Idler Pulley

Summary

An idler pulley is a passive drivetrain component that redirects a bicycle chain without changing the selected gear ratio. On high-pivot full-suspension bikes, an upper idler routes the loaded chain span closer to the suspension’s main pivot or instant-center region. This allows designers to manage chain growth, pedal kickback, and anti-squat while using a more rearward axle path.

An idler does not automatically eliminate chain growth or pedal kickback. Its effect depends on its exact position, diameter, chain wrap, chainline, gearing, and relationship to the suspension pivots.

Key Facts

Category: Drivetrain and suspension component
Also known as: Drive idler, chain idler, idler sprocket
Common use: High-pivot mountain bikes
Other applications: Gearbox bikes, unusual chain-routing systems, and some chain-tensioning mechanisms
Primary function: Redirect the chain path
Typical position: Near the main pivot or suspension instant-center region
Common construction: Toothed pulley with one or more bearings
Does not change: Drivetrain gear ratio
Can influence: Chain growth, anti-squat, pedal kickback, chain retention, drivetrain friction
Typical wear items: Teeth, bearings or bushings, guide plates, axle, and mounting hardware
Historical use: Guide and idler pulleys predate modern mountain bikes; suspension-specific applications became visible on gravity bikes during the 1990s

Overview

A conventional derailleur drivetrain routes the loaded upper chain span directly from the chainring to the selected cassette sprocket. This arrangement works well when suspension movement causes relatively little change in the required distance between those components.

High-pivot suspension complicates that relationship. As the rear axle moves rearward, the cassette generally moves farther from the chainring. The drivetrain then requires a longer chain path. This change is called chain growth, although the chain itself is not physically stretching.

An idler redirects the upper chain span through a point selected by the frame designer. Placing that point near the suspension’s main pivot or moving instant center can reduce the amount by which suspension movement changes the loaded chain span. It also relocates the chain-force line used to create anti-squat.

Modern idler-equipped bikes use the component as part of the suspension system rather than as a simple guide. Moving the idler, changing its diameter, or altering its chainline can affect suspension behavior as well as drivetrain operation.

What an Idler Does

An idler is driven by the chain but does not supply power or select gears. It turns because the chain passes over it.

Its primary functions can include:

• Redirecting the loaded chain span
• Controlling effective chain-path growth
• Establishing the chain-force line used in anti-squat
• Reducing theoretical and felt pedal kickback
• Providing additional chain wrap
• Supporting chain retention
• Creating clearance around a suspension pivot, frame member, or motor

Because the same chain enters and leaves the pulley at the same linear speed, the idler’s tooth count does not alter the bicycle’s gear ratio.

Chain Growth Is Not Chain Stretch

Chain growth refers to an increase in the chain-path length required by suspension movement. It should not be confused with wear-related chain elongation measured by a chain checker.

On a conventional drivetrain, an increase in required chain length may be accommodated through:

• Rear-derailleur cage rotation
• Forward rotation of the rear wheel or freehub
• Backward crank movement
• Suspension resistance created by chain tension
• A combination of these effects

If the rear wheel cannot rotate freely—for example, because the brake is applied—or the freehub engages during rapid suspension movement, the change can be transmitted toward the cranks as pedal kickback.

How the Upper Idler Changes the Chain Path

The loaded chain normally runs from the chainring directly to the cassette. An upper idler divides that route into two sections:

1. Chainring to idler
2. Idler to cassette

The first section is usually fixed or changes relatively little because the chainring and idler are attached to the frame or closely related suspension members.

The second section changes as the rear axle moves. If the idler is located near the rear assembly’s effective center of rotation, the distance from the idler to the cassette can remain more consistent than the original chainring-to-cassette distance.

This can reduce drivetrain-induced resistance to suspension movement. It does not necessarily make total chain-path growth zero.

Idler Position and Anti-Squat

The idler’s position is one of the strongest drivetrain-related controls available to a high-pivot suspension designer.

Chain tension acts along the chain spans entering and leaving the idler. The resulting forces are transferred through the idler mount and suspension structure. Changing the idler’s location changes those force directions and therefore changes how pedaling loads interact with the suspension.

Depending on the design, idler placement can be used to:

• Increase or decrease anti-squat
• Change how anti-squat varies through travel
• Change how anti-squat varies across cassette sprockets
• Reduce pedal kickback
• Balance acceleration support with suspension sensitivity

An idler placed exactly concentric with a fixed main pivot may minimize some upper-span length change. Designers frequently position it slightly away from that point to produce a desired anti-squat curve.

