Mountain Bike Wheels: What to Know
New to mountain bikes? Consider this.
According to Bike Exchange:
The wheels on your mountain bike literally keep you moving and will have a considerable impact on the quality of your ride. Sure the drivetrain will push you along, and the frame will also have a significant impact on ride quality, but the wheels are the first element of your mountain bike to feel and translate changes in terrain and surface imperfections.
One of the most important traits of a quality wheelset is its role in transferring your efforts on the pedals to an outcome on the trail. Matching the right wheels for your riding style, terrain, and preferred riding discipline is important, as is having a reliable set that still offers sound levels of performance.
So before you spend your hard earned money upgrading the rolling stock on your ride, have a read of the buyer’s guide below for everything you need to know about mountain bike wheels.
Anatomy of a wheel
A bicycle wheel consists of four main components, all of which have an influence on weight, performance, and durability. Upgrading (or downgrading) these components can affect the ride quality, your effort output (speed), braking performance and even how well your bike responds to the terrain, so it’s worth knowing a little about each, and how that relates to improved performance on the road.
Rim: The wheel’s rim sits on the outside of the wheel and on the vast majority of mountain bike wheels, has one main function; to hold the tyre. The width of a rim will affect the tyre width which could have a significant impact on rider control and grip on the trail, more on that later.
Hub: The hub sits at the centre of a wheel and provides the axis of rotation. Within each hub, front and back, is the axle which attaches the wheel to the bike. On the rear wheel, the hub features splines which a cassette attaches to. The bike’s chain wraps around the sprockets of the cassette, and in association with the crankset and shifters, forms the bike’s drivetrain which propels it forward. As well as propelling the bike forward, all mountain bike wheels will use a rear hub with a ‘freehub’ mechanism which allows the bike to coast.
Spokes: Material that connects the hub and rim. The number of spokes will vary between front and rear wheels, the rear often featuring more spokes to improve strength and stiffness. More spokes typically mean a stronger wheel but that comes with a weight penalty. Most commonly spokes on mountain bike wheels are made of steel wire but can vary greatly in shape and diameter.
Nipples: Spokes attach to the wheel via a special nut called a ‘nipple’. Nipples are important for adjusting spoke tension which will ‘true’ a wheel, making it straight when spun.
Most entry to intermediate level wheels will feature aluminium rims of varying quality, while high-level wheels will typically feature rims made of carbon fiber which reduces the weight while increasing stiffness.
Aluminium, commonly referred to as alloy, is the more popular choice of rim material used on entry to mid-level mountain bike wheels. This is largely on the back of how much cheaper it is to manufacture. Whilst it is the cheaper alternative, there are still a host of benefits that come with using the material, the main of which being compliance, or comfort that alloy wheels provide out on the trail.
On the flip side, carbon fiber wheelsets are beginning to increase in popularity across the spectrum of mountain bike riding disciplines. This is largely thanks to the falling prices and the increased stiffness to weight ratio, and strength, on offer. This means a wheel can be made super stiff, whilst also being lightweight, particularly helpful given the nature of mountain biking being tough on components. The drawback of this increased stiffness is that typically comes at the expense of comfort, requiring riders to alter tire and suspension settings to suit.
It’s worth knowing the two different tire types that fit onto a wheel as they require a specific rim. Mountain bike tires will either be ‘clincher’ or ‘tubeless’ and the wheel will specify which tire it is compatible with.
Frequently found on entry level bikes, Unless stated, it’s a fair assumption that any new entry-level bike bought will feature clincher tires. Clincher tires require a tube to inflate and hold air, while the tire will feature either a steel or kevlar fiber bead on its edge to hold it in the rim.
As the name suggests, tubeless tires require no inner tube, instead relying on tight tolerances and air pressure to create an airtight seal. It’s a technology very similar to that found in modern cars and motorbikes. Tubeless tires have long been used in mountain biking and its safe to assume that most complete mountain bikes are shipped with tubeless ready wheels and tires fitted. If looking to upgrade your wheels, we strongly recommended seeking wheels with tubeless, or tubeless-ready rims, which will allow you to take advantage of lower tire pressures, lower weights and lower rolling resistance. The lower tire pressure will provide more control, traction, and comfort, while the lack of tube means punctures will rarely be of concern.
