Tires: Rolling Circumference vs Tire Circumference, tire circumference, circumferences

Question
Barry,

I read your answer to a question about rolling circumference of tires in February of 2005, and am curious as to just how the circumferences differ.

On the surface of the issue, it would appear that even if the tire is deflected due to load, all of the tread must come into contact with the ground during a revolution. If this were so, the rolling circumference would be the same as the tire circumference.

The only way I can imagine that this would not be the case is if the tire contact patch has a 'bubble' in which the center of the patch is not in contact with the road surface. If this were the case, it is conceivable that the rolling circumference would change with the load on the tire.

What's really going on with the rolling circumference vs tire circumference?

Thanks,

Andrew

Answer
Andrew,

The way I think about it is like a tank track with rubber pads - and the belt is the linked portions of the track.  No matter how you distort a tank track, it's length will always be the same as measured from link to link - and it will be shorter than the distance around the outside of the track (which includes the tread portion.