What is bicycle chainline and why is it important? Simply put, chainline is the distance between the center of the bike frame and the center of the chain. Chainline can be measured at the crankset as well as at the cassette/rear gear cluster, and if the chainline of both is the same it is said to be perfect. But cassettes consist of multiple cogs, which means that the chain is sometimes closer to and sometimes further away from the frame, so does the chainline change depending on which gear is selected? Also, sometimes cranksets only have one chainring, but how does chainline work if there are two or three chainrings at the front?
The key point, which is often a source of confusion, is that the ideal chainline of a particular bike is actually determined by the rear gear cluster (cassette/reewheel/equivalent rear cog setup), where a ‘perfect’ chainline would measure to the center of the cassette – but as we’ll see in this article that there are other technical reasons which often prevent this chainline from being used. Most other websites talk about chainline in terms of the crankset, because this is where chainline can be adjusted most easily, but what they are really describing is how to set the front chainline in order to best match the chainline required by the rear.
Up until the development of high speed chains, wide-range 1x drivetrains, and disc brakes, the ideal chainline was basically measured to the center of the rear gear cluster, but because most bikes had more than one chainring at the front the question essentially became: how should the center of the gear cluster line up with the chainrings? Since chains were wide and the spacing between cogs was large, the front chainline could actually deviate significantly from the ‘ideal’ and still give good shifting performance, and there are at least three good reasons for why the chainline might not be set to match the ideal chainline demanded by the rear.
The first reason is simply that front derailleurs also have an ideal chainline at which they will give the best shifting performance, and this varies depending on the diameter of the seat tube – a larger diameter tube will push the derailleur further out from the frame and therefore demand a larger chainline to shift properly. The second is that cyclists normally use certain chainrings more than others depending on their riding style – road cyclists will typically use the larger chainrings more than the smaller chainrings, while mountain bikers will spend more time using their smaller chainrings – so the chainline may be tweaked slightly to favor the most used gear combinations. Finally, if the chainline is too low, the front is shifted to the smallest chainring, and the rear is shifted into the smallest cog, there is a chance that the chain angles so much towards the largest chainring that it can actually snag on the teeth of the big ring.
In short, there are a lot of nuances involved with determining an ‘ideal’ chainline, but in practice the combination of the factors described above means that component manufacturers typically specify front chainline to be larger than the theoretical ‘ideal’ chainline required by the rear cogs in order to reduce the possibility of the chain snagging or mis-shifting. I couldn’t find too much information about vintage bike components online, but here are some chainline standards for more modern drivetrains (excluding 1x MTBs, gravel bikes, and disc-brake road drivetrains):
Road Double: 43.5mm
Road Triple: 45mm
MTB Triple: 47.5mm
Because these standards are determined by manufacturers I couldn’t find too much information about what constitutes an ‘ideal’ chainline for the cassette, but with typical 130mm road axle spacing I think that the center of an 8, 9, 10 or 11-speed cassette is around 43-44mm from the center of the bike. This means that a road double actually lines up very well with the ‘ideal’, while a road triple pushes the chainline slightly out in order to avoid some of the issues I described above. Mountain bike chainlines are larger because a classic mountain bike with a triple chainring has 135mm axle spacing so the center of the cassette is pushed 2.5mm further away from the center of the bike.
Modern Mountain Bikes
Things got much more complicated with the development of 1x mountain bike drivetrains. 8 and 9 speed mountain bikes used a 135mm rear hub spacing, but with the development of 10 speed this increased to 142mm – and wider axle spacing directly translates to larger chainline. 11 speed drivetrains also typically use 142mm spacing, while 12 speed drivetrains use 148mm ‘boost spacing’, which increases chainline even further.
Although 1x drivetrains eliminate front derailleurs, they are still subject to the possibility of the chain snagging on adjacent rear cogs at the cassette, which forces the chainline further out from the middle of the bike. In addition, because the axle spacing is wider, the chain stays have to spread further apart between the bottom bracket and the drop out, which means that the chainring can interfere with the bike frame if it is not far enough away from the frame – another reason why the chainline needs to be pushed out further from the ‘ideal’. As a result the industry has largely settled on the following 1x MTB standards:
142mm Non-Boost (10 and 11 Speed): 49mm
148mm Boost Thru-Axle (10, 11 or 12 Speed): 52mm
As axle spacing increases, the necessary chainline is increasingly determined by frame clearance around the crankset – the ‘ideal’ chainline for a 142mm bike is actually 46-47mm, while the ‘ideal’ for a 148mm bike is 49-50mm. This essentially means that you *might* actually be able to run a crankset with a smaller chainline on a bike with boost spacing – especially if you run a smaller 28-tooth chainring rather than a 30 or 32.