For a high virtual-pivot system, the instant center moves throughout the travel. One fixed idler cannot remain concentric with that moving point at every suspension position, so the final behavior is necessarily a compromise.

Pedal Kickback

Pedal kickback is backward crank rotation or crank-loading tendency produced when suspension movement increases the required chain-path length.

An idler can reduce this effect, but the amount actually felt depends on:

• Idler location and diameter
• Main-pivot or instant-center path
• Cassette sprocket
• Chainring size
• Freehub engagement
• Rear-wheel movement
• Brake application
• Suspension speed
• Chain tension

Kinematic software commonly calculates theoretical kickback under specific assumptions, such as a locked rear wheel. The rider may experience less because the wheel or freehub can rotate and release part of the tension.

“Zero pedal kickback” should therefore be treated as a condition-specific calculation rather than a universal operating state.

Idler Types

Toothed Upper Idler

This is the most common suspension idler. Its teeth engage the chain rollers and redirect the loaded upper span.

Characteristics include:

• High chain loading while pedaling
• Positive chain engagement
• Frame-specific diameter and location
• One or more cartridge bearings
• Optional guide plates or covers

Some use narrow-wide tooth profiles for additional retention. Others use chain-specific tooth shaping or lateral float to accommodate the changing chain angle across the cassette.

Smooth Roller

A smooth roller contacts the outside of the chain rather than engaging its rollers with teeth. It is more commonly used on a lower, lightly loaded chain span or as part of a guide.

Smooth rollers are generally less suitable for redirecting a highly loaded drive span because they rely on frictional contact and can create additional noise or wear.

Lower Idler or Guide Pulley

A lower pulley may control the return span beneath the chainring. It can:

• Improve chain wrap
• Reduce chain movement
• Guide the chain around frame structures
• Support chain retention
• Control noise

It does not necessarily perform the same kinematic function as the upper drive idler.

Moving Idler

Not every idler is attached directly to the front triangle. Some are carried by a swingarm or suspension link so that their position changes with the linkage.

The important measurement is the idler’s path relative to the chainring, cassette, and suspension centers—not simply whether it appears frame-mounted.

Idler vs Other Drivetrain Components

An idler-equipped derailleur bike does not automatically require a separate tensioner. In most systems, the rear derailleur cage continues to manage chain slack.

An idler may incorporate guide plates, but that does not make every chain guide an idler.

Pulley Diameter and Tooth Count

A larger toothed idler generally reduces the angle through which each chain link articulates. It also rotates more slowly for a given chain speed, which can reduce some sources of friction and bearing speed.

Larger idlers also require more space and may add weight. Their diameter affects:

• Chain tangency points
• Chain wrap
• Frame clearance
• Guide dimensions
• Required chain length
• Bearing loads
• Suspension kinematics to a small but potentially meaningful degree

Common modern idlers range from the mid-teens to approximately 20 teeth, but there is no universal standard. Norco, for example, specifies an 18-tooth steel idler on some current high-pivot platforms. Norco Sight VLT assembly documentation

A different tooth count should not be installed merely because the pulley fits on the axle.

Bearings, Bushings, and Loads

The upper idler carries substantial chain load while the rider pedals. The bearing must support the combined forces from the incoming and outgoing chain spans.

Common constructions include:

• Single cartridge bearing
• Twin cartridge bearings
• Replaceable bushings
• Sealed axle and bearing assemblies

Twin-bearing systems can improve pulley support and distribute load. Deviate, for example, uses twin idler bearings on the Highlander II. Deviate Highlander II

A pulley that spins smoothly with no chain load may still become noisy or rough under pedaling force if its bearing, axle, or mounting structure is worn.

Chainline

The chain moves laterally as the derailleur shifts across the cassette. A fixed idler must accommodate this angle without forcing the chain against its guide plates or tooth shoulders.

Manufacturers address this through:

• Idler placement near the cassette’s average chainline
• Wider tooth or guide geometry
• Lateral pulley float
• Frame-specific spacers
• Different mounts for 52 or 55 mm chainlines
• Chain-specific tooth profiles

Chainline errors can increase friction, noise, tooth wear, and derailment risk. A crank or chainring should not be selected solely because it is labeled “Boost” or “Super Boost.” The resulting chainline must match the frame and idler specification.

