The guys at AZUB say 26" wheels are not as good as 20" ones for trikes because they are less rigid. I thought it was a far-fetched claim until I saw this video of the Atomic Zombie Delta Wolf.
Right at the 0:20 mark, the driver does a violent maneuver to avoid a puddle and it can be seen how the 26" wheels flex. I guess that's why the Hase Kettwiesel has its 20" rear wheels canted outwards: to minimize that flexing.
Now, those are 36-spoke wheels with a fair amount of dish. The hubs were custom made out of solid steel, so they are not feeble lightweight stuff!
AZUB claims they have solved the flex problem by using a thru-axle in their 26" equipped trikes, and Bryan Ball agrees. ( see his review of the AZUB Ti-Fly here:
http://www.bentrideronline.com/?p=10880 )
My question is, how much of an issue wheel flex on a trike really is? And, what are the ways to increase wheel stiffness to these kind of forces which are not present on leaning bikes?