Another good idea is to track is the number heat cycles your tires experience. Each time your tires get hot they release chemicals from deep within the rubber. As the chemicals are released the tires lose some of their grip. You can see tires that are hardly worn lose their grip due to heat cycles. Count the number of heat cycles that your tires go through and you may find a repeatable pattern that lets you know when to get that new set.
Bearings fail when the oil film breaks down or when the bearing is overloaded.
At rest, the shaft and bearing are in contact. On start up the shaft rubs the bearing briefly. Running, the shaft pulls oil from the clearance space into the wedge shape area between the shaft and bearing. The oil wedge lifts the shaft off its bearing and supports it during engine operation. With normal operating conditions and a continuous supply of clean oil the shaft and bearing surfaces will remain separated.