Summary
A leverage curve shows how the mechanical ratio between a bicycle’s rear-wheel movement and shock movement changes through the suspension travel. It influences the wheel’s effective spring rate, shock-shaft speed, damping response, and the forces transmitted into the shock.
The curve helps establish a suspension platform’s behavior, but it does not determine ride feel by itself. Spring characteristics, damping, friction, sag, bottom-out systems, and chassis geometry must also be considered.
Key Facts
Category: Suspension Kinematics Concept
Instantaneous Leverage Ratio: Incremental rear-wheel movement divided by incremental shock movement
Average Leverage Ratio: Total rear-wheel travel divided by shock stroke
Common Graph: Leverage ratio plotted against wheel travel or percentage of travel
Common Shapes: Progressive, linear, regressive, and mixed
Directly Influences: Wheel rate, shock load, shock-shaft speed, and damping force at the wheel
Interacts With: Air or coil spring, shock tune, suspension linkage, sag, and bottom-out control
Applies To: Full-suspension bicycles
Adjustment: Normally fixed, although some frames use adjustable or replaceable links
Overview
Two bikes can have identical rear travel and use the same shock dimensions yet behave very differently. The leverage curve is one reason: it defines how the frame transforms rear-wheel movement and force into shock movement and force at every point in the travel.
A leverage ratio of 2.8:1 means that, at that position in the stroke, approximately 2.8 mm of rear-wheel movement produces 1 mm of shock movement.
The ratio normally changes as the linkage rotates. Plotting those changing values across the travel produces the leverage curve.
Designers use the curve to control how the shock is loaded, how quickly its shaft moves, and how strongly its spring and damper act at the wheel. It is therefore more informative than rear-travel or shock-stroke numbers alone.
Instantaneous vs. Average Leverage Ratio
Average Leverage Ratio
Average leverage ratio is calculated from total travel and shock stroke:
[
\text{Average leverage ratio} =
\frac{\text{rear-wheel travel}}{\text{shock stroke}}
]
A frame with 150 mm of travel and a 60 mm shock stroke has an average ratio of:
[
150 \div 60 = 2.5:1
]
This value is useful for comparing overall shock loading, but it does not reveal what happens at the beginning, middle, or end of the travel.
Instantaneous Leverage Ratio
The instantaneous ratio describes a very small increment of wheel movement divided by the corresponding shock movement:
[
LR = \frac{dy}{dx}
]
Where:
- (dy) is an incremental amount of rear-wheel travel.
- (dx) is the corresponding shock movement.
The complete leverage curve consists of these instantaneous values plotted through the stroke.
Some software and technical documents use the inverse convention—shock movement divided by wheel movement—often called motion ratio. Always check the graph’s axis and definition before comparing curves. A falling line indicates progression only when the graph uses wheel travel divided by shock travel.
Force and Movement
Ignoring friction, virtual work gives the approximate relationship:
[
F_{\text{shock}} = F_{\text{wheel}} \times LR
]
At a 3.0:1 ratio, a force applied at the wheel produces approximately three times that force at the shock, while the shock moves one-third as far.
For a locally constant ratio, effective wheel spring rate is approximately:
[
k_{\text{wheel}} \approx
\frac{k_{\text{shock}}}{LR^2}
]
This squared relationship explains why small changes in leverage ratio can produce noticeable changes at the wheel.
Because the ratio itself changes through travel, calculating the complete wheel-force curve requires the shock’s actual spring-force curve and the instantaneous leverage ratio at each position. Friction, gas force, bump stops, and damping also affect the real system.
Common Leverage-Curve Shapes
Progressive
Under the usual wheel-travel-to-shock-travel convention, a progressive curve has a leverage ratio that decreases as the suspension compresses.
Example:
- Beginning: 3.0:1
- Sag region: 2.7:1
- End: 2.4:1
As leverage falls, the shock moves farther and gains more mechanical influence for each unit of wheel movement. This generally increases wheel-rate progression and bottom-out resistance.
A progressive curve does not automatically provide excellent small-bump sensitivity. That depends on its starting ratio, shock breakaway force, spring setup, damping, bearing friction, and tire behavior.
Linear
A linear curve maintains a relatively constant ratio through most of the travel.
This can provide predictable loading and makes the shock’s own spring curve more dominant. With a coil shock, the resulting wheel rate may be relatively linear. With an air shock, the air spring can still create substantial end-stroke progression.
Perfectly constant curves are uncommon. “Linear” usually means the ratio changes only modestly.
Regressive
A regressive curve has a leverage ratio that increases as the suspension compresses. This reduces the shock’s mechanical influence deeper in the stroke and can soften the wheel rate.
