A post sparked by recent disappointmenting discoveries whilst fiddling..
Overview
Chainline appears to be another cycling terms that's fairly well-known, but far less understood or quantified.
The term refers to the amount by which the drivetrain sits outboard from the bike's longitudinal centreline - specifically the distance to the lateral centre of the chainrings at the crankset (centreline of the middle ring on a triple, mid-point between the rings on a double and centreline of a single chainring).
Chainline is also applicable to the rear of the bike; being the distance between the bike's centreline and the middle of the cassette - so the centreline of the middle sprocket on odd-numbered cassettes (sprocket 5 on a 9sp for example) or the mid-point between the middle two sprockets on even-numbered cassettes (eg. between sprockets 5 and 6 on a 10sp cassette).
Certain specific examples notwithstanding the chainline should be the same between crankset and cassette - minimising chain deflection / "cross-chaining" at the extremes; which we all know is bad for both drivetrain efficiency and longevity.
Unfortunately it transpires that this is rarely the case.
The Importance of Chainline
While as above an optimal chainline is key to a drivetrain that's functional, efficient, long-lasting and quiet; chainline impacts / is governed by many other facets of a bike's design; including:
- Maximum tyre size: Wider tyres require greater spacing between chainstays; limiting clearance for chainrings and pushing them outboard
- Frame tube size: Larger tubes give less room for front derailleurs; necessitating the movement of chainrings further outboard
- Rear suspension considerations: The presence of linkages limits space for drivetrain components; again pushing them outboard
- Crankset Q-factor: As chainline increases so does the need to space the crank arms wider across the bike to accommodate this
- Chainstay length: While this doesn't directly impact chainline, the shorter the chainstay the greater the angular chain displacement will be and the lower its tolerance for a given offset in chainline between both ends
Chainline Standards
Chainlines for cranksets are relatively well-known if you do a bit of digging. Typically it appears we have the following nominal standards:
Road triple: 43.5 or 45.0mm
MTB triple: 47.5 or 50.0mm
Road double: 43.5 or 45.0mm
Gravel double: 47.0mm
MTB double: 49.0 or 52.0mm
Gravel and MTB single: 52.0, 55.0, 56.5mm and others
Conversely for the most common rear axle standards / spacings we have the following chainlines:
130mm (road quick release): 42.5mm (I think)
135mm (MTB QR, road disc): 45.0mm
142mm (MTB and road through-axle): 45.0mm
148mm (MTB TA - "boost"): 48.0mm
157mm (MTB TA - "super boost"): 52.5mm (extrapolated, TBC)
Gravel bike sit in some weird middle ground; having quickly adopted the 142mm TA standard but some are now using 148mm.
Examples
Use of a particular crankset with a particular axle is not fixed and various different combinations may be found. Picking through the numbers above we can look at a few examples to get an idea of how optimal their chainline is.
For this example I've chosen 11speed to give illustrative numbers; because it's common and has a fairly (physically) wide cassette at around 37.5mm pitch between largest and smallest sprocket, from extrapolating the numbers on the ever-excellent Bike Gremlin site.
Road triple:
43.5 or 45.0mm crankset chainline; typically used with 130mm QR, or 135mm QR / 142mm TA rear ends with 42.5 and 45.0mm CLs respectively.
In terms of chainline this is as good as it gets. It's easy to get the centrelines of each end of the drivetrain perfectly matched; at worst they're only 2.5mm off which is about 2/3ds of a sprocket pitch for 11sp or about half the sprocket pitch of the wider 8sp standard.
In addition the presence of three chainrings further reduces the severity of cross-chaining. Assuming the CLs are aligned the maximum offset / chain deflection from the most outboard ring to the most outboard sprocket during correct use (so big ring / smallest sprocket or small ring / biggest sprocket) would be around (37.5/2)-8.5 = 10.25mm for a 37.5mm wide cassette and a triple with 8.5mm chainring pitch.
