That mark at the bottom of the tire groove indicates that the compound is no longer safe and needs to be changed immediately
The tire must always have a minimum groove of 1.6 millimeters. I don’t know if you’ve noticed, but even in Formula 1, it often starts to rain and people rush to change the smooth tires in the pit for others with grooves, which avoid water between the tire and the asphalt.
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Your car’s tires should also have a minimum groove of 1.6 millimeters. And this minimum groove is defined by some rubber bricks that are at the bottom of the groove, when the tire wears out until it reaches it.
This rubber brick is a sign that it has already arrived at the very least. This rubber band is the TWI, initials in English for Groove Wear Indicator.
