Mountain bike frames have various forms of what we broadly refer to as suspension design, but more specifically could be termed kinematic design. Kinematic design refers to the ways which each member of the suspension linkage move, particularly relative to one another. In other words, the suspension kinematics are the geometric motions of the linkage, before we start introducing force or mass to our considerations.
Anyone who's ridden more than a couple of different mountain bikes is well aware of how different the rear end of the bike can feel, even with the same travel and when using the same shock. Variations in leverage rate, among other things, are a very large factor in determining how the suspension performs, so this week in the Vorsprung Suspension workshop, we're looking at the ways in which variations in the leverage ratio can affect the feel of the bike, and why small changes can result in fairly large differences.
Leverage ratio is typically defined as the ratio of mechanical advantage that the axle has over the shock. This can be an instantaneous leverage ratio at a single point in the travel, or an average leverage ratio obtained by dividing wheel travel by shock stroke. Plotting the instantaneous leverage ratio against travel gives us a leverage rate curve. The difference between ratio and rate is that the rate describes how the ratio is changing throughout the travel. In the motorsports world it is more common to use the term motion ratio, which is the same thing but inverse - it is obtained by dividing shock stroke by wheel travel.
When considering the overall wheel rates - that is, the spring rate and the damping rate when measured at the wheel - it is necessary to understand the spring and damping characteristics of your shock as well as the leverage rate characteristic of your linkage. Some frames work really well with the linearity of coil shocks and relatively poorly with progressive air shocks, and vice versa, and it is this interaction between the leverage rate and the shock's spring characteristics that is primarily responsible for this. An in-depth discussion of how air springs, particularly those of our own products, interact with various leverage rate curves can be found here.
Please note a few things here:
1. As usual, the devil is absolutely in the details. Generalisations, including some of the things we refer to in the video, are not applicable to every variant of any particular linkage design, and this is a long long way from being a comprehensive breakdown of leverage rates.
2. A certain acronym or name for a linkage type does not mean all frames of that type perform or feel the same. They can vary hugely.
3. We're not here to tell you what system is "best". There is really no objective way to assess that - everyone's preferences and priorities vary, and what works well for someone who likes a super firm feel doesn't necessarily work well for someone who wants the plushest thing out there.
4. If you'd like to see what your own bike's leverage rate is like, download a program called Linkage and measure up your own bike to see how it stacks up. Note on using this program: there are a great many existing files out there and a lot of them are a fair way off the mark, particularly on bikes with very short links. Don't rely on clicking on points on a photo for accurate inputs - that can be accurate enough for singlepivot bikes with no linkage, but on anything else it's more than likely going to be misleading. Use a measuring tape to verify distances between points on your suspension if you want accurate results.
Terminology worth knowing:
Linear leverage rate: this would be better termed "constant leverage ratio", as this is what delivers a linear force vs displacement curve at the wheel (if using a linear spring).
Progressive leverage rate: also known as a "rising rate", this creates a force vs displacement curve at the wheel that is progressive, ie an increasing wheel spring rate even if using a linear spring.
Digressive leverage rate: the opposite of the progressive leverage rate.