If your vehicle is showing bald tires with tread wear indicator bars showing, here are the most likely causes and the parts you'll need to fix each one.
Tires naturally wear down over thousands of miles of driving. When the tread depth reaches 2/32 of an inch, the built-in wear indicator bars become flush with the tread, signaling the tire is completely worn out and unsafe.
If the vehicle's alignment is out of specification (camber or toe), the tires will scrub against the pavement rather than rolling smoothly. This causes rapid, premature baldness, usually concentrated on the inner or outer edge of the tire.
The drive wheels (front wheels on a FWD car, rear on a RWD) wear tires much faster than free-rolling wheels. Failing to rotate the tires regularly will result in two tires becoming completely bald while the other two still have usable tread.
Driving with overinflated tires causes the center strip of the tread to wear down to the indicator bars prematurely. Conversely, chronic underinflation causes both outer shoulders of the tire to go bald while the center remains intact.
Worn out shocks, struts, ball joints, or tie rods allow the wheel to bounce or shift dynamically while driving. This leads to accelerated, patchy bald spots (cupping) or rapid edge wear.