How is the best way to tell that your tire nobbies are too worn down?

 

By looking at them.
By riding with them.
By feeling them.

 

Worn out is a matter of opinion. If all you do is trail ride, then your tires are worn out when you no longer get the grip you need to ride in whatever conditions you ride in. For example, if you mainly ride on dry trails with little or no mud then a set of tires that are 50% worn off may still be fine. On the other hand if you ride in a muddy area, that same set of tires that are 50% worn will not give you the same amount of traction as a new set or a set with less wear on them.
If you're racing, then the money factor comes into play. A new set of meats is always going to out "grab" a set with "balled" ****. Meaning, the **** no longer have a sharp edge on them to cut into the track. However, not every weekend racer can afford a new set of tires for each week. The best advice i got from a friend of mine that is now a pro racer is to run your tires for at least two events, then go out and by a new set and see if you can tell a differance. If not, put the old ones back on for another event. Then change back to the newer ones and see if you can tell a differance. Continue this until you can feel a differance in you quads ability to grab the track surface. That will tell you when you need to get a new set.
I sure someone is going to respond and say that you should replace them every so many events. But this is what I've learned to do and it works out that at the places I run, I get about 3 events out of a set of tires. If I put a new set on each week, would I be faster, maybe, but I know for certain that if I go more than 3 events, I'll be noticably slower. Not to mention I'll have a little more cash in my wallet.

 

thank you for the advice i will use it.