MTB triple:
47.5 or 50.0mm crankset chainline; typically used with 130mm QR, or 135mm QR / 142mm TA rear ends with 42.5 and 45.0mm CLs respectively.
Unfortunately since the MTB triple crankset's CL is typically 5mm further outboard than that of a road crankest (while rear-end geometry remains the same) chainlines are mis-matched by 2.5-5.0mm (more likely the latter in more modern setups).
This has the result of biasing the correct chainline towards the smaller / more outboard end of the cassette; meaning the lowest gears (using the smallest chainring and largest sprocket) suffer worse alignment; compared to the road triple example above this combination would give an offset between front and rear of around 15.25mm - 50% worse than that of the road double.
Of course the counter to that is that at the other end of the gearing range (big chainring smallest sprocket) the chain alignment is improved by 5mm; giving only 5.25mm offset. This could be advantageous for stronger riders who spend a lot of time at this end of the gearing range; however despite this it still limits the number of optimal sprockets available to the big ring before a better chainline is achieved by shifting into the middle ring.
Road Double
43.5 or 45.0mm crankset chainline; typically used with 130mm QR, or 135mm QR / 142mm TA rear ends with 42.5 and 45.0mm CLs respectively.
Just like the road triple, the double's chainline alignment has the capacity to be perfect; or at least close. However, the shift from three chainrings to two places the outer rings more inboard on the crank; meaning worse effective chainlines / alignment in the extremes of the gear range. Again, using the 11sp cassette as an example with a perfectly-matched chainline front-rear, looking at each end of the range (big ring / smallest sprocket, or small ring / largest sprocket) we have a maximum chain deflection / offset of (37.5/2)-4.25=14.5mm - around 4.25mm or 40-odd percent more than the triple.
The gravel double is similar; moving the crank's CL outboard by 2.5mm worsens the chainline in the lowest gear (big ring, smallest sprocket) by the same amount and improves it in the same way in the highest gear - giving offsets of around 17.0mm and 12.0mm respectively for the 11sp example.
MTB Double
49.0 or 52.0mm crankset chainline; typically used with 130mm QR, 135mm QR / 142mm TA, 148mmTA and 157mm TA rear ends with 45.0mm 48.0mm and 52.5mm(?) CLs respectively.
Lots of combinations here; with the significant changes in MTB design driving the seemingly constant drive to push chainline ever-more outboard. I suspect that the 49mm crankset CL is / was intended to be used with older rear-end 135mm QR and 142mm TA standards predominently (45.0mm CL), while the 52mm standard was introduced to suit the wider boost and super boost standards with 48 and 52.5mm CLs respectively.
As we can see the 45mm CL of 135mm QR and 142mm TAs is sub-optimal with any crankset; best-case giving 4mm of CL misalignment with the 49mm crank for a worst-case offset / chain deflection in the lowest gear of our 11sp example of (37.5/2)-4.25+4=18.5mm.
Pairing a 49mm CL crankset with a 148mm TA boost rear end's 48mm CL results in an insignificant misalignment of 1mm, giving a worst-case low-end chain deflection / offset of (37.5/2)-4.25+1=15.5mm. Similarly matching a 52mm CL crankset with the (assumed!) 52.5mm CL of a a 157mm super-boost rear end actually offsets the crankset slightly inboard for a change; giving a worst-case chain deflection / offset in the highest gear of (37.5/2)-4.25+0.5=15.0mm
Gravel & MTB Single
52.0, 55.0 and 56.mm crankset chainline; typically used with 135mm QR / 142mm TA, 148mmTA and 157mm TA rear ends with 45.0mm 48.0mm and 52.5mm(?) CLs respectively.
The new messiah that is 1x drivetrains brings a number of undesirable traits to the chainline game. First and most obviously is the need for a single chainring to transmit drive across the entire range of the cassette; rather than the 50-75% of its width you might typically expect from triples or doubles respectively.
Further, clearance issues between the single chainring (which would be larger the inner ring on a double) and chainstays means that rings are typically offset outboard; further worsening chainline.
