Using QGIS with GRASS I have found a route through r.drain but it has many sharp curve. the maximum radius of curve allowed to me is 176m. Is there any to reduce the curve sharpness to 176m or at least to find the radius of a curve?
As a general hint, you can search functionality in the GRASS GIS documentation with g.search.modules.
To your question: maybe the "sinuous: line sinuousity, calculated as line length / distance between end points" functionality of v.to.db may help to approximate this value.
Overall, to reduce the curve sharpness, you can generalize the vector lines with v.generalize.