Why doesn't Blender render the the geometry I've created properly I have a model that consists of the following:


A tire tread: A polygon plane with an array modifer and a curve modifier to bend it into the shape of a wheel and a mirror modifier to mirror it from one side of the tire to the other
A wheel hub (a rim): A polygon plane with a mirror modifier on it and an array modifier set to object with a null rotated to create the other parts of the wheel.
1 &amp; 2 are grouped and instanced 3x to create the other wheels
A simple polygonal box cut in half and mirrored for the body of the car


The model is obviously not done, but why isn't it rendering properly?
Well this is an interesting one. It seams blender displays DupliGrouped objects incorrectly, and renders them correctly.

You made one wheel, using a Curve modifier and a BezierCircle to form the tread of the wheel. Then you distributed the wheel group to the other three places using DupliGroups.

The trouble is you did not group the BezierCircle (you had three and the one being used in the modifier is not in the group), so when you distribute the
wheel around the Curve modifier on the tread is still pointing to the first BezierCircle. Which because the origin of the curve is now in a very different spot then the wheel object, the curve modifier is making a mess instead of a tire.

Simply add the BezierCircle.001 object to your group named "Group" and all the wheels will render correctly.
To add it to the group select the BezierCircle.001, then in  the object tab of the properties panel scroll down to the Groups section and press the large Add to Group button. Choose your group named "Group" from the list.