A bicycle freehub is an integral part of the bike’s rear wheel hub. The freehub’s ability to engage and disengage seamlessly, thanks to mechanisms like pawls or star ratchets, ensures efficient power transmission and reliable gear shifting. Components such as bearings, seals, and axles contribute to durability and performance, requiring regular maintenance to sustain smooth operation. Compatibility with drivetrain components and hub standards further influences the choice of a freehub, whether for road cycling, mountain biking, or commuting. Here’s a comprehensive guide to the workings of a freehub, as well as different types, maintenance and considerations when selecting one.
What does a freehub do?
Coasting Mechanism
Perhaps the most fundamental function of a freehub is to allow the bicycle to coast. When the rider stops pedaling, the freehub disengages, allowing the rear wheel to rotate freely without transferring power from the pedals. This feature is essential for smooth and efficient riding, allowing the rider to rest or adjust position without needing to pedal continuously.
Power Transmission
When the rider pedals, the freehub engages, transferring the rotational force from the pedals to the rear wheel via the cassette. This enables the bike to accelerate and maintain speed efficiently, converting pedaling effort into forward motion.
Gear Shifting
The freehub accommodates the cassette, which contains multiple sprockets of different sizes. Each sprocket corresponds to a different gear ratio, allowing the rider to shift gears for easier pedaling (climbing hills) or higher speeds (on flat terrain or descents). The smooth operation of the freehub ensures that gear changes are reliable and precise.
Compatibility with Drivetrain Components
Freehubs are designed to be compatible with specific cassette types and drivetrain configurations (e.g., Shimano HG, SRAM XD, Campagnolo). This compatibility ensures that the bike functions correctly and efficiently, with smooth shifting and minimal wear on components.
Engagement and Efficiency
Different freehub designs feature a variety of mechanisms (such as star ratchets or multiple pawls) that improve engagement speed and efficiency. For example, the Elitewheels Frequency hub featured on the 29er Gnar Trail has 320 points of engagement, meaning it engages every 1.125°). This can enhance the responsiveness of the bike, particularly in situations requiring quick accelerations or technical maneuvers.
Durability and Maintenance
Freehubs, especially those with sealed bearings and robust construction, contribute to the overall durability and longevity of the bike. They require periodic maintenance, such as cleaning and lubrication, to ensure smooth operation and prevent premature wear.
What components does a freehub contain?
Freehub Body
This is the main cylindrical body onto which the cassette mounts. It contains the mechanism that engages and disengages with the cassette to transfer power from the pedals to the rear wheel.
Pawls
These are small, spring-loaded components that engage with the ratchet ring or star ratchet inside the freehub body. Pawls transmit rotational force from the cassette to the freehub body when the rider pedals. These are on wheels such as the Drive road and gravel wheels.
Ratchet Ring or Star Ratchet
This is the toothed ring or mechanism inside the freehub body that the pawls engage with. It allows the freehub to engage and transfer power while pedaling and disengage during coasting. Wheels with this kind of freehub mechanism include the Velo Six road wheels, the Velo TT triathlon and time trial wheels, as well as the Pro MTB wheels.
Bearings
Bearings are crucial for the smooth rotation of the freehub body around the axle. They are typically sealed cartridge bearings that reduce friction and wear, improving efficiency and longevity.
Seals and Dust Caps
These components help protect the bearings and internal mechanisms from dirt, water, and debris, extending the life of the freehub.
Axle Interface
The freehub body attaches to the hub axle via a specific interface. This could be a spline pattern, thread, or other mechanism depending on the hub design.
Lockring
Many freehub designs feature a lockring that secures the cassette onto the freehub body. This ensures the cassette remains firmly attached and aligned during use.
Springs
Springs are often used to keep the pawls engaged with the ratchet ring or star ratchet, ensuring smooth operation and reliable engagement.
Hub Shell Interface
On some designs, the freehub body interacts directly with the hub shell, contributing to the overall stiffness and durability of the wheel assembly.
What should be considered when choosing freehubs?
Ratcheting Mechanism
The ratcheting mechanism in a freehub allows for the conversion of the rotational motion of the pedals into the forward motion of the bike while enabling coasting. It typically consists of the following components:
Pawls and Ratchet Ring
The most common design features pawls (spring-loaded teeth) and a ratchet ring (a toothed ring inside the freehub body). As the rider pedals, the pawls engage with the teeth on the ratchet ring, driving the bike forward. When the rider stops pedaling, the pawls disengage, allowing the bike to coast. This is found on many road wheels, such as Drive and Marvel.