A fully regressive curve can be difficult to support near bottom-out, particularly with a linear coil spring. However, short regressive sections may be used deliberately to influence sensitivity, sag support, or the transition into a later progressive region.
Mixed Curves
Many real designs use more complex shapes, such as:
- Regressive to progressive
- Progressive to linear
- Linear with increased end-stroke progression
- Progressive through sag with a flatter mid-stroke
- Multiple slope changes created by short links or eccentric pivots
Reducing a complete curve to one progression percentage can conceal these important transitions.
Measuring Progression
A commonly used endpoint calculation is:
[
\text{Progression} =
\frac{LR_{\text{start}}-LR_{\text{end}}}
{LR_{\text{start}}}
\times 100
]
A curve that begins at 3.0 and ends at 2.4 has approximately 20% progression:
[
\frac{3.0-2.4}{3.0}\times100=20%
]
Manufacturers and analysis software do not always use identical endpoints or formulas. Some exclude top-out or bottom-out regions, while others compare average sections. Progression percentages should therefore be compared only when the calculation method is known.
Interaction With Spring Type
Air Springs
An air spring’s force rises nonlinearly as its positive chamber is compressed. Negative air chambers influence breakaway and early-stroke behavior, while positive-chamber volume strongly affects end-stroke ramp.
Combining a progressive leverage curve with a progressive air spring can produce substantial overall progression. This may suit aggressive riding, but excessive combined progression can make full travel difficult to access.
A relatively linear frame curve can still achieve strong end-stroke support through its air spring. FOX and RockShox both use volume spacers to alter air-spring progression without changing frame kinematics. FOX states that air-volume spacers affect mid-stroke and bottom-out resistance, while RockShox says positive Bottomless Tokens primarily increase ramp in the final portion of travel.
Coil Springs
A conventional coil spring has a nearly linear rate. The frame’s leverage curve therefore contributes more of the system’s progression.
A progressive leverage curve often makes coil-shock setup easier by adding end-stroke resistance. A linear leverage curve can also work with a coil, but may require:
- A firmer spring
- More compression damping
- A progressive coil
- A hydraulic bottom-out system
- Greater reliance on the shock’s bottom-out bumper
Coil compatibility cannot be determined solely from shock dimensions. Frame progression, available spring rates, clearance, and manufacturer approval must also be checked.
Influence on Damping
Leverage ratio affects shock-shaft speed as well as spring force:
[
v_{\text{shock}} =
\frac{v_{\text{wheel}}}{LR}
]
For the same wheel velocity, a lower leverage ratio produces greater shock-shaft velocity. Greater shaft velocity generally creates more damping force inside the shock.
That damping force is then translated back to the wheel through the leverage ratio. As a result, a falling leverage curve can increase both spring and damper influence deeper in the travel.
This is why the same shock tune can feel controlled on one frame and underdamped or harsh on another. The damper must be selected around the frame’s ratio range, shaft speeds, intended rider loads, and use case.
Ride Characteristics
Initial Sensitivity
A relatively high starting ratio can reduce effective wheel rate and damping force at the wheel, helping the suspension begin moving. Sensitivity still depends heavily on seals, bearings, shock pressure or preload, compression damping, and static friction.
Mid-Stroke Support
Mid-stroke support is produced by the combined effects of:
- Leverage-curve slope around sag
- Spring-force curve
- Low-speed compression damping
- Shock gas force
- Rider position
- Anti-squat during pedaling
A curve cannot be judged as supportive or wallowy without knowing the shock and setup used with it.
Bottom-Out Resistance
End-stroke control can come from:
- Falling leverage ratio
- Air-spring progression
- Coil or elastomer progression
- Compression damping
- Hydraulic bottom-out
- Mechanical bottom-out bumper
RockShox, for example, uses Hydraulic Bottom Out to increase compression resistance near the end of the shock stroke without relying entirely on spring ramp. RockShox rear-shock development
Relationship to Other Kinematics
Leverage curve is separate from:
- Anti-squat
- Anti-rise
- Axle path
- Chain growth
- Pedal kickback
All originate from the linkage geometry, so changing a pivot or link may affect several simultaneously. However, one curve cannot be used to infer the others reliably.
A bike can have a progressive leverage curve with high or low anti-squat, a rearward or forward-curving axle path, and very different braking behavior.
E-MTB Considerations
An e-MTB carries additional sprung mass from its motor and battery and may be ridden at higher average speeds. Designers may use different spring rates, shock tunes, and leverage curves to support that mass and control repeated impacts.
Motor torque itself is primarily relevant to anti-squat and drivetrain forces, not leverage ratio. The leverage curve governs wheel-to-shock mechanics whether the rider or motor produces the drive torque.