The weight of a wheelset can have a number effects on how your bike feels out on the trail, namely the handling, strength, and comfort of your ride. “Cheap, light or strong – Pick two…” is a famous quote by Industry icon Keith Bontrager, when responding to criticism over the cost of his wheelsets, a saying that continues to hold weight to this day.
Given there are a number of disciplines that fall under the mountain biking umbrella, different ride style will require wheels with different strengths and weaknesses. For racers, or riders frequently scampering uphill, investing in a lightweight wheelset will surely make life easier, whereas enduro and downhill riders will likely see more benefit in a heavier, stronger wheelset. Regardless of your preference of riding style, it must be noted that shedding weight often comes at the cost of overall strength and durability, two items that are foolish to overlook out on the trail.
Wheel Dimensions: Size, Width, and Depth
A hugely topical subject amongst keen riders, there are three main sizes of mountain bike wheels that you will find on the majority of new mountain bikes, plus larger options to cater for specific bikes.
Wheels are referred to by numbers which indicate their size. 26in wheels were the original mountain bike wheel size and are still used sparingly today thanks to being nimble and light. The shift in recent years has been to larger wheels that offer more traction, greater roll-over ability, and a better ride quality, meaning 26in wheels are rarely seen on new mountain bikes.
27.5in, also referred to as ‘650B’ wheels have essentially replaced 26in wheels as the norm in recent years, offering slightly improved roll-over ability, traction and air volume than 26in wheels. 27.5in wheels are lighter, stronger and typically more nimble than 29in wheels so depending on the type of riding you do, these might be a good option.
29in wheels, also called ’29ers’, provide more traction, greater roll-over ability on technical obstacles and a smoother ride thanks to their increased size and air volume. 29er wheels have become the most popular choice in many mountain bike disciplines. Their increased stability also means they’re good on the descent. They do, however, weigh more than the smaller wheel sizes, can be cumbersome in small frame sizes and are slightly limited in the amount of suspension that can be incorporated around them. For this, 29er bikes are best used in cross country and trail-type riding, where generous suspension travel is usually not needed. Although, this is fast changing as many of the world’s best enduro and even downhill racers are making the switch to 29in wheels too.
The trend is for modern rims to be wider, resulting in improved comfort through greater tire air volume. This coincides with the shift to larger tires that are said to improve roll-over ability, traction and control by being able to run at a lower pressure.
Rim width can either be measured internally or externally, which potentially provides some confusing numbers. Traditionally, 17mm was a popular internal width for mountain bike wheelsets, however, more recent times have seen a shift to wider rims thanks to the increases offered to tire stability, footprint and air volume. By current standards, a narrow rim when measured internally is anything under 19mm, a standard modern internal rim width is typically between 21-25mm, while a wide rim is considered anything greater than 28mm.
While closely interrelated, the external rim width will mostly influence the aerodynamics of the wheel, while the internal rim width will influence comfort, rolling efficiency or tire shape.
The depth of the rim will affect the lateral stiffness of the wheel as well as the handling of a bike. Unlike their road-going cousins, rim depth isn’t as much of a sticking point due to the reduced emphasis on the aerodynamic benefits that a deeper wheelset may provide. The majority of wheelset manufactures will typically top out at 30-35mm in depth, however, there are some manufacturers offer increased depths where the material used benefits the strength of a wheelset.
The total number, shape, and material of the spokes on a wheel will vary. High spoke counts (having a lot of spokes) increase the robustness and durability but come with a weight penalty. Spokes come in a variety of materials, including steel, aluminium, carbon fiber and titanium, however, Steel spokes are by far the most common.
The spoke count of the front and rear wheels will vary, rear wheels often have more spokes as more forces are applied (drive forces and additional weight load). Typically a lightweight front wheel will have between 24-28 spokes, while the rear wheel will have between 28-32 spokes. As materials and manufacturing processes have improved, particularly in cross country (XC) racing, spokes counts have gone down, reducing weight.
Normal steel spokes are either ‘straight pull’ or ‘J-bend’. ‘Straight pull’ spokes have no bend at the head and require specific hubs, ‘J-bend’ spokes are the more traditional option, featuring a 90-degree bend at the hub end which looks like a ‘J’.
