In this problem, values for $\mathrm{\tau }$ and $\mathrm{T}$ are given. Since the outside diameter $\mathrm{D}$ is to be determined, neither $\mathrm{c}$ nor $\mathrm{J}$ are known. At first, it may seem that there are too many unknowns since only one equation—the elastic torsion formula—applies here. However, both $\mathrm{c}$ and $\mathrm{J}$ can be expressed in terms of the unknown outside diameter $\mathrm{D}$. After variables $\mathrm{c}$ and $\mathrm{J}$ are reduced to a common variable, the elastic torsion formula can be solved.