February 09, 2023 2 min read
Previously in our series on dampers, we talked about bump and rebound damping as well as how valving is more complicated than “soft” or “stiff.”
A big part of analyzing dampers is understanding how they react to different inputs. Bumps, body roll, really big bumps...each will move the damper piston at different speeds.
Shock dyno plots are one way to gauge damper performance over a range of inputs. A quick look can say a lot, but it takes some experience to understand all the details.
Most shock dyno plots you see are force vs velocity plots. A PVP plot like this is a useful “summary” that gives a basic idea of damper performance for most people.
Other plots can provide more detail and more complexity.
The x-axis is the speed of the shock piston moving up and down in the damper. This is not vehicle speed.
Up to about 50 mm/sec or 2 inches/sec is considered "low-speed.” That’s body roll and slower chassis movements.
High piston speeds are big bumps that really get the damper moving.
The y-axis is damping force. Bump damping is on top and rebound is on bottom (typically). Faster piston speeds usually means more damping force, but the relationship between the two isn’t always constant or linear.
A digressive shock will have a "knee" after which the slope of the line is reduced. Forces will still be higher at higher piston velocities, but the actual damping ratio is lower. We’ll talk about damping ratios in another article.
A linear shock will steadily increase damping force at a constant or near constant rate. It looks like a straight line.
A progressive shock will have a steeper slope in the high-speed range.
What’s best for you? It depends! Digressive shocks are often preferred for track driving, but aren’t the best choice for every car.
A damper may have different characteristics for bump and rebound.
The idea behind a digressive damper is to have plenty of low speed damping to control body roll without too much high speed damping for big bumps. When done right, you can have a responsive suspension that can still soak up bumps without upsetting the chassis.
A progressive damper can be useful off-road or on gravel, where lots of bump damping can prevent bottoming out.
Importantly, a shock with a digressive piston may not always be digressive, or it may only be digressive at certain adjustment settings.
That’s not always a bad thing...but we frequently see popular “digressive” coilovers that only have that distinctive shape when set way too stiff. Too much low speed damping reduces grip and makes a car difficult to drive on track.
A linear damper with proper forces is better than a too stiff digressive damper.
Tweaks to the shape of a curve can improve handling, ride, and driver confidence, but you have to be in the right ballpark.
And getting in the ballpark requires maths. We’ll talk about that next time.