Mountain Bike Gears Explained: The Ultimate Guide (2024)

The groupset comprising the essential mountain bike gears varies based on the type, such as cross-country, downhill, enduro, freeride, etc. Although every kind of mountain bike gear uses the same principle to function, the specifications of the variants have a distinct impact on riding.

The most common mountain bike gears used today are single cranksets with a chainring having up to 38 teeth and 7 to 12-speed cassettes, in which the largest sprocket or cog might have as many as 52 teeth. The drivetrain also includes bottom brackets, chains, derailleurs, shifters, etc.

Unlike road bikes, the versatility of mounting biking isn’t solely about individual preferences and riding skills. The exact nature of a chosen terrain demands appropriate mountain bike gear. In this article, I explain all the standard mountain bike gears and the variations you’ll most likely find.

Types of Mountain Bike Gears in a Groupset

Mountain bike gears are collectively known as a groupset, group, or gruppo. Any groupset has all the mountain bike gears of the drivetrain and the braking and shifting components. Here is a list of the key components that form a groupset of mountain bike gears:

Bottom bracket
Crankset
Cassette
Chain
Derailleur
Shifter
Brake

These components and gears are essential to all types of mountain bikes, such as:

Cross-country
Downhill
Enduro
Freeride
Trail
Fat

However, the total number of gears and their specifications (and thus, functions) are not identical for different mountain bikes. Moreover, the same type of mountain bike can have different specs for the gears on distinct models. Consider the bottom bracket, for example.

Bottom Bracket

The crankset is fitted to a mountain bike frame using a bottom bracket, which is an integral part of the drivetrain. There are different types of bottom brackets, and you can broadly classify all of them into two categories:

Internal
External

This distinction is based on the installation or mounting mechanism of a bottom bracket. You will find both internal and external threaded bottom brackets in mountain bikes. These two types of brackets use threaded internal bearings or external cups to mount the crankset.

You will find bottom brackets rated according to the ISO standards (International Organization for Standardization) or BSA (Birmingham Small Arms). BSA is also known as English threaded. Both ISO and BSA imply the same technical standards for bottom bracket specifications.

Internal Bottom Bracket

An ISO threaded internal bottom bracket uses bearings to mount the crankset. These bottom brackets must be compatible with the shell diameter and width of a mountain bike.

A standard internal bottom bracket with an integrated spindle needs a shell diameter of 34.92 millimeters (1.375 inches) with 24 threads per inch (TPI).

The most common internal bottom bracket shell widths for mountain bikes are:

73 millimeters (2.87 inches) for enduro, freeride, trail, etc.
83 millimeters (3.26 inches) for downhill mountain bikes
100 or 120 millimeters (3.9 or 4.72 inches) for fat bikes

External Bottom Bracket

An ISO threaded external bottom bracket uses cups to install the crankset. The bottom bracket shell’s inner diameter and width requirements are the same for internal and external threaded installations. Also, the threads per inch or TPI count is the same.

The main difference lies in the external cups or the bearings’ inner diameter. Here are a few examples of the internal diameters of bearings of cranksets for external bottom brackets:

Shimano Hollowtech II: 24 millimeters (0.94 inches)
SRAM GXP: 24 millimeters (0.86 or 0.94 inches)
Campagnolo Ultra-Torque: 25 millimeters (0.98 inches)

Some mountain bikes use cranksets with spindles requiring the threaded external cups to have an internal diameter of 30 millimeters (1.18 inches) for the bottom bracket.

Other Types of Bottom Brackets

All bottom brackets use internal or external threaded installation, except the Italian variant. The Italian bottom bracket may use either mounting mechanism depending on a model’s design.

The other types of bottom brackets are usually classified as per their size in millimeters, which is basically the shell width. So, you will find the following bottom bracket sizes for mountain bikes:

BB 73
BB 91.5
BB 92
BB 95
BB 104.5
BB 107
BB 121
BB 132

There are some bottom brackets, whether as a part of a groupset or standalone, with the same brand name for both road and mountain bikes. A fitting example is the Shimano Hollowtech II.

If you match the specs and get a Hollowtech II mountain bike bottom bracket, there shouldn’t be any issues. But if you get Hollowtech II road bike bottom brackets using external cups, you may need spacers to fit the mountain bike crankset. Shimano includes these spacers in the kits.

Crankset

Mountain bike manufacturers incorporate subtle elements in the designs of their cranksets. However, all such cranksets are broadly classified into 3 categories:

Single
Double
Triple

Single Crankset

A single crankset is also known as a one-by or 1x drivetrain. This type of crankset has only 1 chainring. Thus, the single crankset, 1x, or one-by drivetrain doesn’t have any dynamic role to play in a rider’s cadence or revolutions per minute and gear shift.

