Understanding Leverage Curves
In mountain biking, rear suspension design isn’t just about shocks specifically, but involves the design of the frame and how the shock is integrated with it.
Over the years, the development of suspension forks and frames has grown significantly for the better, setting new standards for comfort and efficiency. By understanding frame suspension designs and what they do, we can examine how they affect the performance of our bikes on the trail. It is a very complex topic, but in essence, when we talk about suspension performance we are discussing the interplay of a few key traits - basically, how the combination of frame and shock allows a rider to move forward while keeping the rear wheel tracking over uneven terrain, whilst minimizing harshness to the rider and maximizing control and responsiveness.
Though every suspension configuration (meaning combination of frame, shock, valving and setup) out there is fundamentally trying to achieve the same basic goals, the characteristics, ride quality and “feel” of the bike can vary significantly depending on what the designers prioritize.
Pedalling efficiency, spring rate curve and impact absorption are three of the most important characteristics of suspension design - they’re not the only important considerations by any stretch, but they are arguably the most critical (other than reliability perhaps!). As you can imagine, achieving a good balance of the three can conjure up all sorts of interesting concepts and designs from manufacturers and individuals alike. These days, however, there are a relatively small number of designs that make up the vast majority of the market. There’s plenty of marketing claims out there about every frame, but as always, the devil is in the details - you can achieve a huge variety of characteristics with almost any patented/marketed frame linkage design. The fact that two frames look very similar in their layout says nearly nothing about how they behave - small alterations can create huge differences in the ride.
While we won’t get too nerdy and in-depth about suspension kinematics (anti-squat, brake jack, pedal-kickback, anti-rise, etc) because most of these don’t strongly relate to the shock absorber itself, there is one topic we’d like to cover: leverage rates.
What is a leverage ratio?A leverage ratio is the relationship between the movement of the axle and the shock at any given point throughout the bike’s travel (instantaneous leverage ratio) or as an average across the whole travel of the bike (average leverage ratio). Ie. How far the rear axle moves, divided by how far the shock moves. This is the inverse of a motion ratio - another way of describing the same thing.
For example, a bike that has an average leverage ratio of 2.5:1 means that every 2.5mm of axle movement equates to 1mm of shock movement, on average. As it is a ratio, it does not use units - it is dimensionless.
It sounds relatively simple, however on most frames it will not be exactly 2.5:1 everywhere in the travel - that is only an average based on the overall trajectory. By plotting the leverage ratio at each part of the travel, we get the leverage rate curve.
The ‘leverage rate curve’ is the graph, specifically plotted to the bikes suspension design. These are usually published by frame manufacturers, and can also be found here.
This is an example leverage rate curve below for a 2022 Transition Spire 29”, compared with some other bikes:
Most modern full suspension bikes come with average leverage ratios somewhere between 2:1 and 3:1.
While it is not necessary to break down what is happening at each point in the travel, the average leverage ratio and how it changes can mean different things. Generally there are three different leverage rate curves found on modern mountain bikes: Linear, Rising Rate (Progressive), and 'Falling Rate' (sometimes called regressive, though this term is technically ambiguous). A “rising rate” or progressive design refers to a design that creates a rising spring rate when measured at the wheel, as the suspension compresses. This is more intuitive to visualize from the Motion Ratio (the inverse of leverage ratio) or motion rate curve. A leverage rate curve that begins at 4:1 and drops to 2:1 over the travel, is the inverse of a motion rate curve that begins at 0.25:1 and increases to 0.50:1.
Progressive Leverage Rate Curve
Since everyone in the bike industry seems to use leverage rate rather than motion rate, we’ll discuss it in those terms. A progressive leverage rate curve (or a progressive segment of a leverage rate curve) is where the curve trends down on a graph (like the Transition Spire in the graph above). This means that the amount of force required to move the rear wheel a given increment is increasing because as the suspension compresses further, the leverage that the axle has over the shock is decreasing. An alternative way to consider it is that the mechanical advantage that the shock has over the axle is increasing.
While this can seem confusing as the number seems to be decreasing from 3 to 2.35, hopefully this diagram can explain it:
Frames with a sufficiently progressive leverage rate curve can be better paired with shocks that have linear qualities (coil shocks) because the frame is progressing through travel. If you pair a highly progressive frame with a progressive air shock, it can sometimes be too progressive, or provide too much ramp-up, limiting the use of full travel, or forcing the initial travel to be excessively soft.
Progressive leverage rates give a feel that is relatively soft in the initial travel and relatively stiff later in the travel.
Linear Leverage Rate Curve
A linear leverage rate curve is where the leverage ratio stays the same (or close to) throughout the entire rear wheel travel.
Frames with linear leverage rate curves are better paired with shocks that have progressive qualities (air shocks) because the frame does not have progression built in. If you pair this type of frame with a linear coil shock, the frame’s suspension will not have any progression, meaning that you will likely bottom out often and lack end stroke support.
Linear leverage rates give a more constant spring and damping rate throughout the travel, except for the variation in spring rates that the shock itself may give. If the shock is not progressive enough, it can be difficult to balance initial compliance (i.e. a soft enough beginning stroke) against bottoming resistance.
'Falling Rate' Leverage Rate Curve
A 'falling rate' leverage rate curve is where the leverage rate curve trends upward on a graph. This means that the amount of force required to move the rear wheel each additional millimeter (assuming the shock’s spring rate is linear) is decreasing.
Frames with a 'falling rate' leverage rate curve are commonly paired with highly progressive air shocks. As the force required to move the rear wheel decreases, the shock will need to provide all of the progressiveness in order to resist bottoming out. These leverage rates are not commonly used outside of XC race bikes, as they can make it very difficult to obtain small bump absorption or bottoming resistance.
Why is this important?
The leverage rate gives us an idea of how different areas of the travel will behave relative to one another. It determines the proportional stiffness of one area of the travel to another.
A frame with a higher average leverage ratio will transmit more force to the shock for any given force at the wheel, therefore higher leverage ratio bikes require stiffer springs and/or higher air pressures to function properly. This need for more support can also limit your tuning options.
It is also important to consider a frame's leverage curve when determining if an air shock or coil shock would be best suited.