Efficiency and Drag

An idler introduces another bearing and another point of chain articulation. Its losses arise from:

• Bearing resistance
• Chain articulation
• Tooth engagement
• Chainline angle
• Guide-plate contact
• Contamination
• Chain tension
• Lubrication condition

A clean, aligned idler with a healthy bearing may add little noticeable resistance. A dirty, misaligned, or worn system can create substantial noise and drag.

Efficiency claims should be interpreted carefully because losses change with rider power, selected gear, chain lubricant, idler size, bearing condition, and contamination.

Chain Retention

Idler systems commonly use covers or side plates to prevent the chain from leaving the pulley during suspension movement.

Retention depends on:

• Tooth profile
• Chain wrap
• Guide clearance
• Chainline
• Derailleur clutch condition
• Correct chain length
• Idler alignment

A cover should provide enough clearance for normal chain articulation without allowing derailment. Excessively tight guide clearance creates continuous rubbing and may prevent the chain from shifting cleanly across its lateral range.

Advantages

Greater Suspension-Design Freedom

The chain can be routed independently of the original chainring-to-cassette line, allowing more rearward axle movement without uncontrolled drivetrain effects.

Tunable Anti-Squat

Idler position gives designers another way to shape pedaling support through the suspension travel.

Reduced Pedal Feedback

Correct placement can reduce chain-growth-related crank movement and tension spikes.

Improved Packaging

The chain can be routed around pivots, swingarms, motors, or frame structures that would otherwise interfere with a direct chain path.

Additional Retention

Toothed idlers with guide plates can provide another point of positive chain control.

Limitations

Additional Wear Component

Teeth, bearings, bushings, covers, spacers, and axles require inspection and eventual replacement.

Potential Drivetrain Drag

Every added chain articulation and bearing can contribute resistance, particularly when contaminated or misaligned.

Longer Chains

Some high-pivot bikes require more links than a standard packaged chain contains.

Frame-Specific Parts

Idler diameter, tooth profile, axle, spacers, and guide plates may be unique to one frame generation.

More Complex Chain Setup

Chain sizing, B-gap, chainline, and guide adjustment may use procedures that differ from conventional drivetrains.

Noise

Idlers can generate a characteristic mechanical sound, but grinding, clicking, or repeated guide contact usually indicates a problem rather than an unavoidable trait.

Mechanic’s Perspective

An idler should be serviced as a loaded drivetrain component, not treated like a cosmetic pulley. Correct parts order, chainline, chain length, and bearing condition are essential.

Before Disassembly

Record or photograph:

• Chain routing
• Idler orientation
• Spacer order
• Guide-plate position
• Axle direction
• Washer locations
• Chainline configuration

Some idler axles are keyed into a swingarm or link. Pulling or rotating the wrong part during removal can displace hidden hardware. Forbidden specifically warns that the keyed shaft must remain seated during removal on some Dreadnought systems. Forbidden idler installation guide

Bearing and Tooth Inspection

With the chain removed, inspect for:

• Roughness or binding
• Radial or axial bearing play
• Cracked seals
• Bent axle hardware
• Hooked or pointed teeth
• Uneven wear on one side
• Cracked guide plates
• Embedded dirt or chain debris

Some lateral movement may be intentional on a floating idler. Compare the measured movement with the manufacturer’s specification before condemning the bearing.

Forbidden’s service instructions call for cleaning the assembly, inspecting tooth wear, and checking bearing smoothness. Forbidden idler maintenance

Wear-Pattern Diagnosis

Chain Replacement

Check the idler when replacing the chain. A badly worn idler may not mesh properly with a new chain even if the cassette and chainring remain serviceable.

Confirm:

• Chain speed compatibility
• Correct total link count
• Approved joining method
• Correct idler routing
• Derailleur capacity
• B-gap procedure
• Maximum chain tension through travel

Do not cut the new chain to match the removed chain without first confirming that the previous chain was correctly sized.

Forbidden notes that derailleur B-tension is particularly important on its idler-equipped Druid and Dreadnought platforms. Forbidden derailleur setup guide

Replacement Pulley Selection

Match the original:

• Tooth count
• Tooth profile
• Width
• Bearing dimensions
• Lateral offset
• Guide-plate compatibility
• Axle diameter
• Frame generation

Changing tooth count or pulley diameter can alter chain length, guide fit, clearance, and suspension-related drivetrain behavior.