This single chainring in the crankset may have different numbers of teeth based on the type of mountain bike. A few typical teeth counts are:

Usually, a single crankset or chainring with 38 teeth is more common in cross-country mountain bikes. 26 or 28 teeth are more common in fat bikes. If you go for a downhill or enduro mountain bike, a single crankset is likely to have 30 or 32 teeth on its sole chainring.

Since there is only one chainring in a single crankset, mountain bikes using such a groupset do not require a front derailleur and shifter. Thus, the mountain bike is slightly lighter. Also, there are significantly fewer chances of the chain falling off and no possibility of cross-chaining.

The only instance you can have cross-chaining in a 1x or one-by drivetrain is if the chainring of a single crankset is aligned in the largest or smallest chain line of a double or triple crank setup.

You won’t have to worry about this kind of problem unless you’re switching from one type of crankset to another. A single crankset uses the nominal chain line, not the largest or smallest.

Double Crankset

Double cranksets have 2 chainrings, so they offer a larger range of gears than the 1x or one-by drivetrain. However, double cranksets are not as popular as the single or 1x drivetrains today.

The larger chainring in a double crankset may have 34 or 36 teeth. The smaller chainring might have 22 to 28 teeth. Like a single crankset, the double variant can be used with different types of cassettes, offering several gear ratios.

However, a double crankset has a greater gear range than a single chainring. On the flip side, there’s more potential overlap in a double crankset.

Single cranksets don’t require a front derailleur, so there isn’t a need for its shifter, either. But a mountain bike with a double crankset requires a front derailleur and shifter. These extra parts of the gear system or groupset add weight to the mountain bike and also pose some redundancy.

Triple Crankset

A triple crankset has 3 chainrings. Here’s the typical number of teeth per chainring in a mountain bike’s triple crankset:

Large chainring: 42 or 44 teeth
Medium chainring: 32 or 34 teeth
Small chainring: 22 or 24 teeth

Like a double crankset, the triple chainring setup needs a front derailleur and shifter. This type of groupset has the broadest gear range and the most dynamic shifts involving both the front and rear components of the drivetrain, i.e., crank or chainring and cassette or rear mech.

However, a triple crankset drivetrain doesn’t necessarily offer a lot more gear ratios because of substantial overlap. You can shift sprockets in the cassette for a specific gear ratio rather than switching the crankset’s chainring. Also, a triple crankset poses several practical problems.

A typical problem is cross-chaining. Riding a mountain bike on the largest chainring and gear or sprocket of the cassette stresses the chain. The angular movement of the chain will affect your riding experience and cause the chain to wear out much sooner than otherwise.

These concerns have led to the decline of the triple crankset’s popularity. Although you may find a triple crankset, it is quite rare in new mountain bike groupsets. Even the double crankset is not as popular today as it used to be a decade ago.

Chain

The chain used in any type of mountain bike isn’t fundamentally distinct from those on gravel, road, or touring bikes. However, the exact specifications of a chain depending on the associated parts of the groupset:

Cassette
Crankset
Derailleur

X-Speed Chain

A 12-speed mountain bike cannot use a chain that is cut for a cassette with 7 gears. Likewise, the type of crankset and thus chainring or the number of teeth on them determine the kind of chain you can use, which is the same as the influence of the teeth of a cassette’s sprockets.

Generally, mountain bike chains come in 3 types:

Regular
Half-link
Hollow pin

Chain Specifications

All cassettes and cranksets require specific types of chains, including the length and structural components. For instance, a mountain bike chain may have varying widths for its inner and outer links. However, this type of chain will work only on a 1x or single crankset drivetrain.

A one-by or 1x drivetrain doesn’t require the chain to shift from one chainring to another. So it is always going to find the right alternating tooth once the chain is fitted flawlessly. A 2x (double) or 3x (triple) crankset cannot use such chains because the width should be uniform for both links.

If the inner and outer links have varying widths, the chain may not land in the correct alternating tooth or sprocket when you shift the gear or front derailleur to switch from one chainring to another on a double or triple crankset drivetrain.

This fundamental difference is a reason for many 1x or single crankset drivetrains using tall and un-ramped teeth for their chainrings. The alternating widths of the teeth can work with a chain’s inner and outer links having identically varying dimensions.

Chain Dimensions

Apart from the links, the width of the chain itself is of critical importance. The cassette decides how wide or narrow a chain should be. The size or spacing of the sprockets on a cassette influences the maximum width of a mountain bike chain.

As a rule of thumb, you need narrower chains as you scale up the gears of a cassette. A 12-speed mountain bike needs a narrower chain than a cassette with only 7 sprockets.

The width of a mountain bike chain may vary from 4.9 to 7 millimeters (0.19 to 0.27 inches) as per the requirements of the groupset you have; more precisely, a brand’s cassette.

