Dampers Part 6: An Example

The marketing said all the right things: digressive valving, wide range of adjustment...all the good stuff.

And the shock dyno chart definitely had that digressive shape. Heck yeah. Plus they're coilovers so the springs are perfectly matched to the valving.

But wait…where in that wide range of adjustment are they perfectly matched? We’ve talked about critically damped, underdamped, and overdamped before...let's have a look at how we can use that information.

The graph above is a PVP shock dyno plot for a popular coilover using 8 kg/mm springs on the front of a Subaru BRZ. There’s soft in yellow and stiff in blue. These aren’t RCE coilovers.

It looks like a “bleed” type adjuster with a huge range for low-speed rebound. When set on the stiff setting, they’re digressive with a knee at 20 mm/sec (just under 1 inch/sec). The softer setting is progressive in rebound until higher piston speeds when it blows off.

In our article on critical damping (Dampers part 5), we mentioned that a commonly used but imperfect guideline for a performance shock is to set the rebound valving to around 65% of the critically damped value for low piston speeds (up to about 50 mm/sec of piston speed), and then less than that at higher piston speeds for a digressive curve. All of that takes a little math to work out for a given car and spring rate.

Let’s see how these coilovers compare to a linear plot of 70% critically damped values for 8kg/mm springs.

Focusing on rebound (bottom), the softest setting is under 70% critically damped in low and mid-speed velocities, getting closer at higher speeds. The stiff setting…well the stiff setting is very stiff. Okay, so what does this tell us about how the valving matches the springs?

The soft setting is a bit underdamped and may feel a little sluggish. Maybe a little jiggle or bounce, but it could be worse. Not unusual for a “soft” setting.

The stiff setting is over 700% (yes seven hundred) critically damped at 20 mm/sec, and still over 300% after the knee. That much rebound will hold the shock down, not letting it extend back up after it compresses. It will shock the tire, lack grip, and suck to drive.

A lot of people adjust their shocks to full stiff at the track to improve handling. That’s rarely a good idea, but that’s going to be extra bad in this case.

There might be a middle setting that works fine, but it’s unlikely that anything past halfway on the adjuster is useful.

Sometimes a coilover with a wide range of adjustment means you can swap springs without a revalve, but switching the 8 kg/mm springs here to mega-stiff 16 kg/mm springs and the dampers are still overdamped.

This might not be a “bad” damper but that mega-huge range of adjustment isn’t making things better here. That’s especially true for dampers with an adjuster like this, where the softer settings aren’t very digressive. We often see users struggling to find a “good” street or track setting on dampers like this one. Stiff is too stiff, soft is too soft, and there’s no creamy middle.

Next time, we’ll talk about how to do this type of analysis. This isn’t intended to replace actual testing, but it’s a way to get started. We’ll show the math, too.