What Is a Mountain Bike Drivetrain?

The mountain bike drivetrain is a cluster of parts that work together to get the bike moving forward. As your legs press into the pedals, they add pressure to the cranks (the shafts that connect the pedals to the front gear). This spins the front gear (called a chainring), pulling on the chain connected to the back gears (known as the cassette). The rear cassette is connected to the rear wheel via a freehub, which gives the pedaler momentum. Sounds pretty simple, right? Well, there are a lot of little pieces of technology working in unison to maximize efficiency, and we still haven’t even brought up the derailleurs that allow you to move through different gears. Before we shift things into high gear, let’s start with a quick breakdown of the components that make up the drivetrain and the two main manufacturers of these components, SRAM and Shimano. There is a third manufacturer, Campagnolo, but they make up a tiny percent of the mountain-biking components market in North America.

Drivetrain Components

Crankset

Mountain bike chainrings have seen some change lately, from having two or three rings—also known as 2x or 3x, which would be verbally expressed as “2 by” or “3 by”—to now only one front chainring (1x/“1 by”). This is mainly due to improvements in the rear cassette, which is the next component we will discuss.

Cassettes

Chains

Shifters

These are the controls located in the bike cockpit that enable you to change gears. With a 1x crankset, there will only be a single shifter for the rear cassette, which saves some space for all of the other gadgets that will come into play, like dropper post controls.

The technology behind the shifter has rapidly evolved throughout the years. Some of the newest shifters can now be controlled entirely by your thumb, allowing your index finger to remain on the brake. There are also grip shifters that change the gears as they’re rotated, but this system is quickly losing popularity. Higher-end shifters also allow for faster gear changes by offering multiple gear drops with one thumb push.

One of the hottest pieces of new technology is the electronic shifter or “controller.” First created by Shimano, their Di2 electronic drivetrain was impressive but still required wiring throughout the system. SRAM quickly responded with their own electronic shifter, the wireless AXS system. Both systems can be customized via their individual apps. While relatively expensive, some affordable options are sneaking into the market (check out the SRAM GX AXS upgrade package).

Derailleurs

How To Decide?

As you will see later in the article, for the most part, as the price tag of the groupsets increase, the weight will decrease. This is because the higher-end components are made of more expensive components and more refined, time-consuming manufacturing.   Durability is always a concern when berm bashing on your mountain bike. Again, generally speaking, durability improves with price, but it seems to plateau around the midrange options. The durability can also decline in the super high-end offerings where weight saving is more crucial than longevity.   SRAM and Shimano produce incredible drivetrains that are exceptional at all price points. The technology that was once only in the super high-end pieces of kit is slowly making its way down to the more affordable options, which is a win for everyone. The key when buying a drivetrain is finding one that you can afford. For most cyclists, saving roughly 500g of weight is not worth a $600 jump in price.

SRAM versus Shimano

Time for a bit of a history lesson. Shimano has now been in the cycling game for a century. The company was founded in 1921 by Shozaburo Shimano to produce freewheels for bicycles. Shimano didn’t make it into the US market until the ‘60s. In the 1980s, Shimano introduced indexed shifting, which is now the basis for all mechanical shifters on bikes to this day. They also created “Total Integration,” allowing riders to brake, shift gears, and steer without taking their hands off the handlebars. This game-changer is something we all take for granted now.

Notable Differences

• Shifters

  Both Shimano and SRAM have shifters that can downshift multiple gears in a single stroke. The main difference is that Shimano’s Rapid Fire trigger shifter features “2way-release,” allowing the upshift lever to move in both directions. This gives the rider the choice to upshift using either the thumb or index finger. Personally, I have this on my bike and rarely use the index-finger shift, but it is always nice to know that I have some options.

• Crank Arms

  Fabrication material is the main divergence here. Shimano does not use carbon fiber on their crank arms. What they are known for is HOLLOWTECH, a very durable and lightweight setup that employs hollow aluminum crank arms. SRAM, on the other hand, offers carbon fiber on higher-end models. Carbon fiber does have a higher strength-to-weight ratio, but the trade-off is overall durability.

