I ordered the HOpur wheelset for Roco's V100 Dieselengine after thinking twice about the price of 114, - DM (appr US$75). Three weeks later the set arrived. Wheelsets were completely assembled and adjusted.
The wheel flanges are 0.3 mm (.012") high, back to back distance is 15.6 mm (.614") and the wheels are extremely close to the prototype. It was easy to change wheels and to adjust the springs taking the electric power off the wheels.
First I let the diesel run on a shelf, rails are prefabricated Roco-line. No doubt the engine runs fine, as far as speed is prototypical and not too high. I was very astonished to see the engine round 50 cm (19.7") radius curves with ease.
Also normal HO turnouts aren't a problem provided they are aligned for the straight route. On special turnouts for HOpur wheels the engine runs fine.
However, some turnouts are too imprecise to guide the fine wheels safely. The frogs are not so much the problem as the tips of the switching rails.
After the initial tests, I tried the locomotive on a big layout with flextrack and I found out that the machine doesn't like a track with a gauge lower than 16.5 mm (.649") and the rails sholdn't be twisted too much. Also, with a heavy train in a steep curve the machine derailed at the inner side of the curve.
After some tests the HOpur equipped diesel had some trouble at places it hadn't before: dirt had built up some 0.1 mm (.004") on the wheels and there was only 0.2 mm (.008") flange depth left. I cleaned wheels and rails and it was fine again.
When I check German Federal Railway law I found out that the flange depth may be from 0.3 mm (.012") to 0.44 mm (.017") in HO scale. So 0.3 mm is really the lowest value and there might be 0.14 mm (.005") more.
My personal experience is that 10 wheeler steam engines (Roco BR 57) run as well as the V100 diesel when equipped with wheels of 0.44 mm flangedepth. So my personal solution is, that I shall build my layout according to Proto:87 dimensions having an eye to HO pur driving on it. All wheelsets should have Proto:87 or HOpur dimensions, but on flangedepth as the most critical dimension they are allowed to have 0.1 mm (.004") more. That is prototypically correct.
To summarize, the HOpur wheelsets for the Roco V100 are nice but expensive.
