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 chainline. 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 chainline is quite an abstract concept considering it's a fixed dimension that depends on two variable dimensions : bottom bracket length and crank indentation. 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 threaded square axle bottom brackets for 68mm bottom bracket boxes as they are the most common type of Bottom Bracket used in conversions. We also only will talk about Double-Chainring Cranksets. No Triple.

Theory

Definition of a chainline

The chainline is the line the chain takes in its straight position within the drivetrain. There are multiple chainlines because there are different amounts of chainrings on a crankset. 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. It is physically defined on a bike as a dimension that goes from the plane that cuts your bike in half from one end to the other or in practice to where the middle of your chainring assembly is. The crank forms a more or less concave element, this explains why different cranksets can require bottom brackets of different lengths while keeping the same chainline.

But first, let's try to define what we aim for : a single chainring and sprocket chainline. It can also be called a Track chainline. It is the distance between the middle of your chainring and the same plane mentioned before or in practice, 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. It is the distance between the middle of your two chainrings and still the same plane. 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. And 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 bottom bracket length (and therefore chainline) and still keep a perfectly fine drivetrain.

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

• 
  42,5mm single chainring chainline with single chainring crankset
• 
  43,5mm double chainring chainline with double chainring crankset

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

• 
  42,5mm single chainring chainline with double chainring crankset

Proof of concept

If in theory the concept is pretty simple, the best way to prove that it is a viable way of doing things is an example provided by a credible source : a double chainring crankset being used a single chainring crankset that use both aforementioned chainlines. To build upon this proof of concept, we also need it to give us all bottom bracket dimensions mentioned.

This proof of concept is given to us by the Stronglight 107 crankset, according to the 1984 Stronglight No. 27 catalogue. The catalogue tells us that this crankset can be used as a track crankset or as a road double crankset. It also gives us all the dimensions of the bottom brackets used :

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 (we can assume the 42,5)
68	118,5	30,25	56	32,25	43,5

• 
  Legend of the table

Building upon that proof of concept

The table tells us that if you want to use the Stronglight 107 crankset as a Double Chainring crankset with a 43,5mm chainline, the length of the drive-side length of the bottom bracket axle is 32,25mm. And that when this same crankset is used to have a 42,5mm track chainline, the length of the drive-side length of the bottom bracket axle is 28,5mm. The difference between the two lengths being 3.75mm, we can assume that shortening the drive-side length of a Double Chainring crankset using a 43.5mm chainline by that amount will result in a 42.5mm chainline if you were to use the same crankset as a Single Chainring crankset.

Addressing different chain standards

There are different sizes of chains : 1/2"x3/32" (usually mentioned as 2,38 in french catalogues) and 1/2"x1/8" (usually mentioned as 3,17 in french catalogues). The first size, 3/32", was used for a long time ONLY for multiple sprockets drivetrains and 1/8" ONLY for single speed drivetrains. But nowadays the difference is almost inexistant as using a 3/32" chain for a single speed is totally feasible and even featured since the mid-80s on the Dura-Ace FC-7600 groupset.

The difference in width of the chainring between the two sizes could be mentioned as a possible error in the mathematics. But it's accounted for as, visibly, the catalogue mentions each size of chain for each use of the 107 crankset.

Conclusion

The way of converting a road double crankset to a track crankset is by shorting by 3,75mm the drive-side length of the bottom bracket axle.

Hypotheses

Disclaimer

Issues arise from the dimensions given by the manufacturers and the different types of bottom brackets nowadays. The days of adjustable bottom bracket like the ones the 107 used are long gone, and while dimensions shouldn't suffer from it, as the bottom bracket box dimensions are still the same and the chainline remains the same, the cups and bearings have changed. It's hard to find schematics of a brand new bottom bracket that are as simple and precise as the ones from the Stronglight catalog. So, much of what will follow is strictly theory and there will be trial and error if you seek the perfect chainline.

As Shimano cranks are universally common and cheap and as Shimano have a lot of documentation on them even from decades ago, making educated guesses about converting Shimano cranks is a lot easier than almost anything else. Campagnolo on the other hand makes it extremely difficult as they very rarely provide any information past 1990, let alone specifications of something like a bottom bracket axle.

