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This photo of the XB-70 Valkyrie's main gear shows a small wheel between the two main wheels:

XB-70 MLG
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?

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    $\begingroup$ Ha! I spotted that on the video that was posted the other day, but was too lazy to ask. Thanks! $\endgroup$
    – Jörg W Mittag
    Commented Apr 26, 2016 at 0:58

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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):

enter image description here

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    $\begingroup$ Wow! A quick and authoritative answer! Did you know this or is your Google-fu that good? $\endgroup$
    – FreeMan
    Commented Apr 25, 2016 at 16:54
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    $\begingroup$ Call it a lucky Google. $\endgroup$
    – Fred Larson
    Commented Apr 25, 2016 at 16:56
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    $\begingroup$ Nice "lucky Google", then Fred! I'm sure I could have done the same, but it's good to have information gathered here on what's quickly become my favorite web-site. Thanks for the image embed, @mikeSowsun $\endgroup$
    – FreeMan
    Commented Apr 25, 2016 at 17:31

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