Don’t just replace the bad tire, front or back. Always put the new tire ont he front while rotating the in-use front tire to the back. This keeps the best rubber up front where you need it for control. But don’t take my word for it. Read this article–Corrie
Okay, let’s say your bike needs a new tire. Maybe it has a puncture or a gash in the sidewall. Maybe it has become so worn that the threads underneath are showing through and it clearly needs to be replaced.