Tire rubber wear indicator.
https://s3-us-west-1.amazonaws.com/h..._indicator.jpg
Previously:
https://www.homemadetools.net/forum/...614#post143339
https://www.homemadetools.net/forum/...492#post127847
https://www.homemadetools.net/forum/...034#post129868
https://www.homemadetools.net/forum/...532#post128084
https://www.homemadetools.net/forum/...150#post126075
https://www.homemadetools.net/forum/...176#post149781
