When mountain bikes first crawled off the road and into the ooze there was a huge amount of evolution to adapt to their new habitat, one of the biggest developments was the edition of suspension and although front suspension may have come relatively easily, the implementation of effective rear suspension was a longer process.
This bred an evolutionary stepping stone in the form of the soft tail, which essentially uses the flex of the frame and a damper to provide added compliance. Although the best full-suspension mountain bikes have become the norm in almost every facet of mountain biking, there are still some modern soft tails available.
