The tragedy of modern chainline standards

[wafter]

A post sparked by recent disappointing 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 for both 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 upon / 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 bikes sit in some weird middle ground; having quickly adopted the 142mm TA standard but some are now using 148mm; which is very much the current MTB standard. Seems adoption of 157mm has been somewhat limited.


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 chainlines are.

For this example I've chosen 11speed to give illustrative numbers since 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 at 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 optimum 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 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 about the crank's centreline; 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 chainlines 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 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 to 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 first 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....

 

[fossyant]

It can be a nightmare. Thankfully my CX bike is still 130mm ! Does help with wheel swapability.

 

[DCLane]

One note: it's comprehensive although what about older bikes / fixed with 120/126mm wheels and their chainlines?

 

[fritz katzenjammer]

Building new bikes for Trek I was often surprised by the offset between cassette centerline and chainring(s) centerline, making cross chaining a problem even on 1X systems!

Some 2X road bikes were bad enough that the chain dragged on the side of the large chainring when running on the small chainring and smaller cogs on the cassette.

not everybody appreciates that they should avoid cross chaining and so do it all the time so you would think the factory would aim to minimize it when selecting components but oh no…

 

[Ming the Merciless]

Get a recumbent, problem solved.

 

[Pblakeney]

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.

Campagnolo Athena was available in triple 11 speed. You won't find it new from shops but NOS can be found on Ebay.

 

[FrankCrank]

Yet to have a chain line that I couldn't find a workaround for, usually a bodge of some sort, but that's a benefit of clunkers.

 

[Dogtrousers]

Interesting to know. I learned some stuff.

Not actually useful to me at the moment, but could be useful in future so I have mentally bookmarked this page.

 

[sevenfourate]

Yet to have a chain line that I couldn't find a workaround for, usually a bodge of some sort, but that's a benefit of clunkers.

If it works long-term and achieves what you need: is it a bodge…….

 

[sevenfourate]

Really Informative and interesting read as ever @wafter. Thank you for your time and expertise…. 😁

As someone who’s pondering building a new 1 x 12 as a daily; the thought of all the possible ways of achieving poor chain alignment both stimulates and horrifies me…….

*I also know 12’s are more sensitive to chain - or perhaps drivetrain wear as a whole. If it (Chainline / cross-chaining) fits in with the context of this thread - I’d be most interested to hear people’s experience of a 1 x 12 as a year round rider.

 

[wafter]

One note: it's comprehensive although what about older bikes / fixed with 120/126mm wheels and their chainlines?

The clue's in the title

My experience in this field is limited to the bikes I have access to and what I can dig up on the net; while the motive to investigate this subject that underpins this post comes from the inevitable shortcomings of some of my own gear; so this is where I've concentrated my efforts.

I have no idea about the earlier standards; although I suspect that the narrower CLs quoted for 2x and 3x setups may have originated with the axle spacings you mention.

Refreshing my memory thanks to Sheldon Brown's excellent website, it appears that 120mm is a 5sp standard, 126mm 6/7sp while MTBs started to diverge at this point with 7sp 130mm axle spacing. My Raleigh is 5sp so IIRC 120mm; maybe I'll take a look at this if I get a moment.. although since it doesn't key into the current source of imperfection-driven angst, don't hold your breath

Building new bikes for Trek I was often surprised by the offset between cassette centerline and chainring(s) centerline, making cross chaining a problem even on 1X systems!

Some 2X road bikes were bad enough that the chain dragged on the side of the large chainring when running on the small chainring and smaller cogs on the cassette.

not everybody appreciates that they should avoid cross chaining and so do it all the time so you would think the factory would aim to minimize it when selecting components but oh no…

Indeed - unfortunately being predminently a leisure industry it seems that bike manufacturers can get away with a lot of shonky practices without really being held to account

Campagnolo Athena was available in triple 11 speed. You won't find it new from shops but NOS can be found on Ebay.

Thanks - wasn't aware of that I've only really had experience of Shimano gear.

Good on them for keeping the glorious triple alive for longer than Shimano saw fit to!

Yet to have a chain line that I couldn't find a workaround for, usually a bodge of some sort, but that's a benefit of clunkers.

Great work

I've been thinking along these lines too as currently much of my meagre intellectual capacity is assigned to making MTB triples work properly on my gear.

At the risk of entering a rabbithole that nobody wishes to follow me down, the situation with my Fuji (which is analgous to other setups running 130mm QR / 142mm TA with MTB triples) is an interesting case in point.

When I got the bike I was shocked to discover that the Deore M591 crankset ran a 6.5mm longer axle than standard, along with a 6.5mm thick spacer on the drive side between the crank arm and BB. This resulted in a corresponding-ish offset of the cranks to the DS; causing the DS crank to be further outboard than the NDS crank by around 10.5mm. This is apparently to allow the use of a chaincase (which I've never, ever seen) on the DS.

