Chainline for Marginal Gains

Chainline refers to how straight the chain runs between the front chainring and rear sprocket on your track bike. 

In an ideal world the teeth of your chainring and sprocket should run in the same plane, so that there is no sideways motion causing stress between the links of the chain. When a chain is not in perfect alignment extra friction is created between the links of the chain, and between the chain plates and teeth on the chainring and sprocket as the system rotates. This extra friction is slowing you down, and damaging your equipment.

In current times much research has been done around drive chain lubrication and reducing friction, but without a perfect chainline, all of those marginal gains are useless.

According to the great Sheldon Brown, a track bike chainline should be 42mm ± 2mm.

Depending on manufacturers, and compounding tolerance stack of your equipment, the track bike chainline tolerance standard is ± 2mm. I have seen two bikes built with exactly the same components be 2mm out between one another. You can see how this tolerance difference is possible when you count up all the components that need to be perfectly sized to obtain a perfect chainline between the front chainring and rear sprocket. Below is a list of components that can affect the chainline on your bike. The list begins at the bottom bracket of the frame and travels through the components of the drive chain that can affect the tolerance stack. As you can see, if too many of these features were at the extreme limit of their tolerance allowance, it isn't hard for two identical bikes to have vastly different chainlines.

• Frame paint around bottom bracket shell
• Bottom bracket shell width tolerance
• Bottom bracket shell threads
• Bottom bracket threads
• Bottom bracket axle length tolerance
• Bottom bracket spline diameter (Tapered)
• Bottom bracket spline wear
• Crank spline wear
• Crank forging tolerance
• Crank machining tolerance
• Chainring dimensional tolerance
• Chainring manufacturer (various designed dimensions)
• Chainring teeth width tolerance
• Chain width tolerance
• Sprocket teeth width tolerance
• Sprocket manufacturer (various designed dimensions)
• Sprocket dimensional tolerance
• Sprocket Thread tolerance
• Rear hub thread tolerance
• Rear hub manufacturer (various designs dimensions)
• Rear hub dimensional tolerance
• Rear hub axle assembly stack (including axle, nuts, bearings)
• Frameset dropout insert tolerance
• Frameset dropout spacing tolerance
• Frameset structural makeup (Carbon/steel/aluminium flex) 
• Frameset design (asymmetrical chain/seat stays)

Measuring the Chainline

Chainline is measured from the centerline of the frame to the center of the chain. Runwell tools have developed a handy chainline gauge tool for quickly measuring the chainline on your bike. The card is placed around the seat tube, and an indicator points exactly to 42mm including a ± 2mm tolerance.

You can also measure the front chainline directly with a ruler. Simply hold the ruler against the seat tube and measure the distance from the middle of the seat tube to the middle of the chainring teeth. Using a ruler can be a little more difficult to get an accurate reading due to parallax error or not getting the measurement exactly right in the center of the seat tube.

Measuring the rear chainline is a little more difficult as it can vary depending on the wheel in your frame and needs a bit of math. First you must measure the distance between the inner nuts on the axle of your hub. On a track bike this should be 120mm ± 0.5mm. It is important to measure the hub, because this is what the frameset dropouts will compress down to. Many framesets have a slightly wider dropout width (120-126mm) to make it easier for the rear wheel to slide in and out of the dropouts. When tightened the dropouts will compress down to the width of the hub.

Divide the width of the axle into two (example: 120mm/2 = 60mm).

Measure the distance between the inside of the drive side dropout to the centre of the sprocket teeth. (example: 18mm).

Subtract the dropout to middle of sprocket measurement from the half distance of the wheels axle (example: 60mm-18mm=42mm).

If both measurements are bang on 42mm you are in business for an efficient drive chain. If you are within 2mm this is acceptable, but if you’re outside of the 2mm difference, or you wish to get it even closer to a perfect alignment there are a few steps that you can do.

Methods to Adjust the Front Chainline

Front chainline may be adjusted either by moving chainring, or by moving the crank.