Benefits of the straight pull spokes are that the stress riser of the J-bend is removed, therefore arguably leading to a wheel that’s more durable. The drawback of straight pull spokes is that many companies make proprietary spokes and hubs which means sourcing a replacement spoke can be time-consuming and potentially expensive. Whereas J-bend spokes are often easier to replace and perform close to straight pull spokes in terms of quality and strength.
Butting or ‘butted’ is a term you’ll come across if looking at round spokes. Simply put, butting is the process of varying the thickness, and so a double butted spoke would offer two different diameters along its length. Butted spokes are typically stronger and more durable than ‘straight gauge (one diameter) spokes as they help dissipate stress fatigue better.
Available in either straight pull or J-bend, a flat spoke, often referred to as a bladed spoke, will hold the best mechanical strength to weight qualities. The flat surface allows you to hold the spoke to stop it twisting at high tensions. These spokes are commonly found on high-end wheels.
The hubs are a critical component of any wheel. They perform the task of providing an anchoring point for the spokes, are an anchor point for the wheels attach to the bike and allow the wheels to spin. Where front wheel hubs are typically pretty basic, rear hubs are more complex and feature a freehub. The freehub is found performs two functions: to drive the rear wheel and allow it to coast.
Typically speaking, modern mountain bike rear hubs will feature a width of either 135mm, 142mm or 148mm. 142mm is the standard on many mountain bikes across a range of riding disciplines, however, 148mm (aka Boost spacing) is increasing in popularity. This is largely due to the fact that the increased hub width allows the use of a wider tire and/or shorter chainstays than a regular frame would typically allow.
SRAM vs Shimano: The Differences
SRAM offers a mix of both 11 and 12-speed groupsets in its line-up which require a specific freehub (XD Driver Body) on the rear hub to use the accompanying 10-42 or 10-50T cassettes. Simply put, an XD Driver allows the use of a cog smaller than a 11T on a cassette – the smallest that common Shimano-type freehubs can fit.
All mountain bike cassettes that feature a 11T smallest cog, whether they be eight, nine, ten or eleven speed fit on to the standard Shimano-style splined freehub body. It’s worth noting though that currently Shimano is the only manufacturer to NOT offer XD Driver Body compatibility on its wheelsets, meaning it’s best to avoid Shimano hubs/wheels if you’re planning to run a 1x SRAM groupset.
Six Bolt vs Centre Lock
The two disc brake rotor designs themselves are largely the same and perform the same function as each other, however, it is how they are mounted to the hub that differentiates the two designs.
As per their name, Six Bolt rotors attach to the hub using six bolts whereas the Center Lock rotors attach directly to a spline on the hub and are secured using a special lockring. Whilst Center Lock hubs were initially introduced into the market by industry leader Shimano, they are now more widely accepted, with many brands of wheels (and brakes) available in the design. it’s also possible to use Six Bolt rotors with Center Lock hubs with use of an adapter.
The Six Bolt rotors typically use a T25 Torx wrench to attach them to the hub. When it comes to potential performance benefits between the two differing styles, Center Lock rotors are easier and faster to remove and are centered more accurately on the hub. This is as a result of not having to worry about evenly torquing the six bolts when installing the rotor.
When shopping for new wheels, you’ll likely come across some references to engagement speed. Engagement speed refers to how quickly the rear freehub’s ratchet mechanism activities when you start pedaling after coasting. Engagement is typically referred to in degrees and the general rule of thumb is that the more points of contact in the hub, the faster the engagement. Typically 16 points of engagement are the absolute worst case scenario, offering 22 degree’s of pedal rotation before the rear wheel engages, while some high-end hubs achieve 72 + points of engagement resulting in around 5 degrees or less for more instant feedback when stomping on the pedals.
Lovers of freehub buzz will likely appreciate a hub with more engagement, conversely, if peace and quiet on the trail is more your style, this is something worth considering. It’s also worth noting that faster hubs will also typically require more frequent maintenance as the finer ratchet teeth can’t be as heavily packed with grease, requiring a lower viscosity of lubricant.
As the cost of the wheelset goes up so does the quality of components used. In the case of bearings inside the hub, they typically go from low quality steel to hardened steel, and then to ceramic. A good ceramic bearing is rounder, smoother and harder than an equivalent steel bearing, therefore reducing friction and improving performance as a result. However, a good steel bearing will typically outlast and outperform a cheap ceramic bearing.