Disgusted this was replaced with an M590 crankset which has a shorter axle, used no spacer and had symmetrical cranks about the bike's centreline. While I don't regret this replacement as it was in the name of fitting shorter cranks (the offset issue being discovered by accident) it occurred to me that if the M591 crank was used, the 6.5mm spacer on the DS could be replaced with a 1.5mm job and a corresponding 5mm spacer fitted to the NDS - magically pulling the DS inboard by 5mm and simultaneously correcting both the chainline from 50 to 45mm and evening out the crank spacing across the bike

There are however three potential issues:

The M591 cranksets are extremely thin on the ground; which is why I ended up with the M590 replacement in the first place.

Pulling the CL inboard might be beyond the range of the front derailleur; although (clearances permitting) I could potentially get away with using a road FD as the cable pulls should be the same being 9sp.

Finally the big one; do I actually want to pull the chainline inboard? One the one hand it makes the most sense on paper as everything is balanced, the maximum chain deflection reduced to a minimum and the while gear range spread most evenly across all three chainrings.

However, I ride almost exclusively in the outer two rings and probably outer 2/3ds of the cassette - so it may be that for my particular use case the chainline is actually better as it stands - maybe I need to re-sell it to myself as a narrow-range 7.5sp double with bailout options

One benefit of aligning the chainlines would be that I'd be able to spend more time legitimately in the more efficient big ring (five sprockets versus three) although the chainlines at the extremes of these ranges would be circa. twice as bad for the equal chainlines.

I think I'll keep an eye out for a hen's teeth 170mm M591; and if I get a stab at a decent one for reasonable money might just give it a go out of curiousity..

Really Informative and interesting read as ever @wafter. Thank you for your time and expertise…. 😁

As someone who’s pondering building a new 1 x 12 as a daily; the thought of all the possible ways of achieving poor chain alignment both stimulates and horrifies me…….

*I also know 12’s are more sensitive to chain - or perhaps drivetrain wear as a whole. If it (Chainline / cross-chaining) fits in with the context of this thread - I’d be most interested to hear people’s experience of a 1 x 12 as a year round rider.

Thanks!

Really all I've done is lob a load of information together and it's not entirely altruistic as it helps me get my head around it while of course I always appreciate constructive input and somewhat might spot what I've missed.

I can't speak with authority on the 12sp situation, although did give the Ragley a good coat of looking over when I got back earlier and on paper the situation's not as bad as I'd expected.

It's running a Raceface Ride direct mount crank with 30t ring at a measured 51mm chainline. If the Deore 10(!)-51t cassette has the 48mm CL expected of it's boost setup that's only a mis-match of 3mm; while the comically large sprockets are used less than the rest so again, perhaps this setup's not as bad as it might be.

That said the 148mm TA boost standard and 48mm CL is better in this regard than the 142mm TA and 45mm CL you're most likely to find on a more commuter-centric gravel bikes (unless you plan to stick some fat slicks on a hardtail or monstercross type offering) so my experience might not offer you much relevant insight. In addition the 51mm CL of the Raceface crankset seems to be a bit more daring than the 52mm smallest offering apparently available from Shimano.

All that said, I've just discovered that the GRX cranksets in 1x configuration are slated at just under 50mm CL, so you could claw back a bit there. Also, I've read that (historically at least) Garbaruk (?) offer rings of varying CLs so this might be worth exploring.

I did check the clearance between the Ragley's chainstay and 30t ring at about 8.5mm; so if sticking to this ring size I could in theory fully correct the chainline to the optimum 48mm, were a suitable crankset available.

What sort of bike are you considering for your daily? I've been looking at ATB-type stuff recently for a similar application... while not common there are a few offerings with 148mm axles...

EDIT: Another thought; IIRC some Shimano direct mount 1x chainrings run separate rings and spiders with a bolted interface. It might be possible to add spacers to push the ring inboard and reduce the chainline..

 

[wafter]

If anyone's still reading I had a stab at measuring the chainline of the 12sp 10-51 cassette on the the Ragley's 148mm TA rear end.

Conventional wisdom would suggest this has a 48mm chainline (from the 45mm CL of the 142mm axle plus half the difference) however the actual measurement (which I'm reasonably confident in, plus or minus half a mil) came in at 46mm..

This is a bit disappointing and suggests yet another fairly significant (5mm) mis-match in CL between front and rear.. although again I suppose the real-world implications depend on which sprockets I actually use the most; which truth be told are typically towards the smaller end.

 

[sevenfourate]

What sort of bike are you considering for your daily? I've been looking at ATB-type stuff recently for a similar application... while not common there are a few offerings with 148mm axles...

Yes definitely an ATB. Something infinitely lighter, solid forked and with more / a slightly wider spread of (1 x) gears, hydraulic braked and with flat bars. And really capable of housing 42mm rubber with plenty of clearance for mud, crud and sticks etc. So a chunky’ish Hybrid / Flatbar all terrain machine.

To replace / upgrade from my current Carrera Subway 1 do-it-all machine. The lighter weight, more gears on the rear - with a slightly wider spread than the current 11-32t rear cassette; and hydraulic stoppers being the main areas I’m looking to upgrade to.

A couple of suitable frames I’ve found as a start point are:

https://ebay.us/m/x6PeNW

https://ebay.us/m/XtNLT6

**And yes I’m still reading / following. And worrying.

Can too much info be a bad thing 🤣