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Condensed answer:

8 and 9-speed derailleurs have the same rear shift ratio but only when both models are made either by Shimano or SRAM.

In consequence, a 9-speed derailleur can work just fine with an 8-speed cassette produced by the same brand.

If the bicycle uses index shifting, the shifter will have to be designed for 8 speeds too. If not, the shifting experience will be poor.

Compatibility Factors

The factors that determine whether a 9-speed derailleur can work with an 8-speed cassette are as follows:

1. Rear Shift Ratio

The term rear shift ratio a.k.a actuation describes the lateral movement of the derailleur in relation to the cable length pulled or released by the shifter with each click.

The shift ratio is expressed with a single number which signifies how much the rear derailleur moves for every millimeter of cable pulled or released by the shifter.

The rear shift ratio of Shimano 7,8 and 9 speed derailleur is 1.7.

This means that for every millimeter of cable movement triggered by the shifter, the derailleur moves 1.7mm.

Meanwhile, the rear shift ratio of SRAM 8 and 9 speed derailleurs is 1.1.

In both situations, the rear shift ratios of 8 and 9-speed derailleurs are matching. Or in other words, the derailleurs move equally after receiving a command from the shifter.

Therefore, it’s quite possible to make a 9-speed SRAM or Shimano derailleur work with an 8-speed cassette produced by the same brand.

However, this isn’t always the case for Campagnolo derailleurs. The old 9-speed Campagnolo derailleurs made before 2001 have a rear shift ratio of 1.4 which matches that of 8-speed derailleurs.

But the new post-2001 9-speed Campagnolo derailleurs have a rear shift ratio of 1.5. In consequence, it would be sub-optimal to use one of them with an 8-speed cassette.

FAQ: What happens when the rear shift ratios do not match?

The rear shift ratio determines how much a derailleur moves with each shift. In order for two derailleurs to be interchangeable, they must have the same rear shift ratio, or else the system won’t work optimally when using an index shifter.

For example, a 10-speed MTB Shimano derailleur has a rear shift ratio of 1.2. If you combine it with a 9-speed cassette and subsequently a 9-speed shifter, you will have problems because the derailleur isn’t programmed to move as much as needed per mm of cable pull or release.

When the shifter clicks up or down, the derailleur would end up at the wrong location and fail to guide the chain onto the desired cog.

2. 8-Speed Index Shifter

If the bicycle uses index shifting, then the shifter has to match the number of cogs on the cassette in order for the system to operate as smoothly as possible. In this particular situation, an 8-speed shifter is required.

A 9-speed shifter cannot function properly with an 8-speed cassette because 8 and 9-speed cassettes have different spacing.

The 9-speed cassette has the cogs closer to each other because one extra gear is fitted within the same space. In consequence, the cassette spacing is slightly narrower.

For that reason, 8-speed shifters have a longer cable pull and move the derailleur slightly more.

Therefore, if you try to use a 9-speed shifter with an 8-speed cassette, the shifter won’t be moving the derailleur sufficiently to ensure smooth shifting.

The first table showcases, the cable pull of Shimano, SRAM, and Campagnolo 8 and 9-speed shifters.

Cable Pull

The second table indicates the cog pitch of 8 and 9-speed cassettes.

Note: Cog pitch is the center to center spacing between adjacent cogs.

Cog Pitch

As you can see, in all cases, the 8-speed cassette has a larger cog pitch which necessitates a longer pull too.

Conclusion: If you want to pair a 9-speed derailleur with an 8-speed cassette, you need and 8-speed shifter to acquire shifting smoothness and efficiency. A 9-speed shifter won’t cut it.

3. The Derailleur Capacity Is Important Too

The derailleur has to be capable of encompassing the entire cassette. In most cases, 9-speed derailleurs have an equal or higher capacity than those designed for 8-speeds.

However, there are also 8-speed cassettes with an unusual range e.g., 11-42.

That range is usually found on cassettes with more speeds but some companies put it on 8 and 9-speed models too.

An ordinary 9-speed derailleur does not have the capacity to cover such a cassette without a derailleur hanger extender.

