Sitting Bone Pain
Probably the most challenging source of seat discomfort to conquer is painful "sitting bones." As shown in the diagrams, our sitting bones are called the ischium, and are part of our pelvic structure. These bones carry the bulk of our weight when sitting. Some seats are so uncomfortable that the sitting bones can start hurting within 15 minutes of riding. To get an idea of how much the sitting bones, or ischium, are involved in sitting and riding, archeologists exploring ancient Chinese grave sites supposedly can determine ages of cavalry soldiers based on how much their pelvic bones are worn.
The tailbone, or coccyx, is actually the final segment of the vertebral column (see the picture below). It's an important source of attachment for tendons, ligaments and muscles; and with the sitting bones, forms the weight-bearing tripod structure when we're sitting. It's attached to the sacrum by a fibrocartilaginous joint (the sacrococcygeal joint) that allows it to move a bit. Because this joint is flexible, riding on a bike can cause it to move; and if the joint becomes irritated from this motion, this can trigger pain. If you've ever experienced tailbone (coccyx) pain, you know how debilitating it can be. For some folks, it's almost impossible to sit for more than a brief period of time without serious pain, whether on a bike or office chair. More chronic tailbone pain is called coccydynia (check out this site to get more info on this condition and treatments). It's virtually impossible to have much fun riding with coccydynia. Tailbone pain can be a result of injuries, too much sitting, and even childbirth. Typically, it takes care of itself. Believe it or not, the worst case scenarios result in the surgical removal of the tailbone (coccygectomy). I actually spoke with someone who had this done - not a good time.
Notice that guy in the above pic is in a more upright position with his feet kind of forward – a basic cruiser or feet-on-highway pegs position. This position typically causes more pressure to be placed right on the coccyx. For most riders, this isn't a problem. However, for those with tailbone pain, this situation can be agony. Other than medical treatment for this condition, there are some things that you can do to your seat to alleviate this pain.
Rash is a skin irritation caused by perspiration that becomes trapped beneath the skin, which in turn damages surface skin cells. As sweat continues to build up it causes bumps on the skin. When these bumps burst, sweat escapes into adjacent tissue, causing that stinging, uncomfortable feeling. For obvious reasons, hot, humid weather is often a culprit of butt rash. A sweaty, non-breathing riding suit or leaky rainsuit can also cause it. Some seat pads that increase air circulation, undershorts that absorb moisture, and riding pants that breathe can all help prevent rash. Baby powder and similar products (Anti Monkey Butt) can also help.
Chafing is the irritation and soreness caused from too much friction against the skin. Chafing often is caused by rubbing against seams in your underwear or seat for a prolonged period. It can also happen when your thighs rub the edge of a seat that's not properly shaped for the rider. Bicycle shorts can help, as well as those made for motorcycle riders that eliminate seams and anything else that can cause chafing. If the stock seat has seams that cause irritation, a new seat, seat cover, or a seat pad may be the only fix. The good news on this front is that there are increasingly more nice aftermarket seat covers available.