If you’re lucky enough to get your hands on some bike parts during the pandemic, here are some tips on how to upgrade your drivetrain.
We’re first going to assume not changing speed of the bike. So if you’re on an 8speed cassette, you’re going to stick with an 8 speed upgrade.
Typical equipment you’ll need: lockring tool, chainwhip, chain brake tool, chain pliers, and either allen key set or torx set. There are videos and tutorials on how to do this online.
1. Upgrading cassette to larger sprockets:
This is probably the most popular upgrades for most people as it enables easier climbing. Let’s say you have a 11 speed rear cassette that has cogs ranging from 11-28 and you want to upgrade to an 11-34. You may need a new rear derailleur and new chain.
You can figure out the derailleur type you need by determining the derailleur capacity needed for your upgrade. Capacity is the rear derailleur’s ability to take up chain when moving from the highest gear ratio to the lowest ratio. Capacity is measured as:
C = (T_max – T_min) + (t_max – t_min)
T_max = Largest Chainring
T_min = Smallest Chainring
t_max = Largest Cog
t_min = Smallest Cog
In the picture above, we need a rear derailleur with C = (50-34) + (34-11) = 39. Sram, Shimano, Capagnolo, Sensah, and Microshift all contain derailleur capacity for at least some of their derailleurs. This equation is true whether you have a 1, 2, or 3 front chainrings. The derailleurs that accommodate larger capacities are usually denoted as medium cage or long cage.
As you move to a derailleur that can accept a larger range cassette, it will also prevent you from moving back to a cassette with a smaller range as the derailleur will have minimum largest cog it can accept. For instance the Shimano R7000-GS rear derailleur has a minimum max cog size of 30 so in theory it cannot accept a 11-28t cassette, but in practice, it can run with some minor drawbacks.
The final two pictues show exactly why you need to switch to a different derailleur size when changing your cassette range. As you move from the smallest gear ratio (T_min/t_max) to your largest ratio (T_max/t_min), that difference in chain length must be absorbed by the rear derailleur. In the parallellogram style derailleur (all/most modern derailleurs), the most obvious change to do this is to extend the derailleur cage and the derailleur swinging action during shifting allows it to “take up” or “let out” chain.
Final notes: For a drivetrain with a front derailleur, the maximum cassette size appears to be 40t for any drivetrain 11 speed or lower. This likely has to do with chain angle and chain crossing. So while, yes you probably can get away with running a 2x setup with an 11-42 cassette on the back, it might make a horrible amount of noise.
Sometimes you can get away with “larger cassettes than the derailleur is specified for”. One common upgrade I’ve seen is Shimano 5700 11 speed rear derailleurs taking up to a 30t cassette on on a compact chainring setup. (Shimano rear derailleurs appear to be able to handle at least +2t to capacity).
After you've determined the derailleur you can use, you'll need to size the chain to the right size. Put the chain on the largest cog and largest chainring. Do not run it through the rear derailleur. Pull the chain taught and add 2 full links to the length of the chain. Cut the chain there. If the chain is between two links, round up.
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