Mechanic’s Perspective
A leverage curve is valuable during setup, but it should not replace basic diagnosis.
Before tuning, confirm:
- Exact frame model, year, size, and travel setting
- Correct shock eye-to-eye length and stroke
- Linkage or flip-chip position
- Recommended sag
- Air-can or coil compatibility
- Shock tune and service condition
- Free movement of all frame bearings
Different frame sizes or travel configurations may use different links or shock tunes even when the model name is the same.
Sag Interpretation
Shock sag percentage does not necessarily equal rear-wheel sag percentage when leverage changes through the travel.
For example, 30% of shock stroke may place the rear wheel slightly above or below 30% of its total travel. Use the frame manufacturer’s shock-sag recommendation unless a verified wheel-travel curve is available.
Coil Spring Selection
Coil calculators may use average leverage ratio, ratio at sag, or an internal kinematic model. Results can differ significantly.
Spring selection should account for:
- Rider and equipment mass
- Rear weight distribution
- Ratio near sag
- Desired sag
- Frame progression
- Riding style
- Available preload range
Preload should not be used to compensate for a substantially incorrect coil rate.
Air-Shock Pressure
Frames with higher leverage ratios generally require greater shock force for the same wheel support, often resulting in higher air pressure. Confirm that the required pressure remains within the shock manufacturer’s limit.
Troubleshooting
| Symptom | Check Before Blaming the Curve |
|---|---|
| Frequent bottom-out | Sag, spring rate, air volume, compression damping, hydraulic bottom-out |
| Cannot reach full travel | Excessive pressure or spring rate, too many volume spacers, binding pivots, excess compression |
| Wallowing at sag | Excessive sag, insufficient spring support, air-can configuration, low-speed compression |
| Harsh initial travel | Pivot friction, shock service, excessive pressure or preload, compression setting |
| Packing on repeated hits | Rebound damping too slow, shock overheating, excessive spring energy |
| Inconsistent travel | Air-spring equalization, damaged bearings, shock service condition |
Changing multiple settings at once makes diagnosis difficult. Establish sag and spring rate first, then rebound, compression, and end-stroke tuning.
Changing the Curve
Most riders cannot adjust leverage independently, but exceptions exist:
- Manufacturer progression flip chips
- Alternate shock mounts
- Replaceable rocker links
- Approved long- or short-travel configurations
- Aftermarket links
Yeti’s Sixfinity system, for example, can change leverage-rate progression while retaining the same geometry and primary anti-squat and anti-rise characteristics.
An aftermarket link may also change travel, shock forces, tire clearance, bottom-out clearance, and warranty coverage. Installing a longer-stroke shock to access more travel does not simply improve the curve; it can cause frame or tire contact and should not be done without manufacturer approval.
Common Misconceptions
“Progressive Always Means Plush”
Progression describes how the ratio changes, not how easily the suspension begins moving.
“More Progression Is Better”
Excessive combined frame and spring progression can prevent full-travel use and create harshness late in the stroke.
“A Coil Shock Requires a Progressive Frame”
Progression is often helpful with a coil, but linear frames can work when the spring, damper, bumper, and intended use are properly matched.
“Thirty Percent Shock Sag Equals Thirty Percent Wheel Sag”
This is only true with a constant leverage ratio.
“Shock Tuning Can Completely Change the Leverage Curve”
Shock tuning changes spring and damping behavior, but the frame’s mechanical ratio remains unchanged.
“Single-Pivot Bikes Have Linear Curves”
Shock links, pivot placement, and mounting geometry can create progressive, linear, or regressive behavior on many suspension layouts.
Notable Implementations
Yeti SB120 and SB140: Use relatively linear progression through much of the travel with additional end-stroke control. Yeti SB140
Yeti Sixfinity: Provides rider-adjustable leverage-rate progression through a frame linkage setting.
FOX Air-Volume Spacers: Alter the shock’s spring curve while leaving frame leverage unchanged.
RockShox DebonAir+: Uses different air cans and volume-token options to match frames with different leverage characteristics.
Related Terms
- Leverage Ratio
- Wheel Rate
- Suspension Kinematics
- Shock Tune
- Spring Rate
- Sag
- Anti-Squat
- Anti-Rise
- Axle Path
- Hydraulic Bottom Out
References
- FOX: Rear-Shock Air-Volume Tuning
- RockShox: DebonAir+ Rear Shock
- RockShox: Rear-Shock Development and Hydraulic Bottom Out
- Yeti: Switch Infinity
- Yeti: Sixfinity
- Frame-manufacturer kinematic charts and shock setup manuals