Cleaning

Avoid directing high-pressure water at the bearing seals. Remove packed dirt with brushes and cloths, and use bearing-safe cleaners where required.

The pulley teeth receive lubrication from the chain. Do not pack exposed teeth with grease, which attracts abrasive contamination. Service internal bearings or bushings only according to the manufacturer’s procedure.

Bolt Torque and Spacers

Use the specified torque and thread treatment. A missing spacer can cause the mounting bolt to side-load or clamp the bearing, while an overtightened guide can rub continuously.

Do not assume all idler fasteners use threadlocker. Some require grease, retaining compound, pre-applied threadlocker, or a dry installation depending on the frame.

Full-Travel Check

After changing the chain, chainring, cassette, derailleur, idler, or shock configuration, cycle the suspension according to the manufacturer’s procedure and check:

• Maximum chain tension
• Derailleur-cage position
• Guide clearance
• Idler-to-frame clearance
• Chain contact with the swingarm
• Brake-hose and derailleur-housing movement
• Tire clearance

Maximum chain-path growth may occur before full bottom-out.

Buyer Considerations

Before buying an idler-equipped bike, check:

• Price and availability of replacement pulleys
• Bearing availability
• Recommended maintenance interval
• Required chain length
• Supported chainlines
• Compatible chain speeds
• Availability of guide plates, axles, and spacers
• Whether the idler uses proprietary hardware
• Access for cleaning
• Expected drivetrain noise

A well-designed idler system can be reliable, but its frame-specific parts should be considered alongside shock hardware, pivots, and derailleur hangers when evaluating long-term ownership.

Notable Implementations

Forbidden Druid and Dreadnought

Forbidden uses toothed upper idlers as part of its Trifecta suspension platforms. Idler location, chainline, derailleur setup, and frame generation are treated as parts of the complete kinematic system. Forbidden suspension technology

Norco High-Pivot Platforms

Norco uses integrated upper idlers on HVP and VPSHP bikes, including selected Range, Sight, Optic, and VLT platforms. Idler specifications and hardware vary by model.

Deviate Claymore and Highlander II

Deviate’s high-pivot bikes use covered upper idlers, with current Highlander II documentation highlighting twin bearings and a narrow-wide tooth profile.

Commencal Supreme DH

The Supreme DH uses an idler-equipped high virtual-pivot system developed for downhill racing. Exact idler, guide, and chain setup depends on frame generation.

Common Misconceptions

“Chain Growth Means the Chain Physically Stretches”

False. It means suspension movement increases the required chain-path length.

“An Idler Eliminates Chain Growth”

False. It can reduce effective upper-span growth, but the outcome depends on the complete geometry.

“An Idler Eliminates Pedal Kickback”

False. It can reduce kickback, but gearing, freehub engagement, braking, and wheel movement still matter.

“An Idler Is Just a Chain Guide”

False. A suspension idler redirects the loaded chain span and changes how chain forces interact with the suspension.

“Every Idler Needs a Separate Tensioner”

False. On derailleur drivetrains, the derailleur cage normally manages return-span tension.

“A Larger Idler Changes the Gear Ratio”

False. It changes pulley speed and chain articulation, not the ratio between chainring and cassette.

“Any Pulley With the Same Tooth Count Will Work”

False. Offset, width, bearings, tooth profile, guide fit, and mounting dimensions must also match.

“Idler Noise Is Always Normal”

Some additional chain sound may be characteristic, but grinding, clicking, rubbing, or rapidly increasing noise should be investigated.

Related Terms

• High-Pivot Suspension
• Axle Path
• Chain Growth
• Pedal Kickback
• Anti-Squat
• Chain Guide
• Chain Tensioner
• Derailleur Clutch
• Instant Center
• Drivetrain Efficiency

References

• Forbidden Bike Company: Suspension Technology
• Forbidden: Dreadnought Idler Maintenance
• Forbidden: Dreadnought Idler Installation
• Forbidden: Derailleur Setup Guide
• Norco: HVP Suspension
• Norco: VPSHP Suspension
• Deviate: Highlander II
• Frame-manufacturer service manuals and exploded diagrams
• Bicycle suspension and chain-force simulation references