Tiagra and GRX make for a mean duo, but many dependencies have to be taken into consideration, or else the combo won’t produce good results.
Below I have summarized the main key points. Then, I explain everything in detail.
Super summary (key points):
- Only shifters from the Tiagra 4700 series can be combined with a GRX rear derailleur. The go-to choice is the GRX RD-RX400 model (the Shimano compatibility chart backs that up).
- The Tiagra 4700 shifters can be combined with the 11-speed GRX derailleurs, but you will still be limited to a 10-speed cassette.
- The rest of the Tiagra shifters (4500&4600) are incompatible with a GRX derailleur.
- The 2x front shifter of Tiagra 4700 can operate with the 10-speed GRX front derailleur.
- Tiagra 4700 + a GRX derailleur is a solid gravel drivetrain. The shifting is as good as most people would need. The GRX derailleurs are designed precisely for gravel and can take a beating.
If you want more technical info, please, keep reading.
The Factors That Determine Compatibility Between Indexed Shifters and Rear Derailleurs
Time to get technical.
Modern shifters operate as follows – you press a button/lever, and the chain goes up or down the cassette in an instant.
The only requirement is to time the shift and reduce the tension on the pedals. And sometimes even that’s not needed. If the drivetrain is in good condition, shifting under load can still be done, although it’s not an ideal practice.
How does this happen?
The cassette is a map. Each cog represents a location on that map that the derailleur has to reach with one click of the shifter.
The only way to create that outcome is to exercise complete control over the movement of the derailleur. This is achieved via two values:
Cable pull – the cable pull indicates how much gear cable the shifter pulls or releases to trigger a shift
Rear shift ratio – the rear shift ratio of a derailleur determines how much the derailleur moves per 1 millimeter of cable movement triggered by the shifter.
If you control how much cable the shifter pulls and how much the derailleur travels for that pull, we can fully control the “checkpoints” of the derailleur with each shift.
The cable pull and the rear shift ratio are values that depend on the following factors:
- Bicycle type (road or MTB)
- Number of gears on the cassette
- Year of production (manufacturers sometimes change the pull ratio even for the same number of speeds).
Consequently, it’s not possible to combine all sorts of shifters and derailleurs.
That being said, sometimes there’s a lot of overlap and unusual combinations could be made. For instance, it’s possible to use a 10-speed road shifter with 7/8/9-speed MTB shifters because 10-speed Shimano road derailleurs have the same rear shift ratio as 7/8/9-speed MTB models.
But let’s return to our original story – Tiagra + GRX.
Tiagra = Special Series
Tiagra 4500 is the first Tiagra line. Just like Sora, it’s designed for 9-speed.
Tiagra 4500’s shifters are compatible with Sora’s rear derailleurs, but they can’t operate with a GRX unit.
GRX derailleurs have an 11-speed rear shift ratio – 1.4:1. (The derailleur moves 1.4mm per 1mm of cable pull).
Meanwhile, 9-speed MTB derailleurs have a 1.7 rear shift ratio. Therefore, if a GRX derailleur is combined with a Tiagra 9-speed shifter, it will fail to move the chain sufficiently for a shift to occur.
Tiagra 4600 is the successor of the 4500 series and came with 10 instead of 9-speeds.
Unfortunately, the Tiagra 4600 shifters are not compatible with any GRX derailleur, even the 10-speed model (GRX RD-RX400).
Tiagra 4600 shifters have a pull designed for standard 10-speed Shimano road derailleurs. The GRX models don’t fit the criteria.
Tiagra 4700 = A Special Kid In a Special Family
Tiagra 4700 rear shifters are essentially 11-speed units in a 10-speed envelop.
The derailleurs have an 11-speed rear shift ratio (1.4). But since the series is made for 10-speed cassettes, the shifters don’t have a 10-speed cable pull.
Instead, they have a cable pull that’s neither 10 nor 11-speed as it has to be unique to move a derailleur with an 11-speed rear shift ratio accurately across a 10-speed cassette.
The cable pull of Tiagra 4700 shifters is approximately 2.79mm whereas that of 10 and 11-speed shifters is respectively 2.3mm and 2.7mm.
The above seems a little more complicated than one might expect, so let’s put it all in a small table.
|Rear Shift Ratio
|Shimano 10-speed Road (standard)
|Shimano 11-speed Road (standard)
|Shimano GRX 10&11
|Tiagra 4700 (the special one)
*The cog pitch is the center-to-center distance between two adjacent cogs on a cassette. It can be calculated via the following formula – Cog Pitch = Cable Pull x Rear Shift Ratio.