Star Ratchet
Found in some high-end hubs like those from DT Swiss and Elitewheels, the star ratchet mechanism uses two interlocking toothed rings. When the rider pedals, these rings are pushed together, and their teeth engage to transfer power. During coasting, the rings slide apart slightly, disengaging the teeth.
Point of Engagement (POE)
The point of engagement (POE) refers to the number of positions the pawls or teeth can engage per revolution of the freehub. Higher POE results in quicker engagement, which is particularly beneficial for technical riding and sprinting. Common POE values range from 18 to 120 or more, though the Elitewheels 29er Gnar Trail features 320!
18-24 POE
Lower engagement points, common in budget hubs, suitable for general riding.
36-54 POE
Mid-range engagement points, offering a balance between quick engagement and durability. Our Elitewheels Drive road and gravel wheels fall into this fange.
72-120+ POE
High engagement points, often found in high-end hubs, providing almost instant engagement for technical and performance-oriented riding. The Drive Helix road wheels have 120 points of engagement, which is achieved by using two sets of ratchts.
Spline Design
The spline design of a freehub refers to the grooves or ridges on the body of the freehub that mate with the cassette.
Shimano/SRAM Hyperglide (HG): The most common spline design, featuring multiple splines that align with the matching grooves on the cassette. It supports 8-12 speed cassettes.
Shimano Micro Spline: A newer design for Shimano 12-speed cassettes, featuring more, smaller splines for a more secure fit.
SRAM XD: Designed for SRAM 11 and 12-speed cassettes, featuring a single wide spline and a lockring that threads onto the freehub body.
Campagnolo: Campagnolo uses a unique spline design, incompatible with Shimano and SRAM, supporting their own range of cassettes.
Power Transfer
The efficiency of power transfer in a freehub is crucial for performance. Factors affecting power transfer include:
Engagement Mechanism: Higher POE generally leads to quicker power transfer but can increase friction.
Bearing Quality: High-quality bearings reduce friction and increase the efficiency of power transfer. Some of the best bearings use ceramic balls.
Material and Construction: Lightweight and stiff materials, such as aluminum or carbon, improve power transfer by reducing flex and weight.
Sound
Sound of Pawl Design and Engagement Mechanism
The more pawls a freehub has, the more engagement points it will create per revolution. This typically results in a louder clicking sound as the pawls engage with the ratchet ring. Stronger springs can lead to a more pronounced clicking sound because they snap the pawls into the ratchet ring more forcefully. The material of the pawls and ratchet ring can also affect the sound. Metal components tend to produce a louder, sharper sound compared to softer materials.
Hub sound is all about personal preference. Many MTB riders prefer louder hubs to alert others on the trail. A louder hub is usually attributed to a more expensive one, though this is only sometimes the case.
Sound of Ratchet Design
Star Ratchet Systems: Freehubs like those from Elitewheels and DT Swiss use star ratchet systems, which produce a distinctive buzzing sound. The engagement of the toothed rings can be quieter than traditional pawl systems but still noticeable. The number of teeth on the ratchet mechanism affects the frequency and pitch of the sound. More teeth mean higher engagement points and a higher-pitched, more frequent clicking sound.Elitewheels offers 36-tooth ratchets and 120-tooth ratchets.
Styles of hub
Pawl-Style Hub
This type utilizes several spring-loaded pawls that engage with the notches on the inside of the freehub body when pedaling. The number of pawls and the design of the notches determine the number of engagement points, which affects how quickly power is transferred from the pedals to the wheel.
Advantages
Generally reliable and easy to maintain. Often offers quick engagement, which is beneficial for technical mountain biking and quick acceleration.
Disadvantages
Pawls and springs can wear out over time, requiring maintenance or replacement. Pawl mechanisms can be noisier compared to other systems.
Elitewheels Ratchet System
Uses two interlocking ratchet rings that engage simultaneously. When pedaling, the rings are pressed together by a spring, ensuring consistent engagement. Number of engagement points can vary depending on the ratchet system used (e.g., 36T, 60T, or 120T dual ratchets).
Advantages
The ratchet system is known for its reliability and durability. It requires less frequent maintenance compared to pawl systems and provides consistent and smooth engagement with less risk of slipping.
