This post explains what bicycle chainline is. Why is it important? How is it measured? How to set a “correct” chainline?
Definition is: distance between the middle of chainring(s set/s) and the longitudal axis of the bicycle. It is explained in plain English in this post. 🙂
Middle of the front chainring and middle of the rear sprocket should be at the same distance from the axis that goes along the middle of the bicycle frame from the front to the back. That means the chain will run completely straight from the front to the rear. For bicycles with more speeds, definition is the same, only measurement is taken as middle of the sets of chainrings and sprockets. Picture explains it better:
For single speed bicycles (one chainring at the front and at the back) the answer is more obvious: the straighter the chain, the longer it will take for it to wear out, the less mechanical losses it creates when pedaling and the harder it will drop off the chainrings.
For multi speed bicycles (more sprockets), the answer is a bit more complicated. If the middle of front chainrings is not aligned with the middle of the rear sprocket set, then in some gear combinations, the chain will be too crossed. Picture below explains it better:
Upper half of the picture number 3: middle of the sprocket set is aligned with the middle of the front chainring. That way, in either gear combination, the chain is not at a great angle (green lines).
Lower half: front chainring is too far out compared to the middle of the sprocket set. That means that in top gear (blue line), the chain is almost straight. However, in lowest gear (red line), the chain runs at a sharp angle – bringing more wear and tear to both the chain and the sprockets, more noise and less mechanical efficiency of the whole system.
For double chainring set – the middle between the two front chainrings should be aligned with the middle of the sprocket set. For triples – middle chainring is aligned with the middle of the sprocket set. This is shown in the picture 2.
It can be measured simply with a ruler:
Technically more precise measurement is taken this way:
- measuring diameter of the seat post and the width of the front chainring (set) with vernier calipers. “Set” means the total width of the two, or three chainrings if the crankset is a double, or a triple one.
- measuring distance from the seatpost to the end of the largest chainring.
- add the measured distance to the half of the seatpost width and then deduct half of the chainring (set) width. That’s the front chainline.
For the example in picture 5, the chainline would be:
44 / 2 + 29 – (9.8 /2) =
= 22 + 29 – 4.9 = 46.1 mm
Measure the width of the rear hub. OLD (over locknut distance).
The whole axle is not measured, just the inner locknuts that hold the hub bearings in place. For solid axles – outer nuts that hold the wheel in place are disregarded. Only the part of the hub that sits against the inner part of the rear dropouts is measured.
Most modern rear MTB hubs are 135 mm wide (OLD), most road ones 130 mm, while older road hubs are 126 mm. Ancient road and newer single speed hubs are usually 110 to 120 mm wide.
With help of a picture, the procedure is more easily explained:
- First measure a), like explained in the text above
- Then measure b) – from the hub locknut, to the begining of the smallest sprocket. Measure to the face of the sprocket itself, not to the locknut holding the sprockets in place.
- Finally measure c) – from the beginning of the smallest, to the end of the largest sprocket
Rear chainline = (a / 2 ) – ( (c / 2) + b)
For example: if a = 135 mm. b=4 mm. c = 35.4
Chainline = (135 / 2) – ( (35.4 / 2) + 4)
= 67.5 – ( 17.7 + 4)
= 67.5 – 21.7
= 45.8 mm
By setting a correct chainline it is meant to set front and rear sprockets so that their middles are in line, like shown in pictures 1 and 2. Literally: front chainline (measurement explained in paragraph 3.1.) and rear chainline (3.2.) should be the same.
When doing that, rear sprockets are usually taken as a fixed position, with front chainring position set to match them. That case is first explained under 4.1.
- For single chainring crankset, where chainring is bolted to the spider holding it, the chainring can be moved outwards with adding spacers between the chainring and the spider, then bolting it. To move the chainring inwards, it can be mounted on the inner side of the spider, with fine adjustment made using spacers, as shown in picture 8:
- With triple cranksets, smallest, or largest chainring can be removed, or front derailleur set so it never moves onto one of them, effectively altering the chainline according to needs.
- A spacer can be added between the right bottom bracket cup and the frame, in order to move chainrings outward.
- Longer, or shorter axle (spindle) can be used with square tapper and Octalink bottom brackets to move the chainrings outward, or inward. When using shorter axle, make sure that crank arms aren’t too close to the frame so they don’t hit the rear chainstay when turning.
- Replacing crankset for one with more outward, or inward placed chainrings.
It is important to bare in mind the type of chainrings (crankset) used. This goes especially for square tapper cranksets. On an axle of the same width, some will stick further outwards, while others may be positioned more inwards, towards the frame. This plays an important role when choosing a correct BB axle length. Some crankset manufacturers publish required axle lengths for desired chainlines, while with others it’s more trial and error.
Adjusting the rear chainline is done by rearranging the rear axle spacers.
- Spacers from the left side are placed on the right, to make the sprockets move inward.
- Spacers from the right are moved to the left side to make the sprockets move outward.
It is important to note that this affects:
- Room left for rear sprockets – they might start rubbing the frame if too many spacers are removed from the right side.
- Disc brake alignment – make sure disc brake calipers can be moved laterally enough to match the sprocket rearrangement. If a spacer is added to the left side, disc brake calipers must be moved inwards. If a spacer is removed from the left side, calipers must be moved outwards.
- Rim alignment – rim will no longer sit in the middle of the frame, and must be re-dished. If a spacer is added to the left side, left hand side spokes must be tensioned to move the rim back into the middle. If a spacer is added to the right side, right hand side spokes must be tensioned to center the rim.
It can be also combined by discarding a wide spacer and replacing it with a narrower one from some old axle, then placing a third, narrower spacer on the opposite side to keep the total used spacer width the same.
Setting correct chainline using rear hub is a complicated task that is not recommended, unless it’s not possible to get a satisfactory chainline by adjusting front chainrings. It’s often better to leave a less than perfect chainline, than to mess with the rear hub spacers.
I do most of my riding from the largest front chainring. That’s why I like to have my chainrings more to the inward, than it is perfect. That way, the chain is a bit more crossed when using smallest chainring, but where I do most of my riding, on the big chainring, the chain is straighter than it would be if I had set the correct chainline.
Rule of thumb and safe bet is to set the chainline correct, but if one is certain what they are doing, they can play with imperfect chainlines. Of course, diverting from perfect chainline has it’s drawbacks and consequences, like shown in picture 10:
Important note: when experimenting with imperfect front chainlines, for bicycles with front derailleurs (FDs), it is important to make sure the FD can reach all the chainrings. If they are moved too much outwards, FD might not reach far out enough to shift to the largest chainring, and vice-versa – if moving chainrings too much inward, FD might not be able to move in enough to shift to the smallest chainring.
Axle measurements for BBs with axle (square tapper and Octalink are the most widely used ones) are noted in picture 11:
Measurement d) is always taken from the inside of the flange that aligns with the outer side of the frame’s BB shell. The drive side is always screwed all the way in, until the flange stops at the BB shell wall.
Shimano often notes on a paper inside the package of their (square taper and Octalink) cranks which BB model (and axle length) gives which chainline with that particular crank. Without this, it’s often a trial and error, with various axle lengths.