• 1x Drivetrains

  • SRAM brought the 1x drivetrain back into the mainstream with the XX1 group. Single chainring drivetrains are now the most popular option in modern mountain biking for many reasons. Shimano resisted initially, sticking to the 2x drivetrain, which has a greater gear variety. 
  • SRAM conquered the reduced gear-range problem when they introduced their Eagle 1x drivetrains in 2016. These drivetrains offered a 500% gear range, eliminating the need for 2x front chainrings. 
  • Shimano now offers 12-speed cassettes with a 10-51t range in their top three drivetrain setups. SRAM has a 10-52t cassette, so they still sneak ahead with a slight advantage over the Shimano updates. 

• Electronic Drivetrains

  • Shimano introduced the first electronic drivetrain—the Di2 system—in 2009 (for road bikes only). They now offer it for all of their bike ranges. The Di2 electronic drivetrain is still wired. The single rechargeable battery the Di2 system uses has 1,000-2,000km (621-1,242mi) of battery life. 
  • SRAM released the eTap Red system in 2015 for their road bikes. The AXS system for mountain bikes was introduced in 2019. The AXS drivetrain is wireless and very easy to set up. The downside is that you need separate batteries for each derailleur. These batteries have a life of about 20 hours, so make sure you charge them before every ride! 

• • Freehubs

    • While not officially included in a groupset, it is vital to know which freehub you have on your bike before switching to a new groupset or cassette. For the most part, Shimano created the “standard” freehub called Hyperglide way back in the 1990s. Hyperglide (HG) has become an industry standard for every 8-10 speed Shimano or SRAM cassette; Shimano 11-speed cassettes will also work on an HG freehub. 
    • With the advent of larger rear cassettes, there’s now a need for different freehub setups. To fit SRAM’s GX Eagle and above drivetrains, you’ll need SRAM’s proprietary XD freehub body (10-50t/52t). This freehub is available on many higher-end wheels and is something to note before making a big switch and revamping your gruppo.
    • Shimano also had to create a new freehub to fit the larger cassette sizes. Their new freehub Micro Spline uses a 23-spline interface and will work with a selection of their 11- and 12-speed cassettes. It is essential to know what is on your bike and what the cassette you’re upgrading to will require. 

The Bottom Line

Shimano Groupsets

Starting from least expensive to most expensive:

• • Shimano Altus
  • Shimano Acera
  • Shimano Alivio
  • Shimano Deore
  • Shimano SLX
  • Shimano Deore XT
  • Shimano XTR

Shimano Altus

The Altus is the groupset that you will find on entry-level mountain bikes. With a 2x or 3x crankset (40/30/22t) and a 9-speed cassette, this is a low-cost, low-technology build. The derailleur doesn’t use Shimano’s Shadow Plus clutch but uses similar technology to reduce the chance of damaging the derailleur on the trail. The whole setup is mainly made of steel. It does not even begin to consider weight-saving technology or corrosion resistance.

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Shimano Acera

An Altus step up, the Shimano Acera is built to handle the “rigors of teenage trail riding and continue to perform with minimal maintenance.” Odd to point out the teenage demographic so specifically, but it is a burly gruppo that will work well under harsh conditions and won’t break the bank if something happens.   The Acera group is lighter because it uses a hollow axle compared to the solid Altus axle. There is also a greater variety of materials used for this group compared to the Altus. The shifter is the same, coming with Shimano’s Rapidfire Plus, which offers multiple gear shifts in a single motion.

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Shimano Alivio

A final step up in the hierarchy before getting into the “real deal” drivetrains is Shimano’s Alivio. Even though the Alivio still sports a 9-speed 2x or 3x drivetrain, it is lighter and shifts better than the Acera or Altus. The steel cassette options range from 11-28t to 11-36t and use Shimano’s Hyperglide technology. This tech helps with the tooth positioning on the cog to make shifting smoother and more efficient.

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Shimano Deore

• Deore M6000

  This is where Shimano drivetrains start to get serious, and we see these groups as stock parts on pre-made bikes. The Deore M6000 is only available in 2x or 3x groups, but the cranks get Shimano’s Hollowtech II technology, providing lighter components compared to the previous three. The bottom bracket is integrated with the right crank arm providing more stiffness, weight saving, and more efficient power transfer. The cassette is all steel and is a 10-speed with either 10-36t or 11-42t, and it comes with a rear derailleur clutch that can be turned on or off.

