All tires have a wear indicator. Look on the tire for that indicator between the tread. Once the tire reaches that point they are worn out and need replacing. Another test is to insert a penny with Lincoln's head down. If you can see the top of Lincoln's head they are worn out.
If you're having to put air in your tires frequently then your tires probably need replacing. Also if the tread on the tires is worn down they should be replaced (a symptom of worn tread is the tires squealing on the road for no apparent reason).
Tires are changed during an auto race because the tires get worn down by the track. It is safer to have good tires than to have tires that are worn down that could blow apart and cause accidents.
Generally you replace them every 30,000 miles. You can also tell if the tread is worn down. You can check the depth of the tread with a penny.
Winter tires should be changed out when the tread has been worn down to around 1.5mm to 1.6mm in thickness, after they have been worn down more they are considered worn out. To measure the tread of ones tires one should use a tire gauge which can be bought at websites such as amazon.
the tyres get worn down and it becomes dangerous because the tyres have no gripBadly worn tires can blowout and you can easily loose control of your vehicle.
Tires have molded indicators called "wear bars." If these are visible across the tread pattern, your tires need to be replaced.
Worn tires are worn down to 1.6 mm of their remaining tread depth. They could have bulges, bubbles, slits or holes. For safety reasons they should be changed.
If you are wondering how you can repair your worn out auto tires because your tires went flat, then you should check out Repair Your Own Tires Guide. It is online and free.
You tires are worn out because either the car needs an alignment, you are running them with either too low or too high air pressure, suspension parts are worn, steering parts are worn, or the tires were defective.
Too much camber and the tires aren't aligned right so the tires get worn easily
they are rethreaded
If by tiers you mean rubber, the tread could be worn down so much that they have no grip.