RE: Coilovers vs. Shocks/Springs/Struts

["Belt, Stephen D" <stephen.d.belt@intel.com>]

Khan sez>
> So, what we're looking at (correct me if I stray from reality) is 
> that struts are bolted to the wheel assembly and to the body of the 
> car. The springs are somehow connected to both as well.

Let me see if I can help.  For now, don't worry about what is attached
to what.  Worry about the two key elements:  spring and dampener.  

The spring allows the wheel to move up and down, but resists that 
movement more and more as you move farther and farther away from its 
natural resting point (ie, the more you try to compress a spring the 
harder it gets).  Once the spring gets moved some distance from its 
natural resting point, it will want to bounce back toward that resting 
point, however, it now has a great deal of stored energy that when 
released will launch it beyond the natural resting point.  

This is where the dampener comes in.  To keep the spring from oscillating
back and forth (without energy lost to heat and friction, oscillation
would continue forever), a dampener is added to the system.  The ideal
dampener allows the spring to move to point of travel, but then controls
the spring so that when it moves back to its natural resting spot all
of the stored energy is absorbed by the dampener.  The ideal dampener
would not affect the inital movement of the spring (allowing it to either 
contract or expand without dampening), but then apply its dampening effect 
on the release of the energy.

Unfortunately, such a dampener doesn't exist.  So, most dampeners are
designed with a 50/50 factor that offers equal dampening on compression
and expansion.  Thus when the spring is being compression not only is
the force of the spring acting upon the wheel, but also dampening force
is coming from the dampener (shock).

This is where it becomes difficult to get the correct spring rate and
dampening factor as a combination.  On my truck, I have adjustable shocks
in which I have dampening settings to chose from.  My spring rates are
not adjustable.  

I can attest that have the shocks too "stiff", the ride is very
uncomfortable.  
When you hit a bump, the shock is taking too much control of the spring,
and not allowing it to move upward, and then the ride is very harsh.

If I have the shocks too "soft", its also uncomfortable, though for less 
obvious reasons.  The too soft setting allows the spring to oscillate, and 
thus the vehicle doesn't settle as quickly as you'd like.  A bump is no 
longer an event, but a process as the vehicle continues to bounce around 
for a couple of osscillations until the spring is able to release all of 
the stored energy.  

> The tighter your spring, the harsher the initial impact and the 
> rebound from the spring. The stiffer (more heavy duty) the strut, the 
> more force it exerts on the spring to normalize the spring.

The stiffer the spring, the more force it will take to compress it.
The force comes as a result of the weight of the vehicle acting upon it,
and increases under conditions like speed and cornering.  I think the
A3 Jetta's spring rate is pretty good, though I think it could be made
a bit stiffer.  If I were racing it, however, I'd want it much stiffer,
as the typical speed with which a bump is hit would be much higher.

> The looser your spring (think shock springs) the bouncier the ride. 
> And, if the struts aren't very stiff, the ride will tend to float up 
> and down and up and down for more oscillations than if there were 
> stiffer struts.

A bouncier ride comes from shocks that are too soft or "tired".  The 
problem with soft springs is that they will bottom out under too small
a force.  Of the vehicles I've driven, the Volvo 960 had this problem, 
as did older BMW 525's from the early 90's.  You could hit a speed bump
and the spring would fully compress without fully absorbing the up force
caused by the bump.  The result of the spring being fully compressed is
that you now have no suspension left...not very comfortable.  

> If so, what I'd like to know is how the mechanism actually works... 
> the struts, it seems would simply be bolted between the wheel 
> assembly and the body. The springs are connected... how? To the 
> strut? Around the strut? What stops it's upward and downward motion?

As for how it works, the spring and dampener must each connect to a moving 
part and a fixed part.  The moving part is either the axle or something 
directly connected to the wheel and the fixed part is typically the body 
or frame of the vehicle.  The dampener also must have these same
connections.
The spring and dampener then work together to control the moving part and
attempt to allow the fixed part to stay where it is while the moving part 
performs a single oscillation and returns back to its natural resting spot.

I hope this has helped, as you've about tapped out my limited knowledge
on the subject.

Steve Belt - Scotts, AZ
1998 VW Jetta GLX