Hardtail mountain bikes feature a front suspension fork that is mounted to a rigid, or unsuspended, frame. Soft tail mountain bikes are a form of full-suspension mountain bike but with less suspension travel than traditional full-suspension designs. Instead of using multiple linkages or a single pivot, soft tail bikes use the natural flex of the chainstays to deliver wheel travel. The soft tail design uses a shock to provide different levels of damping, but the movement is provided by natural frame flex as opposed to a swing-arm or intricately designed linkage system.
A hardtail bike is only as forgiving as the material that comprises its frame. There is no suspension attached to the rear-wheel, so it is important to choose a resilient material if you are looking for anything resembling a supple ride. Aluminum is the least forgiving frame material and adds a lot of jolt to your riding experience, while carbon fiber, steel and titanium are more forgiving over sharp edges and offer slight flex. A soft tail mountain bike generally offers an inch of travel, far less than a true full-suspension bike which tends to provide three to four inches and sometimes as many as eight inches of suspension travel. Soft tails do take more of the edge off than hardtails, providing comfort and fighting fatigue, but they hardly open up new trail capability.
A hardtail frame requires no maintenance although the components mounted onto it do require upkeep. A soft tail frame requires service to its rear shock at regular intervals which is an added expense and concern in a head-to-head comparison of the two frame styles. The maintenance required on a soft tail is still less than the service for a true full-suspension ride since soft tails lack the bushings and linkages of full-suspension frames.
Virtually all mountain bike manufacturers offer hardtail mountain bikes. A hardtail mountain bike does not require the excessive engineering, research and development costs associated with a soft tail frame. Soft tail frames are a limited market since most riders will prefer more suspension travel than the pivotless suspension frames offer if they are going to make the jump to a full-suspension bike. Soft tail bikes are generally regarded as a niche market with companies like Trek, KHS, Litespeed and Moots offering the frame style.
A soft tail frame will cost more than a comparably equipped hardtail frame due to the added expense of the suspension system. However, it may be impossible to buy a soft tail at certain price points. Due to the niche nature of the frame style, it has remained a staple of the high-end mountain bike market with few models in the mid-level price range and not being offered at all on entry-level bikes.