Suspension Kinematics

Summary

Suspension kinematics describes the geometry-controlled movement of a bicycle’s rear axle, linkage, and shock as the suspension travels. It determines axle path, leverage ratio, chain-length change, anti-squat, anti-rise, and instant-center location. These characteristics influence pedaling response, braking behavior, support, and how the rear wheel encounters impacts. Kinematics does not include spring rate, damping, friction, chassis stiffness, or tire behavior, so it cannot predict ride quality by itself. Two bikes using the same basic linkage layout can have substantially different kinematics, while different layouts can be tuned to produce similar behavior.

Quick Facts

Category: Suspension engineering
Applies to: Full-suspension bicycles
Defined by: Pivot positions, link lengths, shock mounts, and frame geometry
Common outputs: Leverage ratio, axle path, anti-squat, anti-rise, chain growth
Related concepts: Instant center, wheel rate, pedal kickback, brake response

Overview

In engineering, kinematics describes motion without first calculating the forces that create it. For a bicycle frame, it establishes where the rear axle and shock move as the links rotate.

Anti-squat and anti-rise expand that analysis by applying chain, braking, mass-transfer, and geometry assumptions to the linkage. Suspension dynamics goes further by including spring force, damping, friction, wheel speed, tire behavior, and rider movement.

Designers use kinematic models to balance several competing goals. A bike may need to pedal without excessive movement, remain responsive over rough ground, resist bottom-out, maintain predictable geometry under braking, and fit a shock, bottle, motor, and tire within the frame. No single kinematic curve is ideal for every bicycle or rider.

Core Kinematic Characteristics

Leverage Ratio

Leverage ratio is commonly expressed as incremental rear-wheel movement divided by shock movement. A 2.8:1 ratio means approximately 2.8 mm of wheel travel produces 1 mm of shock stroke at that point. Total wheel travel divided by shock stroke gives the average ratio, but the instantaneous ratio normally changes through the travel.

Using the common wheel-to-shock convention:

Progressive: Leverage ratio decreases as the suspension compresses.
Linear: The ratio remains relatively constant.
Regressive: Leverage ratio increases through part of the travel.

A progressive linkage gives the shock increasing mechanical advantage over the wheel deeper in the stroke. This can add support and bottom-out resistance, especially with a relatively linear coil spring. A linear linkage may work well with the natural progression of an air spring.

Progression alone does not guarantee small-bump sensitivity or good mid-stroke support. Shock tune, spring rate, seal friction, bearing condition, and sag remain important. Some software displays the inverse measurement, called motion ratio, so the same progressive system may appear as a rising rather than falling curve. Vorsprung’s leverage-curve explanation illustrates this distinction.

Axle Path

Axle path is the route followed by the rear axle relative to the front triangle. A single-pivot swingarm follows a circular arc around its main pivot; it does not normally move in a purely vertical line.

A rearward axle path can help the wheel move in the same general direction as it encounters the leading edge of an obstacle. The benefit depends on the amount of rearward travel, wheel size, impact speed, and suspension state.

Rearward movement also lengthens the wheelbase and normally increases the distance between the bottom bracket and rear axle. These changes can affect handling, chain growth, and drivetrain feedback. Many linkages move rearward early and then forward later, but the exact path depends on pivot placement rather than the system’s brand name.

Chain Growth and Pedal Kickback

Chain growth describes an increase in the effective chain path between the chainring and cassette as the suspension compresses. If the chain cannot feed freely from the rotating rear wheel or freehub, that growth can rotate the cranks backward, producing pedal kickback.

Calculated pedal-kickback figures represent a controlled geometric condition. What the rider feels also depends on wheel speed, selected gear, freehub engagement, suspension speed, and whether the rider is resisting crank movement.

High-pivot systems commonly use an idler pulley near the main pivot to control chain growth and separate axle-path goals from drivetrain behavior.

Anti-Squat

Acceleration transfers load rearward and tends to compress the rear suspension. Chain tension and suspension geometry can create an opposing force. Anti-squat expresses how much of the acceleration-induced squat is counteracted.

In a simplified model:

100 percent: The calculated forces balance, producing no squat or extension.
Above 100 percent: The suspension tends to extend under acceleration.
Below 100 percent: Some acceleration-induced compression remains.

Anti-squat changes with suspension position, rear cog, chainring size, wheel size, geometry, and the assumed rider center of mass. A graph calculated in one gear cannot automatically be applied to every gear.

More anti-squat is not always better. High values can make a bike feel firm under power but may increase drivetrain feedback or reduce suspension freedom while pedaling. Lower values permit more suspension movement but may allow noticeable squat. Patents covering variable anti-squat systems illustrate how designers deliberately change it through the travel rather than targeting one fixed number. See US7048292B2.

Anti-Rise

Braking transfers load toward the front wheel, tending to extend the rear suspension. Rear-brake torque acting through the suspension can oppose that extension. Anti-rise describes the calculated balance.

