Chainline is one of the most overlooked numbers on your bike, yet it quietly controls how smoothly you shift, how long your drivetrain lasts, and how much power actually reaches the road. This guide explains what chainline is, how front and rear chainline are measured, how to check your own at home, and how to fix a chainline that is throwing your gears out of alignment. Get it right and the whole drivetrain runs quieter, shifts crisper, and wears more slowly.
What Is Chainline?
Chainline is the horizontal distance, measured in millimetres, from the vertical centreline of your frame to the centre of your chainring or cassette. The frame centreline is an imaginary plane running straight down the middle of the seat tube and bottom bracket. In an ideal setup, the chain would travel in a perfectly straight path from the front of the drivetrain to the back, parallel to that centreline.
Front chainline versus rear chainline
There are two measurements that matter. Front chainline is measured to the centre of a single chainring, or to the midpoint between the two rings on a double crankset. Rear chainline is measured to the centre of the cassette, which falls at roughly the middle sprocket. When the front and rear figures match, the chain sits straightest when it is in the middle of the cassette, which is where most of your riding happens.
Why Chainline Matters
A chainline that is off by just a few millimetres will not stop you riding, but it steadily taxes your drivetrain. Here is what a poor chainline costs you.
- Slower, noisier shifting. When the chain already sits at an angle, the derailleur has to work against that tension to move it across the sprockets, so shifts feel hesitant and rattly.
- Faster wear. A chain running at a constant angle grinds against the sides of the sprocket teeth and chainring. That accelerates wear on the chain, cassette and rings alike. If you already track this, you will notice your chain wear reaching the replacement point sooner.
- Lost efficiency. A straighter chain has less friction. The watts lost to a bad chainline are small but real, and they are free to recover.
- Dropped chains and chain rub. An extreme chainline makes the chain more likely to skip off the chainring or rub the front derailleur cage.
Chainline is closely related to the angle your chain takes across the cassette. Running the extreme gear combinations, such as big-big or small-small, exaggerates that angle. We cover that habit in detail in our guide to cross-chaining, but good chainline is what keeps those angles manageable in the first place.
Front Chainline Figures You Will See
Manufacturers design cranks to a target front chainline that suits the frame standard they are built for. The exact figure varies by brand and model, so always treat these as reference points and confirm against the manufacturer spec for your specific crank:
- Traditional road double: around 43.5 mm.
- Modern road and many gravel cranks: commonly around 45 to 47 mm.
- Mountain bike, non-Boost: around 49 mm.
- Mountain bike, Boost (148 mm rear): around 52 mm.
- Super Boost Plus (157 mm rear): around 56.5 mm.
Notice how the numbers grow as the rear hub gets wider. That is the key point: front chainline exists to match the rear, not the other way round. Chainline is also distinct from Q-factor, which measures the distance between your pedals, though both are affected by the width of your crank and bottom bracket.
Rear Chainline and Axle Standards
Rear chainline is largely fixed by your hub and axle standard, because the cassette body sits at a defined point on the hub. As rear axle widths grew wider to make room for wider tyres, stiffer wheels and 12- and 13-speed cassettes, the cassette moved outboard and the target chainline moved with it. A non-Boost 142 mm rear end places the cassette centre further inboard than a 148 mm Boost rear end, which is why Boost cranks are pushed out to match.
On a modern 1x drivetrain this matching game is simpler and more important than ever. With no front shifting to smooth things over, you want the single chainring sitting directly in line with the middle of the cassette so the chain runs straight in your most-used gears. Understanding how your gears are laid out helps here; if cassette and gearing terms are new to you, start with our beginner’s guide to bike gears.
How to Measure Your Chainline at Home
You do not need special tools. A steel ruler or a set of calipers, plus a few minutes, will get you a usable figure.
What you will need
- A ruler or vernier calipers marked in millimetres.
- A straightedge or a second ruler to find the frame centre.
- A notepad to record your numbers.
Measuring front chainline
- Measure the width of the seat tube at the bottom bracket and divide by two. This gives you the distance from the outer face of the frame to its centreline.