Again, lots of examples here - let's look at the best we can manage with the 45mm CL of the lowly 135mm QR / 142mm TA standard. Off the shelf it appears that the narrowest 1x crankset CL available is 52mm; meaning an offset of 7mm from the CL at the rear end of the bike. If you're unfortunate enough to use the lowest gearing / largest sprocket on our 11sp example there's going to be a massive offset between it and the chainring of (37.5/2)+7=25.75mm - 2.5 times worse than the ideal example set by the road triple.
While tempting to look at pairing the 52mm CL crankset with the super-wide, 52.5mm CL of the 157mm super-boost rear end, in reality the fat chainstays present to support the fat back end will likely preclude the use of anything but the fattest cranksets - meaning 55 or 56mm chainlines for a marginally better, but still not great amount of chain deflection throughout the gears.
While 1x does have some legit advantages, it's also varying shades of bloody awful in terms of chainline while throwing open the door to all manner of difficult-to-quantify variables regarding max chainring size and subsequent chainline.
Unfortunately the alignment problem is likely worsened by manufacturers being conservative with chainlines (pushing them safely outboard) to give clearance with the largest amount of frames.. there are apparently a few smaller brands offering a greater range of crankset chainlines to allow those prepared to throw time and money at finding the least-worst solution to push the crank CL as far inboard as possible; but the result will still be far from ideal.
Conclusion
As usual for the bike industry, what's been touted as "progress" from many angles has resulted in many unavoidable consequences in other areas.
I hope the above ramblings are at least semi-coherent and might help some find the least-worst solution when selecting or building a bike.
This is intended as a broad-strokes overview and I'm aware that not every permutation has been covered; but I hope it's a sufficiently representative snapshot of the most common offerings. Similarly I'm aware that some examples given don't reflect real-world availability (nobody makes 11sp triples for example) although 11sp was used throughout to keep comparisons consistant.
Please let me know if you see any errors or anything that makes less sense than usual; I'm off out on the MTB to "enjoy" its hideously compromised chainlines....
Overview
Chainline appears to be another cycling terms that's fairly well-known, but far less understood or quantified.
The term refers to the amount by which the drivetrain sits outboard from the bike's longitudinal centreline - specifically the distance to the lateral centre of the chainrings at the crankset (centreline of the middle ring on a triple, mid-point between the rings on a double and centreline of a single chainring).
Chainline is also applicable to the rear of the bike; being the distance between the bike's centreline and the middle of the cassette - so the centreline of the middle sprocket on odd-numbered cassettes (sprocket 5 on a 9sp for example) or the mid-point between the middle two sprockets on even-numbered cassettes (eg. between sprockets 5 and 6 on a 10sp cassette).
Certain specific examples notwithstanding the chainline should be the same between crankset and cassette - minimising chain deflection / "cross-chaining" at the extremes; which we all know is bad for both drivetrain efficiency and longevity.
Unfortunately it transpires that this is rarely the case.
The Importance of Chainline
While as above an optimal chainline is key to a drivetrain that's functional, efficient, long-lasting and quiet; chainline impacts / is governed by many other facets of a bike's design; including:
- Maximum tyre size: Wider tyres require greater spacing between chainstays; limiting clearance for chainrings and pushing them outboard
- Frame tube size: Larger tubes give less room for front derailleurs; necessitating the movement of chainrings further outboard
- Rear suspension considerations: The presence of linkages limits space for drivetrain components; again pushing them outboard
- Crankset Q-factor: As chainline increases so does the need to space the crank arms wider across the bike to accommodate this
- Chainstay length: While this doesn't directly impact chainline, the shorter the chainstay the greater the angular chain displacement will be and the lower its tolerance for a given offset in chainline between both ends
Chainline Standards
Chainlines for cranksets are relatively well-known if you do a bit of digging. Typically it appears we have the following nominal standards:
Road triple: 43.5 or 45.0mm
MTB triple: 47.5 or 50.0mm
Road double: 43.5 or 45.0mm
Gravel double: 47.0mm
MTB double: 49.0 or 52.0mm
Gravel and MTB single: 52.0, 55.0, 56.5mm and others
Conversely for the most common rear axle standards / spacings we have the following chainlines:
130mm (road quick release): 42.5mm (I think)
135mm (MTB QR, road disc): 45.0mm
142mm (MTB and road through-axle): 45.0mm
148mm (MTB TA - "boost"): 48.0mm
157mm (MTB TA - "super boost"): 52.5mm (extrapolated, TBC)
Gravel bike sit in some weird middle ground; having quickly adopted the 142mm TA standard but some are now using 148mm.
