill yourself, no joke. Cutting one coil (approximately 3.5 inches
worth of compression at 250-500 lbs/in) would release 825-1750 lbs of
potential energy. Imagine converting that much energy into kinetic energy
and applying it to a sharp, hot piece of metal... can you say "FLYING
SHRAPNEL"? That's great, I knew you could :) Take the time to remove the
strut from the car and remove the spring from the strut, then clamp the
spring in a vise and use a cutoff tool to cut one coil.

You will not shorten the life of the springs significantly by cutting them,
in fact, you could lengthen their life... shot peening metal is like cold
working the surface; it decreases ductility (defined as a material's
willingness to bend without causing microstructural damage) while increasing
tensile strength (defined as resistance to stress or load). Therefore, the
more a shot peened spring bends, the more quickly it will fatigue and fail.
By cutting a coil off the spring, you will actually stiffen it (spring rate
equation has three factors- wire diameter, spring diameter, and number of
effective coils), therefore it will bend less and cause less fatigue. Harder
material is more conducive to stiffer springs, so no worries there. As a
final note, most springs are hardened in some way, whether by shot peening,
pre-stressing, or some other method.

Clamping coils instead of cutting them has a similar effect but without as
much of a height loss... see the above comment about spring rate equation?
One of the factors is number of EFFECTIVE coils. Clamping down one coil onto
another reduces that number by 1 to 2 coils. In my case, I was faced with
the opposite situation... I put 240 sport springs on the back of my 780 and
it was TOO low, so I had to spread the coils using coil spring clamps in
reverse... they were also progressive rate springs which complicated things
further. I chose to spread the second widest coil which raised the height
and stiffened the spring. If I had spread some of the lower coils which were
closer together, I would have gotten "coil slap" as the coils closed quickly
(rather than gradually) to their "shut height," defined as the height a
spring is when it becomes fully compressed, coil against coil.

One final issue you addressed- the spring being too short so that it can
possibly bounce out of it's mounting... that is one tradeoff of stiffer
springs. Before you place a spring under load, if you measure from one end
to the other you get it's uncompressed length. When you load that spring
with a constant load (in this case, the weight of the car), you get it's
compressed length. The difference between the two is called the spring's
pre-load length. When a spring has a lot of space between coils and is not
too stiff, there is plenty of room to compress the spring (say, with a speed
bump) and not get the coils to come together, and there is also enough room
to allow the spring to decompress (say, with a pothole) and not get the
spring to reach its uncompressed length. This is because softer springs have
longer preload lengths. The downside, of course, is that 1/4 mile after you
passed the pothole or speed bump, you're still bouncing up and down. With a
stiff spring, the problem is compounded. You don't have a lot of coils and
the spring is very stiff, so you've got a smaller margin for error. Clamping
a spring makes it even worse. It's shorter, so the compressed length is
closer to the uncompressed length, and you've got less coils working to
absorb a disturbance so you're more apt to reach your shut height,
effectively turning your spring into a rigid steel cylinder. Granted,
hitting a speed bump too hard and causing the coils to slap together is not
nearly as bad as having the car hit a pothole and having the spring come out
of its seat, but neither are a desireable suspension characteristic.

One caveat- this mostly applies to the rear suspension, as there is enough
freedom of movement to cause both problems. When you put a spring in a
MacPherson strut, however, you already know (b/c the strut is out of the car)
the MAXIMUM length the spring will be extended to. So, as long as the spring
is compressed even a little bit when you put the top of the strut on, you
know it won't bounce out of the mount. By the same token, because of the
pre-designed length of the strut, if your spring is taking up the whole
thing, it's probably got enough travel in it to absorb any bumps without
causing the coils to touch. Still, be wary about clamping front springs in
struts.

Hope this helps, if you need further help deciphering the techno-engineer
babble, please write back.

Good luck!
-Jon

--
2LT Jonathan T. Belmont