- For a single chainring, measure from the frame’s outer reference face to the centre of the chainring teeth.
- For a double, measure to the midpoint between the two rings.
- Subtract half the seat-tube width from that measurement. The result is your front chainline.
Measuring rear chainline
- Measure the rear hub’s over-locknut width (the full axle width, for example 142 mm) and divide by two to find the hub centre.
- Measure from the outer face of the cassette’s smallest cog to the centre of the cassette stack.
- Compare that centre position to the hub centre to find how far the cassette centre sits from the frame centreline. That is your rear chainline.
- Compare your front and rear figures. If they are within about a millimetre or two of each other, your chainline is healthy.
Common Chainline Problems and How to Fix Them
The wrong bottom bracket or spacers
The most common cause of a bad front chainline is a bottom bracket that does not match the crank, or spacers installed in the wrong order. Many press-fit and threaded systems rely on precise spacer stacks to position the crank spindle. Rebuilding the bottom bracket with the correct spacers, in the sequence the manufacturer specifies, usually brings the chainring back into line.
A mismatched axle standard
Fitting a non-Boost crank to a Boost frame, or vice versa, guarantees a chainline mismatch. Some brands sell Boost conversion kits, chainrings with extra offset, or longer spindles to bridge the gap, but the cleanest fix is to use components designed for your frame’s standard.
Chainring offset and spindle length
On 1x setups, chainrings are often sold with different offsets (for example 3 mm or 6 mm) that shift the ring inboard or outboard. Choosing the correct offset ring for your frame is frequently all it takes to dial in a 1x chainline. On some cranks a different spindle length does the same job.
Once your chainline is correct, any remaining shifting roughness is usually a simple adjustment away. Work through our guide to adjusting your derailleurs to fine-tune the indexing and limit screws.
Chainline on Modern 1x and Boost Drivetrains
Wider axle standards and single-ring drivetrains have made chainline both simpler to reason about and more important to get right. With one chainring and a wide-range cassette, the chain spends its life swinging across every sprocket, so a well-centred chainline keeps the average angle as small as possible. Drivetrain makers now publish a specific chainline for each groupset, and matching your crank, frame and chainring to that number is the single best thing you can do for quiet, durable 1x shifting.
Does Chainline Matter on Every Type of Bike?
Chainline applies to every chain-driven bike, but how much it matters and how you adjust it changes with the drivetrain.
Singlespeed and track bikes
On a singlespeed or track bike, chainline is arguably more critical than anywhere else. With only one gear, the chain has no derailleur to pull it into place, so a perfectly straight line between the chainring and the rear cog is the only thing keeping the chain from rubbing, running loudly, or throwing itself off under hard efforts. Riders fine-tune it with bottom bracket spacers and, at the rear, by repositioning the cog and spacers on the hub or flipping the rear wheel. A straight singlespeed chainline is quiet, efficient, and far less likely to drop the chain during a standing sprint.
Internally geared and belt-drive bikes
Internally geared hubs and belt-drive systems also depend on straight alignment, and belt drives are especially unforgiving: a belt that is not running true will ratchet, wear quickly, or skip teeth. These systems typically use the same spacer and offset principles, but because there is no derailleur to hide a small error, getting the line dead straight is essential rather than merely helpful.
Geared road and mountain bikes
On a derailleur-equipped bike you have the most tolerance, because the rear derailleur constantly adjusts the chain’s path. That forgiveness is exactly why chainline errors here go unnoticed for so long, quietly wearing the drivetrain until shifting degrades. A quick check pays off even when nothing feels obviously wrong.
The Bottom Line
Chainline is a small measurement with an outsized effect on how your bike feels and how long it lasts. You now know that it is the distance from your frame’s centreline to the centre of the chainring and cassette, that front and rear figures should match, and that most problems trace back to the wrong bottom bracket spacers, a mismatched axle standard, or the wrong chainring offset. Spend ten minutes measuring yours, compare it to your drivetrain’s published spec, and you will have a quieter, longer-lasting bike for the cost of a ruler and a little attention.
