XT Shadow RD+ Medium Cage Rear Derailleur

component-RD-M8050-GS

The XT DI2 Medium Cage Rear Derailleuris perfectly suited to a wide variety of MTB applications, it delivers race-worthy performance without breaking the bank. 

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Di2 Rear Derailleur User Manual

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XT M8050 Dealer Manual

Note that if you want to use **synchronized shifting** with mountain bike derailleurs you will need either the [SC-MT800 display](/component/xt-display-junction-sc-mt800/) or its XTR equivalent, the [SC-M9051](/component/xtr-display-junction-sc-m9051/).

Its **low sprocket max** capacity is 42T for 2x11-speed setups and 46T for 1x11-speed setups. The **max front difference** is 10T and its **total capacity** is 35T.

This page has moved to [Installation guides > What do Shimano derailleur specifications means?](/installation-guide/understanding-di2-derailleur-specifications/). You should be redirected automatically.

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Shimano front derailleur and rear derailleur specifications... what do they mean, really? This page explains all of them, and why they matter. I'll also briefly talk about derailleur hanger extenders and swapping derailleur cages.

Derailleur specifications - what do they mean?

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:

**Front derailleur**
- [Chain line](#frontchainline)
- [Total capacity](#fronttotalcapacity)
- [Top gear teeth](#fronttopgearteeth)

**Rear derailleur**
- [Top sprocket min/max](#reartopminmax)
- [Low sprocket min/max](#rearlowminmax)
- [Total capacity](#reartotalcapacity) 
- [Derailleur hanger extenders](#hangerextenders)
- [Swapping derailleur cages](#swapderailleurcage)

## Front derailleur
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.

### Chain line {#frontchainline}
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](/installation-guides/), you cannot turn a 43.5mm chain line front derailleur into a 51.8mm one. 

### Front derailleur Total capacity {#fronttotalcapacity}
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 {#fronttopgearteeth}
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](/component/ultegra-front-derailleur-fd-r8050-f/) 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.

## Rear derailleur

### Top sprocket min and max {#reartopminmax}
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 {#rearlowminmax}
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 {#reartotalcapacity}
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.

Basically:

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](/component/ultegra-short-cage-shadow-rear-derailleur-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 {#hangerextenders}
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](https://www.wolftoothcomponents.com/products/roadlink) 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 {#swapderailleurcage}
Shimano's 12-speed rear derailleurs, the [RD-R9250](/component/dura-ace-shadow-rear-derailleur-rd-r9250/) and [RD-R8150](/component/ultegra-shadow-rear-derailleur-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. 

![RD-R8050-GS and RD-R8050-SS](https://assets.bettershifting.com/r8050_gs_ss_5690cccfd6.png)

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](/contact/).

Understanding Di2 derailleur specifications

In another post I wrote how to [micro-adjust your road di2 rear derailleur](/micro-adjust-your-road-bike-di2-rear-derailleur/) and [front derailleur](/micro-adjust-your-di2-front-derailleur/). This time I’ll explain how to do the same with your mountain bike rear derailleur. The process is similar, but slightly different.

## Set up the rear derailleur
Make sure your rear derailleur is set up correctly – especially the distance between the guide pulley and largest sprocket:

- On a 2x bike, shift the front derailleur to the small chainring.
- Shift to the largest sprocket

Now check the distance between the guide pulley and the largest sprocket is within the specified range. Turn the B-screw if it isn’t – have a look at the image below for directions on how which direction and what the acceptable ranges are.

![Indexing and limit screws](https://assets.bettershifting.com/adjust_mtb_rd_index_b_screw_ebb86a429f.png)

##   Adjustment mode
In order to make adjustments to your rear derailleur you’ll have to enter adjustment mode. When you’re in this mode the shift buttons will only make adjustments – they won’t let you change gears. This is how it’s done:

![Fifth rear sprocket](https://assets.bettershifting.com/adjust_mtb_rd_rear_5th_sprocket_2e79255a90.png)

- Shift the rear derailleur to the 5th largest sprocket.
- If you use a **road Junction A**: Press and hold the button on your junction box until the red LED illuminates, indicating you’re in adjustment mode.
- If you use **System Information Display**: Press (read *note* below!) the button on the lower left part of the display once to enter adjustment mode. The adjustment indicator “**R**” will flash (“R” stands for Rear)
- Note: how long this ‘single press’ is varies, this can take anywhere from 0.5 to 2 seconds according to Shimano.

Easy, right? Now you can micro-adjust your rear derailleur using the shift buttons:

![MTB RD buttons](https://assets.bettershifting.com/adjust_mtb_rd_buttons_2d050595fc.png)

- Press shifting **button Y** to move the rear derailleur one step towards the inside (larger sprocket)
- If you’re using the System Information Display you’ll see the current adjustment setting on the screen.
- While turning the crank/front chainring keep moving the rear derailleur towards the 4th largest sprocket until it makes a **slight noise**
- The chain now touches the **4th sprocket**. Press the **X button five times** to move the rear derailleur 5 steps to the outside.
- Press the Junction Box/Information Display button until the red LED turns off or the blinking R stops blinking. You have now left adjustment mode.

Your rear derailleur should (almost) be set up perfectly now. Of course you can always enter adjustment mode again and make small changes, but this should be your baseline setup.

The only thing left to do now is to check and correct the high/low side stopper adjustment bolts. We’ll do the low stopper bolt first (largest sprocket).

![low stopper mtb](https://assets.bettershifting.com/adjust_mtb_rd_low_side_stopper_3f2aa42238.jpeg)

- Shift the rear derailleur to the largest sprocket
- Tighten the low-side adjustment bolt until it just touches the outer link
- (if it’s tightened too much it either won’t shift, make noise or shift back by one gear after about 5 seconds)

Now let’s do the high stopper bolt:

![high stopper mtb](https://assets.bettershifting.com/adjust_mtb_rd_high_side_stopper_1cb3d4072d.jpeg)

- Shift to the smallest sprocket
- Tighten the high-side adjustment bolt until it just touches the left link at the position where the rear derailleur stops.

Note that while you could do the micro-adjusting on the bike, it’s probably best to do it with the bike on a bike repair stand. This will allow you to notice when the chain is touching the 4th sprocket more easily. Questions? Problems? Just [send me a message](/contact/)!



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Indexing Di2 rear derailleurs is done through micro-adjustment. The MTB Di2 rear derailleur process is slightly different from that of road RDs. Read how to set up your MTB rear derailleur and stop that chain rubbing noise.

Shadow RD+ Medium Cage Rear Derailleur

XT RD-M8050-GS

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Related pages

Derailleur specifications - what do they mean?

Understanding Di2 derailleur specifications

Micro-adjust your 11-speed Di2 MTB Rear Derailleur

Manuals & other documents