Friction Shifters = High Compatibility

Index shifters should always be paired with their corresponding cassette because they move the cable a predetermined length with every click. Or in other words, they’re locked in a pattern and cannot operate out of it.

There’s another option, however. It’s called friction shifting.

Friction shifters allow the mix of MTB and road bike parts which is why they’re so popular among touring cyclists. The bar-end version is the most used one.

Unlike indexed gearing, friction shifters are “unlocked” and move the derailleur as much as the rider wants. In consequence, the rear shift ratio of the derailleur isn’t nearly as important.

Are There Any Benefits To Using a 9-Speed Derailleur With an 8-Speed Cassette?

While it’s possible to use a 9-speed derailleur with an 8-speed cassette in the aforementioned situations, this combo will not result in improved performance even though 9-speed derailleurs are technically considered an upgrade.

Honestly, the only situation when this combination is appropriate is when you don’t have all the parts needed for an 8 or 9-speed drivetrain.

To make full use of a 9-speed rear derailleur, you will also need a 9-speed shifter and cassette.

Frequently Asked Questions

Can I use a 9-speed chain on an 8-speed cassette?

Most of the time, chains have +/- 1-speed tolerance. Thus, a 9-speed chain should work just fine with an 8-speed cassette.

The thickness of 9-speed and 8-speed cogs is the same. Consequently, the inner width of 9-speed and 8-speed chains is also identical.

The cassette spacing, however, is different. As already mentioned, 9-speed cassettes have a smaller cog pitch. To reflect that, 9-speed chains come with thinner side plates and shorter rivets. This property makes them inherently weaker than 8-speed chains.

The downside of using a chain designed for more speeds is that the shifting could be a bit slower because the chain is thinner and the front derailleur needs to travel ever so slightly more to reach it.

However, there’s a positive side too – a thinner chain should, at least theoretically, minimize cage rubbing.

Can I use a 10-speed derailleur with an 8-speed cassette?

A 10-speed derailleur can work with an 8-speed cassette only in one situation – if it’s a road model designed by Shimano and not part of the Tiagra 4700 series.

Why? Because the rear shift ratio of Shimano’s 10-speed road derailleurs is 1.7 and matches that of 8 and 9-speed Shimano derailleurs. The Tiagra 4700 series are an exception as they use a different rear shift ratio.

In all other cases, a 10-speed derailleur wouldn’t operate all that well with an 8-speed cassette, and the shifter that it requires.

Can I use a 9-speed derailleur on a 7-speed cassette?

The answer is yes because the rear shift ratio of 9-speed and 7-speed derailleurs is the same if you stay within the same brand.

Shimano 7-speed derailleurs have a rear shift ratio of 1.7 whereas that of SRAM 7-speed derailleurs is 1.1.

Therefore, a 9-speed derailleur should work fine with a 7-speed cassette if the shifter and the cassette match the brand of the derailleur.

Will a 9-speed derailleur work on a 10-speed cassette?

A 9-speed Shimano derailleur will work on a 10-speed cassette if the shifter is also made by Shimano and designed for road bikes.

However, a 9-speed MTB derailleur won’t be able to operate properly with a 10-speed MTB index shifter and subsequently with a 10-speed cassette.

For a more detailed answer to this question, consider reading this post.

Summary 

A 9-speed derailleur can work just fine with an 8-speed cassette if: 

1. The derailleur is made either by Shimano or SRAM. 

2. The brand of the cassette and shifter match that of the derailleur. 

3. The derailleur has to be capable of reaching the largest cog.

4. An 8-speed index shifter or a friction shifter will be needed.  

Key Takeaways  

The factors that determine whether a derailleur is compatible with a cassette are: 

1. Rear shift ratio 

The rear shift ratio of the replacement derailleur has to match that of the original derailleur. If the rear shift ratios are dissimilar, indexed gears won’t move smoothly. However, an old school friction shifter will operate just fine. 

2. Total capacity 

If the cassette has a massive range, the derailleur will have to reflect that.