More progressive, more better?

Many riders seem to think a more progressive system is always better. This is not true:

1. There's a limit to everything
2. Many suspension applications outside of mountain biking have dramatically more variability in the curve shapes, demonstrating it's certainly not a universal truth
3. Few riders understand the effects of progressive/digressive/regressive curvature in the spring, damper, and linkage rates, or how these things interact

I gave a light-duty overview of this topic in the comments section of a news article, so I might as well repost it here.



First, we have to think about springs and dampers separately. Each one can, independently, be progressive, linear, or digressive. These properties are added together for a given point in the stroke and shaft speed to create the total support from the shock.

Next, we have to multiply the spring and damper linkage properties by the linkage properties, if present (a telescoping fork has no linkage). A linear spring and damper can produce a highly progressive system if it's mounted to a highly progressive linkage, or any combination of properties.

These things describe only the nature of the curvature, not the slope or the values. Some applications favour digressive or even regressive systems. A linear system could have a shallow slope ("soft") or a steep slope ("firm"). If there was a universally correct set-up, no others would exist. That said, not every set-up is appropriate - many are just inappropriate and bad.

Let's start with springs and compare a linear and a progressive system. We have to keep something constant to compare in a meaningful way, so let's use to total area under the force-displacement curve (total energy absorbed). The linear system gets firmer, faster, so it has a higher ride height (less dive), gives more platform to push against (more responsive), and compresses less when hitting typical impacts (less "plush"). Obviously, the progressive system is the opposite: more dive, squishier, and more plush. Good and bad points to each.

Dampers are similar. Digressive systems offer more support at lower shaft speeds (less dive, more responsive, less plush most of the time but can be softer on intense impacts), while linear or progressive systems have inverse characteristics.

Imagine an extremely (excessively) progressive system: There would be almost no resistance for most of the stroke, most of the time. Incredibly plush, but incredibly squishy. When you hit a large and/or fast impact, the end of the travel is like hitting a wall - your suspension hasn't bottomed out, but it feels almost like it has. Obviously, this is not a good set-up.

Now imagine an extremely digressive system: It barely moves until it reaches an extremely high level of support, then it opens up. It feels almost like a fully rigid bike, then - just when your hands and feet are about to blow off - it opens up and absorbs the entire obstacle or impact. It's super responsive and never bucks and dives like the previous example, but it's a brutal experience.

Clearly, the ideal set-up is somewhere between these two - but where? When we consider the range for all existing suspension systems (beyond mountain biking), the ideal for most people, in most situations, is pretty close to linear for both the spring and the damper. Fast and strong riders usually benefit from a more progressive set-up to provide enough support for the maximum forces they can produce, without making the system too firm the rest of the time.