Cassette Compatibility Tutorial

This post looks at replacing an 8-speed cassette to down gear your bike, however, the issues discussed are equally relevant to other speed drivetrains. Down gearing a bike can make cycling up hills easier. Buying a wider range cassette is a relatively inexpensive way to upgrade your bicycle.

If it’s hard to pedal, you’re in the wrong gear.

By the end of this article, you will be able to choose a compatible wide range cassette. If you want me to double check your conclusions, simply leave a comment with specifics.

The cassette’s brand is not a key consideration. Shimano and SRAM I suspect you are familiar with. However, SunRace may be an unfamiliar brand. I consider these three manufacturer produces durable and well-engineered cassettes. Furthermore, they have computer designed sprockets, allowing shifting to be efficient and quiet.

I could only afford an 8-speed Claris groupset on my Cannondale CAAD8. The CAAD8 came with a SunRace 12T – 28T 8-speed cassette and a 50/34T compact crankset. The remainder of this post will discuss compatible cassettes from Shimano because they are widely available and great value.

Know your drivetrain speed

8 sprockets making up a cassette defines an 8-speed cassette. Especially relevant is having 8-speed shifters to make sure of correct gear shifts. Always make sure your chain is the same speed as your cassette. Most chains labelled 5, 6, 7 or 8 speed with a pin width around 7.2 mm will work. But the chainrings and derailleurs have some flexibility. This Sheldon Brown article explains drivetrain speed compatibility.

Know your current 8-speed cassette range

A clean cassette should let you see the tooth number marked on each sprocket. The low gear is the biggest sprocket, nearest the spokes. The high or top gear, therefore, is the smallest sprocket. However, an alternative way to find out the number of teeth on your lowest and highest gears is just to carefully count them.

The sprockets on an 8-speed cassette are marked with their number of teeth
Markings showing the tooth count are on each sprocket i.e. 11T or 32T

Rear derailleur specifications

My CAAD8’s rear derailleur is the Claris 2400, which will happily accept an 11T to 13T highest gear. Almost all cassettes you’ll come across have an 11T top gear, therefore the larger the cassette range the lower your gearing.

Both the short cage (SS) and medium cage (GS) variants of the Claris 2400 can accept the same range of low sprockets (25T – 32T).

2016 Claris RD-2400 rear derailleur specification
2016 Claris RD-2400 specification

Finding derailleur specifications

It is useful for you to check with your derailleurs manufacturer to check the following four key specifications. Our article on Shimano component searches may be helpful.

Max. front difference

Your crankset’s difference in teeth between the smallest and largest chainrings is called the front difference. Cage length refers to the separation between the guide and tension pulleys of a rear derailleur. As a result, a longer cage accepts bigger front differences.

Keeping the chain under tension is one function of the rear derailleur. Inadequate chain tension and you risk chain slap on your frame and chain drops. However, having your chain too long can also cause these problems.

Total Capacity

The total capacity of a rear derailleur has to be equal or more than the sum of the cassette’s range (rear difference) and the crankset’s range (front difference).

The following worked examples explain the relevance of Total Capacity.

Can I fit an 11 – 32T cassette on my CAAD8?

I have a compact double crankset. The biggest chainring is 50T and the smallest chainring is 34T. This is a front difference of 16T.

If I have the Claris RD-2400-SS short cage derailleur then my total capacity is 37T. 37T minus 16T means I only have 21T left for my max rear difference.

If I buy an 11-32T cassette the range is 21T and therefore it is compatible.

Could I fit an 11-32T cassette on my CAAD8, if I had a triple crankset?

If I had a 50 / 39 / 30T triple crankset the front difference would be 20T.

I would need the medium cage Claris RD-2400-GS because it has the 20T max. front difference required by my crankset. In addition, remember the GS has a total capacity of 41T.

41T minus 20T means I only have 21T remaining for my rear difference. Since we know the 11-32T cassette’s range is 21T, again it is compatible.

Top sprocket range

Confirm your top sprocket is compatible. Here an 11T top sprocket is within the range 11 – 13T for the Claris RD-2400.

Low sprocket range

Confirm your low sprocket is compatible. Here any low sprocket from 25T – 32T will work with the Claris RD-2400.

Choosing between Shimano’s 8-speed  cassettes

My goal is finding an 8-speed cassette with the widest range, hence resulting in lowest possible gearing. My CAAD8 can accept an 11-32T cassette without changing my rear derailleur. Finally, should I go for a Claris HG50 8-speed cassette, perhaps an HG51, HG41 or HG31? Making this choice is discussed in the next post.

Choosing between Shimano 8-speed HG Cassettes

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Author: David Jackson

Hello, I'm a 40+ yo cyclist trying to get back into cycling after the Covid-19 pandemic. I have hardly been on a bicycle in the last 2 years.