The hub is the central part of your bike’s wheels (front and rear), which connects to the wheel’s rim via the spokes and through which the axle is fitted, enabling the wheel to freely spin on two sets of bearings. Our best bike hubs buying guide is the place to find out more!
As bikes have front and rear wheels, so too have they front- and rear-specific hubs. The front hub is simply designed to enable the wheel to spin, while the rear hub is a little more complex as it also forms part of the bike’s transmission – the cassette or sprocket which drives the rear wheel is attached to the rear hub, which on most bikes also features mechanism to allow you to coast or freewheel (exceptions being fixed-gear or ‘fixie’ bikes and track bikes).
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Front hubs consist of a tubular body (usually metal) with a set of bearings at each end, either loose ball bearings packed into a bearing race and secured via locknuts, or cartridge-type bearings which press in as a single unit.
The wheel axle threads through the body and spins on these bearings. The axle may be secured to the fork/frame using bolts or quick-release clamps, depending on the bike and hub type.
The hub will typically feature a raised lip at either end (the flange) which is drilled with holes for the spokes, again the number of holes/spokes depending on the wheel type.
Many MTB and some road hubs will also feature disc mounts for disc brake rotors (on bikes running disc brakes, or with the potential to have them fitted).
Most road, MTB and BMX rear hubs will also feature a freewheel on the drive side of the hub. This is a splined metal (aluminium or steel) tube onto which you slide the cassette or sprocket, and which contains the spring-loaded pawls responsible for making the clicking sound you hear when you are coasting on the bike.
NOTE: The ratchet-and-pawl mechanism is what enables the wheel to spin freely when you are coasting, but to engage the transmission when you turn the pedals. The ratchet is a toothed gear wheel inside the hub body which engages with the spring-loaded pawls in the freewheel to allow rotation in one direction only. When you are freewheeling, the pawls click past the teeth (that’s the noise you hear), but when you start pedalling they engage (catch on the teeth and transfer your pedal power to the wheel).
The number of teeth/pawls is a consideration for some riders when upgrading their rear hubs as while fewer/bigger teeth can deal with more torque (e.g. for BMX or gravity riders) smaller/more teeth mean better engagement rate and more immediate power transfer.
Which hub is right for you?
A replacement or upgrade hub can give your bike a performance advantage with lighter weight and better quality bearings available in aftermarket units.
However the type of hub you choose will depend on many factors including bike type, axle dimensions, riding type and budget.
MTB hubs come in a wide variety of dimensions and types to fit multiple bike/fork/axle standards and to suit different off-road disciplines.
The right one for you will depend on the type of riding you do – a lightweight alloy racing hub, for example, will not be able to cope with the punishment dished out by a dirt jump or downhill rider – your frame and fork dimensions, your rims and your budget.
Which MTB hub is right for you?
Aside from the obvious decision – front or rear? – there are a number of things to take into acccount when choosing a new hub or hubset, among them bearing type, materials, disc mount standard, axle type and rims.
• Bearing type: Good-quality bearings, sealed and protected from dirt and water, are the key to long and healthy hubs. Older or less expensive hubs may use cup and cone bearings, which contain two rings of loose ball bearings that sit in ‘cups’ inside the hub body, and are secured in place by conical nuts known as ‘cones’. The balls can rotate freely between the cup and cone with each unit making up one bearing.
Cup and cone bearings have the advantage of being easily serviced at home – new bearings and a dollop of grease once in a while are all you need to keep them running smoothly – but they can be tricky to adjust perfectly. More expensive hubs may use cartridge bearings, where the steel balls or needles rotate in a single cartridge unit that can be easily removed for servicing or replaced once worn out. Many riders prefer the convenience of cartridge bearings – no more tedious micro-adjusting or hunting for stray ball bearings mid-service – but they are more expensive to replace.
• Materials: Most MTB hubs, front and rear, will be made of lightweight aluminium alloy although carbon fibre units are available at the top end of the market. Rear hubs for trail, jump and gravity bikes may use steel rather than aluminium freewheels to prevent the softer alloy splines being chewed up by the cassette under heavy load.
• Brake rotor compatibility: Bikes running disc brake systems need hubs to which the disc rotor can attach. There are two main standards for rotor mounting, six-bolt and Centrelock. As the name suggests, six-bolt systems have hubs with six evenly-spaced holes to bolt on a matching rotor. Rotor bolts generally use Torx heads so you will need to make sure you have a Torx tool to tighten or loosen them – don’t try to fudge the job with an Allen key or you will risk stripping the bolts.
The Centrelock system used by Shimano foregoes rotor bolts in favour of a splined hub-rotor interface secured with a locking ring (special tool needed). This is claimed to reduce installation time but it does put limits on mixing and matching hubs and brakes as most other manufacturers stick with the six-bolt system. Again however, adaptors are available to mount six-bolt rotors to Centrelock hubs, and vice-versa. Before investing in new hubs check which disc interface you are currently using and buy accordingly.
• Axle size (front): You must also ensure that your hubs are compatible with your fork axle. Many entry-level or shorter-travel cross-country forks attach to the hubs via quick-release (QR) skewers, with a 9mm diameter axle clamped into the fork and frame dropouts via a 5mm QR skewer. However recent years have seen the growth of the ‘bolt-through’ or ‘thru-axle’ standard on MTB front forks for trail, enduro and gravity riders. This uses a larger-diameter (12mm, 15mm or 20mm) axle for extra stiffness and security, with the axle threading right through the fork ends and being secured by two QR clamps integrated into the fork lowers.
