A helical compression spring is wound using $2\mathrm{.}74$ mm$-$diameter music wire. The spring has an outside diameter of $25$ mm with plain ground ends, and $18$ total coils. What should the free length be to ensure that when the spring is compressed solid the torsional stress does not exceed the yield strength, that is$,$ that it is solid$-$safe? Give your answer in $($m$)$