Examples
Use of a particular crankset with a particular axle is not fixed and various different combinations may be found. Picking through the numbers above we can look at a few examples to get an idea of how optimal their chainline is.
For this example I've chosen 11speed to give illustrative numbers; because it's common and has a fairly (physically) wide cassette at around 37.5mm pitch between largest and smallest sprocket, from extrapolating the numbers on the ever-excellent Bike Gremlin site.
Road triple:
43.5 or 45.0mm crankset chainline; typically used with 130mm QR, or 135mm QR / 142mm TA rear ends with 42.5 and 45.0mm CLs respectively.
In terms of chainline this is as good as it gets. It's easy to get the centrelines of each end of the drivetrain perfectly matched; at worst they're only 2.5mm off which is about 2/3ds of a sprocket pitch for 11sp or about half the sprocket pitch of the wider 8sp standard.
In addition the presence of three chainrings further reduces the severity of cross-chaining. Assuming the CLs are aligned the maximum offset / chain deflection from the most outboard ring to the most outboard sprocket during correct use (so big ring / smallest sprocket or small ring / biggest sprocket) would be around (37.5/2)-8.5 = 10.25mm for a 37.5mm wide cassette and a triple with 8.5mm chainring pitch.
MTB triple:
47.5 or 50.0mm crankset chainline; typically used with 130mm QR, or 135mm QR / 142mm TA rear ends with 42.5 and 45.0mm CLs respectively.
Unfortunately since the MTB triple crankset's CL is typically 5mm further outboard than that of a road crankest (while rear-end geometry remains the same) chainlines are mis-matched by 2.5-5.0mm (more likely the latter in more modern setups).
This has the result of biasing the correct chainline towards the smaller / more outboard end of the cassette; meaning the lowest gears (using the smallest chainring and largest sprocket) suffer worse alignment; compared to the road triple example above this combination would give an offset between front and rear of around 15.25mm - 50% worse than that of the road double.
Of course the counter to that is that at the other end of the gearing range (big chainring smallest sprocket) the chain alignment is improved by 5mm; giving only 5.25mm offset. This could be advantageous for stronger riders who spend a lot of time at this end of the gearing range; however despite this it still limits the number of optimal sprockets available to the big ring before a better chainline is achieved by shifting into the middle ring.
Road Double
43.5 or 45.0mm crankset chainline; typically used with 130mm QR, or 135mm QR / 142mm TA rear ends with 42.5 and 45.0mm CLs respectively.
Just like the road triple, the double's chainline alignment has the capacity to be perfect; or at least close. However, the shift from three chainrings to two places the outer rings more inboard on the crank; meaning worse effective chainlines / alignment in the extremes of the gear range. Again, using the 11sp cassette as an example with a perfectly-matched chainline front-rear, looking at each end of the range (big ring / smallest sprocket, or small ring / largest sprocket) we have a maximum chain deflection / offset of (37.5/2)-4.25=14.5mm - around 4.25mm or 40-odd percent more than the triple.
The gravel double is similar; moving the crank's CL outboard by 2.5mm worsens the chainline in the lowest gear (big ring, smallest sprocket) by the same amount and improves it in the same way in the highest gear - giving offsets of around 17.0mm and 12.0mm respectively for the 11sp example.