• Deore M6100

  12-speed performance makes its way into the more economical drivetrains with the Deore M6100 series. The Deore 12-speed is a 1x only groupset but is also available in 2×11 or 2×10-speed. The front chainring is directly mounted to the crank and is available with 30t or 32t. The 12-speed cassette has 10-51t cogs made entirely of steel, not alloy, but does get the Hyperglide+ technology for smoother shifting. The shifters come with Rapidfire Plus and 2way-release; these are technologies that are found on the more expensive pieces of kit as well. Both are great examples of the trickle-down technology that more budget-friendly bike components receive after the work goes into their more expensive counterparts.

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Shimano SLX

The Shimano SLX group is most commonly found on reasonably priced mountain bikes. You will save a little weight from the Deore groupset with the alloy cassette. The SLX can be purchased as a 1×12 or 2×12 drivetrain. The 2×12 options are a 36-26t chainring with either a 12-speed 10-45t cassette or a 12-speed 10-51t cassette. The 1×12 uses the 10-51t cassette, and options for the front chainring range from 28-34t. The addition of the 1×12 cassette has also introduced the need for Shimano’s Micro Spline freehub, so it will no longer fit on the fairly standard Shimano Hyperglide freehub.   The cranks are directly mounted to the chainring and are hollow-forged with HOLLOWTECH II. The chainring uses Shimano’s Dynamic Chain Engagement for chain retention to deliver smoother pedaling along uneven terrain. The shifters on the SLX come with I-SPEC EV, enabling the rider to join the shifters and brakes on one clamp for a cleaner cockpit and dial in your riding setup. Shimano has done lots of work to offer a high level of customization to suit each rider’s preferences and terrain.

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Shimano Deore XT

The Deore XT is similar in setup to the SLX drivetrain but again with overall lighter weight. The Deore XT 32t chainring and crankset claim a weight of 620g while the SLX crank and chainring weigh 638g—already 18g of weight saving in a similar component. The technology is very much the same in the XT compared to the SLX, so you’re really just seeing the weight savings. The XT 10-51t cassette is 64g lighter than the SLX cassette. The 10-51t cassette does require Shimano’s Micro Spline freehub, so keep that in mind when making the upgrade or switch. Many riders have noticed a more refined feel to the shift in the Deore XT compared to the SLX. With the technology you get for the price, it is hard to argue with the value.   With the Deore XT, we also see the first compatibility with Shimano’s Di2 electronic groupset. The Di2 groupset is 11-speed only and uses the Deore XT 11-speed parts with the addition of specific shifters and the handlebar-mounted display.

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Shimano XTR

The XTR is Shimano’s top-of-the-line group for mountain bikes. It’s similar to the XT but with an added R (which stands for racing). Suffice it to say, you are looking at some serious kit. Shimano did everything possible to lighten the XTR groupset without compromising quality or performance. The cranks are two-piece alloy that are bonded together for a lighter weight. The chain is directional and has a slight curve to better follow the profile of the teeth.   The shifter comes with a non-slip rubber face and 20% quicker engagement, as claimed by Shimano. The double upshift is still possible, and with the two-way shifting, you can shift up with either your thumb or index finger. The lever itself is adjustable as well, allowing the rider to choose their preferred starting point.   The cassette uses the same ranges as the SLX and XT but features an alloy “beam spider” carrying 3 alloy, 5 titanium, and 4 chromoly sprockets—saving weight and adding a cool aesthetic to the rear cassette at the same time. If you are looking to get XTR’s 12-speed cassette, you will have to upgrade to Shimano’s Micro Spline freehub to accommodate the larger cassette. The front chainring is available in sizes from 30 to 38t for the single and 28/38t for the double. The cranks are interchangeable between the 2x or 1x chainrings, so you can switch without getting a new set of cranks. The XTR is also compatible with the Di2 but again, only in the 11-speed version.

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SRAM Groupsets

• • SRAM X3, X4, X5, X6, X7, X8 and X9
  • SRAM SX Eagle
  • SRAM NX, SRAM NX Eagle
  • SRAM GX, GX Eagle, GX Eagle AXS
  • SRAM X01, X01 Eagle, X01 Eagle AXS
  • SRAM XX1, XX1 Eagle, XX1 Eagle AXS

SRAM X3, X4, X5, X6, X7, X8 and X9

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SRAM SX Eagle

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SRAM NX, SRAM NX Eagle

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SRAM GX, GX Eagle, GX Eagle AXS

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SRAM X01, X01 Eagle, X01 Eagle AXS

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SRAM XX1, XX1 Eagle, XX1 Eagle AXS

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