Get a tire gauge from the auto parts store. Your manual will tell you the minimum tread depth.

For instance - the 97 F3 is 2/32nds on the front and 3/32nds on the rear. The tread depth on a new tire is about 6/32nds.

 

Are tires bad if there are many cracks in the rubber in/around the grooves?

 

I have been told by several shops that the tires are indeed bad. The cracks mean the rubber has gone bad and lost the oil an is rotting. (dry rot) Both my front and back tires were like this on my bike when I purchased. I went out and bought a set of Pilot Road tires and they have a ton of tread depth to work with before the wear indicators show. Plus I don't take the bike to the track and on the streets around here I won't be making any turns at 50mph to need sticky tires.

 

There are many indicators that tells you when to change your tire.

Tread (or lack thereof) is the most obvious. The short of it is don't wait until your tires are bald...much less have corded spots. Cords = no traction. Enough said. Your tires were never designed to be run while bald...don't let it get there. Technically what denhaou said is true and the ideal way to do it. I don't really measure though...when the tread pattern gets to be more of a suggested design, it's time to change, imo.

Now one thing that many people don't think of is the hardness of the tires and/or dry rot. Tires can keep their tread for a very long time, especially if you don't ride very many miles each year. The tires on my F4i when i bought it were about four years old (you can tell when they were manufactured by the number molded in the sidewall - four digits with the first two being the week and the last two the year), but they still had a good 1/2 tread left on them. They even warmed up ok and were fairly sticky...so i didn't *have* to replace them, but i did and it made a world of difference. They were so flat spotted...

Anyways, my point is that an old tire can get so hard that it needs replacement even if it has a decent amount of tread left. And how do you know when to replace it? Well, you'll just have to make that call on your own. If there are visible cracks, replace it for sure...the rubber's breaking down. Other than that, well...how safe do they make you feel?

 

when they get slippery...

 

I change mine when it hits the wear indicators, I'm not going to dick around with a real safety issue for a few bucks. Besides, when I ride I want to be in a good mental state (rather than a state mental) & not worry about tyres (as in grip), I'm even **** enough to check the tyre pressures before I ride (I have a digital tyre gauge key ring) Plus I really enjoy scrubbing in a new tyre.

 

I change them once the tire gets to the tread wear bars. $125 bucks for a rear tire is NOTHING when it comes to what could happen if the tire were to fail.

 

I change my tires before the tread shows. I have had a few tires at the wear limits start to bubble and I've had one tire blow on acceleration... they just aren't made to be worn past the wear limits and certainly not on the thread. I have limped home on a tire showing bubbles and thread... but I can tell you that I was using next to no throttle and no rear brake. I took a long time to get home.

I change tires if they are 3 years old - even if they aren't cracking. On the flip side I've never had a tire I've installed last 3 years.