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1x Drivetrains Explained - Gearing, Mechanics, and Optimization

Posted by Chelsea Strate ·

Wolf Tooth CAMO with elliptical chainring installed on bike
The simplicity and effectiveness of a 1x (pronounced "one-by") drivetrain, which refers to a single front chainring with multiple sprockets and a rear derailleur, has many people converting their multi-chainring setups.  At Wolf Tooth, we were at the forefront of this 1x revolution and wanted to share a summary of the most important points for 1x gearing, mechanics, and optimization.

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How to measure Bolt Circle Diameter (BCD)

Posted by Wolf Tooth ·

How to measure Bolt Circle Diameter (BCD)

Bolt Circle Diameter (BCD) is the diameter of the circle that goes through the center of all of the bolts on your chainring. On bicycle chainrings this dimension is usually measured in millimeters. It is critical to know the BCD of your crankset when you are selecting a new chainring for your bike. In many cases the BCD is printed right on the chainring like in the example below.  Sometimes it is stamped or engraved on the back side of the chainring.

Three Wolf Tooth Chainrings

If it is not labeled on your chainring you will need to measure it. On a chainring with 4 bolts the BCD is the distance between two bolts across from each other. It can be somewhat difficult to make this measurement if you don't have a caliper since the crank arm gets in the way. The image below shows this measurement on a 104mm BCD chainring.

Caliper measuring distance between opposing bolts on chainring on bicycle

Alternatively you can measure the distance between two adjacent bolts and use the table below to determine the BCD. This is the easiest method to use for chainrings with five bolts.  Note that there are a few crank sets that have non-standard arm spacing. If this is the case then the distance between adjacent bolts will be different as you move around the chainring and standard chainrings will not fit.

4 Bolt Chainrings - Measuring 2 adjacent bolts

BCD        Distance (mm) Distance (in)
64mm 45.3mm 1.78in
76mm - asymmetrical 49.0mm; 62.4mm; 53.9mm 1.93in, 2.45in; 2.12in
88mm 62.2mm 2.45in
94mm 66.5mm 2.62in
96mm - symmetrical 67.9mm 2.67in
96mm - asymmetrical		 55.2mm; 78.8mm 2.17in; 3.10in
102mm 72.1mm 2.84in
104mm 73.5mm 2.90in
110mm Shimano - asymmetrical 63.6mm; 90.6mm 2.50in; 3.56in
110mm SRAM - asymmetrical		 64.8mm; 89.3mm 2.55in; 2.51in
120m 84.9mm 3.34in

 

5 Bolt Chainrings - Measuring 2 adjacent bolts

BCD Distance (mm) Distance (in)
94mm 55.2mm 2.18in
110mm 64.7mm 2.55in
130mm 76.4mm  3.01in

 

A caliper measuring the distance between two adjacent bolts on a chainring on a bicycle
 

The equation for the BCD for any symmetric bolt pattern is given by:  BCD = d / sin(180/n)

where d is the center to center distance between adjacent bolts, n is the number of bolts, and the angle is in degrees.

BCD Guide

Another method is to remove the chainring from your bike and place it on a printout of the BCD guide in the link below. Make sure to print it out in the actual size with no scaling. Then just find the circle pattern that lines up with the holes on your chainring. 

Measure BCD Bolt Circle Diameter

 

Exceptions

 There are always exceptions.  Be aware of a few specific examples:

  • For SRAM X7 (new version), X9, XO, XO1, XX1 our solution is the direct mount rings. These rings on these cranksets are 94 BCD. With our direct mount solution you remove the original chainring and spider and replace it with our ring that interfaces directly with the splines on the crankset. You will need a T25 Torx L-wrench to remove the three bolts that secure the chainring to the crankarm.
 
Backside of a direct mount chainring installed on bicycle cranks
  
  • SRAM Red Exogram has a hidden bolt that screws into the crank arm. The chainring bolt may be too long to firmly secure the chainring so you will need to add a washer (see photo) or carefully file down a bolt to the proper length.
 
SRAM Red Exogram bicycle cranks and chainring                 Hidden chainring bolt on SRAM Exogram cranks
 
  • 11 speed Shimano (so far DuraAce and Ultegra) has a non standard arm spacing. We do not currently offer a solution for this application.

Direct Mount Crank Interface

Chainring Diameter

Some bicycle frames have an upper limit for the maximum chainring size you can use due to clearance between the chainring and chainstay. We created a separate page with information on Chainring Diameter by Tooth Count that will help you determine how large of chainring will fit on your bike.

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