# How much can you exceed/abuse rear derailleur limits?

5 min read

##### Asked by: Sharon Catchings

## What does total capacity mean for rear derailleur?

Rear derailleur Total capacity

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.

## How do you calculate rear derailleur capacity?

**Subtract the number of teeth on the smallest chainring from the number of teeth on the largest chainring**. 52-36= 16. Now subtract the number of teeth on the smallest cassette cog from the number of teeth on the biggest cassette cog.

## What is chain wrap capacity on derailleur?

Chain-wrap capacity: **up to 33 teeth**. Click for an explanation of chain-wrap capacity. An important part of a rear derailleur’s job is keeping the chain taut as you change gears. Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on.

## What happens if you exceed derailleur capacity?

So if you exceed the specs, it won’t be dangerous for the derailleur, but **the shifting won’t be optimal, or it will rub, or may not work at all**, because the cage will be too big/small or the grooves won’t be at the right position, or the parallelogram won’t be aligned properly.

## How do I know if my rear derailleur is short cage?

Quote from video: *So look for those markings on your derailleur. If you can't eyeball it to see if it's long or short again short is SS medium GS long is s GS.*

## Do I need a new derailleur if I get a bigger cassette?

Derailleur Adjustments

When putting a big cassette, the derailleur jockey wheel it may not be able to be under that big cogs, therefore the shift can’t be made. For that, **it requires to use a specific rear derailleur compatible with these big ratios** and this can be expensive upgrade.

## Which is better long cage or short cage derailleur?

It’s generally best to go for **as short a derailleur cage** as you can get away with, as long as you can avoid the chain contorting on the extremes of the cassette, such as using the largest on both sprocket and chainring. Smaller cages tend to have snappier gear changes, they’re lighter, and also less exposed to damage.

## How many teeth can acera handle?

Chain-wrap capacity: **45 teeth**.

## How many teeth can Alivio handle?

It can work with **up to a 36-tooth rear cassette sprocket and down to an 11-tooth smallest ring**.

## What is Max chainring size?

Standard double cranks – the most common combination is 39 tooth inner and 53 tooth outer. Older chainrings will be 42T inner and 52T outer. The smallest inner chainring possible is to 38T. The largest outer chainring possible is **55T** (without changing to a different front mech).

## How do you work out chain capacity?

To calculate Chain Wrap Capacity **determine the difference between your largest and smallest chain ring.** Then determine the difference between your largest and smallest cog. Add the differences and you have Chain Wrap Capacity. Example: 42/52 chain rings and a 12/21 cassette yields a CW of 19.

## Do I need long or medium cage derailleur?

If you are in-between the capacity ranges of the two, which is often the case with mountain bikes, then note the following: **long cage derailleurs will accommodate a wider gear range and deliver a better chain line, while medium cage derailleurs are less likely to be hit by rocks on the trail**.

## How can you tell the difference between a short cage and a medium cage derailleur?

Quote from video: *If we go right up to the big ring at the front and all the way up to the biggest cog. At the back you can see there's a lot of tension in a chain. So that cage has had to move right forward.*

## Does Cage length affect chain length?

**Cage length doesn’t impact the chain length**, but a longer cage can help with a small-small. Because the longer cage pulls the chain further back the derailleur is less likely to fold on itself in extreme ratio situations.

## Which is better long cage or short cage derailleur?

It’s generally best to go for **as short a derailleur cage** as you can get away with, as long as you can avoid the chain contorting on the extremes of the cassette, such as using the largest on both sprocket and chainring. Smaller cages tend to have snappier gear changes, they’re lighter, and also less exposed to damage.

## How do I know what derailleur I need?

If you are replacing an existing derailleur on a bike, simply **count the number of cogs on your cassette** and you’re good to go. If your drivetrain’s speed is an unknown quantity, you can count the number of steps that your shifter runs through and add ‘one’ to determine the number of gears your drivetrain has.

## Do I need a new derailleur if I get a bigger cassette?

Derailleur Adjustments

When putting a big cassette, the derailleur jockey wheel it may not be able to be under that big cogs, therefore the shift can’t be made. For that, **it requires to use a specific rear derailleur compatible with these big ratios** and this can be expensive upgrade.

## How many teeth can acera handle?

Chain-wrap capacity: **45 teeth**.

## How many teeth can Altus handle?

Specifications: Cage Length: Long. Cassette Spacing: Shimano/SRAM 8. Chain Wrap Capacity (teeth): **43 teeth**.

## How many teeth can Altus have?

Total Capacity: **43 Tooth**.