Re: Question about stiffer/shorter springs for 5k/200...

[Larry Mittell <lmittel@ibm.net>]

While I haven't BT or DT, I know a little about helical compression
springs. By "stiffness," I assume you are referring to the spring rate,
commonly expressed in lbf/inch the U.S., perhaps in dynes/cm (newtons/m?)
elsewhere. The rate of any given helical spring is a function of wire
diameter, spring mean diameter, number of active coils, the flexural
modulus of the spring material, and nothing else. It's pretty clear that
the only parameter the spring works can change is the number of coils.
While cutting coils will certainly increase spring rate (stiffness), it
will also force the entire removal of at least one of the "dead" coils at
one end of the spring. "Dead" coils are so termed because they're "closed,"
meaning that there is no space between them and the beginning of the
adjacent coil. They are thus not active, not being able to flex: they're
"shorted out," to use an electrical analogy. Their purpose is solely to
provide an axial seating surface for the spring and a means for capturing
it radially. They are generally ground perpendicularly with respect to the
spring centerline to allow the spring to seat squarely.

Removing coils would thus appear to be a bad idea, since, among other
things, only a portion of the coil on the modified end would be seated at a
time and the spring wouldn't be well guided radially. Some *really bad
stuff* could result. OTOH, if all you're after is a lowered ride height,
the spring works could cause the spring to take a set by heating it while
it's loaded (in effect, overstressing it), then giving it the proper heat
treatment to restore its strength. Mind you, a disproportionate portion of
its fatigue life may be used up in doing so, unless the springs are
entirely annealed prior to bringing them to final temper.

On reflection, I guess the spring works could remove a dead coil, "set" the
next coil so it becomes the new dead coil, grind it square, then re-heat
treat the result. This would by definition remove at least two entire
coils. Intuition tells me that would be too much. Also, it would seem to me
that the cost would rival that of making new springs from scratch.

I expect that the aftermarket folks juggle the wire diameter and the number
of coils to arrive at the desired result. They may make their springs from
a different alloy steel and/or use a different temper if the stress winds
up too high for the OEM alloy and temper. Variance in flexural modulus
amongst the various steel alloys is essentially insignificant. Ride height
can be adjusted by simply winding the springs to an appropriate free length.

The bottom line is that you'll most likely never make the stock springs
behave like aftermarket springs. It would be expensive to try and fraught
with peril to do *anything* to them, really, lacking some good solid BTDTs.
*Find* a guinea pig, if there is one; don't *be* one. Otherwise, I'd advise
leaving well enough alone or ponying up the cash for aftermarket equipment.

HTH 

At 06:13 PM 3/1/98 +0200, Gerard wrote:

--much snippage--
>The questions go along this line: what difference is there between
>the stock spring and the sport spring kits that lower the car by 30mm?
>How can I specify to the spring works to take the stock spring and
>retension/recoil them to give the same characteristics as the sport
>springs? How does one describe the spring parameters? Are the Eibach
>or other springs made of a very different material compared to the
>stock springs? Basically, I'd like to hear from someone who does work
>on spring systems and has to specify difference between springs when
>having them made up by spring works.
>
--more snippage--
Cheers,
Larry Mittell
87 5kcstq
89 200tqw
lmittel@ibm.net