[jspatz1]

Unlike timing belts, roller chain tends to grow slightly in length as it "breaks in". This is due to normal wear in the hundreds of axial joints present in the chain. Due to this, it is really not possible to machine the "perfect" center-to-center distance between sprockets. For short chains, you can arrive at a spacing that will work, and simply be somewhat more slack later in life than it was initially. Short chains can accomodate some slack without whipping or tracking probems. For most chains of typical length however, some tensioning method is needed to accomodate variations and growth in chain length. This can be adjustable C to C length, or an idler or tensioning glide that deflects excess slack in the chain.