Questions arise about tire sizing. For the recumbents I ride and sell, fortunately, diameters are pretty straightforward; the 26 and 20 inch wheels are well-established standard sizes.
Rim widths are another matter. Questions are bound to arise because recumbent manufacturers frequently supply bikes with narrow rims and relatively wide tires. That's alright as long as the components are high quality and in top condition. But a rider who tries to cheat the system, by using lower quality rims or tires, or not replacing worn out rims or tires -- is risking an equipment malfunction and possible injuries.
Volae Centuries and Tours usually come with Alex DA 16 rims (with roughly a 16mm internal dimension) and generally mount 1.35" tires on them. Though that's pushing the capacity of the rim (in the opinion of a Velocity wheel builder I spoke with at Interbike) with high quality rims and tires (e.g., Alex or Velocity), and high-pressure tires at full pressure, riders are likely to be okay. The NYC Urban Centuries comes with a stronger-than-stock wheel build, with Velocity rims and Schwalbe Marathon tires.
The stock HP Velotechnik Grasshoppers and Street Machines also use Alex DA 16s but mount 1.5" Schwalbe Marathon Racers. Again, this is okay only as long as the rims and tires are in top condition.
If you are thinking about trying different tires, for example mounting Marathon Winters for the icy months, be sure the tires match the capabilities of your rims. In the case of Winters, the main issue will arise when you run the tires at low pressure in order to have maximum traction on icy roads. With the DA 16s, you could be asking for trouble -- the tire may come off the rim while you're riding.
Sheldon Brown has a good page on the subject of matching rims and wheels. It includes an extremely useful chart from cyclist Georg Boeger. This will assist you in making intelligent and safe decisions about fitting tires on your rims.
New York City Recumbent Supply (TM)
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copyright 2009 Robert Matson