Other Chain Features

Some branded groupsets cannot use the same chain for both road and mountain bike cassettes or cranksets, even if the latter two have the same configuration. A 10 or 11-speed chain for a road bike may not suit a corresponding cassette for a mountain bike.

This issue is usually due to the demands of the configuration or specification and sometimes the design. You have to check the features or specs of a brand’s groupset, including the following:

Chain
Cassette
Crankset
Derailleur

Furthermore, mountain bike chains may or may not be directional. SRAM chains usually don’t have a forward and a reverse side. On the other hand, Shimano mountain bike chains often have an inner and an outer side, so they are directional. An example is Shimano’s HG-X chain.

The 10-speed HG-X chain has the branding and model name on the outside, which you can use as a reference to get the alignment or direction right during installation.

Cassette

Mountain bike cassettes are usually in the 7 to 12-speed range. Downhill mountain bikes tend to have fewer gears, so you are likely to find a 7-speed cassette. On the flip side, enduro mountain bikes have up to 12-speed cassettes. Most cassettes are compatible with select chains.

The number of sprockets or gears determines the speed range of a cassette. The full range of gears or speed of your mountain bike also depends on the crankset. A 2x crankset drivetrain with a 10-speed cassette gives you a range of 20 gears.

X-Speed Cassette

Downhill mountain bikes are generally used for descents, not ascending or uphill riding. Having more gears or a higher-speed cassette adds to the weight of a mountain bike without serving any purpose. So, a 7-speed cassette is usually sufficient.

In contrast, enduro or freeride and cross-country or trail mountain bikes require a greater gear range, so the cassettes have more sprockets for a rider to conveniently maintain the appropriate or desired cadence on every kind of off-road condition and sloping terrain.

However, enduro or freeride mountain bikes don’t necessarily have a 12-speed cassette. A 10 or 11-speed cassette may be sufficient for most cyclists.

Cassette Gearings

A mountain bike cassette’s sprockets have different gears or tooth counts. The smallest cog of a cassette may have only 10 teeth. The largest sprocket of an 11 or 12-speed cassette may have up to 52 teeth. On the other hand, a 7-speed cassette won’t have as many teeth on its largest sprocket.

The largest sprocket of a 7 or 8-speed cassette on a mountain bike may have 32 or 34 teeth, while the smallest may have 11. Moreover, the size of each sprocket isn’t identical for all cassettes or mountain bikes.

Furthermore, a particular cassette model or groupset may have variants. These variants might not have the same number of gearings. For instance, the Shimano Deore 10-speed cassette has 11 to 36 teeth. So the smallest and largest gearings on these 10 sprockets are 11 and 36.

But the Shimano SLX 10-speed cassette for mountain bikes has 3 sets of gearings:

11-32T
11-34T
11-36T

Larger sprockets with more teeth usually facilitate better acceleration, but the top speed is lower than smaller cassettes or rear mechs. Thus, you can choose the gearings suited to your riding skills and selected terrains.

Cassette Materials

Mountain bike cassettes are usually made of the following materials:

Steel
Aluminum
Titanium
Carbon fiber

The cassette is one of the bulkiest and heaviest mountain bike gears in the drivetrain, with a significant impact on weight. The most common material used for a mountain bike’s cassettes is steel, which is the most inexpensive but also the heaviest of all available options.

Aluminum and titanium are lighter materials with greater efficiency than steel, but both are not as durable as the latter. You will find many companies offering mountain bike cassettes with a few sprockets made of aluminum or titanium and the remaining being steel.

Generally, the larger gears of such cassettes are aluminum or titanium, and the smaller ones are made of steel. Since the smaller sprockets have fewer teeth, there’s less friction between the chain and the cassette. Additionally, the smaller steel sprockets add less weight to a mountain bike.

Derailleur

Derailleurs are essential mountain bike gears to switch the chain from one chainring (crankset) or sprocket (cog) to another. If you have a 1x or one-by drivetrain or a single crankset, the mountain bike won’t require a front derailleur. A rear derailleur is an indispensable tool, though.

Derailleurs aren’t universal for any type of bike, whether mountain or road. Also, the features or specs of a groupset shall determine the type of derailleur you need. Here are the distinguishing features of mountain bike rear derailleurs:

Cage: short, medium, long
Teeth: under 30 to over 40
Cable pull, e.g., 3.4 mm (0.13 inches)
Rear shift ratio, e.g., 1.2 (Shimano 10 MTB)
Mounting system: hook, standard, direct

Derailleur Cage

Both front and rear derailleurs can have a short or long cage. The cage determines how many teeth the largest and smallest sprockets and cranksets can have.

