What is a bicycle cassette?
A bicycle cassette is the cluster of sprockets located on the rear hub of your bike, slotting onto a freehub body and held firmly in place with a threaded cassette lockring. A typical cassette can have anything between five and 12 sprockets, although most modern bicycle drivetrains use either 9, 10 or 11.
Why are cassettes important?
Cassettes provide a range of gearing options for your chain to run on. The range of gear ratios allows you to vary your pedalling cadence (revolutions per minute) to achieve optimum efficiency.
Running your chain on one of the larger sprockets - those with more teeth - will provide an 'easier' gear. This reduces the torque required to turn the back wheel - perfect for climbing hills - but it also means you have to complete more pedal revolutions to cover the same amount of ground.
Running your chain on a sprocket with a lower number of teeth maximises the torque, meaning less frequent but harder pedalling is required. This will allow you to keep adding power through your drivetrain without 'spinning out' (pedalling at an uncomfortably high number of revolutions) on a downhill section or sprint.
A good range of gears on your cassette, therefore, allows you to select the optimal level of torque versus pedal speed to transmit your total effort to the wheels in the most effective way.
How do you choose the right cassette for your bike?
The choice of a cassettes can appear overwhelming at first glance. There are different combinations of sprockets to suit different tastes, terrains, and disciplines; for example, there is a significant difference cassettes for triathlon bikes compared to a mountain bikes.
The main thing to consider is the spread of gears on the cassette. The smaller the difference between the highest and lowest number of teeth, the smaller the jump between gears; facilitating a smoother gear change. However, having closer-geared sprockets will normally decrease the size of the largest sprocket on the cassette, leaving you with a gear ratio that may be less suited to climbing and tough terrain.
Mountain bike cassettes
Mountain bike cassettes have a larger range of sprocket sizes due to the wide variety of gradients encountered on an off-road trail. Riding a flat forest track and then hitting a steep technical climb requires a major jump in gears. To accommodate this, the sprocket sizes on mountain bike cassettes require bigger gaps, which means sacrificing some of the smooth, tight shifting enjoyed by road versions.
The creation of 10, 11, and now even 12 speed cassettes was a significant development for mountain biking. The larger number of sprockets means the biggest gear can have a huge number of teeth - providing easier gears for impossible climbs - while reducing the scale of the jump between each gear. This evolution allowed mountain bikers to do away with their triple chainsets, including the small get-out-of-jail chainring, often derided as "the granny ring". Instead, mountain bikers can run double or even single chainsets, reducing weight, clutter, and the frequency of mechanical problems.
Now, 10 speed mountain bike cassettes come in sprocket ranges such as 11-32, 11-34 and 11-36. Wiggle's best selling 10 speed mountain bike cassette is the Shimano Dyna-Sys Deore XT 10 Speed Cassette
Meanwhile, 11 speed mountain bike cassettes come in even larger sprocket ranges, providing even greater gear ratio choice, such as 11-40 and even 10-42! Wiggle's best-selling 11 speed mountain bike cassette is the Shimano Deore XT M8000 Cassette (11-42)
Road bikes cassettes
The largest sprocket on a road bike cassette is generally smaller than those on mountain bikes, providing smaller jumps between gears. Most road bike cassettes have an 11, 12, or 13 tooth smallest sprocket, then between 21 and 32 teeth on the largest sprocket.
The vast majority of road bikes come with a 12-25 cassette, which is suitable for most cycling terrain when paired with a compact or standard chainset.
If you ride a lot of hills or struggle with hill climbing, a cassette with a lower ratio largest sprocket (one with more teeth, such as 27 or more) may be beneficial. It will allow you to keep spinning for longer, rather than grinding.
When selecting a cassette for your road bike, ensure your derailleur can accommodate the largest sprocket. A longer cage rear derailleur is needed for larger sprockets because more chain is required to go around the greater number of teeth. Use a small cage derailleur with a large sprocket cassette and you'll risk over-stretching the derailleur. You may also see a lot of slack chain when riding in the smaller sprockets on the cassette.
Compatibility between different cassettes
It is possible to run a cassette from a different brand than the rest of your drivetrain, such as a SRAM cassette with a Shimano derailleur.
However, this is not always the case. You should note the following cross-compatibility of cassette options:
- SRAM and Shimano cassettes, on either road or mountain bike, are interchangeable with each other as the spacing is the same between the sprockets.
- Campagnolo road cassettes will only work with Campagnolo drivetrains.
A freewheel threads directly onto the hub shell rather than onto a freehub body. The ratchet system is built into the freewheel mechanism. Freewheels are becoming less common on bikes and are now usually only found on entry level cycles with 6, 7, or 8 speed groupsets. The choice is not vast and only a few brands produce freewheels, such as the Shimano MF TZ21 7 Speed Freewheel.
How do I fit my new cassette?
Once you've found the perfect cassette for your bike, how do you fit it? You'll be glad to know the process is relatively simple.
You'll need a chain whip, a Shimano or Campagnolo specific removal tool, an adjustable spanner, and maybe some gloves.
Similar to the cross-compatibility above, a Shimano removal tool will work with a Sram cassette.
After you've removed your rear wheel, brace the cassette using the chain whip, attach the removal tool, and remove the lockring with the adjustable spanner. Slide off the old cassette and replace, then reattach the lockring using the chain whip and spanner.
The below video from the team at Pro-Lite makes it look easy.