Dampers Part 5: Critical Damping

We’ve talked a lot about dampers and coilovers. Last time, it was shock dynos. Linear, digressive, and progressive damper profiles, the difference between compression and rebound damping, low and high speed damping, and so on.

All important information, but while a lot of people focus on the shape of a shock dyno curve, they skip over the very basic question:

How much damping do I want?

A shock dyno graph can tell you if a damper is digressive, linear, or progressive. But that’s only part of it…the actual forces are crucial to shock performance.

You may have heard of the concepts of overdamped, underdamped, or critically damped suspension systems. To understand these, it helps to think about what happens after hitting a bump in a car without dampers at all (only springs). That car will bounce for a very long time. You’ve probably seen a car with blown shocks bouncing over and over down the highway.

Adding a damping force will reduce the bounce and eventually the system will return to a steady state.

A critically damped suspension has the least amount of damping force that returns the system to a steady state without overshooting (no bounce).

More damping force than critical is considered overdamped. The system will not overshoot (no bounce), but will take longer to settle (return to steady state) than a critically damped system.

Less damping force than critical will overshoot (some bounce) and is considered underdamped. It will return to steady state at some point.

A shock’s damping ratio is the actual damping force divided by the critical damping force. So a damping ratio of 1 means the system is critically damped. There’s no overshoot.

A ratio of 1.2 is overdamped (more damping force than critical), while a ratio of 0.5 is underdamped. These can also be expressed as percentages (i.e. 100%, 120%, 50%).

Minimizing overshoot (bounciness) is a good thing…we need to control the spring and reduce tire load variation. We want to the system to settle quickly.

But too much damping can take too long to return to steady state as well. A shock that’s overdamped in rebound can pack down, meaning after a bump it stays compressed. That’s bad news when you hit the next bump.

Underdamped systems (within reason) tend to feel less harsh over bumps.

A useful but oversimplified rule for performance cars is to use a damping ratio of around 65% for low piston speeds. Rebound and compression generally have different ratios.

Digressive dampers allow you to have different damping ratios for low and high piston speeds. A linear damper will have the same damping ratio for all piston speeds.

So how do you calculate critical damping force? It depends on a few factors: spring rate, motion ratio, and corner weights (sprung mass). It also varies with shock piston velocity.

There’s some math and you have to be careful with your units. Making a spreadsheet or an R script can help.

We’ll walk through an example next time and talk about how we can choose compression and rebound damper forces through low and high piston speeds.