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Shop Talk | Chainline Theory | 1x Drivetrain
When you sign up for a 1x (one by) drivetrain, as in you only have on gear up front and 10, 11, or 12 out back, you need to pay attention to your bike's rear axle spacing as there are odd outcomes with your chainline. Rear axle spacing will determine what offset chainring you end up using. The chainring offset brings the chainring closer to the bike and corrects the angles which affect the chain. Finally, a properly set chainline can be the difference between a nice riding, quiet drivetrain, that wears slower, and a noisy, high-wearing one. NOTE: If you're on a newer bike, chances are this isn't something you need to worry about unless you are planning to replace your own chainring.
Below, I will describe the different chainring offsets and hopefully guide you to get the right one for your bike. I also think it worthwhile to describe why all this is so important. Aside from messing around with your bottom bracket or crank spindle spacing, which you probably shouldn't do, an offset chainring may be the only way to improve your chainline.
The Direct mount ring here replaces the spider system and has a 6mm offset which makes for better riding and smoother chain movement. The spider system has 0mm offset and is noisy. This rider was chronically replacing his chain, had 'noisy chain', and was probably creating unnecessary wear on his cassette.
Chainline
Chainline is often overlooked by all but seasoned mechanics. However, many drivetrain and bike manufacturers took chainline into account when making their direct mount chainrings for 1x drivetrain applications. It's important to pay attention to Boost vs non-Boost spacing.
What is Chainline?
The plain on which the chain travels from cassette (the largest cog) to chainring is your bikes chainline. This has to be optimized with hardware selection to prevent really abrupt shifts in angles as the chain leaves the cassette and engages the chainring. Hard angles in the chainline cause progressively more noise the harder the climb. If you look down on your chain while it sits on the climbing gear, you can see this line, and remark that there is an unnatural bend in the chain at the cassette and chainring.
This illustration shows chainline in reality. You can see this on almost any drivetrain, not just 1x drivetrains.
The idea is to get the chain to travel as straight as possible when it is under the most force (climbing gear). In order to correct a crazy angle, the teeth of the chainring need to be aligned with the plain of the cassette's climbing gear. As in the picture above, the 0mm offset spider sat 6mm further outboard than the replacement direct mount of the same tooth count. The angles are bending the chain against its construction. This is accepted as part of the recipe; however, if a mechanic can reduce those angles, the bike will be happier, quieter, and chains will last longer.
Notice the less abrupt angles in the chain, with the 6mm offset or corrction.
So we want our chainline to be pretty straight in the climbing gear. That's the basis for equipment selection. And the chainring is the final part of the equation because it's really the only aspect of the configuration over which you have any control. The driving factor will actually be your rear axle spacing.
Axle Spacing? The Heck?
Mountain bikes have several types of rear axle spacing. Below are listed a few of the most common. There's another volume of writing describing why there are so many varying widths. But that's not important right now. What's important is knowing how to measure this. The further back in time we go the more we have to consider narrower and narrower axles widths.
In the beginning, mountain bikes were not mountain bikes. They were adapted "other bikes". And that is why so much was shared between road bikes and their trail-oriented counterparts. These days, we're mostly looking at non-Boost, Boost, and Super-Boost (Pivot).
| Axle Type/Size | Description |
|---|---|
| 135mm x 10mm | QR or quick release, old school, the normal for over 2 decades |
| 142mm x 12mm | the newer standard, thru-axle, and 7mm wider axle; however, still not good enough |
| 148mm x 12mm | Boost, the standard that revolutionized everything, the most common spacing as of 2020 |
| 157mm x 12mm | Super Boost (Pivot), not common at all |
Making use of this information: The wider the axle spacing, the further outboard your cassette sits. Which means it is closer to acceptable if the chainring sits further outboard as well. Take a look at my 3rd grade MS Paint illustration below:
Maybe the rule of thumb could be, the narrower the axle, the further inboard that chainring needs to sit. Which today means more offset.
By understanding how wide your axle is, you'll have a better understanding of what chainring offset you need. This will determine the type of chainline you're going to have to live with.
Remember, more or less offset does not necessarily correspond to your axle spacing, no matter how much I wish it could be a hard rule. Be careful and research how much room you have to wiggle. Some frames don't allow for 6mm offset and the teeth will chew right into your carbon chainstay.
As an example, these offset are offered from SRAM. Do the research with your bike manufacturer and get it right. This is what I've found to work in most scenarios so I've paired the axle spacing with chainring offsets; thus, this is not a fail-safe cheat-sheet.
| Axle Type/Size | Chainring |
|---|---|
| 135mm x 10mm | 6mm offset |
| 142mm x 12mm | 6mm offset |
| 148mm x 12mm | 3mm offset |
| 157mm x 12mm | 0mm offset (these may require negative offset...) |