Just curious what makes a front derailer work with only a specific amount of chainring teeth? Is it the curve of the metal cage? Or is the throw different? Was wondering b/c I was going to buy a cheap one off ebay to use with my compact crank, but many of what I was looking at must've been for mnt bikes as they were maxed at 46 and 48T. Was looking at top pull FD's, they must be more common in mnt bikes I take it. Building a cx bike and need one for that application.