I felt that this was going to be an obvious fit, but met some doubters on the Yahoo Xtracycle forum.

To cut to the chase: A front 160mm adapter + 185mm rotor = sweet harmony as a rear brake on a Free Radical. This is all a trial-fit mock up as I wait for parts to complete my build.

<img src="http://www.booboodog.net/images/bikes/xtra/xtra-185-1.jpg">

"For 203mm Rotor"
<img src="http://www.booboodog.net/images/bikes/xtra/xtra-185-2.jpg">

160F = 140R = 185FR
<img src="http://www.booboodog.net/images/bikes/xtra/xtra-185-3.jpg">