Pinewood Derby Wheels Inner Bore Flaws

Part of the injection molding process is the EJECTION portion of the process of the wheel.  This is where the void space that is the inner hub is subjected to some forces while the wheel is still cooling from the liquid plastic.  Mass production flaws that might go unnoticed to the once a year racer.    I also have a feeling this is also where wheel wobble is produced.  The failure of the center bore staying perpendicular to the outer wheel tread.  I believe as the mold is cooling it sags a  bit and this is the cause of the alignment issue.

There are failures that happen that need to be sorted before we spend any more time on our wheels.  After all, we want to only start with the very best wheels that we can use.  The internal flaws can cause binding, unnecessary wobble at high speeds.  We want our bore to be smooth, error free.  Some companies ream the inner bore, but I have found that reaming it larger does not help it much.

Using angled lighting, the inner bore needs to be inspected for flaws.  Rotate the wheel a full 360 degrees, then flip around and look at the bore the other way.  There is a flawed lip towards the inner most part of the wheel that sometimes goes unnoticed.