The norm of a skew polynomial

Let $D$ be a finite-dimensional division algebra over its center and $R=D[t;\sigma ,\delta ]$ a skew polynomial ring. Under certain assumptions on $\delta$ and $\sigma$, the ring of central quotients $D(t;\sigma,\delta) = {f/g ,|, f \in D[t;\sigma,\delta], g \in C(D[t;\sigma,\delta])}$of$D[t;\sigma,\delta]$isacentralsimplealgebrawithreducednorm$N$. We calculate the norm $N(f)$ for some skew polynomials $f\in R$ and investigate when and how the reducibility of $N(f)$ reflects the reducibility of $f$.