As well as the material of the bearings, proper lubrication will influence how they roll and the amount of friction that is produced. Friction in the bearings reduces performance and slows the wheels down. Excessive friction occurs if the bearings aren’t appropriately lubricated if debris or other substances get into the bearings, or if the bearings are flushed by high-pressure water courtesy of a pressure washer. Higher quality hubs offer improved sealing from the elements, which keeps them rolling smoother, for longer.
Bearings are either cartridge (sealed) or loose ball (cup and cone). The cartridge or sealed system features an inner and outer race, with the bearings sitting in between them, all enclosed within a single unit. The cartridge bearing is then pressed into the hub shell with the axle going through the middle. A sealed bearing is a single unit, if it wears out, replacement of the whole bearing cartridge is required, but fairly cheap to do so.
Loose ball or cup and cone bearings are most commonly found on Shimano products and entry-level wheels. Cup and cone bearings have multiple pieces and loose bearings. They are not enclosed like cartridge bearings, instead, loose ball bearings are sandwiched between a fixed outer race (usually part of the hub shell), with an adjustable cone-shaped inner race threaded onto the axle. If the bearings are worn too far, they can wear the hub surfaces causing enough damage to require complete replacement of the hub. On the positive side, they’re easy to maintain to prevent this from happening. To service such a hub, you’ll often need a couple of special thin spanners known as ‘cone wrenches’.
Axle standards are one area that can be cause for a bit of confusion as there is no industry standard. Simply put, thru axles are frame specific, not hub specific meaning you will have to choose your wheels based on your frame and fork’s compatibility. The table below summarises common axle hub standards and their uses.
HUB WIDTH AXLE DIAMETER TYPE / COMMON USE
100mm 9mm Quick Release (QR)
100mm 15mm QR15 – Cross Country (XC) and MTB
110mm 15mm Boost 15 – For Boost compatible forks, allows the use of stiffer wheel and wider tyres,
110mm 20mm Boost 20 – Oversized Thru Axles, popular on long travel bikes requiring additional stiffness
HUB WIDTH AXLE DIAMETER TYPE / COMMON USE
135mm 10mm Quick Release (QR)
135mm 10mm Oversized QR Axle – Works in standard open-dropout quick release frame, Increased strength and stiffness
142mm 12mm Standard Thru Axle – The most common thru-axle spacing found on non Boost compatible frames
148mm 12mm Boost Spacing – For Boost compatible frames, allows the use of wider tyres
150mm/157mm 12mm Most downhill bikes will use this wider hub spacing for a stronger wheel build
Proprietary and Non-Proprietary
Although technically many ‘factory built’ wheels are built by hand, the characteristics of each wheelset are very different.
Factory built wheels are mass produced to exact specifications and very often feature proprietary spoke and rim designs. They are designed to be bought off the shelf or paired with a manufacturer’s bike. Extensive research, development, and marketing mean these wheels dominate this space. Examples of these include wheels from Shimano, Mavic, DT Swiss and Bontrager.
Conversely, hand built wheels are unique, featuring individual hubs, spokes, nipples and rims that are made to order. Hand built wheels are custom creations to suit a riders exact preferences and needs. There are some off-the-shelf wheelsets that fall under the hand-built designation, for example, wheels from carbon-specialist Enve feature commonly sourced steel spokes, and a choice of hub prior to being built. A professional wheel builder would be able to replicate this same wheel by buying the individual parts from their respective sources.
‘True’ wheels: True refers to your wheel tracking in a straight line without deviation. If your wheel is out of true it may rub on your frame or negatively affect the bike’s handling. Adjusting spoke tension is a process to true your wheel that any local bike shop can do.
Clean your bearings: Your hub bearings may need to be cleaned out and repacked with grease on a semi-regular basis. The exact time frame will depend on the quality of your hubs, how much you ride and the conditions you ride in. Unless you are proficient with bike maintenance this is one job probably best left with your local bike shop, but be sure to mention it to them when you take it in for its regular service if you think it needs doing.
Inspect the rim: It’s worth routinely inspecting your rims for chips, scrapes, and dents (most common). Being vigilant and ensuring your rims are in primo condition before hitting the trail can often mean the difference between reducing the likelihood of you succumbing to rim failure out on the trail.