The next table indicates compatibility and incompatibility:
|9-speed Road RD
|10-speed Road RD
|11-speed Road RD
|GRX RD 10&11*
|Only Tiagra 4700
What Does The Shimano Chart Say?
Shimano’s compatibility chart indicates that the Tiagra 4700 rear shifters are compatible with GRX’s RX400 rear derailleur.
The chart doesn’t indicate that the Tiagra 4700 shifters are compatible with the 11-speed GRX rear derailleurs, because Tiagra 4700 shifters cannot be used in an 11-speed system. (You can’t use a 10-speed shifter on an 11-speed cassette.)
However, an 11-speed GRX mechanical derailleur can be integrated into a 10-speed Tiagra 4700 drivetrain because the original 4700 derailleurs have the same rear shift ratio.
At the end of the day, derailleurs are not speed-limited. In other words, if they have the needed capacity and the required rear shift ratio, they are interchangeable.
That being said, an important question arises – why go for an expensive 11-speed GRX derailleur when the 10-speed model gets the job done?
The main incentives are:
- Higher capacity
From a practical standpoint, however, the motivation to do so is indeed low. The 10-speed model will be more than good enough for most people.
What About Front Derailleur Compatibility?
You thought I was going to skip front drivetrain compatibility, didn’t you? (Just kidding).
This is not how brainybiker.com operates. I always try to make as complete posts as possible and answer the question from every possible angle.
The front shifters of Tiagra 4500 and 4600 are not compatible with GRX’s front derailleurs.
However, the 2x front shifters of Tiagra 4700 can operate with the 10-speed GRX front derailleur, FD-RX400 as shown in Shimano’s table.
Note: The 3x front shifter of Tiagra 4700 is not compatible with a GRX front derailleur, only the 2x model.
The 11 and 12-speed front GRX derailleurs are not compatible with any of the Tiagra series.
The Advantages of Combining Tiagra 4700 with GRX
- High IQ Gravel Transition
GRX derailleurs are simply better when it comes to gravel riding than road models. Of course, road group sets can get the job done too, but if you want an upgrade, GRX will provide it.
GRX derailleurs are tougher, and come with a high capacity thanks to their longer cage. The 11-speed versions have a clutch too.
- Minimal Investment
If you already have a Tiagra 4700 drivetrain, you will only have to buy a rear derailleur to make your bike more gravel-oriented.
- Mechanical Brakes
The GRX brake shifters are designed for hydraulic brakes only. Some cyclists, such as myself, are not the biggest fans of hydraulic brakes due to the higher maintenance cost.
Mechanical brakes are good enough for most people and are much easier and simpler to maintain and re-adjust on the go.
The Tiagra 4700 series has a mechanical set of shifters that will allow you to keep the braking system simple.
- Flat Shifters
The Tiagra 4700 family has flat bar shifters (SL-4700 and SL-4703). Meanwhile, GRX only offers brake-shifters that are suitable solely for drop bars. If you want to use flat or riser bars, you won’t be able to.
That said, most gravel bikes use drop bars.
- High Capacity
Tiagra 4700 rear derailleurs come in two versions – short and medium cage.
The maximum sprocket capacity of the short cage model is only 28T. This isn’t surprising because Tiagra is designed for the road after all.
The medium cage derailleur maxes out at 34T but only when using a 2x at the front. If you run 3 chainrings, the limit drops to 32T.
Meanwhile, GRX derailleurs have a maximum low sprocket capacity ranging from 34 to 42T.
As a result, you will be able to engineer a drivetrain with significantly lower gearing than what a Tiagra derailleur can control.
Also, if you want to run a 1x drivetrain (single-chainring at the front), the extra range of a GRX derailleur will give you access to lower gears that would reduce the limitation coming from the absence of additional chainrings.
But let’s organize everything in a table once again, as it’s much easier to learn visually:
|Number of Speeds*
|Maximum Large Cog
|RD-4700-SS (Tiagra short cage)
|RD-4700-GS (Tiagra medium cage)
The Disadvantages of Combining Tiagra 4700 with a GRX Rear Derailleur
There’s only one notable disadvantage that I could think of.
The whole upgrade may simply be unnecessary in your case. Yes, GRX derailleurs are tougher, but if your riding doesn’t need that extra toughness, what’s the point of investing extra time and money in parts and bicycle tinkering?
If the Tiagra derailleur has the needed capacity and is in good shape, upgrading to GRX won’t improve your performance by a lot. The Tiagra components are tough enough for most people. Truth be told, even Sora will satisfy the gravel needs of an average cyclist even though it’s not ideal for the task.