What is the purpose of the extra wheel on the XB-70 main gear? This photo of the XB-70 Valkyrie's main gear shows a small wheel between the two main wheels:

Courtsey of AirPigz.com

This smaller wheel doesn't touch the ground, though I can see where it might as the side walls of the main tires compress under landing weight. 

The AirPigz site promised an explanation, but none has been forthcoming (in the 4+ years since that was posted), and I couldn't find any information on it on Wikipedia, either. Does anyone have any information (real or speculative) on what it's for?
It is a sensing wheel that is part of the antiskid braking system. It contains an electronic sensor to measure the true groundspeed of the airplane. See NASA Technical Note D-4836, Landing Loads and Accelerations of the XB-70-1 Airplane (1968), bottom of page 3 of the document (page 5 on the PDF):