Modern Road & Gravel Bikes
Things also get more complicated with higher speed road and gravel bikes, and much less standardized than they are in the mountain biking world. In contrast to mountain bikes, where axle spacing has steadily increased, 10, 11, and 12-speed road bike cassettes still fit within a standard 130mm rear hub spacing, so for the most part the cassettes are approximately the same width and in theory their chainlines are approximately the same. However, the introduction of road bike disc brakes has complicated things because brake discs take up space, which means that the hub spacing had to increase to accommodate them.
It appears that there was a very short-lived 135mm standard for disc brake road bikes, but as of 2023 the industry seems to have settled on a 142mm thru-axle, which makes room for the brake disc but also causes the cassette to sit further out from the frame and the chainline of road bike cranksets has obviously had to increase to accommodate this. I am not a road cyclist, so I am much less familiar with road bike components and groupsets, but I did some googling and dug through some manufacturer’s websites and it appears that although chainline has not been standardized between manufacturers for their disc brake compatible road bike drivetrains, most modern disc compatible road cranksets have a chainline of around 44.5mm – 45.5mm.
This suggests that bike manufacturers are increasing the chainline of their road bike cranks by 1-2mm which seems like a very small amount considering that in theory the cassette sits 6mm further from the frame with 142mm spacing compared to 130mm spacing. The simple reason for this is Q-factor, which is essentially the horizontal distance between the pedals.
Road cyclists very strongly prefer a narrow Q-factor because this keeps their feet aligned with their hips and knees, supposedly resulting in increased pedaling efficiency and lower chance of injury. Because manufacturers make a point of increasing Q-factor as little as possible, they increase the chainline as little as possible, and there are reports that road bike drivetrains for disc brakes (especially 2×12 drivetrains) are therefore somewhat more sensitive to derailleur rub when cross-chaining. Extreme gear combinations are never recommended so this isn’t necessarily a problem, but this definitely sheds light on some of the compromises involved with making road bikes with disc brakes possible.
Gravel bikes are another interesting category because they are essentially hybrids between road and mountain bikes. Older gravel bikes simply adapted the available road and mountain bike components as best as possible, often using 135mm rear spacing combined with road double cranksets with standard 43.5mm chainline. More modern gravel bikes typically use 142mm spacing but the trend towards wider tires means that standard road cranksets are often unsuitable in this case because they don’t have enough clearance around the chainstays – in addition, as with disc brake road bikes, the standard 43.5mm chainline is also significantly off from the 46-47mm ideal demanded by the cassette.
The solution is gravel-specific cranksets, which Sram calls ‘wide’ cranksets, whereas Shimano simply has a specific gravel groupset called Shimano GRX. Both of these systems increase the chainline compared to standard road cranksets in order to both provide more clearance around the bottom bracket and in order to correspond more closely with the ‘ideal’ cassette chainline of 46-47mm:
Sram Wide: 47.5mm
Shimano GRX 1×11 : 49.7mm
Shimano GRX 2×10/2×11: 47mm
It is important to note that although these cranksets should work with standard road cassettes and spacings, they are not compatible with standard road derailleurs – the Shimano GRX 2x cranksets only work with Shimano GRX derailleurs.
Adjusting Chainline
So how can chainline be adjusted? Although chainline is technically ‘determined’ by the cassette or freewheel, it is normally the crankset which is adjusted to match the rear. This can traditionally simply done by using a bottom bracket with a different spindle length, but even vintage bikes were designed around a certain chain line, and each crankset was specified with a certain spindle length to match and also to allow the front derailleur to work properly. There is some leeway here, and there are also complications such as asymmetrical bottom brackets, but for the most part this only needs to be adjusted if there is a problem with the shifting or if some sort of unusual custom drivetrain is being built.
