I visited the Jura region last year, camping for a few nights near Doucier before we headed south to Courchevel.
In the market square in Doucier I came across a map of local VTT routes and I did a couple, one out towards Chambly and one round Lac de Chalain.
There was a mix of trails – very little singletrack, mostly fire-road and some farm tracks. Also some sections were on the local back roads.
Navigating around was difficult. The routes are signed but it was easy to miss a sign and take the wrong route. I had a GPS with me which helped, and I knew roughly where I was to be going so getting lost just added to the adventure.
As stated above, they call it VTT in France, and the region’s tourist info site has a page http://www.jura-tourism.com/jura/jura-vtt.html
They also have a Carte des Circuits VTT which lists the routes and distances.
Cycling through the high meadows in Jura was one of my highlights of last year…..
EG