If you intend to upgrade your wheels make sure the hubs match whichever axle standard you are using, or alternatively consider hubs that can switch between QR and bolt-through with the insertion of a simple adaptor to enable the wheels to be used with different fork types.
• Axle size (rear): Rear hubs are sized according to the diameter and length of the rear axle (measured in millimetres). The standard MTB rear axle dimensions are 135x5mm (for a QR rear hub) but jump and gravity bikes may use wider and thick axles for extra strength and stiffness, with the available sizes being 135×10, 142×12, 150×12, 157×12 and 165×12. Ensure your rear hub is the correct size to fit your frame.
• Rims: If choosing a new hub to rebuild a wheelset you will need to match the number and type of spoke holes on the hub’s flange to the number and type of spoke holds drilled into the rim, e.g. a 32h hub is designed for a 32-spoke wheel, and needs to be matched to a 32h rim.
The Boost hub is an alternative standard width for MTB front and rear hubs. Introduced by manufacturers Trek and SRAM, the Boost standard widens the rear axle by 6mm and the front axle by 10mm. This creates 148 x 12mm rear hubs, and 110 x 15mm front hubs.
The aim of the new standard was to keep up with the increasing demand for bigger 29″ wheels. A wider hub means wheel spokes are fitted at an increased angle, adding stiffness to the rims. The lack of rigidity in 29″ wheels had, until the invention of the Boost standard, been a big drawback of the larger wheel size.
The hub is the engine room of the road bike wheel, usually consisting of an alloy body with two sets of bearings through which the axle is threaded (a quick-release skewer runs through the axle to secure the wheel to the frame or fork). Rear hubs also feature a freewheel mechanism onto which the cassette fits, and which incorporates the ratchet into the hub body.
Good-quality bearings, sealed and protected from dirt and water, are the key to long and healthy hubs. Older or less expensive hubs may use cup and cone bearings, which contain two rings of loose ball bearings that sit in ‘cups’ inside the hub body, and are secured in place by conical nuts known as ‘cones’. The balls can rotate freely between the cup and cone with each unit making up one bearing.
Cup and cone bearings have the advantage of being easily serviced at home – new bearings and a dollop of grease once in a while are all you need to keep then running smoothly – but they can be tricky to adjust perfectly. Many newer hubs use cartridge bearings, where the steel balls or needles rotate in a single cartridge unit that can be easily removed for servicing, or replaced once worn out.
Many riders prefer the convenience of cartridge bearings – no more tedious micro-adjusting or hunting for stray ball bearings mid-service – but they are more expensive to replace.
Road wheelsets being generally subject to less punishment than their MTB cousins often tend towards the minimalist in order to shed weight. You may find road hubs designed for fewer spokes (24h as opposed to 32 or 36h) and for straight-pull spokes, which shave vital grams by being shorter than the j-shaped standard spoke. If building a wheel with a hub like this at the heart, ensure that your rims are compatible.
Manufacturers of high-end wheelsets aimed at competitive riders have also in recent years taken advantage of developments in materials technology to offer carbon fibre hubs, superlight axles and lighter-weight, long-lasting ceramic bearings in place of steel balls. These improvements offer weight savings and corresponding performance advantages under the right rider, but come at a price.
BMX hubs consist of an alloy housing containing two or more sets of bearings through which the axle is threaded, and secured to the frame and fork dropouts via bolts at either end. Good-quality bearings, sealed and protected from dirt and water, are the key to long and healthy hubs. Most high-end BMX hubs use cartridge bearings, where the steel balls or needles rotate in a single cartridge unit that can be easily removed for servicing, or replaced once worn out.
The standard BMX axle size is 14mm (especially for rear axles), although some race wheelsets or front wheels may use a 3/8” (10mm) axle that is lighter, but more prone to bending. Your axle choice will be determined by the size of the dropouts in your frame and forks, so check before you buy or upgrade.
An important element of your rear BMX hub is the driver or freewheel, which is turned by the chain and translates your pedal stroke into movement.
BMX hubs can be further divided into three types depending on the driver/freewheel type used: cassette, freewheel and freecoaster.
• Cassette hubs – These use an internal, independent driver that presses into the hub shell, and can run sprockets as small as 8 teeth, offering a weight saving owing to less materials
• Freewheel hubs – The use an external freewheel that threads on to the outside of the hub shell, requiring the use of larger-circumference bearings and freewheel. Because of this, the smallest gear size available with a freewheel is 13t, necessitating a larger front chainwheel in order to achieve optimum gearing, and adding weight.
• Freecoaster hubs – These use an internal clutch system to enable a bike to roll backwards without the need to back pedal at the same time.
Other BMX hub variations include the option of switching between left-hand and right-hand drive (as per rider preference), or ‘flip-flop’ hubs which carry different-sized freewheels on each side, and can be ‘flipped’ to offer a choice of gear ratios (e.g. 15t on one side, 16t on the other).
Keep your front and rear hubs running smoothly with our selection of spare parts including replacement bearings, freewheel bodies, sprockets, ratchets, pawls, springs, axles, conversion kits and more.