Ask a Mechanic: Adding an Extended Range Cog

Ed wants to know how to ad an extended range cog to his cassette.

Adding an extended-range cog is a great way to get a low gear for climbing, without making the investment in an 11 speed drivetrain. They are offered in both 40 and 42 tooth options depending on which cassette you currently have. The 40 tooth cog is designed to work with a 34 tooth cassette and the 42 tooth cog is designed to work with a 36 tooth cassette. Although they are primarily fitted to 1×10 drivetrains, I have seen them on 2×10 systems as well. It’s important that you have a cassette that allows you to remove either the 15 or 17 tooth cog, and a medium or long cage derailleur. Once you’ve confirmed that you have the proper components to make the switch, here’s how you do it.

Start by pulling off the old cassette. Once the cassette is removed, put the new cog on the freehub body by itself. The cassette will go on top of the new cog after you remove either the 15 tooth or the 17 tooth cog from the cassette. I’ve had better success removing the 17 tooth than the 15 tooth, but it depends on which gear you’re more likely to use. When removing the cog, whichever you choose, it’s very important to make sure that you remove one of the spacers for the cog as well. When replacing the cassette onto the freehub body, make sure the spacers are in the correct spot and you don’t have 2 cogs flat against each other or 2 spacers stacked on each other. Making either one of those mistakes will prevent the bike from shifting properly in those gears.

Once you are sure the spacing is correct, tighten down the cassette and put the wheel back in the bike. Now we can check the shifting. Adding the extended cog shouldn’t change the spacing of the cassette, but it never hurts to re check your high and low limits and make sure you can’t over-shift into the spokes or into the frame. We’ve covered adjusting the rear derailleur in other videos so I won’t go over that process here, but the adjustments should be very minor if any are needed. It’s best to check the shifting in the smaller cogs before going up the cassette into the extended cog. When shifting up the cassette to the extended cog, be sure to pay attention to the chain length. If you were running a 1×10 set up before and had the chain as short as possible, you will likely need to replace the chain to accommodate for the added teeth on the extended cog.

The last thing you’ll need to do is dial in the b-tension screw on the rear derailleur. Add b-tension by turning the screw clockwise until the top pulley wheel clears the extended cog and has the required clearance. Check to make sure that the derailleur pulley cage isn’t hitting the extended cog, which is a problem I’ve seen before. If the original b-tension screw isn’t long enough for you to add enough b-tension to the derailleur, replace it with the longer screw that is included with the extended-range cog. After setting b-tension double check the shifting throughout the cassette and you’ll be ready to ride.