Advanced Double Crankset Conversion

The main hurdle to converting a road bike to a single sprocket bicycle (fixed gear or single freewheel) is to get the correct chain line. While many simply put their main chainring on the innermost placement of their double chainring and call it good enough, chainline is something that can be calculated and the aforementioned practice can be avoided. The issue about chainline is the fact that it's quite an abstract concept considering it's a fixed dimension that depends on two variable dimensions. Manufacturers don't make it easier because they usually tell both dimensions determine the chainline. For exemple, you will find the chainline in the specifications of either Bottom Brackets or Cranks depending on manufacturer, crankset, bottom bracket and even then, there is no real standard to how a chainline works and what it depends on. In this article, we're going to theorise the length of the bottom bracket needed to turn a double crankset into a single chainring crankset. We will focus on square axle bottom brackets as they are the most common type of BB used in conversions.

Theory

What are chainlines ?

The basic gist of a chainline is that it is the straight line from the crankset to the back sprockets. There are multiple chainlines because there are different use cases and different cranksets. Chainlines make for a starting point for the chain to pivot from on a multiple sprocket bike. It is meant to be a "middle-ground" amidst all possible combinations of gears and chainrings.

But first, let's try to define what we aim for : a single chainring chainline. The single chainring chainline is the distance between the middle of your chainring and the plane that cuts your bike in half from one end to the other or to put it in a more telling way, the middle of your seat tube. It is meant to be as straight as possible from the chainring to the sprocket as these two points are the only possible positions.

The most common Single Chainring chainline is 42,5mm.

Now let's try to define what we're starting from : a double chainring chainline. The double chainring chainline is the distance between the middle of the space between your two chainrings and the same plane as before. Where it deviates in definition to the Single Chainring chainline is that there are two pivot points in a double chainring drivetrain. The chainline serves the same purpose : a middle-ground, but as the chain is a flexible component and as there are multiple sprockets, the double chainring system allows for multiple positions. Even in the optimal use cases, the line from the chainring to the chosen sprocket may not be straight. In fact, you can deviate from the recommended chainline and still keep a perfectly fine drivetrain.

The most common Double Chainring chainline is 43,5mm. It is the Shimano standard.

How are chainlines ?

Chainlines are physically defined by the drive-side length of the axle from the middle of axle (same plane mentioned before) and where the chainring is situated on the crank, whether the crank forms a more or less concave element. Which is why different cranksets can require bottom brackets of different lengths while keeping the same chainline.

As bottom brackets come in many different lengths, we can shorten the required length of the bottom bracket from a double chainring crankset to account for the difference in the concave-ness between a single-chainring crank and a double-chainring crank and therefore give a double-chainring crank a single-chainring chainline.

Where to start from ?

The start of the journey to theorise this entreprise is a double chainring crankset being used a single chainring crankset, with all specifications mentioned. In this case, we're going to take the case of the Stronglight 107 crankset, according to the 1984 Stronglight No. 27 catalogue :

Width of the bottom bracket box	Length of the bottom bracket axle	A (mm)	B (mm)	C (mm)	chainline
68	113	28,5	56	28,5	Track
68	118,5	30,25	56	32,25	43,5