Kidney Belts Kidney belts are among the most frequently bought items of clothing for bikers. Their main purpose is to protect the sensitive area around your kidneys against wind-chill on your motorbike. A kidney belt also provides support and relieves strain on the lower back muscles on long tours. The Louis kidney belt range includes many different shapes and materials, reflecting varying emphasis in their function. The following basic rules apply to all models: The kidney belt should be worn over your base layer and under your motorcycle jacket. That way it can do its job most efficiently.