Consider the example of the Shimano Deore M6000 Rear Derailleur. This 10-speed derailleur is compatible with the Shimano HG-X chain. The SGS or long cage derailleur has these specs:

Total capacity: 43T
Large sprocket teeth (max): 36T
Large sprocket teeth (min): 32T
Crankset teeth difference (max): 22T
Small sprocket teeth (max): 11T
Small sprocket teeth (min): 11T
Pulley teeth: 11T
Maximum rear sprockets: 10

You cannot use this rear derailleur if the largest sprocket on your mountain bike’s 10-speed cassette has more than 36 teeth. Likewise, the difference between the largest and smallest crankset in a 2x or 3x drivetrain can’t be more than 22 teeth for a 10-speed 11-36T cassette.

Derailleur Mount

The Shimano Deore rear derailleur has a direct mounting system, which is an improved version of the standard installation. Standard mounts often use a derailleur hanger to screw the tool into the frame. A direct frame mount doesn’t use the b-link of standard installations.

If a frame isn’t compatible with a direct mount derailleur, you need to use the b-link of a standard mounting system. The hook mounting system is distinctly different. Such derailleurs have hooks fastened to a frame with bolts.

Rear Shift Ratio

The rear shift ratio is the displacement of the derailleur divided by the cable pull. Shimano’s rear derailleurs for mountain bikes with a 10-speed cassette have a rear shift ratio of 1.2:1. This ratio drops to 1.1:1 for 11-speed cassettes. However, these ratios are rarely accurate in the real world.

For instance, Shimano advertises a derailleur shift ratio of 2:1 in some cases when the actual effect of the cable pull on the displacement is around 1.7:1. The cable pull or derailleur shift ratio is another reason why this mountain bike gear isn’t an interchangeable or universal component.

Beyond the basic features, derailleurs may use electronic shifters. An electronic groupset uses a different kind of setup that doesn’t work with regular derailleurs and shifters, irrespective of all other factors, such as mount. This incompatibility applies to all mountain bikes and fat, road, etc.

Shifter

The shifter, also known as the shift lever, on a mountain bike changes the gear on the cassette or crankset through the corresponding derailleur. The conventional shift lever uses a cable with varying tensions to move the derailleur’s position, which then lifts and toggles the chain line.

The traditional shift lever is no longer the only option on mountain bikes today. Trigger shifters are available that you can use with either your thumb or index finger. There are throttle shifters that aren’t technically a simple lever. Also, there are electronic shifters with switches.

Electronic shifters don’t use the old cables to maneuver a derailleur and its pulley. Instead, small motors operate the derailleur to change the chain line and switch gears.

Right Shift Lever for Rear Gears

Generally, the right shift lever on a mountain bike is for the cassette or rear mech. This shifter is specifically made for the cassette and rear derailleur you have, which is another reason why groupsets are not universal in most cases.

Manufacturers have their take on the gear-shifting technology they use. Shimano has named its shift lever technology for mountain bikes Rapidfire Plus. Apart from the efficiency of shifters, some models have a few special features, such as the Shimano Deore SL-M6000 – R.

This 10-speed right shift lever for the rear mech or cassette allows riders to downshift 3 gears in only one stroke. The model also has an optical gear display feature unless any variants clearly state that they don’t.

While rear gear shifters are always mounted on the right side, you can choose to install one on the left if your mountain bike has only a single crankset.

Left Shift Lever for Front Gears

The left shift lever is usually for the crankset if your mountain bike has a 2x or 3x drivetrain. This shifter doesn’t manage as many gears as the right shift lever. However, the gear change with a stroke of the left shift lever is more significant than switching from one sprocket to the next.

Double or triple cranksets have a larger size and teeth difference among the two or three gears, respectively. So, one shift of the crankset makes your riding twice to thrice as harder or easier than toggling one gear level on the cassette.

Shimano’s Deore SL-M6000 – IL features the same Rapidfire Plus efficiency of the right shifter for the left shift lever. This model is for mountain bikes with 2 or 3 cranksets. The left shifter has a 2-way release feature enabling relief shifts with a push of the thumb or pull of the index finger.

Electronic Shifters

Electronic shifters are more accurate and efficient. However, they are costly, and you need compatible derailleurs to work with the electronic shifters. Plus, electronic shifters run on batteries, so those should be charged if you want to continue changing gears.

Electronic shifters require less physical maintenance than mechanical shift levers. Also, these high-end electronic shifters often use derailleurs with a clutch and can record data. You can find out how long or frequently you use a particular sprocket or gear combination.

Final Thoughts

Every type of mountain bike and its compatible groupsets have different gear specifications. If you aren’t changing an entire groupset, always check the compatibility of each kind of mountain bike gear before shortlisting the best options for replacements or upgrades.

Mountain Bike Gears Explained: Everything You Need To Know