Modern bikes really are designed as a system, and if you buy a new bike everything will have been properly specified by the manufacturer, but it is important to keep chainline in mind if you change your crankset or if you are building a custom bike. Here are a few general guidelines valid as of May 2023:
Road bike cranksets are pretty much always compatible with each other, unless disc brakes are involved – if you use a non-disc crankset on a disc brake road bike you may have poor shifting performance and your chain may snag in extreme gear combinations. Disc cranksets will work on non-disc bikes but will often have a slightly increased Q-factor.
Mountain bike cranksets are specific for whether or not the frame has ‘boost’ spacing. Boost cranksets will work on non-boost frames, but the shifting performance may be slightly reduced due to the increased chainline. Non-boost cranksets may work on a boost frame, especially if you run a smaller chainring, but it is possible that you will run into clearance issues around the bottom bracket if you attempt this.
Gravel bike cranksets have a higher chainline and Q-factor than road cranksets, and many new gravel frames are specifically designed to work with ‘wide’ cranksets such as Sram Rival 1 wide or Shimano GRX. Standard road cranksets have a suboptimal chainline for gravel wheels and also may not provide adequate frame clearance.
Chainring Offset
But what if you are designing a custom build or creating a mullet drivetrain, like I did with my Fairlight Faran 2.0? Well, modern cranksets do not use square taper bottom brackets with a variable spindle length, instead using external cup bottom brackets with a fixed spindle integrated with the crankset. In this case, what actually changes to adjust chainline is something called chainring offset, which is how much a chainring is dished towards the frame.
Some manufacturers simply use chainrings with a different offset to create say boost vs non-boost cranksets, while others use a different crankset design entirely, but the chainline of many cranksets can be adjusted by either changing to different OEM chainrings with a different offset, or changing to 3rd party chainrings with a different offset.
The key point here is that offset is measured towards the frame, so a lower offset corresponds to a higher chainline. This depends a little on the manufacturer, but the standard offset is normally 6mm, which results in whatever the manufacturer has specified as the chainline for a road or gravel crankset, 49mm chainline for standard mountain bike cranks, and 52mm chainline for boost cranks. For example, Wolf Tooth offers their Camo direct mount spider for Sram cranksets in offsets of 6mm, 3mm, 0mm, and +4mm, which results in a wide range of chainlines for different applications.
The idea with my Fairlight Faran 2.0 was to create a 1×12 drivetrain with a range of gears suitable for touring on a drop bar bike using 142mm hub spacing. I chose a 10-51T Shimano cassette, which is normally specified with a chainline of around 49mm on a non-boost frame (the ideal for the cassette is actually 46-47mm but the drivetrain is designed around 49mm for chainstay clearance). I could have used a standard mountain bike crankset, but I also wanted to use as small a Q-factor as possible.
I noticed on the Wolf Tooth Camo page for Sram cranks that the Sram spider is also compatible with cranks from Praxis Works. I found out that the Praxis Works Alba crankset is a road crankset which works with Sram mountain bike chainrings, and the standard chainline of this crankset is 45.5mm with a standard 6mm offset. I was working towards a 49mm chainline, so I chose a 3mm offset spider which results in a 1×12 drivetrain with a 48.5mm chainline and a narrow 147mm Q-factor.
Conclusion
Chainline is an important concept when designing a drivetrain or building a custom bike, but it is normally specified by component manufacturers and any stock bike will be set up for proper frame clearance and shift performance. All groupsets are designed to work as a system and one of the design criteria of these is proper compatibility between the crankset and the gear cluster.
It is important to consider chainline when replacing a bottom bracket or crankset. Replacement square-taper bottom brackets should be chosen with the same spindle length as the original bottom bracket, while cranksets should be chosen to have the same chainline as that which they are replacing.
When building a custom bike or designing a drivetrain, it is very important to do some research and figure out what chainline the chosen cassette or gear system uses and then choose a compatible crankset. There are also other factors at play here which need to be considered such as frame clearance and Q-Factor. The best source of information for this is the technical documentation for your frameset.
I hope this article helped you understand some of the nuances associated with bicycle chainline – if you’d like to get in touch with me about anything feel free to send me a message on instagram @crazycyclingchannel.