Disadvantages
While the engagement is smooth and reliable, it may not be as instant as some high-end pawl systems. Generally it is more expensive than standard pawl systems.
Popular freehubs on the market:
Shimano HG (HyperGlide)
This is one of the most common freehub designs, used by Shimano and compatible with many other brands. It features spline grooves and is known for smooth shifting.
Advantages
Widely compatible with a variety of drivetrains and components.
Smooth shifting performance due to spline groove design.
Generally more affordable compared to proprietary designs.
Disadvantages
Limited to Shimano and compatible drivetrains.
Not compatible with SRAM XD or Campagnolo cassettes without adapters.
SRAM XD (X-Dome)
Introduced by SRAM, XD freehubs have a larger diameter and use a cassette with a 10-tooth smallest cog. They are commonly used for SRAM’s 11 and 12-speed cassettes.
Advantages
Allows for a 10-tooth smallest cog, expanding gearing options.
Durable design with easy maintenance.
Disadvantages
Primarily compatible with SRAM drivetrains.
Limited aftermarket compatibility with other brands without adapters.
Higher cost compared to Shimano HG.
Campagnolo
Campagnolo uses its own unique freehub design, which is not compatible with Shimano or SRAM cassettes without adapters.
Advantages
Precision shifting and durability tailored for Campagnolo drivetrains.
Disadvantages
Proprietary design not compatible with Shimano HG or SRAM XD without adapters.
Higher cost and limited availability of components compared to Shimano and SRAM.
Micro Spline (Shimano)
Shimano introduced Micro Spline freehubs for their 12-speed MTB drivetrains, offering a smaller spline pattern and allowing for a 10-tooth smallest cog.
Advantages
Supports 10-tooth smallest cog for 12-speed MTB drivetrains, enhancing climbing capabilities.
Secure interface design.
Disadvantages
Limited compatibility outside Shimano’s 12-speed MTB drivetrains.
Requires adapters for non-Shimano components.
Maintenance of freehub:
Maintaining a freehub is essential for ensuring smooth operation, longevity, and optimal performance of your bicycle’s drivetrain. Here are some key maintenance steps for a freehub:
Cleaning:
Regularly clean the exterior of the freehub body and surrounding area to remove dirt, grime, and debris that can affect performance.
Use a soft brush or cloth along with mild detergent or bike-specific cleaner. Avoid using harsh chemicals that could damage seals or bearings.
Lubrication:
Proper lubrication is crucial for the internal components of the freehub to function smoothly and reduce wear.
Use a lightweight, bicycle-specific grease or oil recommended by the manufacturer. Apply a small amount to the pawls, ratchet ring, or star ratchet mechanism as per the maintenance instructions.
Inspect Seals and Bearings:
Check the condition of the seals regularly to ensure they are intact and free from cracks or damage. Seals protect the bearings from water, dirt, and contaminants.
Inspect the bearings for smooth rotation and signs of wear or roughness. Replace bearings if they feel gritty or show excessive play.
Check Pawls and Springs:
Inspect the pawls and springs inside the freehub body. Ensure that the pawls move freely and engage smoothly with the ratchet ring or star ratchet.
Clean the pawls and springs if necessary and replace any worn or damaged components to maintain reliable engagement.
Adjustment and Tightening:
Periodically check the tightness of the lockring that secures the cassette onto the freehub body. Ensure it is snug to prevent cassette movement or play.
Check for any play or wobble in the cassette, which could indicate a need for spacer adjustment or replacement.
Professional Servicing:
For complex freehub mechanisms or if you’re unsure about maintenance procedures, consider taking your bike to a professional mechanic. Elitewheels has a worldwide service center network which makes servicing convenient.
Professional servicing may include disassembling the freehub, cleaning all internal components thoroughly, and re-greasing to factory specifications.
Storage and Usage:
Store your bike in a clean, dry environment to prevent corrosion and damage to the freehub and other drivetrain components.
Avoid riding in extreme conditions (mud, sand, water) if possible, as these can accelerate wear and require more frequent maintenance.
Conclusion
A freehub not only enables smooth coasting but also efficiently transfers power from pedaling to forward motion, crucial for speed and control. By exploring its workings, various styles, maintenance needs, and different options, cyclists can make informed choices to suit their preferences.
