Understanding Di2 derailleur specifications
When deciding what derailleur to buy, most people will just look at the largest sprocket/cog they can fit, and make a decision based on that.
This works fine for most of us, but Shimano list a lot of other specifications. Why do they do this, and what do the specifications mean?
What is a rear derailleur's total capacity, really? Why is there a top sprocket minimum?
And what about front derailleurs? Chain line, total capacity, Top gear teeth?
I'll explain all of these, and why they matter. If you're interested in one of these in particular, feel free to skip ahead:
- Top sprocket min/max
- Low sprocket min/max
- Total capacity
- Derailleur hanger extenders
- Swapping derailleur cages
Shimano Di2 road groupsets come with just in front derailleur version, so there's not much to choose there. Are you running Ultegra Di2? Then you'll probably use the Ultegra front derailleur. Is your bike equipped with DURA-ACE? Then it'll have a DURA-ACE front derailleur.
Nonetheless, these still have specifications like total capacity, chain line, and top gear teeth. Let's discuss what these mean.
A bicycle's chain line is the distance (in mm) between the frame's center and the point right between the two chain rings.
Shimano 10-speed and 11-speed road front derailleurs have been designed with a 43.5mm chain line in mind. Their 12-speed road front derailleurs were designed for a 44.5mm chain line.
The chain line for Shimano's gravel Di2 groupset (GRX) is 47mm, and the MTB chain line is even more - 48.8mm and 51.8mm.
If you find the midpoint between the two rings a bit hard to measure, then just measure the distance from the frame center to each chain ring, and then average that.
A specific crankset will have a chain line specification as well, and you can use this to determine whether or not a front derailleur and crankset will work together.
While some mismatch can be accounted for using adjustment and setting the limits, you cannot turn a 43.5mm chain line front derailleur into a 51.8mm one.
Front derailleur Total capacity
The total capacity specification of a front derailleur indicates the maximum difference (in teeth) between the largest and smallest chain ring that the derailleur can handle.
You don't need to take the installed cassette or rear derailleur into account.
Front derailleur Top gear teeth
You'll usually see a couple of numbers listed here - this is the range of sizes of large chainrings compatible with the front derailleur. For example, FD-R8050 has a top gear teeth spec of 46T-53T, meaning that it is designed to handle large rings sized 46T, 53T, and anything in-between.
Top sprocket min and max
When talking about Shimano derailleurs and top sprocket, this refers to the smallest sprocket, or simply the 'hardest' gear. While going outside the specified range will not likely damage your derailleur, it usually results in poor shifting, and can lead to a skipping chain.
Low sprocket min and max
The low sprocket is the easiest gear, or the largest sprocket (or cog). Most people will pay attention to only the max specified sprocket size, but there is a minimum too.
Again, running a cassette with a low sprocket that is smaller than the minimum will generally not damage the derailleur, but it can affect shift performance.
Exceeding the low sprocket max is definitely a lot riskier. Go over too far and you'll experience difficulty shifting into the easier gears, and run the risk of damaging your derailleur, wheel, chain, and frame.
Rear derailleur Total capacity
The rear derailleur total capacity spec is often overlooked, but it is in fact pretty important. It is the maximum amount of chain slack a derailleur can take up or let out, expressed in a number of sprocket teeth. Exceed the total capacity, and your chain will likely be too long or too short for some combinations of gears.
When you exceed a derailleur's total capacity and you shift to the smallest chainring and smallest sprocket, you'll find that the chain is longer than it should be. This can cause chain suck and skipping chains, or worse.
Depending on how you sized the chain, it'll likely be a bit on the short side in the largest chainring - largest sprocket gear. This can make shifting into the bigger sprockets difficult, and your chain might even get stuck.
And yeah, I do realise that this will only be a problem at either end of the cassette - so it's usually fine to exceed the total capacity if you're riding a single event like the Marmotte - but do try to stick to the total capacity for your day-to-day gearing though.
Determine a bike's capacity
To determine the capacity you need for a cassette and chainrings combination, take the differences between the largest and smallest sprockets, and the smallest and largest chainrings. The rear derailleur's total capacity must be greater than or equal to the sum of these two numbers.
Required RD total capacity >= (Large front ring - small front ring) + (Largest cassette sprocket - smallest cassette sprocket)
So if you have an 11-28 cassette installed and a 50x34 crankset, the total capacity required would be 17 + 16 = 33T.
Looking at the Ultegra RD-R8050-SS, this has a total capacity of 33T, and the low sprocket max it supports is 30T. You can probably see why it doesn't support a 32T max sprocket - it simply doesn't have the total capacity to do so.
Wolftooth and other derailleur hanger extenders
Looking for a way to increase your rear derailleur's max (low) sprocket, or simply the largest sprocket your derailleur can clear? Then you can use a derailleur hanger extender.
These increase the low sprocket max, but do not increase a derailleur's total capacity. This makes them ideal for 1x setups since those aren't usually limited by total capacity as much.
Add a front derailleur though, and it's a whole other story. Sure, a derailleur hanger extender will move the derailleur further away from the cassette and will let the guide pulley do its job... but you'll easily exceed your rear derailleur's total capacity.
Wolf Tooth mention this on their RoadLink page too:
RoadLink repositions your derailleur to provide clearance for larger cassette cogs but it does not increase derailleur capacity. It is critical to find and understand your derailleur’s capacity before attempting to expand its gear range.
Swapping out a rear derailleur's cage
Shimano's 12-speed rear derailleurs, the RD-R9250 and RD-R8150 only come in one size - there's no short/medium cage for you to choose from.
However, previous generation derailleurs came in different versions - a short cage version (SS) and a medium cage (GS) one. Some users swapped out the short SS cage for a longer GS cage, in order to fit larger cassettes.
This works to some extent, however: modern derailleurs are not just defined by their cages. There are also subtle differences in the derailleur body and parallelogram.
More on derailleurs, and chain sizing
That concludes this little page on derailleur limits. I appreciate that this probably doesn't answer all your questions - this page will get updated as time passes.
I'd love to hear your feedback and questions, so don't hesitate to send them in. Either via the comments section below, or the contact form.