Axle boots / Constant-velocity Joint boots are used to protect the Constant-velocity joints. This is usually filled with molybdenum disulfide grease. Cracks and splits in the boot will allow contaminants in, which would cause the joint to wear quickly as grease leaks out. Wear of the boot often takes the form of small cracks, which appear closer to the wheel, because the wheel produces most of the vibration and up and down motions. Cracks and tears in the areas closer to the axle are usually caused by external factors, such as packed snow, stones or uneven rocky off-road paths. Aging and chemical damage can also cause boot failure.