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Revised: Bicycle Tech Note -
How Chainline Affects Drivetrain Performance

05-16-23
New revisions include:  1) Using taut string instead of yard sticks to determine centerline of bicycle at rear axle.  2) Recommend procedures to adjust chain line to become parallel.

05-30-19
The chainline of a bicycle is the line the chain makes from the front sprocket (chain ring) to the rear sprocket (gear or gear cluster).  Ideally, the chainline is parallel to the bike’s centerline when measured from the middle front sprocket to the middle rear sprocket.  If the bike has three chainrings, then it is measured from the second or middle chainring.  If it has two chainrings, then it is measured in between the two chainrings.  Likewise, for the rear gear cluster it is measured from the middle gear, or in between if there are an even number of gears.

The idea to locate the chainline parallel to the bike’s centerline is intuitively obvious. It will cause the straightest and most efficient drivetrain.  It will optimize the shifting of all the gears and eliminate any extreme angles of the chain which can contribute to “cross-chaining”.  Cross-chaining is the interference caused by the chain with either the derailleur cage, another chainring, or the chatter caused by too great an angle with a gear tooth.  The first condition of interference with the derailleur cage is the one most familiar to people.  Too much cross-chaining in a drivetrain reduces the number of effective gears.  For instance, for a 7-speed gear cluster, if there is cross-chaining when shifting into the lowest or highest gear, then only five of the seven gears are actually effective, and only these five gears should be used.  However, by eliminating cross-chaining more gears can be made available.  This is accomplished by ensuring that the rotating components of the drivetrain are true and round, and positioning the chainline correctly.

A bicycle's chain line may not be parallel due to several reasons.  Over the bike's history, it may have undergone upgrades to the drivetrain such as a new gear cluster, new freehub, new crank or chain rings, new bottom bracket, or a combination thereof.  But also, surprisingly, a new bicycle may not have been built with a parallel chain line, including higher-end bikes.

But also, surprisingly, a new bicycle may not have been built with a parallel chain line, including higher-end bikes.

Checking the position of a chainline requires measuring the aforementioned distances.  Measuring distances on a bicycle may be easier said then done, simply because the round tubing makes it difficult.  The first problem is locating the centerline of the bike near the locations of the sprockets, both front and rear.  For the front, it may simply be a matter of measuring from the tooth of the middle chainring to the middle of the seat tube.  The seat tube is the one most vertical on the bike, and as it name implies, where the seat and seat post are installed.  If you don’t have measuring calipers, you can lightly close a C-clamp about the seat tube, remove the C-clamp, and then measure the gap the C-clamp made about the diameter.  Divide this by 2 for the radius.  Then measure from the tooth of the chainring to the outboard surface (RH side) of the seat tube, and add this to the radius.  This will be the chainline distance for the front sprocket.

For the rear, if you don’t have sophisticated tools like a bike shop for measuring bike parameters, you can invert the bike onto a floor with the rear wheel removed.  Use two straight-edges, such as two wooden yard sticks, clamped to the top-tube and the seat-tube and angled such that the ends are near the rear axle line.  The two straight-edges should be checked beforehand on a flat surface, and if using metal straight-edges, wrap a layer of tape around the bicycle tubes to protect the paint.

 
  

To obtain a bicycle's centerline, invert the bike onto a floor and clamp two yard sticks to the front triangle with one end of the yard sticks near the rear axle line (quick release skewer shows axle line).

 

Instead of yard sticks, it is possible to stretch a taut string from both sides of the headset to the rear axle.  This method is preferred when there is some difficulty obtaining flat yard sticks or slats - or perhaps worse - it becomes difficult to find a flat surface to check the flatness of a slat. 

From your bike specs, see if you can obtain the dimension of the rear hub called the “O.L.D.”, which is the “Over-Locknut Dimension”.  If you can’t find this spec, it is essentially the space between the rear chainstays when the wheel is installed.  Since the rear chainstays of the bike frame can typically spring outward when the wheel is removed, it is only accurate to obtain this dimension when the wheel is clamped in place.  Otherwise, the O.L.D. can be obtained directly by measuring it from the hub itself.

When you have the O.L.D., place a quick-release skewer in the rear lugs and gradually close it (e.g., turn CW) so that the space between the chainstays match the exact dimension of the O.L.D.  Wrap some legible tape about the middle of the skewer, then measure the space between the two straight-edges, divide by two, and mark this on the tape with a pen.  This will coincide with the bike’s centerline, as referenced from the “front triangle”.  The front triangle of the bicycle is formed by the top tube, down tube, and seat tube - and is the only true reference on the bike for obtaining the centerline.  Don’t be surprised that the centerline is not the same as the midpoint between the rear chainstays.  The bike is made unsymmetrical in this way to accommodate the driveline components.

When using string, tape the string to the front of the headset allowing the string to normally contact each side of the headset when stretched.  One side at a time, stretch the string taut from the headset, gradually bringing it inboard across the skewer in the rear axle.  When the string first contacts the seatpost tube, mark the location on the skewer (legible tape).

 

        

To obtain the bike's centerline using a taut string, tape the string to the front of the headset as shown above and stretch it to the rear axle, sliding the string inboard across the skewer until it first contacts the seatpost tube.

 

Now pick up the rear wheel and measure from the tooth of the middle gear to the end of the hub, which is to the end of the overlock nut (the right end of the O.L.D.).  On the skewer (still clamped in the bike), measure from the centerline that was just marked to the inside of the right chainstay (also the right end of the O.L.D.).  Subtract the two and this will be the rear chainline distance.  If the gear teeth have a staggered design (to aid shifting), take the midpoint of the gear-plate thickness visible in the trough.

Either the front and rear chainline measurements match and you have a parallel chainline, or it may show itself as an unparallel chainline.  A parallel chainline is not the final say, however, and it can be the choice to have a slightly angled chainline in favor of lower or higher gearing.  In other words, if the front chainline distance is slightly outboard of the rear distance, it favors lower gearing (mountain biking); and vice versa to favor higher gearing (road biking).  If you are interested in overall optimization and achieving the most number of effective gears, then a parallel chainline is the choice.  If the chainline distances don’t equal each other, you can adjust the spacing of the rear gear cluster or cassette on the hub, or change the spindle length of the bottom bracket.  The components affecting chainline are known to be adjustable on just about all bikes.

To adjust the chainline to become parallel, the first recommended choice is to change the spindle length of the bottom bracket assembly.  Changing the spacing of the rear cluster may be rather limited, and in some cases, there may be no room at all to make this adjustment.  An example is a 7-gear cluster assembled to a 7-gear freehub (instead of a 7/8-gear freehub).  But bottom bracket spindle lengths come in quite a variety of sizes.  You will most likley always be able to adjust the chainline by way of the bottom bracket.  For  instance, a cartridge bottom bracket has a 122mm length spindle and you discover that to make the chainline parallel you need a 115mm spindle.  Simply swap-out the existing bottom bracket with one that has a 115mm spindle length.  If the chainline is unparallel in the other sense, change the bottom bracket to a longer spindle length to bring the chain rings outboard.  If not using a cartridge bottom bracket, the adjustment can be less costly by changing the spindle only.

 

The picture above shows a multitude of spindle lengths for a size 68mm bottom bracket.  The lower diagram indicates an unsymmetric spindle, in which case you will need to pay close attention to the drive-side of the spindle dimension when adjusting the chainline.