Shimano Altus is an underrated group set. I have it on my hardtail, and it has operated just fine for 10-15000 kilometers. The only maintenance I’ve done is to occasionally clean up the gunk on the jockey wheels.
That’s why I am not surprised when people want to install it on entry-level road or gravel machines. It’s solid, strong and affordable.
The question is can you do that in the first place?
The short answer is yes.
The long answer is…longer.
Rear Derailleur and Shifters Compatibility
To understand the compatibility dependencies between a rear shifter and a rear derailleur, it’s necessary to become familiar with two values:
- Cable Pull
- Rear shift ratio
On an indexed drivetrain, each click of the shifter equals a shift up or down the cassette. А shift happens after the shifter has pulled (or released) a certain amount of gear cable. That amount is called cable pull.
The rear shift ratio is a property of the derailleur and shows how much the derailleur moves per 1mm of cable pulled or released by the shifter.
The cable pull in conjunction with the rear shift ratio results in predetermined movement of the derailleur that corresponds to the so-called cog pitch of a cassette. (The cog pitch is the center-to-center distance between two cogs on а cassette.)
The outcome is automatic shifting that requires only a click.
The values of the cable pull and the rear shift ratio depend on:
- Bike type
- Brand
- Year of production
- Number of speeds
If two derailleurs have the same rear shift ratio and the capacity to operate in a particular drivetrain, they are interchangeable regardless of their type (road, MTB…etc.) and the number of speeds that they were originally built for.
Shimano Altus is an MTB groupset serving 7,8 and 9-speed drivetrains. The rear shift ratio of Shimano’s 7,8 and 9-speed derailleurs is 1.7:1. In other words, the derailleur moves 1.7 millimeters per 1 millimeter of cable movement triggered by the shifter.
The derailleurs that match these criteria (have the same rear shift ratio) are 8 and 9-speed road/MTB models as well as 10-speed road models. (Yes, 10-speed Shimano road derailleurs also have a 1.7:1 rear shift ratio.)
Therefore, if you have a road bike equipped with a Shimano Sora (9-speed model) or Claris (8-speed model), you can replace it with a Shimano Altus model.
Technically, you can integrate an Altus derailleur in a 10-speed road drivetrain using Tiagra 4600 or even Ultegra 10-speed, but a similar move makes sense only if you have no other options (e.g., You’re touring in the middle of nowhere and your derailleur needs replacement after a crash.)
Tiagra 4600 and Ultegra are higher-end road group sets sitting above Sora and Claris (the road equivalents of Altus). Thus, it’s unlikely that someone owning a bike equipped with either groupsets would want to transition to Altus.
By the way, Tiagra 4700 is a 10-speed road derailleur with an 11-speed rear shift ratio and doesn’t fit the bill.
Front Derailleur Compatibility
Like the rear drivetrain, front shifters and derailleurs have a specific cable pull and shift ratio. However, front derailleurs don’t have to be as precise since there are only 2-3 chainrings and the space between them is larger.
It’s not recommended to use MTB derailleurs on a road bike for the following reasons:
- Dissimilar shift ratios
The shift ratios of MTB and road front derailleurs are different. They were changed during the 1990s.
- Dissimilar cable pulls
The cable pulls of STI and MTB front shifters don’t match either.
- Different Pulling Architectures
Typically, road front derailleurs are bottom pull.
This means that the gear cable controlling the front derailleur goes under the downtube and then up.
When the rider presses the lever, the cable pulls the derailleur’s swing arm. And since the cable is under the derailleur, we refer to those units as bottom pull derailleurs.
Meanwhile, MTB front derailleurs are top pull. In that case, the gear cable goes along the top tube and then towards the bottom bracket.
Since the shifter/cable pulls the swing arm of the derailleur from the top, the derailleur is defined as top pull.
The reason why most MTB front derailleurs are top pull is the higher chance for the downtube area to come in contact with an obstacle (e.g., a tree branch) during off-road riding.
Since top and bottom pull derailleurs require different pulling leverages, the shift ratios and the cable pulls don’t match either.
There are also dual pull front derailleurs that can be set to operate as either top or bottom pull depending on the bike’s cable routing. Shimano Altus offers a dual-pull front derailleur too (FD-M313), but it doesn’t have the correct pull ratio for a road system.
- Cage Shape
Road derailleurs are designed for larger chainrings and have a longer cage with a more accentuated arc. As a result, they shift fast between large chainrings and provide better chain retention.
For the reasons above, it’s advisable to avoid using an Altus front derailleur on a road bike.
Cassettes
Altus cassettes are 7,8 and 9-speed. Within those gear ranges, Shimano’s cassettes are interchangeable between road and MTBs. Compatibility gets more complicated as the number of speeds increases.
However, the range of MTB cassettes is larger. For instance, a 9-speed MTB/Altus cassette could have a large cog with 36 teeth.
The extra low gearing could be beneficial for gravel riding and hilly terrain, but it comes at a price – the jumps between the cogs are larger in comparison to road cassettes designed for the same number of speeds.
Larger jumps between the gears hurt the rider’s cadence. Cadence is a term indicating the rotations of the cranks per minute. A higher cadence such as 90RPM or more is associated with greater efficiency during long-distance rides.
The table below compares the gradation of 8/9-speed road and MTB cassettes.
| Speeds | Road | Jumps | MTB | Jumps |
| 8 | CS-HG50-8 | 11-13-15-17-20-23-26-30T | CS-HG31 | 11-13-15-17-20-23-26-34T |
| 9 | CS-HG50-9 (11-30Т) | 11-12-14-16-18-20-23-26-30T | CS-HG400-9 | 11-13-15-17-20-23-26-30-34Т |
As shown in the table, the last few jumps on MTB cassettes are slightly larger. The discrepancies could get even bigger if we include extra large cassettes such as 11-40T.
However, an Altus derailleur cannot operate with an 11-40T cassette. It maxes out at a 34T large cog. Some people have used an Altus derailleur with a 36T cog, but it’s technically not recommended.
If you want the extra low-gear range, you will have to use a derailleur hanger extender.
Cranks
Installing MTB cranks on a road bike is a tricky subject and usually a non-necessary procedure. The key points that you have to know are:
- MTB cranks can be installed directly on a road bike only if they’re designed to be used with a square taper bottom bracket and the bike is equipped with one. There are Shimano Altus cranks that use a square taper bottom bracket, but there are also OctaLink and Hollowtech models.
- MTB cranks and bottom brackets are wider than the road versions. If you’re trying to install Altus (or other MTB models) on a road bike with a bottom bracket other than a square taper, you will have to replace the bottom bracket with an MTB version and add two spacers to make up for the extra width.
- Road and MTB Octalink bottom brackets are different and cannot be mixed.
In general, the trouble of installing Altus cranks on a road bike is not worth it. If you want lower gearing, use smaller chainrings and a large cassette at the back.
For instance, you can use a set of compact cranks and an 11-34/36 cassette. Compact cranks have 50/34 chainrings and will give you a 34:36 low gear with a 0.94 gear ratio.
Truth be told, most trained cyclists don’t need a gearing that low on a road machine. It can be helpful, however, if you are touring or gravel riding with a customized road bike.
