The largest commercially available Z-Scale curve has a 270 mm radius-- and for some of our layouts that is perfectly fine. But we also make layouts where we want, on a 10' 6" long layout, to run trains with up to 25 cars. And, for that, we think 270 mm will fail.
 
Why will it fail? Our experience is that, IF a train has  any kind of "slinky" effect as it goes into and then out of a curve, you should expect to see the back cars bouncing at least a bit back and forth (which is factor even for real railroads and is commonly known as the "slinky effect. And, in Z-Scale, if you have the slightest bit of "slinky," you should expect decouplings or derailments.  Our experience is that you can run SHORTER TRAINS on a 270 mm radius-- but we believe that it is impossible to extra long trains without ever having a problem (especially in MictroTrains or AZL).
 
     THE SOLUTION: OUR CUSTOM 3-D PRINTED ROADBEDS THAT TRANSITION FROM ROKUHAN TO THE 3-D CURVE AND BACK TO ROKUHAN.
 
     We only make them in 90 degree segments -- for a 180 curve, that means you would need to buy TWO of them.
 
    They are not cheap -- but we think we have priced these at a bargain level compared to YOU trying to make your own 320 mm curves.