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    • Home  / Supercross BMX | News | BMX Education
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    How to Fix a Crooked Chain Line

    4 min read

    Chain alignment is the relationship of the sprockets and chain to the centerline of the bike. The centerline of the bike is an imaginary line that runs straight through the middle of your bike. We want a chain that sits perfectly parallel to this imaginary line.

    We want to ensure that our chains are running along the gearing in a perfectly straight line. Why? Because an out of line chain results In premature drivetrain wear. Since the drivetrain is the engine of the bike, we want to optimize and protect this system to the best of our ability; without it, we aren’t going anywhere. Along with premature wear, an out of line chain dampens power transfer efficiency.

    Let’s talk about how you can check your chain alignment and how to fix a chain that’s out of alignment.

    Checking Chain Alignment 

    No fancy tools required here. We found that a yardstick or ruler works best. Position yourself behind the rear end of the bicycle. Place the ruler on top of the rear cog. If the chain retains a straight line in relation to the ruler, the chain is aligned correctly.

    You can also slowly rotate the cranks and watch as the chain rolls from rear cog to chainring. As the chain turns across the drivetrain, ensure the chain’s path is straight.

    Here’s an example of an offset chain to chainring relationship. The chain approaches the chainring from the side. In this example, the chain is far more likely to grind down the chainring teeth over time. We want a perfectly straight chain line that hit’s the chainring head-on.

    How to Check Chain Alignment

    So you’ve done the test and found out that your chain is indeed out of line. Now what?

    There are multiple solutions for fixing chain alignment. Unfortunately, we can’t provide you with a one-way fix-all solution. Depending on your situation and root cause for the chains’ lack of alignment, trial and error are needed until you achieve proper alignment.

    So here are a few methods you can try to get your chain rolling straight!

    Method #1 Flip the Rear Cog Over

    Certain cogs feature an outdent that pushes the chain line further away from the hub. If your chain is too far to the left or right and traveling on a diagonal path, flip the rear cog over. Positing the outdent along either side can aid in centering the chain. It’s an easy fix too. Loosen the lock ring on the cog, flip the cog over, and re-tighten the lock ring.

    Method #2 Adding / Removing Crank Spindle Spacers

    Adding or removing crank spacers will impact the chain line in a similar way as flipping over the cog. The difference is that the adjustment of spacers will move the drive side crank arm and chainring. Don’t stress the idea of moving the crank arm; these will be millimeter adjustments that will not impact the pedaling experience. However, these millimeter adjustments will aid in aligning your chainring with the rear cog. Add a spacer to push the chainring away from the frame, and remove spacers to bring the chain ring closer. 

    Method #3 Ensure the Rear Wheel is Straight 

    This method may seem like a no-brainer, but it’s pretty common. If your wheel does not sit straight in the dropouts, it will impact chain alignment. Ensure that both the left and right side of the hub is even within the dropouts. A more primitive way of checking is to place your middle finger within the space between your rear tire and bottom swing arm. You can tell the wheel is out of alignment if one side is tighter for your middle finger to fit into than the other side. Keep adjusting until both sides have an equal gap.

    Method #4 Check the Size of your Hubs & Cranks 

    In recent years, companies within the Mountain bike world have produced hubs and cranks that push a 45-50mm chain line. There are a few bmx race specific frames that can sport a 45mm chain line, however, no bmx frame can be equipped with a 50mm chain line. Essentially you want to check the size and dimensions of your cranks and hubs. Ensure that they both feature the same chain line measurement. If your cranks are a 45mm chain line but your rear hub is 42.5mm you are bound to have a crooked drivetrain. We want to avoid this at all cost so use caution when purchasing new components and ensure that they are BMX race specific as the majority of BMX race parts with feature similar chain line measurements. 

    The Wrap Up

    Generally speaking, at least one of these 4 methods should correct your chain alignment. Having a straight drive-train is the easiest way to ensure you’re fully optimizing power output. The last thing you want is to have your hard training efforts undermined by the limitations of your equipment. Head out to the garage and make sure your drivetrain is dialed in before your next session!

    Got any questions? Feel free to reach out as we would be more than happy to provide you with the nece