Just to recap on the choice of dampers ... the advantage of the Koni's is that they are progressive, and the other two are more similar to the OEM?
Hi Mike,
Truth be told, I'm not that well informed about the Monroes and the Bilsteins. I believe the Bilsteins are a mono-tube design, rather than twin-tube.
The main advantage of mono-tube dampers is that the pistons are much larger than those of twin-tube dampers, creating more flow through the valves than the smaller twin-tube piston as it can displace more oil through the valve; this makes the mono-tube damper more sensitive to small suspension movement. But it's a more costly design.
The Monroes are, I believe, an uprated, higher-quality implementation of the OEM stuff, but I've not really looked into them.
Progressive suspension has a non-linear characteristic. Basically, it's easy to compress the suspension by a small amount, but the more you compress, the greater the force you need to exert in order to compress any further. This means the suspension travels easily within the first portion of its range, but with greater difficulty in its later range. Koni's FSDs are not progressive but frequency-selective. I'll do my best to explain without the aid of graphs...
When a car turns a corner, the body rolls to one side, slowly compressing the dampers on one side of the car. As the car comes out of the bend, the damper returns to its equilibrium position. In our heads, let's plot compression as a function of time, where the equilibrium position is at y=0 (the x-axis). The resultant graph is a low-frequency sine wave (albeit only a half wavelength).
When a car drives over cobbles, the suspension undergoes rapid compression and rarefaction. Again, taking the x-axis as the equilibrium position, let's plot the suspension's position as a function of time. The resultant graph is a high-frequency sine wave.
The principle of FSDs is in their name: frequency-selective damping. Basically, FSDs are 'stiff' to low frequencies and 'soft' to high frequencies. Body roll is resisted when cornering, but any bumps in the road are taken up nicely.
For all that it's been done by many members on the forum, I don't think the Weitec springs are a particularly good complement to FSDs. The Weitec springs are quite stiff, meaning that the FSD's high-frequency softness is stifled. With the softer Spidan springs, the Koni's frequency-selective nature is free to work as it should, keeping the car flat in the corners whilst being loose and bouncy over the rough stuff.
Cheers,
Tom