Database

Here is a list of a few cranksets, the associated bottom bracket and its length, the length of the drive side of the axle, the recommended length for a conversion with an exemple of available bottom brackets and what the measured chainline is. This will serve as a database for the fixed gear cyclist on a budget that has a totally functionning double-chainring crankset and wants to use it for his fixed-gear bike conversion.

Model of crankset	Associated bottom bracket	Length (drive side length)	Converted Length (new drive side length)	Recommended bottom bracket length (drive side length)	Measured chainline of the conversion
Shimano FC-A551	BB-UN52	107 (?)	102,5	103mm Neco B910/B920/B940CT (28mm)
Shimano FC-A550	BB-A550	113 (31)	105.5 (26.25)	103 mm Neco B910/B920/B940 (28mm)
Shimano FC-A415	BB-UN52	107 (?)	99.5
Shimano FC-1050	BB-1050	113 (30.5)	105.5 (26.75)	103mm Neco B910/B920/B940 (28mm)
Shimano FC-1051	BB-1050	113 (30.5)	105.5 (26.75)	103mm Neco B910/B920/B940 (28mm)
Shimano FC-1055	BB-1055	113 (31)	105.5 (26.25)	103mm Neco B910/B920/B940 (28mm)
Shimano FC-1056	BB-UN52	107 (?)	99.5
Shimano FC-4400	BB-UN52	110 (?)	103.5	103mm Neco B910/B920/B940CT (28mm)
Shimano FC-7400	BB-7400	112 (32.5)	104.5 (28.75)	108mm Neco B910/B920/B940 (29mm)
Shimano FC-7402	BB-7400	112 (32.5)	104.5 (28.75)	108mm Neco B910/B920/B940 (29mm)
Shimano FC-7402-SG	BB-7400	112 (32.5)	104.5 (28.75)	108mm Neco B910/B920/B940 (29mm)

Attention put to the different dimensions given

Most, if not all, of the recommended bottom brackets come from Neco. Neco give a plethora of dimensions for their bottom brackets, it's really useful for our hypotheses but it is necessary to be wary of some of them : if you look at the B920HAL, the first two Bottom Bracket Axle lengths are 107.5mm and 110.5mm. Yet both drive side axle length are 20.5mm, the box being the same 68mm, the difference comes from the length of the non-drive side : 19mm vs 22.5mm.

In general, it is better to go with a matching (or closer) drive-side axle length than a matching (or closer) total axle length and then go with the non-drive side as close to the drive side as possible, to get closer to a symmetric bottom bracket.

Conversion table between 68mm and 50mm B-dimension bottom bracket dimensions

As nowadays, open ball bearings become rarer and rarer, manufacturers don't give the same dimensions. Open ball bearing bottom bracket will give a B dimension between 50 and 56mm when Industrial ball bearings bottom brackets will give a B dimension that is 68mm. The following table was made with the dimensions given in the Neco catalogue. The 103 (50) and 107.5 (50) lines were calculated using the difference between the 108 (68) and 108 (50), they are estimations.

total length	A	B	C
103 (50)	25	50	28
103 (68)	16.5	68	18.5
107.5 (50)	27.5	50	30
107.5 (68)	19	68	20.5
108 (50)	29	50	29
108 (68)	20.5	68	19.5

Practice

In reality this practice is a LOT more common than we can imagine. We saw it with the Stronglight 107 but Cheap "fixie" bikes also very often use double-chainring cranksets with the sole chainring being on the outer placement. Good exemples stem from Bianchi's offering of fixie bikes. The Bianchi "Pista Steel" uses a "FSA Tempo CK-200S" crankset (and the following year, this becomes a "FSA Tempo Track") and the "Pista Sei Giorni Alu" uses a "Truvatic TOURO Track 1.1" crankset. Both crankset are in every possible sense not designed to be used as either a single chainring crankset (despite the Track name used sometimes) or as a crankset with a 42,5mm chainline. Yet, I am sure that neither bike have a bent 42,5mm chainline. If anything both bikes are Proof of Concept that it works.