A quick guide to the mountain bike drivetrain

We take a look at the parts that make up the bicycle drivetrain

The humble drivetrain is greater than the sum of its parts, but what are those parts and what does each part perform? Here we simply break down the drivetrain for you as simply as possible, but if you're looking for something more in-depth, check out our guide to road bike groupsets, guide to mountain bike groupsets or our complete guide to rear derailleurs.

1. Shifters and derailleurs

Some bikes have a front and rear mech
Some bikes have a front and rear mech

The right-hand shifter actuates the rear derailleur, which moves the chain across the cassette to select different gears.

Some bikes also have a left-hand shifter and front mech, to move the chain between two or three chainrings on the cranks.

Rear mechs operate on the lower half of the chain, while front mechs shift the upper.

2. Cassette

Differect cassettes offer different gearing options and sizes
Differect cassettes offer different gearing options and sizes

The cassette is a replaceable cluster of cogs located on the rear wheel. The number of cogs your cassette has — multiplied by the number of chainrings you have if you have a double or triple setup — defines the number of 'speeds' your drivetrain has.

10- and 11-speed cassettes are the most common these days, with a typical 10-speed cassette having a range in the region of 11>36t, with and an 11-speed, 11>42/44t.

Cassettes mount onto something called a freehub. Shimano uses a simple splined affair that limits the smallest cog to 11t, while SRAM which requires a special freehub that can have a cog as small as 10t, allowing you to use a smaller chainring without sacrificing top-end speed. 

3. Mind the gaps!

The size of the gearing gap can affect shifting
The size of the gearing gap can affect shifting

Most cassettes are designed so that the change in gear ratio between sprockets (e.g. 11-13t, 32-36t) is as consistent as possible, usually around 16 percent.

Engineering a wider range without increasing the number of sprockets means bigger gearing gaps. This can make shifting clunky or leave you wanting an ‘in-between’ gear.

4. One ring to rule them all?

Should you move to one ring for simpler shifting?
Should you move to one ring for simpler shifting?

A 12 or 11-speed system (or a 10-speed with an expander cog) and one front chainring offers a spread of gears suitable for most riders.

That means less weight and maintenance (you can ditch the front derailleur, etc), and simpler shifting.

Shimano still offers 2x11 options but SRAM, with its dedicated 1x11 and 1x12 transmissions, has fully committed to the ‘one ring’.

This article was originally published in Mountain Biking UK magazine, available on Apple Newsstand and Zinio.

Seb Stott

Technical Writer, UK
Seb is a geeky technical writer for BikeRadar, as well as MBUK and What Mountain Bike magazines. Seb's background in experimental physics allows him to pick apart what's really going on with mountain bike components. Years of racing downhill, cross-country and enduro have honed a fast and aggressive riding style, so he can really put gear to the test on the trails, too.
  • Discipline: Mountain
  • Preferred Terrain: Steep!
  • Current Bikes: Focus Sam 3.0, Kona Process 111, Specialized Enduro 29 Elite
  • Dream Bike: Mondraker Crafty with Boost 29" wheels, a 160mm fork and offset bushings for maximum slackness.
  • Beer of Choice: Buckfast ('Bucky' for short)
  • Location: Bristol, UK

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