MTB Double
49.0 or 52.0mm crankset chainline; typically used with 130mm QR, 135mm QR / 142mm TA, 148mmTA and 157mm TA rear ends with 45.0mm 48.0mm and 52.5mm(?) CLs respectively.
Lots of combinations here; with the significant changes in MTB design driving the seemingly constant drive to push chainline ever-more outboard. I suspect that the 49mm crankset CL is / was intended to be used with older rear-end 135mm QR and 142mm TA standards predominently (45.0mm CL), while the 52mm standard was introduced to suit the wider boost and super boost standards with 48 and 52.5mm CLs respectively.
As we can see the 45mm CL of 135mm QR and 142mm TAs is sub-optimal with any crankset; best-case giving 4mm of CL misalignment with the 49mm crank for a worst-case offset / chain deflection in the lowest gear of our 11sp example of (37.5/2)-4.25+4=18.5mm.
Pairing a 49mm CL crankset with a 148mm TA boost rear end's 48mm CL results in an insignificant misalignment of 1mm, giving a worst-case low-end chain deflection / offset of (37.5/2)-4.25+1=15.5mm. Similarly matching a 52mm CL crankset with the (assumed!) 52.5mm CL of a a 157mm super-boost rear end actually offsets the crankset slightly inboard for a change; giving a worst-case chain deflection / offset in the highest gear of (37.5/2)-4.25+0.5=15.0mm
Gravel & MTB Single
52.0, 55.0 and 56.mm crankset chainline; typically used with 135mm QR / 142mm TA, 148mmTA and 157mm TA rear ends with 45.0mm 48.0mm and 52.5mm(?) CLs respectively.
The new messiah that is 1x drivetrains brings a number of undesirable traits to the chainline game. First and most obviously is the need for a single chainring to transmit drive across the entire range of the cassette; rather than the 50-75% of its width you might typically expect from triples or doubles respectively.
Further, clearance issues between the single chainring (which would be larger the inner ring on a double) and chainstays means that rings are typically offset outboard; further worsening chainline.
Again, lots of examples here - let's look at the best we can manage with the 45mm CL of the lowly 135mm QR / 142mm TA standard. Off the shelf it appears that the narrowest 1x crankset CL available is 52mm; meaning an offset of 7mm from the CL at the rear end of the bike. If you're unfortunate enough to use the lowest gearing / largest sprocket on our 11sp example there's going to be a massive offset between it and the chainring of (37.5/2)+7=25.75mm - 2.5 times worse than the ideal example set by the road triple.
While tempting to look at pairing the 52mm CL crankset with the super-wide, 52.5mm CL of the 157mm super-boost rear end, in reality the fat chainstays present to support the fat back end will likely preclude the use of anything but the fattest cranksets - meaning 55 or 56mm chainlines for a marginally better, but still not great amount of chain deflection throughout the gears.
While 1x does have some legit advantages, it's also varying shades of bloody awful in terms of chainline while throwing open the door to all manner of difficult-to-quantify variables regarding max chainring size and subsequent chainline.
Unfortunately the alignment problem is likely worsened by manufacturers being conservative with chainlines (pushing them safely outboard) to give clearance with the largest amount of frames.. there are apparently a few smaller brands offering a greater range of crankset chainlines to allow those prepared to throw time and money at finding the least-worst solution to push the crank CL as far inboard as possible; but the result will still be far from ideal.
Conclusion
As usual for the bike industry, what's been touted as "progress" from many angles has resulted in many unavoidable consequences in other areas.
I hope the above ramblings are at least semi-coherent and might help some find the least-worst solution when selecting or building a bike.
This is intended as a broad-strokes overview and I'm aware that not every permutation has been covered; but I hope it's a sufficiently representative snapshot of the most common offerings. Similarly I'm aware that some examples given don't reflect real-world availability (nobody makes 11sp triples for example) although 11sp was used throughout to keep comparisons consistant.
Please let me know if you see any errors or anything that makes less sense than usual; I'm off out on the MTB to "enjoy" its hideously compromised chainlines....