• Prior to doing anything drastic to change your front chainline try a few other brands of chainrings to see if there are any differences in their dishing. Manufacturers make their components to different specifications as there is no official standard for chainring spacing.

• Chainring Spacers
  Adding chainring spacers is the quickest and easiest way to alter the chainline. These can sit between the crank and chainring to shift out the chainring by an extra 1mm. These spacers are available through our store.

• Crank Axle Spacer
  On more modern cranksets with a one piece drive crank and axle, a spacer may be added between the crank and bottom bracket.

• Crank Spider Spacer
  For a crankset with a removable spider, a spacer may be added to space the spider away from the crank.

• Bottom Bracket Spacer
  If you need to increase the chainline (move the chainring to the right) you can usually add a spacer washer between the right-side bottom bracket mounting ring (or cup) and the bottom bracket shell of the frame. Metal spacers without internal lugs used with Shimano cassettes work well. These fit all threaded bottom brackets except Italian size. Adding a spacer on the right side will move the left bottom bracket cup or mounting ring inward -- a lockring might not then engage enough threads, so check before you commit to this.

• Change the Bottom Bracket
  Most cartridge type bottom brackets made since the mid-'90s are symmetrical, they stick out the same amount on each side. If you replace your present bottom bracket with one that has a shorter axle - say, 4 mm shorter, it will move the chainline 2 mm to the left, because it will be 2 mm shorter on each side, and the cranks will remain equally spaced from the bicycle's centerline. If you go this route to a narrower chainline, make sure that there won't be a clearance problem between the frame and the crank or chainring.

• Replace the Cranks
  Sometimes it is not worth fighting. Do some research on cranks, bottom brackets and their chainlines and mounting systems. Your local bike shop should be able to help you out.
  
  

Methods to Adjust the Rear Chainline

• Prior to doing anything drastic to change your rear chainline try a few other brands of sprockets to see if there are any differences in their dishing. Some manufacturers make their components to different specifications as there is no official standard for sprocket spacing.

• Sprocket Spacing
  We make a stainless steel washer shims of various widths to space the sprockets out. Multiple spacers can be used to increase the offset required.
  

• Flipping the Sprocket
  Fixed-gear sprockets are usually intended to be installed with the flange facing inward, so the teeth are outboard. However, in some cases it is possible to flip a fixed-gear sprocket over if you need to move the rear chainline to the left. In combination with sprocket spacers (mentioned above) you can adjust the chainline spacing inwards at various increments. When fixing the sprocket in this method, make sure you have enough sprocket thread engagement or you risk stripping the threads on your hub.

• Axle Spacing
  Chainline may be adjusted at the rear wheel by rearranging spacers on the rear axle. Hubs with conventional threaded axles, can be spaced by removing the locknuts and adding M10 spacer washers to offset the centre line of the hub within the dropout. As dropouts on many frames are wider than 120mm, there is often room to add in some extra axle width if required to create this offset.
  
  If you add equal thicknesses to both sides, the chainline is unaffected, since it's measured from the middle outward. If you add more spacers to one side, or move them from one side to the other, you can change the chainline, but this will render a double-sided (flip-flop) hub unusable on one side or the other, since you'll be increasing the chainline on one side while decreasing it on the other - unless you also use a different chainring depending on which way the wheel is inserted.
  
  Adding spacers on one side only will disturb the central alignment of the wheel in the frame. If only 1-2mm it might be ok, but if major adjustments are required, the wheel spokes may require re-dishing by tightening the spokes on one side of the wheel, and loosening them on the other. For one piece carbon wheels, re-dishing isn't an option - so make sure there is enough clearance between the wheel, tire and frame beforehand.
  
  

One last technique used by professional teams is to run 3/32” chainrings and sprockets with ⅛” chains. The extra width available for the chain to slop from side to side means there is greater tolerance for the chain to be misaligned without the negative effects. Some testing has been done in this field to identify that some friction losses are negated with the less chance of chain plates sliding across the teeth on the sprocket and chainring.

Have we missed anything out? Let us know!