Re: Need new susp...help me decide (Kritter)
You'll definitely want to get the Bilstein Sports instead of the HDs for this application.
I'm considering a new setup myself and went so far as to call the tech hotline at Bilstein and talk to their support folks. According to them, the HD and Sports dampers basically give similar damping rates. The Sports simply have a bit shorter shaft.
Bilstein's recommendation was that any drop equal or greater than 3/4" (~18mm) should use the Sport, anything less than 3/4" should use the HDs.
The 3/4" rule is easy to remember and follow but some folks (like me) are looking for a more in-depth explanation, in order to understand why the rule is what it is, and what happens if you violate it.
First, these are position sensitive dampers. As the shaft is driven deeper into the shock body, the compression damping rate increases. This is analagous to progressive rate springs, except here it applies to the damping effect of the shock and not the resistance to compression of the spring.
For example, at the beginning of a compression/rebound stroke the damper applies a minimum compression damping force, which increases (stiffens) as the suspension compresses, finally reaching the peak of a curve (or hitting a very abrupt plateau as the bumpstomp kicks in and the suspension bottoms out) and begins decreasing, now follwing the rebound dampening curve.
So, you've got a variable rate damper and maybe you've got a progressive spring as well. Each of these devices doesn't really have a single rate at all, it has a rate curve, with compression travel (inches or millimeters) on the vertical
axis and compression rate (resistance or stiffness) on the horizontal (x) axis. So, you can begin to see that matching springs and shocks isn't simply a matter of lining up a single spec printed on the side of a box.
Adding further complexity is the fact that the rate of damping is also sensitive to the upward velocity of the shaft. So a sharp, squared-off bump will generate a higher damping rate and not be taken up as smoothly as a gentle wave. This is due to inertia in the suspension and the way the oil in the damper reacts to changing forces.
Now, back to the original question: "Why shouldn't you mess around with the 3/4-inch rule?". For instance, what happens when you take a Bilstein HD and mount it with a 1.5-inch lowering spring? Well you've basically mounted the shock so that the shaft is about 3/4-inches into the shock body when it's at rest. So, when you hit a bump and the suspension compresses, rather than starting at the softest point in damping curve, you're already a good bit into the stiffer portion of the curve and into the section of the curve that is rising most sharply.
The net result is a harsher ride than if you chose the Bilstein Sport.
With the Sport shock, the curve has to rise from the same beginning point to the same ending point over a shorter stroke, so the curve IS somewhat steeper. But, the initial portion of the curve (say from the zero point of the stroke to the 3/4" point) will be noticeably softer than the portion of the HD's curve that you would be in if you had mounted them on the shorter springs (the 3/4" point of the stroke to the 1.5" point).
This first 3/4" or so of suspension travel is where most of the real world work is done. It's certainly where most road irregularities that produce what we'd call vibration or harshness are dealt with. This is why progressive rates springs can offer good cornering performance with reduced harshness.
The fact is that if a suspension has more travel, it can use less damping rate (damping force per unit travel) to get the job done. This fact is too often ignored, particularly when the currently popular fashion calls for cars to appear noticably lowered in order to seem "sporty".
So, don't fall into the trap of thinking Sporty=Firm=Harsh AND HD=Softer=Comfort, because in this instance, the Bilstein Sports will be the more comfortable choice, as well as giving you better handling and balance, due the fact that they are a better match for your springs.
--keihin