100 percent: Rear braking produces no theoretical compression or extension.
Above 100 percent: Rear braking tends to compress the suspension.
Below 100 percent: The rear suspension tends to extend.

Higher anti-rise can reduce chassis pitch and preserve geometry under braking, but excessive compression bias may reduce the suspension’s ability to respond to additional impacts. Lower anti-rise allows more extension and movement but permits greater geometry change.

Anti-rise is normally calculated for rear-brake operation under defined assumptions. Actual hard braking also involves the fork, front brake, tire grip, rider position, and changing wheel loads. The definitions are outlined in this bicycle-suspension patent.

Instant Center

The instant center is the point about which the rear axle assembly is effectively rotating at one particular suspension position.

On a conventional single pivot, it is the physical main pivot. On multi-link designs, it is usually a virtual point established by the link geometry and may move substantially as the suspension compresses.

Instant-center migration helps designers shape axle path, anti-squat, and braking response. It is a calculation point, not necessarily a pivot or component that exists physically on the frame.

Common Layouts

Layout	Kinematic characteristic
Single pivot	Fixed instant center and circular axle path
Linkage-driven single pivot	Fixed axle path with greater freedom to shape leverage ratio
Horst-link four-bar	Moving instant center and broader control of axle and braking behavior
Dual short-link	Rear triangle guided by two links, allowing a highly tunable moving instant center
High pivot with idler	Greater potential for rearward axle travel with drivetrain forces managed by an idler
Translating-pivot or six-bar	Additional tuning freedom, with more links, bearings, and packaging requirements

VPP, DW-Link, Maestro, Switch Infinity, and similar names describe particular implementations. They do not define one universal leverage, anti-squat, or axle-path curve. Yeti, for example, describes Switch Infinity as a four-bar system with a translating pivot, but each frame receives its own kinematic and shock tune.

Why Kinematics Matters to Riders

Riders may notice kinematics through several behaviors:

How much the bike moves during seated or standing acceleration
Whether the rear wheel feels free or firm while pedaling over rough ground
How readily the bike uses mid- and end-stroke travel
Whether the wheelbase changes noticeably through deep compressions
How the chassis pitches and the rear tire tracks under braking
Whether drivetrain feedback reaches the pedals

These sensations cannot be assigned to one graph alone. A progressive bike can still feel soft if undersprung, while a high-anti-squat bike can bob because of rider movement or insufficient damping. Tire pressure, shock setup, frame stiffness, and geometry can be more noticeable than relatively small kinematic differences.

Mechanic’s Perspective

Kinematics is built into the frame. Air pressure, compression damping, rebound, and volume spacers change suspension response but do not change the linkage’s axle path or leverage curve. Flip chips and adjustable pivot positions may alter kinematics when the manufacturer specifically designs them to do so.

Workshop inspection should include:

Pivot-bearing play or binding
Loose linkage hardware
Worn shock bushings and mounting hardware
Correct shock eye-to-eye length and stroke
Frame and tire clearance through the permitted travel
Manufacturer-specified pivot torque and assembly order

Bearing friction can make a theoretically sensitive linkage feel reluctant to move. Excessive play changes alignment and creates knocking that may be mistaken for a shock problem.

Installing a shock with a different length or stroke can alter geometry, travel, leverage operating range, and frame clearance. It should not be done without manufacturer approval. Coil-shock suitability also depends on the frame’s leverage curve, available clearance, and approved shock dimensions—not simply the linkage category.

If a bike’s behavior changes suddenly, its kinematics have not changed unless hardware has moved or failed. Mechanics should first inspect shock pressure and damping, pivot condition, fasteners, tires, and suspension service status.

Buying Considerations

Kinematic information is useful when comparing bikes intended for similar riding, but graphs must use comparable assumptions. Anti-squat curves may be calculated in different gears or with different rider center-of-mass positions.

The suspension layout name is a poor shortcut for ride quality. Shock tune, geometry, tires, frame stiffness, fit, and component condition may matter more than small differences between curves.

Riders who climb technical terrain may value controlled anti-squat and traction under power. Riders prioritizing high-speed impacts may place more emphasis on axle path and progression. Braking behavior matters most on steep or rough descents. These goals involve trade-offs rather than a universally correct design.

Engineering Trade-Offs

Greater kinematic freedom often requires additional links, bearings, hardware, and frame space. This can increase manufacturing cost, weight, and service requirements. Simpler layouts offer fewer independent tuning variables but can still perform well when the pivot locations, leverage curve, and shock tune suit the bike’s purpose.

Common Questions

Is more progression better?

No. Too much combined frame and air-spring progression can prevent full-travel use. Too little may require greater spring or compression support.

Is more anti-squat better?

No. Higher anti-squat can improve support under acceleration but may increase drivetrain influence or reduce suspension movement under power.

Can shock tuning change the kinematics?

No. It changes the forces and speeds within the system, not the geometric paths defined by the frame.

Can two bikes with the same suspension layout ride differently?

Yes. Small changes in pivot and shock-mount positions can produce different axle paths, leverage curves, anti-squat, and anti-rise.