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.
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.
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
|76mm - asymmetrical
|49.0mm; 62.4mm; 53.9mm
|1.93in, 2.45in; 2.12in
|96mm - symmetrical
|96mm - asymmetrical
|110mm Shimano - asymmetrical
|110mm SRAM - asymmetrical
5 Bolt Chainrings - Measuring 2 adjacent bolts
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.
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.
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.
- 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.
- 11 speed Shimano (so far DuraAce and Ultegra) has a non standard arm spacing. We do not currently offer a solution for this application.
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.