All of them are made in standard sizes, the most common have a distance between the axes of 170 and 175 mm. The connecting rods always come complete with sprockets, which are collapsible and non-collapsible. The former should be bought if you mostly ride in the same gear and only occasionally switch to others. Then, as you wear out, you can always replace the star you are most familiar with. If you want to buy a part and forget about it forever, it is better to take a non-separable connecting rod. For athletes, a connecting rod with two stars is more suitable, and for owners of bicycles with an urban bias, one star will be enough. The body of the connecting rod arm itself can be made of different materials: steel, aluminum or carbon. Modern technology has reduced to zero the likelihood of connecting rods breakage from metal fatigue, but if the aluminum part is bent as a result of any impact, then it must be replaced, since alignment can lead to an emergency situation later.