Once upon a time, chainrings were only a little complicated. You typically had 4 or 5 holes of some common measurement apart. Teeth were approximately the same and if they fit, they were fine. But with newer cranksets and chainrings, there are more options, and I see people get confused. I am sure there are some more systems out there than I'm listing, but here are a few: Conventional Bolted Rings These have been used for decades. The chainrings use some number of bolts to mount to the spider of the crankarm. The critical dimensions are the number of holes and the diameter of a circle that passes through them all. If your old one is not labeled (or the crank isn't) you need to measure and that can be complicated. Measure BCD on 4 Bolt (Source: Wolftooth) 4-bolt patterns are easy, whether symmetrical or not. Just measure across an opposing pair. In theory, you're measuring the centers of the holes, but edges on the same side is usually easier. That distance IS your BCD. 5-...
Writings on experiments building, maintaining, upgrading bikes with various parts ranging from vintage to modern electronic