I am forwarding to you instructions on how to set up navigation from our developer Radim:
Once you set new navigation, just before start, there is the Settings option. It gives you the possibility to choose from online or offline routing engines and configure them.
Currently, out of the box there are two online routing engines. (Graphopper, Yours). The offline option is coming soon.
Following this link you can install an advanced offline routing engine, the Brouter. It provides more profiles. Some of them do favor unpaved roads (MTB, Cycle short). It is also possible to use community maintained profiles - see the other link too.
Even more profiles for nerds:
Anyway we don't think there is such a profile as 'dirtbike' and using any MTB or cycling optimized profiles on a dirtbike is the rider responsibility. You certainly know this, but we have to say it.
Have a nice day
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