Friends in Köln. Anywhere within circa 50miles for good mountain biking?
Pfalzerwald (although I've not actually been there... yet)
http://www.mtb-park-pfaelzerwald.de/en_index.php oh hello? 🙂
http://www.bikepark-winterberg.de/
They are bike parks, but there are plenty of trails in the areas as well. especialy around Willingen.
Check out http://www.gps-tour.info/ for lots of routes all over Germany, the site has an English option, but the route descriptions are not translated.
Navagation in germany can be difficult as it all forests, and one tree looks pretty mush like all the rest, a gps is advisable if you have access to one.
What's the travel distance limit?
Must be something in the Eifel Hills / Ardennes? which is just SW of Köln. No idea of any bike parks, xc routes, etc. that way.
Ardennes- Good point. It has to be closeish as any true distance would test mrshora's patience.
Nurburgring Nordschleife in Eiffel Mtns has an mtb route around the outside - not great (mostly rough fire road) but quite interesting depending what is running on the track.
There are also lots of waymarked hiking / nordic walking singletrack trails in the centre of the track - maybe scope for some German cheeky trails midweek?...
This may help with some options hora, looks like there is a bikepark near Dortmund but not been there myself so don't know for sure:
[url= http://www.bikeparkmap.de/de/map?PHPSESSID=47cf1f7a45aad747136ef7776e1fc485 ]German riding places map[/url]
Bream- thank you. Welchenberg Trails looks good. 🙂