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Condensed Answer: Even long cage 105 Shimano derailleurs cannot “climb” a cassette with a 42T large cog. Consequently, it’s necessary to use a derailleur hanger extender when integrating an 11-42 cassette into a road drive train.

Derailleur Cage Length

The cage is the arm that goes from the body of the derailleur to the lower jockey wheel. It comes in three sizes – short, medium and long.

The length of the cage determines the cassette size that the derailleur can cover/climb. The longer the cage, the larger the rear cogs can be. That said, short cage derailleurs are used for small cassettes to ensure snappy shifting.

The table below contains the maximum cog capacity of Shimano 105 derailleurs depending on cage length:

Conclusion: 105 rear derailleurs do not have the capacity to operate with an 11-42 cassette by default. This isn’t surprising because the 105 group set is designed for road bikes. And road bikes come with higher gearing than MTBs.

Derailleur Hanger Extenders

Rear derailleurs attach to the frame via a derailleur hanger. The hanger is made of soft aluminum. The choice of material is strategic – during a fall, the hanger is supposed to break or bend first without transferring stress to the frame.

By attaching an extender to the derailleur hanger, one can effectively increase the length of the derailleur cage and make it possible to cover very large cassettes.

One of the most common derailleur hanger extenders is the Wolf Tooth link, but there are other possibilities too.

Possible Problems

The most common issue with a similar setup is inadequate chain tension when combining the small rear cog and the small chainring. Technically, this gear could easily be avoided, but it’s possible to shift into it by accident.

Another problem may occur when putting the chain on the largest rear cog and the large chainring. In some cases, this combo won’t even be possible due to the short chain necessary for adequate chain tension throughout the rest of the gears.

Chain Wrap Capacity

The problems above occur due to the low chain wrap capacity of the derailleur.

The function of the rear derailleur isn’t just to move the chain up and down the cassette. It also has to maintain adequate chain tension in each gear. To learn if a derailleur has adequate chain wrap capacity one needs the following data:

• The derailleur’s total capacity
• The needed chain wrap capacity

The formula for calculating the needed chain wrap capacity for a particular drivetrain is:

Chain Wrap Capacity = (Large Front Ring – Small Front Ring) + (Largest Rear Cog – Smallest Rear Cog)

If the bike has 50/34 cranks and an 11-42 cassette, the chain wrap capacity amounts to: (50-34) + (42-11) = 16 + 31 = 47.

None of the 105 rear derailleurs have that kind of chain wrap by default. Hence why the chain is too slack when riding in the small cog and the small front chainring or too short when riding in the large cog + large chainring. This issue can be mitigated via the alternative solutions below.

FAQ: Doesn’t a derailleur hanger extender increase the derailleur’s total capacity?

No. The hanger increases the max cog capacity but has no effect on the derailleur’s total capacity. Hence why the issues above will still manifest.

Alternative Solutions

If you want to install an MTB cassette on a road bike, there are two other paths that one can take:

Option 1: Large 8 or 9-speed MTB Cassette + MTB Derailleur

The rear shift ratio of 8/9-speed Shimano MTB derailleurs and 8/9/10 Shimano road derailleurs is the same – 1.7.

(Side note: The rear shift ratio is a term that describes how much a derailleur moves per 1mm of cable pulled or released by the shifter. If the rear shift ratio is 1.7, then the derailleur moves 1.7mm for every 1mm pulled or released by the shifter.)

Since the rear shift ratio of 8/9 MTB and road Shimano derailleur overlaps, one can integrate an 8/9 MTB derailleur in an 8/9/10 road system. The matching rear shift ratio ensures that the derailleur moves as much as needed per each click of the shifter.

Consequently, one can also use an 8/9 11-42 MTB cassette with an MTB derailleur that’s long enough to cover it.

The benefit of this approach is that you won’t need a hanger extender, and the derailleur will not be strained to its limits.

The downside is that 8/9 cassettes come with large jumps between the gears. This makes it harder to maintain optimal cadence (rotations of the cranks per 1 minute) and consequently reduces the rider’s efficiency. (High cadence such as 90 RPM is directly associated with higher average speed and efficient output.)

Another issue would be finding 8/9 speed cassettes of that size. The options are highly limited.

Option 2: Wolf Tooth’s Tanpan

Wold Tooth sells an adapter called Tanpan installed before the derailleur to adjust a road shifter’s cable pull so that it can work with an MTB derailleur.

The Tanpan is designed for 10/11 road shifters and 10/11 speed road derailleurs. Or in other words, with it, you can use a 10/11 MTB derailleur with a large cassette such as 11-42 on your road bike.

What are the advantages of running an 11-42 cassette on a road bike?

• Lower Gearing

The main incentive to install an 11-42 cassette is to acquire low gearing that isn’t characteristic of a standard road bike. People often do this when they plan to use their road bike for touring or even gravel.

Another benefit of low gearing is that it reduces the stress on the knee joints.

• 1x Drivetrain Friendly

A large cassette such as 11-42 is suitable for a 1x drivetrain (single chainring).

What are the disadvantages of installing an 11-42 cassette on a road bike?

• Slightly problematic installation

As already explained, the installation of an 11-42 cassette is not straightforward and requires the use of a derailleur hanger. Moreover, in some gears, the user can experience less than ideal-performance.

• Extra weight

Larger cassettes are heavier.

Summary: What You Need To Know

• The maximum rear cog capacity of 105 derailleurs is insufficient to combine them with an 11-42 cassette right away. Consequently, it’s necessary to use a derailleur hanger extender for the combination to work.

• This conversion works best with a medium or long cage 105 derailleur.

• The hanger extender increases the maximum cog capacity but doesn’t affect the total capacity of the derailleur. As a result, the chain is too slack when riding in the smallest cog and the smaller chainring. Also, the chain will be too short when combining the biggest cog with the largest chainring. Of course, those gears can easily be avoided.

• Another way to acquire the same gearing would be to use an 8/9-speed MTB derailleur with an 8/9-speed 11-42 MTB cassette.

• Wolf Tooth makes an adapter that adjusts the pull ratio of 10/11-speed brake shifters so that they can work with 10/11-speed MTB derailleurs. This combination will allow you to run a wide cassette too.